PROPERTIES AND MICROSTRUCTURE OF CO 2 CURED CONCRETE BLOCKS. Caijun SHI (1), Fuqiang HE (2)

Transcription

1 PROPERTIES AND MICROSTRUCTURE OF CO 2 CURED CONCRETE BLOCKS Caijun SHI (1), Fuqiang HE (2) (1) College of Civil Engineering,Hunan University, Changsha, China (2) School of Civil Engineering and Architecture, Central South University, Changsha, China caijunshi@yahoo.com Abstract CO 2 curing of concrete seems a very efficient process in both absorbing CO 2 and producing quality concrete products. Experimental results indicated that a dry pre-conditioning before CO 2 curing was very critical for concrete specimens to achieve desirable strength or degree of CO 2 curing. the strength of concrete mixtures for block manufacture after CO 2 curing was close to that after conventional steam curing. The specimen pre-conditioned in a moist environment hydrated more than those pre-conditioned in the dry environment during the pre-conditioning period. However, much less CaCO 3 formed in the former than that in the latter after the CO 2 curing. Thus, the CO 2 curing is mainly contributed by the reactions between CO 2 gas and cement clink minerals. Keywords: CO 2 curing, concrete block, reaction products, microstruct 1. INTRODUCTION CO 2 is the dominant greenhouse gas resulting from human activities and comes mostly from centralized sources, such as thermal power plants, building and transportation, etc. Cement plants release about 5% of the total released CO 2. Use of CO 2 for concrete curing can consume a large amount of CO 2. It can make a significant contribution to climate changes and sustainable development of concrete technology. Several studies in the 1960 s found that CO 2 could be used to cure hydraulic and non-hydraulic calcium silicate to achieve very high strength in a short period of time [1-4].Compacted C 3 S mortar cylinders could reach a compressive strength of 14 MPa within 3 minutes in a CO 2 gas atmosphere with a pressure of 0.4 MPa. Afterwards, several 96

2 publications reported the use of supercritical CO 2 to mix with cement to cure cement-based products [5-6]. CO 2 curing of concrete functions based on the chemical reactions between CO 2 and cement clinker minerals so to achieve strength development. Young et al. [1] proposed the following steps for the CO 2 curing of portland cement based products: (1) CO 2 is dissolved into water to form CO 3 2- ; (2) CO 3 2- reacts with Ca 2+ to form less soluble CaCO 3, which precipitates on the surface or between cement particles;(3) CO 3 2- diffuses through precipitated reaction products and arrive in reaction zone;(4) the reaction continues until CO 3 2- or Ca 2+ is depleted or the system is depleted. Compared with steam curing, CO 2 curing of concrete consumes very little energy. Since the main reaction product CaCO 3 from CO 2 curing is stable, CO 2 cured concrete products are expected to have good dimensional stability. However, the past researches deal with the compacts with very low water to cement ratio [1-4, 7-9] or the use of supercritical CO 2 [5-6], which make them difficult to be used in practical production. This study investigates strength development and microstructure of concrete after a simple pre-conditioning of conventional concrete block mixtures. 2. RAW MATERIALS AND TESTING TECHNIQUES 2.1 Raw Materials Portland cement was used in this project. The chemical composition and physical properties of the cement are given in Table 1. The expanded shale lightweight aggregate (LWA) used in this project was block mix with loose unit weight of 945 kg/m 3 and a moisture content of 17.0%. Sieve analysis of the LWA is given in Table 2. Its grading meets both ASTM C330 and ASTM C331 [10, 11]. A natural siliceous fine sand with a fineness modulus of 2.85 was used. CO 2 gas with a concentration of 99.5% was used in this study. Table 1: Chemical Composition of Portland Cement Oxide SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 K 2 O Na 2 O R 2 O f-cao LI [%] Table 2: Sieve Analysis of Expanded Shale Lightweight Aggregate Sieve No. No. 4 No. 8 No. 16 No. 30 No. 50 No. 100 Sieve Size (mm) %, Retained 5.5% 32.5% 55.9% 69.9% 78.6% 84.4% ASTM C330 specification (%, Retained)

