Effect of Relative Humidity and CO 2 Concentration on the Properties of Carbonated Reactive MgO Cement Based Materials

Transcription

1 Effect of Relative Humidity and CO 2 Concentration on the Properties of Carbonated Reactive MgO Cement Based Materials by Yaroslav Bilan A thesis submitted in conformity with the requirements for the degree of Master of Science Graduate Department of Civil Engineering University of Toronto Copyright by Yaroslav Bilan (2014)

2 Effect of Relative Humidity and CO 2 Concentration on the Properties of Reactive MgO Cement Based Materials Yaroslav Bilan Master of Applied Science Graduate Department of Civil Engineering University of Toronto 2014 Abstract Sustainability of modern concrete industry recently has become an important topic of scientific discussion, and consequently there is an effort to study the potential of the emerging new supplementary materials. This study has a purpose to investigate the effect of reactive magnesia (reactive MgO) as a replacement for general use (GU) Portland Cements and the effect of environmental factors (CO2 concentrations and relative humidity) on accelerated carbonation curing results, The findings of this study revealed that improvement of physical properties is related directly to the increase in CO2 concentrations and inversely to the increase in relative humidity and also depend much on %MgO in the mixture. The conclusions of this study helped to clarify the effect of variable environmental factors and the material replacement range on carbonation of reactive magnesia concrete materials, as well as providing an assessment of the optimal conditions for the effective usage of the material. ii

3 Acknowledgments This important study would have not be possible without the financial support provided by Ministry of Economic Development and Innovation for support by Professor Daman Panesar's Early Researcher Award (ERA) which provided the funding for this research. In addition to this, the author of this study wants to express his most sincere appreciation to the support, advice and guidance of Professor Daman Panesar, who was a research supervisor for this study. The council of other Concrete Material Group Professors, namely Prof. Doug Hooton, and Karl Peterson has been also immensely important for the successful outcome of this research. The support of the whole Concrete Materials Group was surely significant and is much appreciated. Finally the author wants to thank deeply from his heart to the selfless and dedicated support and care from Olga Perebatova. Without a doubt, this study would not be possible without her critical help during the long and hard work in a laboratories when conducting the tests for this study. iii

4 Table of Contents Effect of Relative Humidity and CO 2 Concentration on the Properties of Reactive MgO Cement Based Materials i Abstract Acknowledgements Table of contents List of Figures List of Tables ii iii iv vii ix 1.Introduction Motivation for Study Research Objectives 4 2. Literature Review Conventional Cement-Based Materials and Carbonation Reactive Magnesia Cements Hydration, Chemical Composition and Physical Properties Mechanics of Carbonation of Reactive MgO as Cement Replacement and the Effect on Compressive Strength, Porosity and Durability Properties of the Material Competition Between Hydration, Carbonation and Influence of Environmental and Material Conditions 15 3.Experimental Program Materials and Mix Design Sample Preparation and Curing Testing Procedure Sample Preparation and Drying Procedures Carbonation Front Compressive Strength 21 iv

5 3.3.4 Chemical Composition Tests Mercury Intrusion Porosimetry (MIP) Results and Discussion Chemical Analysis XRD, DT and TGA Carbonation Front X-ray Diffraction Results DTA/TG Results Mechanical Properties Effect of Reactive MgO on Compressive Strength Strength-Porosity Correlation Effect of CO 2 on Compressive Strength Effect of Relative Humidity on Compressive Strength Porosity Interplay between Chemical Analysis and Mechanical Properties Interplay between Chemical and Physical properties and the Effect of CO 2 Concentration Compressive strength Porosity Carbonation front Interplay between Chemical and Physical properties and the Effect of Relative Humidity Compressive Strength Carbonation and Chemistry of the Carbonation Zone Carbonation and Porosity Conclusions 79 v

6 7. Recommendation for Future Research Appendix References 96 vi

7 List of Figures Figure 2.1: Compressive Strength- Total Porosity Relationship for Conventional Concrete 9 Figure 2.2: Compressive Strength- Total Porosity Relationship for Conventional Paste and Mortar 10 Figure 2.3: Compressive Strength- Permeability Relationship for Conventional Concrete 10 Figure 2.4: Compressive Strength- Total Porosity Relationship for Carbonated Concrete 11 Figure 4.1: Condition [75%RH, 50%CO 2 ] XRD Pattern of Mortar a) 3d, b) 28d 32 Figure 4.2: Condition [75%RH, 50%CO 2 ] XRD Pattern of Paste a) 3d, b) 28d 33 Figure 4.3: Condition [75%RH, 75%CO 2 ] XRD Pattern of Paste a) 3d, b) 28d 34 Figure 4.4: Condition [75%RH, 99%CO 2 ] XRD Pattern of Paste a) 3d, b) 28d 35 Figure 4.5: Condition: [75%RH, 99%CO 2 ] Influence of MgO on DT at Day 3 38 Figure 4.6: Condition: [75%RH, 99%CO 2 ] Influence of MgO on DT at Day Figure 4.7: Condition: [75%RH, 99%CO 2 ] Influence of MgO on TGA at Day 3 39 Figure 4.8: Condition: [75%RH, 99%CO 2 ] Influence of MgO on TGA at Day Figure 4.9: Condition: [75%RH, 50%CO 2 ] Effect of MgO on 3, 7, and 28d compressive strength 45 Figure 4.10: Condition: [75%RH, 75%CO 2 ] Effect of MgO on 3, 7, and 28d compressive strength 45 Figure 4.11: Condition: [75%RH, 99%CO 2 ] Effect of MgO on 3, 7, and 28d compressive strength 46 Figure 4.12: Condition: [50%RH, 50%CO 2 ] Effect of MgO on 3, 7, and 28d compressive strength 46 Figure 4.13: Condition: [50%RH, 75%CO 2 ] Effect of MgO on 3, 7, and 28d compressive strength 47 vii

8 Figure 4.14: Condition: [50%RH, 99%CO 2 ] Effect of MgO on 3, 7, and 28d compressive strength 47 Figure 4.19: Correlation between Compressive Strength and Total Porosity for 0%MgO mortar 49 Figure 4.20: Correlation between Compressive Strength and Total Porosity for 20%MgO mortar 51 Figure 4.21: Correlation between Compressive Strength and Total Porosity for 40%MgO mortar 52 Figure 4.22: Correlation between Compressive Strength and Total Porosity for 60%MgO mortar 52 Figure 4.23: Effect of CO 2 Concentration on Compressive Strength Development of 0%MgO mortar, a) Relative Humidity - 75%, b) Relative Humidity - 50% 52 Figure 4.24: Effect of CO 2 Concentration on Compressive Strength Development of 20%MgO mortar, a) Relative Humidity - 75%, b) Relative Humidity - 50% 53 Figure 4.25: Effect of CO 2 Concentration on Compressive Strength Development of 40%MgO mortar, a) Relative Humidity - 75%, b) Relative Humidity - 50% 54 Figure 4.26: Effect of CO 2 Concentration on Compressive Strength Development of 60%MgO mortar, a) Relative Humidity - 75%, b) Relative Humidity - 50% 55 Figure 5.1: Correlation between Compressive Strength and Carbonated Area for 0%MgO mortar 62 Figure 5.2: Correlation between Compressive Strength and Carbonated Area for 20%MgO mortar 63 Figure 5.3: Correlation between Compressive Strength and Carbonated Area for 40%MgO mortar 63 Figure 5.4: Correlation between Compressive Strength and Carbonated Area for 60%MgO mortar 64 Figure B.1: Condition: [75%RH, 50%CO 2 ] Effect of MgO on 3, 7, and 28d carbonated area 76 Figure B.2: Condition: [75%RH, 75%CO 2 ] Effect of MgO on 3, 7, and 28d carbonated area 76 viii

