An Investigation of Blended Cements with High Volume Interground Limestone

Transcription

1 An Investigation of Blended Cements with High Volume Interground Limestone A. R. Kotwal 1, A. T. Winters 2 and J. J. Schemmel 3 1 Texas State University, Materials Science, Engineering and Commercialization Program, 601 University Drive, San Marcos, TX 78666; PH (512) ; FAX (512) ; ak1348@txstate.edu 2 Capitol Aggregates, 2330 North Loop 1604, San Antonio, TX 78249; PH (210) ; FAX (210) ; rusty.winters@capitolaggregates.com 3 Texas State University, Concrete Industry Management Program, 601 University Drive, San Marcos, TX 78666; PH (512) ; FAX (512) ; jjs143@txstate.edu Abstract The production of portland cement is one of the largest sources of energy consumption and carbon dioxide emissions in the world. Although cement is critical to the development of most infrastructure, regulations and permitting make it extremely cumbersome to increase production capacity. Consequently, the investigation of cement replacement in concrete mixtures is considered to be a worthwhile challenge in the construction industry. By intergrinding limestone with clinker, cement manufacturers can increase overall production volume and decrease material costs. The present research takes advantage of these benefits through the development of innovative and sustainable blended cements with limestone content up to 25%. To determine their feasibility in concrete applications, it is crucial to evaluate the characteristics of the materials and their effect on fresh and hardened properties. An experimental program was performed to support the development of portland limestone cement. Results indicated a lower rate of heat generation during cement hydration as limestone content increased. Reduced reactivity delayed the setting time and decreased compressive strength, although drying shrinkage measurements displayed improved volumetric stability. Conclusions suggest that additional mechanical and durability testing should be performed to determine the commercial viability of portland limestone cement and provide recommended applications in the construction industry. Introduction Urban land in the United States is projected to double by 2030 (Seto et al., 2012). Many of the structures and pavements built in the expanding urban environment will include large volumes of concrete. Consequently, concrete will continue to be the most widely used construction material and the second most consumed resource in the world. Thus, the demand for the basic constituents of concrete will also grow (Atakan et al., 2014). Texas is the largest producer and consumer of cement in the United States. As shown in Figure 1, projections for the demand of cement in Texas quickly outpace production and import capacity. Including announced production expansions, it is estimated that native 2015 International Concrete Sustainability Conference 1

2 production capacity and foreign import capacity will be exceeded as early as 2017 and 2024, respectively (Prusinski, 2014). Figure 1. Texas Cement Capacity and Projected Demand (Prusinski, 2014). To increase the overall production volume of cement while reducing energy consumption and hazardous emissions, manufacturers produce blended cements using supplementary cementitious materials. Another option is to produce portland limestone cement conforming to ASTM C595 (2013) which is manufactured by blending limestone at levels currently limited to 15% by mass. However, to produce a more sustainable material, larger amounts of mineral admixtures can be used. It is commonly known that the manufacturing of conventional portland cement consumes large quantities of energy. Figure 2 illustrates an example of a cement manufacturing plant. Although there are different layouts and equipment at every plant, the operational steps of the modern process are generally the same. Raw material sources of calcium carbonate, silica, alumina and iron are quarried, crushed, blended and milled into a fine powder. The material is then fed into a preheater tower and rotary kiln that are used for the chemical conversion process (Kosmatka et al., 2003). As the temperature increases, calcium carbonate is converted to calcium oxide, and carbon dioxide is released. Silicates begin to combine with calcium oxide to form round belite crystals, and the reaction between belite and calcium oxide forms angular alite crystals. Agglomeration of crystalline particles forms clinker nodules as the rotary kiln temperature reaches approximately 1370 C (2500 F). When the clinker cools, it is combined with a small amount of gypsum and ground into a fine powder (Kosmatka et al., 2003) International Concrete Sustainability Conference 2

3 Figure 2. Cement Manufacturing Plant (WBCSD, 2012). The thermal energy from burning coal, oil, natural gas, rubber tires or byproduct fuels is used during the manufacturing process to heat the rotary kiln and preheaters. The thermal processing stage is the most energy-intensive, accounting for over 90% of the energy used during cement production. With an efficient preheater tower and rotary kiln, it requires approximately 2.6 GJ (2.5 MMBtu) to produce 900 kg (1 ton) of clinker. The energy consumed during the finish grinding process is dependent on the additives used and the particle size required for the final product. Conventionally, grinding is performed with ball mills, which use an average of about 190 MJ (180 MBtu) per 900 kg (1 ton) (Worrell et al., 2013). Blended cements reduce the carbon footprint of concrete by using less clinker. They are produced either by intergrinding mineral admixtures with clinker at the manufacturing plant or by blending them with cement powder after production. Intergrinding limestone with clinker has been shown to produce cement with a wider particle size distribution. Since limestone is softer than clinker, portland limestone cement is often finer and has higher surface area. These characteristics impact constructability, water demand, packing density and durability (Tennis et al., 2011) International Concrete Sustainability Conference 3