3 2.2 Selection of Concrete Mixtures The mixture proportion used for manufacturing the lightweight concrete was based on lightweight concrete block production and is shown in Table 3. The mixing procedure used in the lab was as follows: mix lightweight aggregate with sand first for 30 seconds, then add all mixing water and mix for 30 seconds, and finally add all cement and mix for two minutes. The concrete mixture was filled into and compacted in a steel cylinder with an internal diameter of 5 cm. The uniformly mixed concrete mixture was added into the steel mould in three layers with approximately equal volume, slightly compact each layer, then compacted using a compression machine until the specimen reaches 10 cm in height and hold for additional three minutes. The fresh density of the specimen was controlled at 1840 kg/m 3 to ensure not exceeding the maximum oven dry density of 1680 kg/m 3 as specified for lightweight concrete. The lightweight concrete blocks used in this study were obtained from a concrete block manufacturer. The size of the block is 20x20x40 cm in nominal dimension and 19.5x19.5x40 cm in actual dimensions. Table 3: Mixing Proportions of Lightweight Masonry Blocks Portland Cement (kg/m 3 ) Expanded Shale Lightweight Aggregate (saturated surface dry) (kg/m 3 ) Manufactured Limestone Sand (kg/m 3 ) Pre-conditioning and CO2 Curing of Lab Specimens In order to accelerate the reactions between CO 2 and cement minerals during CO 2 curing, pre-conditioning was designed to adjust the moist content in the specimens so to reach higher degree of CO 2 curing. Several pre-conditioning environments were used: fog curing room and dry environments with a RH around, 25%, 50% and 75% with circulated air. The moisture losses from specimens were measured after preconditioning periods of 1, 2, 3, 4, 6, 8, 16 and 24 hours. The laboratory CO 2 curing set-up for small cylinders is illustrated in Fig. 1. The pressure inside the curing chamber was monitored by a pressure gauge attached on the curing chamber. After the designed pre-conditioning period, the specimens were placed into a curing chamber. Then the curing chamber with specimens was closed, vacuumed to a pressure of around 600 mmhg and maintained for two minutes before CO 2 was injected. The CO 2 pressure was set as 0.14 MPa. The pressure of CO 2 inside the chamber was controlled by a regulator on the CO 2 tank and kept constantly during the CO 2 curing period. 98

4 Figure 1: Illustration of Laboratory CO2 Curing Setup 2.4 Steam Curing of Lightweight Concrete Blocks Some steam cured lightweight concrete blocks were randomly taken from production line in the plant. They typically have 2-6 hours of pre-setting before they are steam cured. The recorded temperature profile inside the steam kiln in the plant is shown in Fig. 2. Once cured and discharged from the steam curing kiln, these blocks were transported to the laboratory for testing ( C) o re 60 tu ra e 40 p m e 20 T Time (hours) Figure 2: Temperature Profile of Steam Curing Kiln (max. curing temp. = 55.8 C ) 2.5 CO2 Curing of Lightweight Concrete Blocks Freshly molded lightweight concrete blocks were placed on a rack for a period of time between 3 to 6 hours, depending on the temperature and relative humidity, to gain sufficient strength for transportation purpose before being transported to the laboratory for further pre-conditioning, CO 2 curing and testing. After they were delivered to the lab, they were preconditioned in a relative dry windy environment for about 4 hours to lose some moisture before they were placed into a tank for CO 2 curing. The moisture evaporation during the pre-conditioning was controlled based on the laboratory small cylinders. The set-up of the CO 2 curing tank is as shown in Fig. 3. The tank has an internal diameter of 80 cm and a length of 90 cm and can hold six full size blocks in two layers. Blocks were laid on a metal grid, which could be easily slid on the surface of metal sheet for 99