9 Figure B.3: Condition: [75%RH, 99%CO 2 ] Effect of MgO on 3, 7, and 28d carbonated area 77 Figure B.4: Condition: [50%RH, 50%CO 2 ] Effect of MgO on 3, 7, and 28d carbonated area 77 Figure B.5: Condition: [50%RH, 75%CO 2 ] Effect of MgO on 3, 7, and 28d carbonated area 78 Figure B.6: Condition: [50%RH, 99%CO 2 ] Effect of MgO on 3, 7, and 28d carbonated area 78 Figure C.1: Condition: [75%RH, 50%CO 2 ] Effect of MgO on 3, 7, and 28d total porosity 79 Figure C.2: Condition: [75%RH, 75%CO 2 ] Effect of MgO on 3, 7, and 28d total porosity 79 Figure C.3: Condition: [75%RH, 99%CO 2 ] Effect of MgO on 3, 7, and 28d total porosity 80 Figure C.4: Condition: [50%RH, 50%CO 2 ] Effect of MgO on 3, 7, and 28d total porosity 80 Figure C.5: Condition: [50%RH, 75%CO 2 ] Effect of MgO on 3, 7, and 28d total porosity 81 Figure C.6: Condition: [50%RH, 99%CO 2 ] Effect of MgO on 3, 7, and 28d total porosity 81 Figure D.1: Condition [50%RH, 50%CO 2 ] XRD Pattern of Paste a) 3d, b) 28d 84 Figure D.2: Condition [50%RH, 75%CO 2 ] XRD Pattern of Paste a) 3d, b) 28d 85 Figure D.3: Condition [50%RH, 99%CO 2 ] XRD Pattern of Paste a) 3d, b) 28d 86 Figure D.4: Condition: [50%RH, 50%CO 2 ] Influence of MgO on TGA at a) Day 3, b) Day Figure D.5: Condition: [50%RH, 75%CO 2 ] Influence of MgO on TGA at a) Day 3, b) Day Figure D.6: Condition: [50%RH, 99%CO 2 ] Influence of MgO on TGA at a) Day 3, b) Day ix

10 Figure E.1: Effect of Relative Humidity on Compressive for Mixture with 0% of MgO a) 50% CO 2, b) 75% CO 2, c) 99% CO 2 91 Figure E.2: Effect of Relative Humidity on Compressive for Mixture with 20% of MgO a) 50% CO 2, b) 75% CO 2, c) 99% CO 2 92 Figure E.3: Effect of Relative Humidity on Compressive for Mixture with 40% of MgO a) 50% CO 2, b) 75% CO 2, c) 99% CO 2 94 Figure E.4: Effect of Relative Humidity on Compressive for Mixture with 60% of MgO a) 50% CO 2, b) 75% CO 2, c) 99% CO 2 95 x

11 List of Tables Table 3.1: Chemical Composition of Cementitious Materials...18 Table 3.2: Mortar and Paste Mixture Design Proportions...19 Table 3.3: Carbonation Curing Scenarios...20 Table 4.1: Carbonated Area of Specimens cured in 75%RH...23 Table 4.2: Carbonated Area of Specomes cured in 50%RH...24 Table 4.3: X-ray Diffraction Main Peaks Data...26 Table 4.4: Influence of Age, Percentage of MgO and CO 2 Concentration on X-ray Diffraction Results...27 Table 4.5: DTA/TG data for Separate Peaks, Condition [75%RH, 99%CO2]...38 Table 4.6: Mean Compressive Strength (f'c) and Coefficient of Variation (COV) for 75% RH...42 Table 4.7: Mean Compressive Strength (f'c) and Coefficient of Variation (COV) for 50% RH...43 Table 4.8: Percentage Change in Compressive Strength Due to MgO used as Cement Replacement...44 Table 4.9: Percentage Change in Compressive Strength Due to MgO used as Cement Replacement...57 Table 4.10: Total Paste Intruded Porosity...58 Table 4.11: Percentage Change in Total Mortar Porosity Due to MgO used as Cement Replacement...59 Table 4.12: Percentage Change in Total Paste Porosity Due to MgO used as Cement Replacement...60 Table A.1: Percentage Change in Compressive Strength Due to MgO used as Cement Replacement Compared to 0%MgO Mixes...74 Table A.2: Percentage Change in Compressive Strength Due to MgO used as Cement Replacement (Incremental)...75 Table C.1: Percentage Change in Total Mortar Porosity Due to MgO used as Cement Replacement (Incremental)...82 xi

12 Table C.2: Percentage Change in Total Paste Porosity Due to MgO used as Cement Replacement (Incremental)...83 xii

13 1. Introduction Reactive magnesia (r-mgo) is a relatively new binder type that has cementitious properties, similarly to Portland Cement (PC) under certain conditions. This new binder can potentially partially replace ordinary PC cements, which are known for their significant carbon dioxide (CO 2 ) emissions during production. This material is not to be confused with ordinary magnesia (also known as periclase) that can be found in low-quality PC and is known for its low reactivity that leads to material's dimensional instability. Reactive MgO has a relatively smaller (~120 microns, and less) particle size compared to conventional magnesia and resultantly a larger surface area. Hence, the reactivity of reactive magnesia is improved drastically compared to periclase and is also whilst lower is still relatively comparable to that of PC. This process is discussed more in depth in section The binder is produced at lower ( C) temperature, and it means that CO 2 emissions for a mass unit are reduced. (Thomas et al, 2007). Reactive magnesia-based materials do not just have lower carbon dioxide emission costs, but can also be used to sequester CO 2 inside its structure more effectively, thus sealing and removing this major greenhouse gas from the atmosphere. While the overall emissions during the production of ordinary magnesia are around 1.4 t for 1 t of material produced (almost double of 0.85 t PC production emissions), new production technologies together with sequestration can potentially decrease the emissions. The environmental benefits from such solutions should not be underestimated (Liska and Al-Tabbaa 2008; Taylor 1990). Sequestration of CO 2 inside concrete material is possible through carbonation reactions. This in turn changes concrete's properties such as compressive strength, density, permeability but also its alkalinity. As the carbonation reaction is exothermic, it also affects hydration kinetics, and consequently strength gain of the material. It is possible to use these features of the reaction to cure the concrete. This curing regime is known as accelerated carbonation curing. Using higher than normal atmospheric (~400 ppm) carbon dioxide environment, controlling relative humidity and temperature, the carbonation is facilitated significantly. However, it should be noted that several researchers who have examined carbonation curing of reactive MgO, have used various CO 2 concentrations, relative humidity and temperatures. Mo and Panesar (2012, 2013) studied reactive MgO pastes in conditions of 99% CO 2, and 98% relative humidity. Vanderper and Al- 1