5 loading and unloading the blocks. CO 2 can also easily access and penetrate into the blocks from their all surfaces. After the pre-conditioned blocks were loaded into the curing tank, the tank was closed and vacuumed to a pressure of 600 mmhg. Then CO 2 was injected into the tank at pressure of 0.07 MPa (10 Psi) or 0.14 MPa (20 Psi) by adjusting regulator on the CO 2 cylinder. The selection of the CO 2 pressure was also based on the laboratory study on small cylinders [12]. (a) Setup for CO 2 Curing of Blocks (b) Loading Blocks into the CO 2 Curing Chamber Figure 3: Equipment Setup for CO2 Curing of Full Size Concrete Blocks 2.6 Continued Curing after Steam and CO2 Curing Two continued curing methods were used for blocks after both CO 2 and steam curing: (1) curing in a room at a temperature 22±3 C and a relative humidity of 55%±10%; (2) moist curing in sealed plastic bags by spraying water at 22±3 C. Compressive strength was measured after continued moisture curing of 28 and 90 days to evaluate the strength development. The blocks cured in sealed plastic bags were taken out for drying 48 h prior to strength testing. 2.7 Compressive Strength Measurement The specimens for compressive strength test were prepared according to ASTM C140 [13], i.e. conditioned in air for not less than 48 hrs at 24±8 C and a relative humidity of less than 80 % The blocks were saw-cut along the middle web into halves. The top and bottom surfaces of these half-cut blocks were then ground to smooth using a grinder. Two rubber pads were used between specimen and steel bearing blocks so to distribute the load uniformly over the testing sectional area. The average net area of compression specimen was measured according ASTM C140 for strength calculation. 100

6 3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1 Effect of Pre-conditioning Period on Strength of Small Concrete Cylinders Fig. 4 shows the effect of pre-conditioning time on compressive strength after moist pre-conditioning and two hours of CO 2 curing. After a total of four to six hours of curing, the specimens exhibited very low strength, which is lower than one MPa. As the total curing time was increased to six hours, the strength of these specimens increased very significantly. The specimens after CO 2 curing showed much higher strength than these without CO 2 curing. The strength of the specimens after CO 2 curing increased as the pre-conditioning time increased, especially with 18 hours of pre-conditioning time. Actually, the specimens after 18 hours of pre-conditioning and two hours of CO 2 curing exhibited strength only slightly lower than those through 2 hours of pre-conditioning time and 16 hours of steam curing. Fig. 5 shows the effect of pre-conditioning time and CO 2 curing on compressive strength after dry pre-conditioning and two hours of CO 2 curing. The specimens cured for 4 to 14 hours under dry condition exhibited almost the same strength as those cured under moist condition. However, the specimen cured for 18 hours under dry condition had obvious lower strength than those cured under moist condition. This difference might be attributed to the water evaporation of the specimens under dry conditions, which decreased the hydration of cement and caused lower strength. One interesting phenomena is that all the specimens preset under dry condition showed almost the same strength after 2 hours of CO 2 curing, which was only slightly lower than those cured in steam. This means that the total curing time could be reduced to four hours with the CO 2 curing, compared with a total time period of hours for steam curing, which may be of significant economic benefits in addition to these technical benefits. Compressive Strength (MPa) h of CO2 Curing afetr Pre-conditioning Period Immediately after Pre-conditioning Steam Curing Pre-conditioning Time (h) Figure 4: CO 2 Curing for 2 hours at 0.14 MPa (20Psi) after Different Periods of Moist Pre-conditioning 101