14 Tabbaa (2007) used carbonation environments of 0.04, 5, and 20% CO 2, and 65 and 95% relative humidity for reactive MgO pastes. Liska and Al-Tabbaa (2008) studied masonry concrete containing up to 10% reactive MgO and cured samples at 40 C, 98% relative humidity and 20% CO 2. Very recently, Unluer and Al-Tabbaa (2014) reported on reactive MgO concrete blends with fly ash and they cured the specimens in CO 2 ranging from 5 to 20%, and a relative humidity ranging from 55 to 98%. Therefore, to date there is no consensus on the most desirable curing conditions in terms of CO 2 concentration and relative humidity in order to achieve optimum material mechanical and transport properties. This is currently a recognized gap in the knowledge reported in published literature. Reactive magnesia materials are specifically interesting in this regard, as the carbonation reaction has a number of features that are significantly different from ordinary PC-based materials. These features affect resulting properties such as amount of CO 2 sequestrated, carbonation speed, porosity and the resulting strength and in generally may have a greater practical usefulness. A potential of such practical application largely depends not just on the properties of reactive MgO, but also on different environmental variables in accelerated carbonation process. While the carbonation of ordinary PC materials has been a subject of very many different research publications, reactive MgO material's carbonation and especially the effect of the aforementioned variables remains being covered very lightly. 1.1 Motivation for study The environmental effect of CO 2 emissions on the atmosphere temperature and ocean acidity has been and is a very important subject in many spheres of science (Scott and Levine 2006). The concerns of global warming and ocean acidification were brought back again as a new serious warning has been received when in May 2013 the atmospheric CO 2 concentration have passed the 400 ppm (parts per million) point, and became the highest in the whole 55 years history of measurement, and possibly even more than 3 to 20 million years. (Tans 2013). Being a major greenhouse gas, CO 2 content increase is believed to be a main reason of recent temperature increase, commonly known as global warming. As this correlates with both human- 2

15 made CO 2 emission increase and glaciers retreat, it is indicated by many analyses that a climate change on a global scale is indeed a possible scenario (Stocker, 2013). It is now widely accepted that the driver of such sudden change are carbon fossil fuel, cement production and also deforestation. Thus, there is an international initiative to reduce man-made carbon dioxide impact through various means. Emission control is the primary solution to mitigate the impact. However, as industry is without a doubt remains a vital part of civilization, and as the industry currently largely depends on fossil fuels, this reduction has a negative impact on economy, especially in developing countries (Ghosh and Kanjilal, 2014). This means that if any other emission process, such as cement production, can be controlled, the pressure on the industry, and thus on economy of developing countries may be reduced significantly. Apart of that, sustainability doctrines almost inevitably lead to the more effective use of available resources that may in its turn improves the stability of economical development. The cement industry, as mentioned, is responsible for at least 5% of global man-made CO 2 emissions (Mukherjee and Cass. 2012). The amount of Portland cement produced today exceeds 2.5 billion tonnes per year (Taylor 1990) It is estimated that for each ton of cement produced there is 0.85 t of CO 2 emitted (Taylor 1990). As sustainability becomes one of the most important tasks that lies before the scientific community in this century, a concrete in the environmental perspective has become the subject of major discussions around the world. One of the solutions to this problem was found in reducing the amount of conventional Portland cement concrete used by replacing it with the so-called supplementary cementitious materials - mostly byproducts of different industries. These products include pulverized fly ash (PFA), ground-granulated blast furnace slag (GBFS) and silica fumes (Johari et al. 2011). In addition to these, there are other binders that can potentially replace a part of PC in use, thus further reducing CO 2 output. One of such binders is reactive magnesia that as many other current and potential supplementary materials may actually improve the physical properties of the concrete in addition to being more sustainable alternative to conventional Portland cement concretes. As mentioned above in section 1.0, accelerated carbonation curing is another potential solution towards more sustainable construction materials. It allows to rationally use the available CO 2 to facilitate carbonation, hydration of the concrete, but also to seal a gas inside its structure. 3

16 This essentially removes this greenhouse gas from the atmosphere and outer environment. The effectiveness of this process would largely depend on the amount of CO 2 being captured (Shao et al., 2014; Caijun and Yanzhong, 2009). This leads to the conclusion, that it is possible to effectively combine both material replacement and accelerated carbonation, and, possibly, reaching truly effective sustainability results. 1.2 Research Objectives The overall purpose of this research is to investigate the effect of carbonation curing conditions (relative humidity and carbon dioxide concentration) on the chemistry (formation of carbonation and hydration products); and physical properties (microstructure and compressive strength) of cement based materials containing up to 60% of reactive MgO. Outcomes of this study will reveal optimum carbonation curing conditions for reactive MgO products and the sensitivity of the carbonation curing conditions on the reactive MgO materials properties In order to achieve these research goals the following sub-tasks are identified: 1) Create a controlled curing chamber with appropriate curing conditions in which concrete materials can be subjected to the specified environment necessary for accelerated environment; 2) Examine the effect of six different curing conditions based on relative humidity and carbon dioxide concentration on sequestration efficiency, chemical and physical properties; 3) Study the chemical composition of a carbonated material and assess the hydration and carbonation reactions products using X-ray diffraction and DT\TGA techniques; 4) Establish the most optimal environmental conditions and material replacement ranges for the practical use. In addition the optimal size and shape of concrete elements can be assessed based on carbonation depth achieved. The conclusions on the most effective setup are essential to facilitate the integration of reactive MgO products as a construction building material such as in industries that are dealing with precast concrete elements - porous masonry blocks, bricks and other similar products. 4

17 2. Literature review The subject of carbonation of conventional Portland Cement based materials, and those blended with Supplementary Cementitious Materials (commonly known as SCMs) are well published in scientific publications (Johari et al, 2011). In contrast carbonation of cement materials containing reactive magnesia remains less understood, since relatively few studies have been conducted on the subject. When comparing Portland cement and reactive magnesia in terms of their carbonation processes one must identify the key differences in the carbonation reaction mechanics. Furthermore, it is necessary to understand the implications of carbonation on other properties, such as hydration kinetics, initial porosity and chemical composition, and also how do all those properties affect and compete with each other in the process must be studied extensively. 2.1 Conventional Cement-Based Materials and Carbonation A topic of concrete carbonation is known to be a recurring theme of many building materialsrelated studies. Today, the outcomes and conclusions of these studies are taken into account in building materials development. However, carbonation of concrete is deemed to be such an important topic because of its degrading effect on a steel reinforcement. As the bulk material carbonates a major drop in alkalinity occurs, leading to depassivation of oxide protective film on the steel reinforcement. As it is established, ph level drops from around 13 down to 9 and less as a result of CaCO 3 formation and further oxidation occurs, degrading the metal rebar. The resulting corrosion leads to the loss of mechanical properties of the reinforcement, but also to the formation of rust that induces local stresses and consequently damages the concrete. Such a loss of predesigned properties that can potentially occur because of carbonation process was a reason why for most of the time carbonation studies were concentrated on its negative tendencies. Carbonation processes have been well researched and it is well recognized that they have potential to alter the microstructure of hydrated OPC (Johannesson and Utgenannt 2001). The extent to which carbonation alters the microstructure is significant largely because of the well recognized implications it could have on mechanical and durability properties. The general 5

18 strength-porosity relationship of solids is very well established. materials it is commonly expressed as (Mehta and Montiero 2006): For basic homogeneous S=S o e -kp Where, S = strength of the material which has a given porosity, p S o = intrinsic strength at zero porosity k = constant For cement based materials, several decades ago, Powers (1958) found that the 28-day compressive strength was related to the ratio between the solid hydration products (gel) and the total space in the system (space) (gel/space ratio). Certainly, the exact compressive strength to porosity relationship when plotted for paste, mortar, or concrete of different mix proportions, materials, curing regime etc. will vary, however the general trend of increasing compressive strength with decreasing porosity holds. This is apparent in Figure 2.1, which presented the concrete compressive strength to total porosity as measured by mercury intrusion porosimetry (MIP) by Das et al (2012), Kumar and Bhaltacharjee (2003), and Poon et al (2006). Figure 2.2 shows the compressive strength vs total porosity relationship for pasted and mortars using Portland cement (Guneyisi et al 2008, Chindaprasirt et al 2005, Rossler and Odler 1985, Li et al 2006, and Kuo et al 2006). In all cases it is apparent that strength increases with reduced porosity. Recognizing that durability performance is largely controlled by permeability and not total porosity, many researchers also evaluate permeability and incontext with its relationship to compressive strength it is generally recognized that the less connected the capillary pores, the lower the strength and this is apparent in Figure 2.3 based on the study by Das et al (2012). However, it should be noted that the relationship is not linear. Carbonation processes alter the microstructure of hydrated OPC as a result of the reaction of CO 2 with the Ca(OH) 2 present in hydrated paste or mortar (Johannesson and Utgenannt 2001). The formation of CaCO 3 product results in a densification of the paste s microstructure, a decrease in capillary pore volume and a decrease in total porosity. Several studies have validated this observation as a result of various types of tests to measure changes in the microstructural form such as: scanning electron microscopy images, mercury intrusion 6

19 porosimetry measurements, desorption isotherm determinations and chloride penetration tests (Johannesson and Utgenannt 2001, Tumidajski and Chan 1996, Kropp and Hilsdorf 1995). Figure 2.4 presented the compressive strength vs. total porosity relationship for carbonated ordinary Portland cement concrete based on (Chang and Chen, 2005 and Liska and Al-Tabbaa 2008). In general it is observed that the strength-porosity relationship holds. The ability of carbonation processes to occur and the implications of these process is influenced by the type of cementing material used and its chemical composition. For example, considering the surface layer of concrete containing GGBFS, the occurrence of carbonation reactions yields an increase in capillary porosity in particular, the volume of very large capillary pores (>100 nm). The increase in porosity has been identified to be due to the formation of soluble metastable calcium carbonates (Stark and Ludwig 1997, Tumidajski and Chan 1996). Beyond the vulnerability of reinforced concrete elements to be subjected to higher likelihood of corrosion due to the effect on the ph of the pore solution, other durability mechanisms can also be affected such as freeze-thaw and de-icer salt scaling damage (Stark and Ludwig 1997, Tumidajski and Chan 1996). Carbonation of ordinary Portland cement paste is expected to decrease the amount of freezable capillary pore water owing to the more refined microstructure whereas mixtures containing GGBFS will permit a greater amount of freezable pore water in comparison to a non-carbonated specimen (Stark and Ludwig 1997). The details of the chemistry of the carbonation processes are described. One of hydration products that is affected by CO 2 in the concrete material is Ca(OH) 2, which comprises 25 to 50% wt of the total hydrated product (Muntean et al. 2008). In its simplest, the reaction can be written as the following: Ca(OH) 2 + CO 2 CaCO 3 + H 2 O The nature of this physiochemical reaction is more complicated. First, CO 2 dissolves in water, forming carbonic acid - HCO 3 : H 2 O + CO 2 = HCO 3 + H + 7

20 The resulting acid can react with calcium hydroxide alkali, and the following neutralization reaction occurs: Ca(OH) 2 + 2H + + CO 2 3 = CaCO 3 + 2H 2 O The reaction s main product is calcite. Since neutralization reactions are exothermic in nature the standard free energy G o r and volume expansion of solid V are: G o r = kj.mol-1 V = 3.22 moles The reaction is highly energy intensive, and results in densification of the hydrated paste (Guo et al. 2013) It must be taken into account although that this reaction effectively removes water, and while this facilitates further carbonation, it also results in carbonation shrinkage (Garcia- Gonzales et al. 2006). In addition to this the primary carbonation reaction, CSH and unreacted cement also react with CO 2 to form carbonate compounds: (3CaO 2SiO 2 3H 2 O) + 3CO 2 -> (3CaCO 3 *2SiO 2 *3H 2 O) and (x CaO *SiO 2 ) +2CO 2 +nh 2 O -> (SiO 2 *nh 2 O) +2CaCO 3 where x is number of moles (2 and 3 for C 2 S and C 3 S respectively), and n is a variable molar content of the water. These reactions contribute to around 30% of CO 2 sequestrated during the carbonation and have much smaller impact compared to the primary reaction (Muntean et al. 2008). In general, the carbonation process follows a pattern: 1) Calcium bearing compounds undergo decalcination during which they are gradually dissoluted, and Ca + ions start to fill the water in pore solution; 2) CO 2 is absorbed into water and both carbonate (CO 2 3 and bicarbonate(hco 3 ) ions form; 8

21 3) As these ions form a supersaturated solution of CaCO 3, calcite starts precipitating, filling the pore space gradually; As mentioned earlier, as the CO 2 penetration front forms calcite layers, the resulting densification of microstructure gradually obstructs the intake and in turn slows the carbonation reaction (Rostami et al, 2012). Concrete Compressive Strength (MPa) Das (2012) Kumar (2008) Poon (2006) Trend Line Total Porosity (%) Figure 2.1: Compressive Strength- Total Porosity Relationship for Conventional Concrete 9

22 Compressive Strength (MPa) Guneyisi (2008) Chindaprasirt (2005) Rossler (1984) Li (2006) Kuo (2006) Trend Line Total Porosity (%) Figures 2.2: Compressive Strength- Total Porosity Relationship for Conventional Paste and Mortar 60 Das (2012) Concrete Compressive Strength (MPa) Permeability (index) Figure 2.3: Compressive Strength- Permeability Relationship for Conventional Concrete 10

23 Concrete Compressive Strength (MPa) ACI-102-M36 Liska (2008) Trendline Total Porosity (%) Figure 2.4: Compressive Strength- Total Porosity Relationship for Carbonated Concrete 2.2 Reactive magnesia cements Hydration, Chemical Composition and Physical Properties. Reactive magnesia is known to react with water and harden into a solid bulk material. When magnesia reacts with water, the following reaction occurs: MgO + H 2 O ->Mg(OH) 2 While brucite is the only product of this reaction, it has been reported that a formation of brucite hydrate (Mg(OH) 2 nh 2 O ) may occur if a sufficiently high amount of water is available for the reaction (Harrison, 2003). In addition to these reactions, if magnesia is used in conjunction with Portland Cements the formation of M-S-H gel can happen, however it is influenced by alkalinity of the material (Zhang et al. 2011). The overall effect of brucite on strength development was found to be negative, both because of bigger water-to-cement ration needed for the reaction to occur (that in its turn increases the amount of water needed for a 11

24 specific workability), and also because of slower hydration rate of magnesia (Cwirzen and Habermehl-Cwirzen, 2013). Various crack-producing dimensional instabilities are traditionally linked with brucite formation in concrete. Indeed, the presence of dead-burn magnesia in cement, also known as periclase has been found to affect mechanical properties of concrete in a degrading way (Nokken 2010). This primarily occurs because of very low reactivity of periclase, that means that most of reaction-produced expansion happens at later ages. This expansion induces tensile stresses in the concrete material, which may lead to uncontrolled cracking and overall material deterioration. On the other hand, reactive magnesia has a hydration rate similar to that of PC, and thus the mentioned instabilities can be avoided. The topic of reactivity of various reactive magnesia products and the extent of various factors influencing that has been a subject of major debates and discussion in the literature due to its importance in refractory applications (Salomao et al, 2007). It has been defined that the hydration of magnesia is crucially dependent on surface area of a particle that is undergoing solvation when contacting with water molecules (Salomao and Pandolfelli, 2007; Harrison, 2010; Pera and Soudee, 2001). Therefore, in a bulk hydrating material the degree of saturation and thus the speed of a reaction is also dependent on how quickly various magnesia particles react with water. The effort has been done to define the effect of surface area, and thus specific surface area or SSA has been used to explain the process (Stumm, 1992). SSA takes into account the available surface for the dissolution and is affected not just by the size of a particle but also structural defects and disorderly packing of the molecules in the crystalline structure. Other factors, such as ph have also been identified to influence both rate and mechanism of the reaction (Souza et al, 2014; Filippou et al, 1999). Various factors affecting the initial SSA have been defined in the research. Most important of these are the calcining temperature (e.g. a temperature at which magnesite is decomposed during the production process) and grinding size (Suvorov and Nazmiev, 2007). The relevance and importance of these factors was a subject of several scientific studies. It was found that the effect of calcining temperature is a dominant in this regard. That is primarily because of a different entropy and enthalpy energy induced into a forming magnesia crystal (Harrison, 2010; Salomao et al, 2007; Rocha et al, 2004). Higher temperatures are giving in excess energy, and 12