7 Compressive Strength (MPa) h of CO2 Curing after Pre-conditioning Period Immediately after Pre-conditioning Steam Curing Pre-conditioning Time (h) Figure 5: CO2 Curing for 2 hours at 0.14 MPa (20Psi) after Different Periods of Dry Pre-conditioning Under moist pre-conditioning condition, only a small amount of moisture (0.51%) was lost during the pre-conditioning. The different pre-conditioning times did not show an obvious effect on the CO 2 consumption. Under dry pre-conditioning condition, the average moisture was 4.13%. However, the specimens exhibited much higher CO 2 consumption than those preset under moist condition. Also, the CO 2 consumption decreased slightly as the pre-conditioning time increased. 3.2 Strength of Full Size Concrete Blocks Fig. 6 shows the strength comparison of the blocks after steam and CO 2 curing. Table 4 is the ANOVA comparison of the two sets of data. It can be seen that P=0.197>0. This means that, statistically, there is no significant difference between the two sets of data, or no difference in strength between the steam cured and CO 2 cured blocks. However, the strength after CO 2 curing varied in a broader range than that after steam curing. This can be contributed to the variation of moisture content in the blocks, which affects the CO 2 curing. 102

8 Figure 6: Strength Comparison of the Blocks after Steam and CO2 Curing Table 4: One-way ANOVA Strength Comparison of the Blocks after Steam and CO2 Curing Source DF SS MS F P Factor Error Total S = R-Sq = 6.84% R-Sq(adj) = 2.96% Individual 95% CIs For Mean Based on Pooled StDev Level N Mean StDev Steam Curing (SC) ( * ) CO 2 Curing ( * ) Fig. 7 compares the strength development of blocks after steam and CO 2 curing. The strength of blocks after steam curing continued to develop quickly in a moist environment. The strength of blocks after CO 2 curing also increases with time, but at a lower rate than steam cured blocks. Blocks after steam curing still contain a high content of moisture and their strength can continue to increase as unhydrated cement particles within the blocks hydrate after the steam curing. Many results have confirmed that only a small portion of cement particles react with CO 2 during the CO 2 curing. However, blocks lost a very 103

9 significant amount of water during the pre-conditioning before and during the CO 2 curing. The reaction products formed during the CO 2 curing also slow the transport of water toward unreacted cement particles. All these contribute to the slow strength development of blocks in a moist environment after CO 2 curing. Figure 7: Strength Development of the Blocks after Steam and CO2 Curing 3.3 Reaction Products after Pre-conditioning and CO2 Curing Fig. 8 is the XRD patterns of cement pastes after pre-conditioning and CO 2 curing. From Fig. 8(a), it can be seen that after 4 hours of pre-conditioning period, hydration products such as Ca(OH) 2 and Aft can be clearly identified. Based on the intensity of these products, it can be justified that the specimens pre-conditioned in the moist environment contain more reaction products than those pre-conditioned in the dry environment. After CO 2 curing, peaks of calcite and aragonite appeared on the spectrum with very strong peaks, as shown in Fig. 8(b). On the other hand, peaks of cement hydration products such as Ca(OH) 2 and Aft become very weak or disappeared. This means that these reaction products also react with CO 2, which results in the formation of CaCO 3 and some other products. 104

10 (a) Before CO 2 Curing (b) After CO 2 Curing Figure 8: XRD Patterns of Minerals in Specimens Before and After CO2 Curing Fig. 9 shows SEM pictures of specimens before and after CO 2 curing. It can be seen that the specimens pre-conditioned in the dry environment have less amount of hydration products than those pre-conditioned in the moist environment. In the specimens pre-conditioned in the dry environment, only amorphous hydration products could be observed. In addition to amorphous hydration products, some prism and plate hydration products could be clearly observed in the specimens pre-conditioned in the moist environment. After 2 hours of CO 2 curing, a lot of tiny particles ranging from 1 to 2 um could be observed in the specimens cured in CO 2 gas after the dry-preconditioning. EDAX analysis indicated that these tiny particles are calcium carbonate. Compared with the specimens cured in CO 2 gas after the dry-preconditioning, much less tiny particles could be observed in the specimens cured in the CO 2 gas after moist pre-conditioning. This is in agreement with the measurement of the degree of CO 2 curing. However, the former demonstrated significantly higher strength than the latter. Thus, the CO 2 curing is mainly contributed by the reactions between CO 2 gas and cement clink minerals. 105