25 this allows more orderly crystalline lattice to form. Since a highly ordered lattice of periclase crystal has high (>3795 Kj*mol-1) lattice energy this kinetic barrier is is significantly harder to overcome by the energy of solvation. Several studies have underlined the importance of low calcining temperatures, and how is this factor more relevant in comparison to grind size (Harrison, 2010; Blaha 1997) Mechanics of carbonation of reactive MgO as Cement Replacement and the effect on compressive strength, porosity and durability properties of the material. Carbonation in r-mgo based mixtures is more complex and depends on several of variables. The primary carbonation reaction in materials containing only r-mgo yields nesquehonite: Mg(OH) 2 + CO 2 ->MgCO 3 3H 2 O Same as with PC analogue, the reaction requires water to be present for the reaction to occur. In contrast though it directly binds water molecules, instead of just removing them from solid phases. This directly means that the reaction is considerably more water demanding (Vandeperre and Al-Tabbaa, 2007). The reaction is much different in comparison to its PC counterpart in other aspects as well. It is much less energy intensive. In addition the solid volume change is significantly larger: G o r = kj.mol-1 V = moles Apart from nesquehonite, many other compounds can form. They include landsfordite ( MgCO 3 *5H 2 O), hydromagnesite (Mg2CO 3 (OH) 2 *3H 2 O) and others (Harrison, 2003). These numerous compounds may only form in certain specific relative humidity, temperature, partial CO 2 pressure among them. The carbonation proceeds in a similar to PC pattern, when brucite (Mg(OH) 2 ) dissolves and yields Mg 2+ ions to the solution, which then reacts with CO 2 based ions in the water. However, magnesium hydroxide is considerably less soluble (0.009/100 ml compared to CH 0.185/100 ml) in water, so in normal conditions the amount of material carbonated will most likely be comparatively low (Mo and Panesar 2012). This itself is also 13

26 influenced by acidity of the pore solution, that may also regulate the overall speed of the reaction (De Silva at al, 2009). A special case is represented by Ca-Mg carbonates that readily form in r-mgo-pc mixes. Main product of such reaction is magnesium calcite: Ca(OH) 2 + Mg(OH) 2 + CO 2 (Ca, Mg)CO 3 Because of low solubility of brucite means that under normal conditions only a fraction of magnesium ions will precipitate in the solution, similarly as in nesquehonite formation. However as the amount of MgO increases, this changes considerably (Mo and Panesar 2013, Liska and Al-Tabbaa 2008). This compound is often taking most of Mg 2+ ions from the solution, and it may explain why very small amount of nesquehonite forms in calcium bearing mixes. Moreover, because of uneven solubility of brucite and portlandite the ratio of Mg 2+ and Ca 2+ ions in the precipitated and also because of nucleation and separation of both ion's enriched zones, solid material is heterogeneous. This in its turn affects the impact of bulk magnesium calcite on the physical properties of a carbonated material (De Silva et al, 2009, Mo and Panesar, 2012). While the carbonation process is varying under many different factors, it itself affects physical properties of the material where it occurs. As discussed in 2.1, the subject has been well covered in a literature for ordinary PC materials. On the other hand, several studies has assessed the changes in microstructure due to carbonation in reactive magnesia containing materials (Mo and Panesar, 2012, Vandeperre and Al-Tabbaa, 2007). This effect of carbonation on microstructure can at first be summarized by reviewing the change in molar volume of solid phases that occur due to carbonation. If the sole formation of nesquehonite is considered, there is a change of 24.3 g/l -1 to around 75 g/l -1 which results in ~400% expasion. This is a significant increase compared to ~3% expansion of 33 g/l -1 to 36.9 g/l -1 when calcite forms from portlandite. When Mg-PC blends are considered, the amount of nesquehonite product is very variable, and depends highly on the mix composition and curing conditions. Instead a formation of magnesium calcite was reported to be prevalent. In its turn magnesium calcite has a different molar volume due to varying Ca:Mg ratio in the forming product, that depends on several factors (Mo and Panesar, 2013). Thus, the effect of carbonation cannot be estimated accurately 14

27 by empirical means. Studies of porosity using different techniques, such as MIP and SEM have been conducted, and the decrease of porosity was found to be generally consistent with presence of carbonated phases(mo and Panesar, 2012, Shi and Wu, 2009, Liska and Al-Tabba, 2009 and others). It has not been entirely clear though what input each particular carbonate phase has on such densification on microstructure. Only small fraction of reactive magnesia studies have made an effort to cover a durability aspects and properties. In general, an increase in early shrinkage resistance has been reported for magnesia containing products, while the freeze-thaw resistance was found to be decreasing due to %MgO (Choi et al 2014, Cwirzen and Habermehl-Cwirzen, 2013 and Choi et al, 2014). 2.3 Competition between hydration, carbonation and influence of environmental and material conditions Irrespective of if Portland cement is used alone or in the presence of reactive MgO or other cement replacement materials, there are several competing processes involved when the cement-based material is being subjected to carbonation. First and the foremost, the cement hydrates, yielding more and more hydrated products that quickly fill the gaps in the initial pore structure. Second, a parallel process is a carbonation of a material, first hydrated and then nonhydrated if conditions allow them to be included in the process. Carbonation in its turn densifies the pore structure of a material, when molecules of carbonate compounds precipitate at walls inside the pores. Both reactions are exothermic, and both increase the speed at which cement hydrates and further fills the pores. Considering these factors, as the path for becomes more and more obstructed so the absorption slows, and so carbonation gradually decrease its intensity (Shi and Wu, 2009). Furthemore, the obstruction of pore network due to both carbonation and hydration leads to another phenomena. As more and more [Mg 2+ ] and [Ca 2+ ] dissolves into the pore solution, and more carbonates precipitate on the sides, the flow of each ion is becoming more and more restricted, and thus several zones start to occur where the ratio of each ion is more prevalent. In its turn this leads to formation of nesquehonite, and high-mg magnesium calcite, both of which have different morphology and impact on the material properties (De Silva et al, 2009, Mo and Panesar, 2012). 15

28 Moreover, there is an increased water demand for both hydration and carbonation reactions to occur, but even more so in case of magnesium containing mixes (He et al, 2003). Although CH releases water, and magnesia compounds consume them, there is still need for the water solution to be present in order for the reaction to occur. As the relative humidity changes, so does the amount of water in the solution. However if there is too much water, it will block the pore system thus only allowing the much slower diffusion to take place of absorption. All these factors indicate that the single most important period for the efficient carbonation is early age. The governing condition in this case would be initial porosity - a porosity at the moment of a beginning of accelerated carbonation curing. The properties such as density of the material, degree of compaction, amount of free water in the pore network, and finally speed of early hydration and carbonation - would all affect early age carbonation considerably (Mo and Panesar, 2013). The optimal ranges for relative humidity needed for an effective accelerated carbonation of conventional concretes were extensively studied. Some reports indicate that optimal conditions lie in the range of 60 to 75% (Shi, and Wu, 2009, Sulapha et al, 2003), while others reported broader ranges from 40 to 80% (Sisomphon and Lutz, 2007). In some researches, ranges of 65 to 70% were successfully used to carbonate specimens based on plain Portland cements. However, the more complicated chemistry of brucite carbonation means that these values might not be sufficiently accurate for cements containing larger amount of r-mgo. Since the penetration of a carbonating material by CO 2 is primarily governed by permeability, it is justified to mention the effect of pores other than the capillary. It is known that there is a negative effect of entrapped air pores on durability that may occur during the mixing process if inadequate compaction has been achieved (Lomboy and Wang, 2009). These entrapped air voids that have a size greater than 1000µm are frequently penetrated by a capillary system thus increasing the interconnectivity of pores and consequently permeability of the concrete (Scherer, 2008; Kim et al, 2007, Balaguru and Ramakrishan, 1989). Thus, when high water demand of magnesia mixes comes into play, it is possible that the amount of these voids that occur during the mixing will be relatively greater with increasing %MgO. 16

29 As mentioned before, both hydration and carbonation processes in these concrete materials have considerably larger water demand. Therefore while lower humidity values may allow a better ingress of CO 2 deeper inside the material's structure, higher amount of water in pores may facilitate the onset of a reaction. CO 2 concentration is also known to have a considerable effect on efficiency of carbonation curing. Typically the concentrations of 20-30% and higher are to be used for the carbonation curing to be accelerated (Liska and Al-Tabba, 2009, Vandeperre and Al-Tabbaa, 2007). However in many studies concentrations have been close to maximum % (Mo and Panesar, 2012, Monkman and Shao, 2006). It is, however, not entirely clear, what overall effect do intermediate concentration ranges have on the subject. In addition to that, no study has been conducted to examine possible interplay between different humidity and CO 2 concentration, which is as pointed out even more important with the r-mgo case. The effect of cement composition is also known to have a significant influence on the rate mechanism and also on the effect of carbonation. The reactivity of magnesia or conventional cement, SCM replacement effect and other have been studied extensively (Mo and Panesar, 2013, Borges et al, 2012). It was indicated in several studies that chemical composition is affected by ratio between PC and r-mgo. For instance nesquehonite formation was found to increase dramatically when this ratio is around 1:1 and higher (Vandeperre and Al-Tabbaa, 2007). The possibility of formation of other compounds such as landsfordite was also been found to relate to the cement composition (De Silva et al, 2009). Therefore it is evident that more extensive approach towards the examination of effect of these material and environmental variables. The replacement ratios up to 60% of MgO were chosen in this research to further see its role on the carbonation and also its dependency over different environmental conditions that must be replicated to cover the gaps in the existing research. Early age must be more carefully examined, and this means the effect of the variables in this period must be related to time needed to fully carbonate the concrete specimen. In this way it might be possible to narrow the choice of both optimal conditions and more effective r-mgo replacement ratios for the most effective CO 2 sequestration without compromising physical properties and microstructure. 17

30 3.Experimental program 3.1 Materials and Mix Design For this study, general use (GU) Portland cement was supplied by Holcim Canada. Reactive magnesia cement from Liyang Special Materials Company, China, that was prepared by calcining of magnesite under the temperature of 800 C. The chemical composition of cementitious materials can be viewed in Table 3.1 Table 3.1: Chemical composition of cementitious materials Oxide General Use (GU) Reactive Composition Portland Cement Magnesia MgO (%) CaO (%) SiO 2 (%) Al 2 O 3 (%) Fe 2 O 3 (%) Na 2 O (%) K 2 O (%) SO 3 (%) 4.07 LOI (%) Sample preparation and curing Four mortar mixes were tested with the same water-to-binder (w/b) of Sand-to-cement ratio of 2 was also identical in all mortar mixes. These mixtures were containing 0% MgO, 20% MgO, 40% MgO, and 60% MgO, as a Portland Cement replacement in a mixture, and designated as M-0, M-20, M-40 and M-60. For each mix a set of twelve 50mm cubes was prepared, and it was cured in an environment with relative humidity of 90% and temperature of 23±2 C for two days. After that they were demoulded and immediately placed into an 18

31 environmental chamber with a variable relative humidity (RH) and CO 2 concentration and a temperature of 23±2 C and atmospheric pressure. Additionally, six paste 50 mm cubes were casted for each magnesia replacement level with the same w/b, designated similarly to mortar (M-0p, M-20p, M-40p and M-60p). CO 2 concentration was monitored using DCS inc. M400 CO 2 sensor and controlled manually. By adjusting CO 2 concentration each day (or more often at early curing ages, when CO 2 consumption is more rapid) specific concentration of 50, 75 or 99% has been achieved. Relative humidity and temperature was monitored using two different hygrometers. In order to maintain uniform CO 2 concentration in the chamber, a ventilating fan was used to create air circulation, and therefore maintain uniformity in CO 2 distribution. Salt solutions were used keep the humidity inside the chamber constant. More specifically, NaCl was used for ~75% RH, and a combination of CaCO 3 and LiCl 2 were used to establish ~50% RH. Table 3.2: Mortar and Paste Mixture Design Proportions Mixture designation General Use (GU) Reactive Fine Water Portland Cement (g) Magnesia (g) Aggregate (g) (g) Mortar M M M M Paste M-0p M-20p M-40p M-60p

32 Table 3.3: Carbonation Curing Scenarios Environmental Condition CO 2 Concentration (%) Relative Humidity (%) Temperature ( C) 50%RH, 50%CO 2 50±5 50±5 23±3 50%RH, 75%CO 2 75±5 50±5 23±3 50%RH, 99%CO ±5 23±3 50%RH, 50%CO 2 50±5 75±5 23±3 50%RH, 75%CO 2 75±5 75±5 23±3 50%RH, 99%CO ±5 23±3 3.3 Testing procedure The 3, 7 and 28 day testing was conducted on both mortar and paste specimens. Pore structure analysis on mortar and paste specimens was done using Mercury Intrusion Porosimetry. Paste specimens were used for X-ray diffraction (XRD) and DTA testing for the purpose of chemical composition analysis. In addition, the carbonation front was examined using 1% phenolphthalein alcohol solution ph-indicator on a fresh-split specimen surface. Additionally, compressive strength tests were conducted only on mortar specimens Sample preparation and drying procedures Following the splitting procedure material from the specimens was prepared for the chemical and porosimetry testing. First, material was taken from a carbonated area, or the area being as close as possible to it. It was then crushed to a size of between 1.50 to 1.25 mm. A material was then immersed in isopropyl alcohol for 24 hours, then vacuum dried for 24 hours, then put in a desiccators with 20% RH for an additional period of 24 hours. The half of the material was used for MIP testing, the other half was further grounded to powder with size of no larger than 35 microns and used in DT\TGA and XRD tests. 20

33 3.3.2 Carbonation front After the designated accelerated carbonation curing time, specimens were taken out of the curing chamber. Phenolphthalein solution in alcohol of 1% was then applied to fresh split surface of the corresponding mortar and paste cube specimens Compressive strength The mortar cube specimens were tested on compressive strength on the corresponding testing dates. For each testing day and mix tested a set of 5 cubes was used. Average compressive strength values and coefficients of variation were then calculated Chemical composition tests X-ray analysis was performed using Analytical X-Ray Powder Diffractometer with Cu Kα radiation (λ= Å), 2θ range of 5 80, and a step size of 0.02 to investigate crystalline phases in the powdered samples. Same powder was used in DTA\TG test using NETZSCH STA 409, with a temperature range of 25 to 1050 C, a heating rate of 10 C/minute and nitrogen flow rate of 50 ml/minute Mercury Intrusion Porosimetry (MIP) Autoscan 60 mercury intrusion porosimeter was used to perform a test. Specimens to be tested were crushed with particle size being in a range from 1.5 to 2 mm. The average mass of a tested sample was 2.5 g. Pore-size distributions, apparent densities and threshold values were collected for each sample tested. 21