11 (a) After 4 hours of Dry Pre-conditioning (b) After 4 hours of Moist Pre-conditioning (c) CO 2 Curing after Dry Pre-conditioning (d) CO 2 Curing after Moist Pre-conditioning 4 CONCLUSIONS Figure 9: SEM Observation of Specimens Before and After CO2 Curing Based on the research from this study, the following conclusions can be drawn: (1) Pre-conditioning has a great effect on the reactions between CO 2 and cement minerals, and the strength of CO 2 curing of concrete products. (2) After proper pre-conditioning, the strength of conventional concrete blocks after 2 hours of CO 2 curing could be similar to that of concrete blocks after about 20 hours of steam curing. (3) A small portion of cement hydrates during the preconditioning period. The specimen pre-conditioned in the moist environment hydrates more than those pre-conditioned in the dry environment. However, much less CaCO 3 formed in the former that that in the latter. Thus, the CO 2 curing is mainly contributed by the reactions between CO 2 gas and cement clink minerals. ACKNOWLEDGEMENT Financial support from National Science Foundation of China ( ) is greatly appreciated. 106

12 REFERENCES [1] Young, J. F., Berger, R.L. and Breese, J., Accelerated Curing of Compacted Calcium Silicate Mortars on Exposure to CO 2. Journal of the American Ceramic Society, Vol.57, No.9 (1974), p [2] Bukowskiand, J.M. and Berger, R.L., Reactivity and Strength Development of Activated Non-Hydraulic Calcium Silicates. Cement and Concrete Research. Vol.9 (1979), p. 57. [3] Goodbrake, C.J., Young, J.F. and Berger, R.L., Reaction of Beta-dicalcium Silicate and Tricalcium Silicate with Carbon Dioxide and Water Vapor. Journal of the American Ceramic Society, Vol.62, No.3 (1979), p [4] Sorochkin, M.A., Shchrov, A.F., Safonov, I.A., Study of the Possibility of Using Carbon Dioxide for Accelerating the Hardening of Products Made from Portland Cement. J. Appl. Chem. Vol. 48 (1975), p [5] Short, N.R., Purnell, P. and Page, C.L., Preliminary investigations into the Super-critical Carbonation of Cement Pastes. J. Mater. Sci., Vol.36, No.1 (2001), p. 35. [6] Purnell, P., Short, N.R. and Page, C.L., Super-critical Carbonation of Glass Fiber Reinforced Cement: Part1. Mechanical Testing and Chemical Analysis. Composites Part A, Vol.32, No.12 (2001), p [7] Shao, Y. and Shi, C., Carbonation Curing for Making Concrete Products An Old Concept and a Renewed Interest, Proceedings of the 6 th International Symposium on Cement and Concrete, Vol.2 (2006), p [8] Monkman, S. and Shao, Y., Assessing the Carbonation Behavior of Cementitious Materials. Journal of Materials in Civil Engineering,Vol.18, No.11 (2006), p [9] Steinour, H. H., Some Effects of Carbon Dioxide on Mortars and Concrete: A Discussion, Journal of the American Concrete Institute, Vol.56, No.4 (1959), p [10] ASTM C330 - Standard specification for Lightweight Aggregates for Structure Concrete, Annual Book of ASTM Standards, Vol , Aggregates, Concretes, American Society for Testing & Materials, Philadelphia, [11] ASTM C331 - Standard specification for Lightweight Aggregates for Concrete Masonry Units. Annual Book of ASTM Standards, Vol , Aggregates, Concretes, American Society for Testing & Materials, Philadelphia, [12] Shi, C. and Wu, Y., Studies on some factors affecting CO2 curing of lightweight concrete products, Resources, Conservation and Recycling, Vol.52, No. 8-9, pp , [13] ASTM C140, Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units, Vol.04.05, American Society for Testing & Materials, Philadelphia,