34 Results and Discussion 4.1 Chemical Analysis XRD, DT and TGA Carbonation front Tables 4.1 and 4.2 show the results of carbonation front tests for the conditions with 75% and 50% relative humidity respectively, both mortar and paste. The visible difference in the rate of carbonation of each mixture can be observed. The only mixture to consistently carbonate completely is M-60. This mixture is regularly penetrated almost completely as early as at 7d. In contrast, mix M-0 is consistently the least carbonated at all ages. Other mixes, irrespective of CO 2 concentration show the increase in zone carbonated with time. CO 2 concentration was found to improve the rate of carbonation in many cases, however on the contrary the rate was reduced for 75% RH cured mixtures M-20 and M- 40. The reasons for this are unclear. In general, most of the mixtures have carbonated more readily in the environments with 50% RH compared to those cured in 75%RH. As CO 2 concentration increased, all the specimens carbonated more, same as with the case of 75% RH environments. The difference in carbonation due to CO 2 concentration was smaller, mostly because of much higher carbonation rates of [50%RH, 50%CO2] compared to that of [75%RH, 50%CO2]. Mix M-0 has achieved markedly quick and high carbonation rate in 50% RH conditions. Even though its complete carbonation achieved in [50%RH, 50%CO2] has not been repeated at elevated CO 2 concentrations, it has still carbonated more than both M-20 and M-40, in contrast to 75% RH environments where it was consistently the least carbonated mix. 22

35 Table 4.1: Carbonated Area of Specimens cured in 75%RH [75%RH, 50%CO2] Mortar carbonated area, % Paste carbonated area, % %MgO 3d 7d 28d 3d 7d 28d 0 8% 12% 26% 4% 23% 28% 20 8% 13% 42% 51% 80% 87% 40 15% 51% 84% 10% 54% 93% 60 50% 84% 100% 57% 71% 100% [75%RH, 75%CO2] Mortar carbonated area, % Paste carbonated area, % %MgO 3d 7d 28d 3d 7d 28d 0 21% 25% 31% 25% 30% 77% 20 4% 9% 29% 23% 33% 51% 40 14% 42% 65% 26% 42% 52% 60 98% 100% 100% 51% 64% 100% [75%RH, 99%CO2] Mortar carbonated area, % Paste carbonated area, % %MgO 3d 7d 28d 3d 7d 28d 0 24% 30% 66% 24% 50% 69% 20 6% 19% 26% 14% 28% 63% 40 5% 17% 46% 42% 59% 80% 60 74% 96% 100% 66% 94% 96% 23

36 Table 4.2: Carbonated Area of Specimens cured in 50%RH [50%RH, 50%CO2] Mortar carbonated area, % Paste carbonated area, % %MgO 3d 7d 28d 3d 7d 28d 0 29% 51% 100% 21% 26% 66% 20 14% 19% 28% 15% 42% 50% 40 6% 15% 58% 32% 39% 66% 60 74% 88% 100% 36% 74% 94% [50%RH, 75%CO2] Mortar carbonated area, % Paste carbonated area, % %MgO 3d 7d 28d 3d 7d 28d 0 24% 30% 66% 25% 30% 77% 20 6% 19% 26% 23% 33% 51% 40 5% 17% 46% 26% 42% 52% 60 74% 96% 100% 51% 64% 100% [50%RH, 99%CO2] Mortar carbonated area, % Paste carbonated area, % %MgO 3d 7d 28d 3d 7d 28d 0 23% 33% 75% 34% 39% 88% 20 13% 22% 42% 23% 32% 47% 40 11% 24% 77% 39% 48% 71% 60 73% 90% 100% 66% 79% 86% 24

37 4.1.2 X-ray Diffraction results Figures 4.1, 4.2, 4.3 and 4.4 show the example of XRD patterns of a carbonated paste material after 3d and 28d of accelerated carbonation curing. For the curing condition of 75%RH and 50% CO 2, Figure 4.1 and 4.2 shows the influence of percentage of MgO on the XRD pattern of carbonated mortar and paste, respectively. The key compounds have been identified based on matching their characteristic peaks as shown in Table 4.3. Although XRD is a proven technique since it provides accurate phase identification, interpreting the results of cementbased constituents does pose some challenges. Peak overlaps and a masking effect generated by the presence of both coarse and fine aggregate has been reported by various researchers (Sarkar and Cheng 1994; Beaudoin and Ramachandran 2001; Glasser and Sagoe-Crentsil, 1989). Even if some aggregates are removed during the sample prepartation stage (a technique used by some researchers), some finer rock fragments tend to remain together with the fine aggregates such as quartz and feldspar. This creates a permanent unavoidable masking effect in the diffraction pattern. These observations also apply to the results reported in this study where both mortars and pastes were examined. Consequently, the reminder of the curing condition scenarios beyond 75%RH and 50% CO 2, all XRD and chemical analysis was conducted on pastes alone. In all mixes, irrespective of age and amount of MgO, calcite was the main carbonate compound. It should be noted though that it was expected that magnesian calcite would form through [Mg 2+ ] precipitation in the pore solution. Calcite and magnesian calcite have similar 2θ values, and thus are hard to be distinguished from each other in XRD patterns. In addition to Calcite, peaks of uncarbonated calcium silicate hydrate, portlandite and Brucite were found, along with ettringite and unreacted cement - calcium silicate and MgO. Nesquehonite has been also found in mixes with 40% and 60% of r-mgo content. 25

38 Table 4.3: X-ray Diffraction Main Peaks Data (Catti et al. 1995) Chemical compound X-Ray Diffraction Pattern Identity (2θ) Calcite Portlandite Magnesia Ettringite Calcium Silicate Brucite CSH Nesquehonite Quartzite Figure 4.2, 4.3, and 4.4 present the XRD patterns for the paste mix designs for the curing conditions of [75%RH and 50%CO 2 ], [75%RH and 75%CO 2 ], and [75%RH and 99%CO 2 ], respectively. There are several influencing variables such as age (3d and 28d), percentage of reactive MgO (0 to 60%) and the CO 2 concentration which ranges from 50% to 99%. Some general observations for all three curing scenarios are that, irrespective of age and amount of MgO, calcite was the main carbonate compound In addition to calcite, peaks of uncarbonated calcium silicate hydrate, portlandite and brucite were found, along with ettringite and unreacted cement - calcium silicate and MgO. Nesquehonite has been also found in mixes with 40% and 60% of r-mgo content. Although it is recognized that there are some similarities in the predominant compounds that are present in all mixtures, the chemistry of the carbonated material is influenced by age (3 d to 28d), the percentage of reactive MgO which ranges from 20 to 60%, and the differences in curing condition (CO 2 increases from 50 to 99%). In order to understand the effect of all these variables on the formation of the various compounds, namely calciute, portlandite, magnesia, ettringite, calcium silicate, brucite, calcium silicate hydrate, and nesquehonite, Table 4.4 summarizes the general trends observed for each variable (age, %MgO, CO 2 concentration and relative humidity) in XRD results conducted in this study, and the effect of age, r-mgo replacement and CO 2 concentration on peaks intensity. 26

39 Table 4.4: Influence of Age, Percentage of MgO and CO 2 Concentration on X-ray Diffraction Results Chemical Compound Age % MgO CO 2 Concentration Relative Humidity 3d to 28d 20 to 60% 50 to 99% 50 to 75% Calcite Increases Similar Similar Decreases Portlandite Decreases Decreases Similar Increases Magnesia Decreases Increases Similar Decreases Ettringite Decreases Decreases Similar Increases Calcium Silicate Decreases Decreases Similar Increases Brucite Decreases Increases Similar Increases CSH Decreases Decreases Similar Increases Nesquehonite* Increases Increases* Similar Increases * only apparent in mixtures with 40 or 60% MgO and relative humidities of 75% All peaks that correspond to the non-carbonated material in the carbonation zone show a visible decrease in their intensity with age. The substantial reduction of CSH, portlandite and brucite, along with simultaneous increase for carbonates may indicate the continuing carbonation reaction, even though the material has already been penetrated by CO 2. As the amount of reactive magnesia in the mix increases, the reduction of Calcium bearing phases has been observed. This can be attributed to overall reduction in Portland Cement content as it is replaced by magnesia. Magnesia bearing phases, such as unreacted r-mgo, brucite and nesquehonite in turn increase their intensity. Finally, no apparent effect of CO 2 concentration has been observed, as all phases remain relatively the same irrespective of changes in CO 2 concentration. The indepth analysis of each particular phase is described below. There is marked difference in chemical composition of the material when relative humidity decreases from 75% to 50%. First of all, the intensities of calcite/magnesium calcite peaks increase. In addition to this, an overall decrease in all hydration products, such as portlandite and brucite is also visible. The intensity of reactive MgO increases significantly, while calcium silicate peaks show small decrease. Finally, nesquehonite has not been found in any sample 27

40 which may indicate that it has not formed or only formed in the negligent quantities. The detailed discussion on each group of phases is below. Calcite/Magnesium Calcite Table 4.4 indicates that the calcite peaks intensity increase through 3d to 28d for all mixtures irrespective of the curing condition and magnesia content. It was expected that more hydrated material in a zone where CO 2 has penetrated the pore network will carbonate. It is observed that the intensity of calcite peak is similar irrespective of if the percentage of MgO increases even though it is expected that as reactive MgO percentage increases, calcite would decrease due to lesser amount of Ca-bearing reactants. The calcite peak appears to be unaffected by the concentration of CO 2. One of the reasons for that might be a negligible effect of CO 2 concentration on chemical structure of the carbonation zone (in contrast to its influence over the size of this zone in a bulk material). As discussed earlier in the section, magnesium calcite has 2θ values very similar to calcite, and these two phases cannot be distinguished from each other on XRD pattern. Calcite\magnesium calcite are also the main phase in 50%RH conditions. Same as in conditions with 75%RH, even though there is the increase in intensity of this phase, it cannot be reliably correlated with CO 2 concentration rise cause of the small significance. On the other hand there is a marked, visible increase in calcite's intensity related to change in humidity (from 75% to 50%). Portlandite With age the intensity of the Portlandite peak decreases which is expected because as a result of hydration processes portlandite or CH is consumed during hydration and carbonation reactions. Similarly, as the percentage of reactive MgO increases in the 28

41 mixtures, the portlandite peak decreases because the cement content becomes diluted which reduces the formation of portlandite. The portlandite appears to be unaffected by the concentration of CO 2 in a similar manner to calcite. In specimens cured in 50% RH environments the intensities of portlandite have declined greatly. For instance, almost no portlandite has been found in mix M-0 under the conditions [50%RH,75% CO 2 ] and [50%RH,99% CO 2 ]. Magnesia From 3d to 28d the peaks associated with magnesia reduce as a result of ongoing hydration reaction. As the reactive MgO content increases in the mixtures, the magnesia peaks increase owing to the increasing presence of unreacted material. The formation of zones in the material with a higher than normal MgO concentration during the mixing, which was observed to increase consistently with % of MgO may also be attributed to this effect. Similar to the other compounds discussed the intensity of the magnesia peaks are not affected by the CO 2 concentration. Unreacted magnesia is also present in low-humidity environments. The intensity of this phase is severely greater under these conditions. While it also undergoes the decrease over time, and increase over %MgO it still remains a major phase even at later ages. Ettringite With age from 3d to 28d ettringite intensity decreases as it is consumed in hydration reactions. In addition to that, with increasing reactive MgO content ettringite also decreases because less [Ca 2+ ] -bearing cementitious materials are present for its formation to occur. In contrast to 75% RH environments, ettringite has not been found in those with 50%RH. This can be related to both quicker hydration and reduced free water content in pore network. 29

42 Calcium Silicate The change in intensities of calcium silicate phases were observed to be decrease with age, similarly to another unreacted cementitious compound, magnesia. Under the accelerated carbonation conditions, the decrease is owing to hydration reactions that occurred during that time. Although unaffected by the CO 2 concentration, the increasing presenece of reactive MgO tends to decrease the intensity of calcium silicate as a result of cement content being dilluted by reactive magnesia. Almost no visible change in calcium silicate intensity has occurred due to decrease in relative humidity. Brucite Brucite's intensity was found to decrease with age, which can be explained by the fact that this compound is used in carbonation reaction. Less dramatic (compared to that of portlandite) decrease is because of lower solubility of brucite compared to portlandite (Mo and Panesar, 2012). As reactive magnesia content increases, brucite was found to also increase. Brucite being the main product of magnesia hydration is a reason for such increase. Under lower relative humidity brucite's intensity has declined, same as that of other non-carbonated phases. Calcium Silicate Hydrate Calcium silicate hydrate is another hydration production of calcium silicate (with the other being portlandite). Small wide peaks of amorphous CSH can be observed on most patterns. Sharp peaks of crystalline material have also been identified. CSH intensities decreased with age, and it may mean that the compound was consumed in carbonation reaction. It also decreases with %MgO increase, due to the same reasons as with other compounds described above. CSH has also decreased in lower humidity, however this decrease is less significant than that of brucite. 30

43 Nesquehonite Nesquehonite, also known as magnesium hydrocarbonate (MgCO 3 *3H 2 O), is the only pure magnesium carbonate that has been found in the carbonated material. The amount of this carbonated phase was markedly low compared to other carbonates - calcite/magnesian calcite. Generally nesquehonite only occurred in mixes with high r- MgO% (40 and 60%) and only at later ages (28d). While there has not been a visible variability in nesquehonite intensities based on CO 2 concentration, main peaks of this compound were different and it is hard to accurately estimate the difference in nesquehonite content based on %CO 2. Another probable influencing factor were zones in a material with higher than normal reactive magnesia concentration, that occur regularly during the mixing procedure in high r-mgo containing mixtures and that occur as more often as more magnesia there is in a mix. As discussed in section 2.3.7, nesquehonite regularly forms in places where precipitating carbonates have blocked the outflow of Mg2+ ions, and so the nucleation of this compound occurs in high-mg2+ zones.nesquehonite has not been found in any mixture cured under 50%RH. This is attributed to the insufficient water content for the formation to occur. Based on this it can be concluded that the formation of this particular phase is indeed extremely dependent on water supply. Quartzite As already mentioned in this section, quartzite peaks have been present in XRD patterns of mortar samples tested only for condition [75%RH,50% CO 2 ]. These peaks occured in the range of other important phases, such as nesquehonite, which can be seen on Figure 4.1. This way these compounds are effectively masked by quartzite presence, and make it harder to establish a more accurate picture of chemical composition of a carbonated material. Thus the testing of all other mixes was conducted on paste only. 31

44 a) Degrees (2 ) b) Degrees (2 ) Figure 4.1: Condition [75%RH, 50%CO 2 ] XRD Pattern of Mortar a) 3d, b) 28d 32

45 Degrees (2 ) a) b) Degrees (2 ) Figure 4.2: Condition [75%RH, 50%CO 2 ] XRD Pattern of Paste a) 3d, b) 28d 33