4 The study was based on an investigation of the cement types shown in Table 1. Nominal values of interground limestone and Class F fly ash are shown by mass percentage. Table 1. Cement Types. Cement Type Limestone Content Class F Fly Ash Content (% by Mass) (% by Mass) I 5 0 IL(15) 15 0 IL(25) 25 0 IT(L15)(P25) Research Significance Previous studies of portland limestone cement have focused mainly on cement containing up to 15% limestone. In addition, limited research has been conducted on the volumetric stability of portland limestone cement and its interaction with fly ash. Original contribution to the scientific literature was made by the research team to fill gaps in the existing body of knowledge regarding the characteristics and physical properties of cement with limestone content up to 25%. Further contribution was made by quantitatively analyzing the hydration, hardening behavior, compressive strength and volumetric stability of portland limestone cement. Material Characteristics Cement performance is directly related to its constituent crystalline phases. The use of limestone in cement requires thorough knowledge of its characteristics, which are primarily influenced by particle size and chemical composition (Schiller & Ellerbrock, 1992). There are three primary effects of incorporating limestone in cement: 1. Limestone particles fill the interstitial sites between the aggregates and cement particles, creating a denser binder matrix (Sprung & Siebel, 1991). 2. Limestone affects cement hydration by dispersing cement grains and acting as a crystallization nucleus (Lothenbach et al., 2008). 3. Limestone is mostly inert but can react with cement to form carboaluminate phases, which is likely to occur with finer particles at low concentrations (Matschei et al., 2007). Blaine Fineness of Cement. As specified in ASTM C204 (2011), the Blaine fineness of the cement was quantified using an air permeability apparatus. Comminution of the cement in the finish mill resulted in the Blaine fineness values reported in Table 2. With a consistent milling duration, the surface area increased as more limestone was interground with clinker International Concrete Sustainability Conference 4

5 Table 2. Blaine Fineness of Cement. Cement Type Blaine Fineness (cm 2 /g) I 3760 IL(15) 4390 IL(25) 4840 IT(L15)(P25) 4170 X-Ray Diffraction of Cement. A Bruker D8 Advance Eco A25 was used for measuring the X-ray diffraction pattern of the cement powder. A thin and level layer was placed on the stage for determination of crystalline structure, and low angle noise was blocked with a blade. Figure 3 illustrates the X-ray diffraction patterns of the blended cements. The primary crystalline component was alite, which is characterized by the Bragg peaks shown for Type I cement. As the limestone content increased, the sharp peak at 29.4 intensified. This increase in intensity is indicative of calcium carbonate, the main crystalline component of limestone. A small peak at 26.6 was detected in the Type IT(L15)(P25) cement due to the crystalline silica contained in fly ash Type I Type IL(15) 300 Intensity Type IL(25) Type IT(L15)(P25) θ ( ) Figure 3. X-Ray Diffraction Pattern of Cement International Concrete Sustainability Conference 5

6 Specific Gravity of Cement. The specific gravity of the blended cements was calculated based on the specific gravity and mass percentage of the constituents. The results are shown in Table 3 based on the interground limestone and Class F fly ash having a specific gravity of 2.71 and 2.32, respectively. The specific gravity decreased as a consequence of incorporating larger quantities of the mineral admixtures. Table 3. Specific Gravity of Cement. Cement Type Specific Gravity I 3.15 IL(15) 3.09 IL(25) 3.05 IT(L15)(P25) 2.89 Particle Size Distribution of Aggregate. A sieve analysis of the fine aggregate used to make mortar was performed in accordance with ASTM C136 (2006). Figure 4 shows the results of the sieve analysis. The gradation of the Ottawa sand conformed to the requirements for graded standard silica sand as per ASTM C778 (2013) % Passing Grain Size (mm) Figure 4. Sieve Analysis of Ottawa Sand. 0.1 Specific Gravity and Absorption of Aggregate. The specific gravity and absorption of the fine aggregate was measured in accordance with ASTM C128 (2012). The test results are presented in Table International Concrete Sustainability Conference 6

7 Table 4. Specific Gravity and Absorption of Aggregate. Oven Dry Saturated Surface Dry Aggregate Absorption (%) Specific Gravity Specific Gravity Ottawa Sand Constructability The constructability of fresh mortar is dependent on its ease of placement, compaction requirements and hardening behavior. The rheology of portland limestone cement was investigated by Tezuka et al. (1992), concluding that the flow improved with the inclusion of 5% limestone. In cement containing 15% limestone, Neto & Campiteli (1990) found that consolidation occurred more easily due to the fine particles that displaced water from the voids between the coarser particles. Thus, the water content can be decreased while making a mixture that retains adequate constructability. Previous research has shown that water retention also improves with the addition of limestone, thereby decreasing segregation (Schmidt, 1992). Additionally, the curing of cement can be affected due to an increased number of nucleation sites depending upon the fineness of the limestone particles (Livesey, 1991). Isothermal Calorimetry of Mortar. A Calmetrix I-Cal 8000 isothermal calorimeter with CalCommander software recorded the heat flow generated by the early hydration reaction of cement as the ambient temperature around the samples was controlled. As per Figure 5, the more prominent initial rate of thermal energy generated by Type I cement after a hydration time of 0.5 hours resulted from a higher content of tricalcium aluminate. This heat flow peak exhibited decreased power as the amount of limestone increased. A predictable trend was observed during the hydration of the calcium silicate phases after a hydration time of about 7 hours. More limestone and fly ash resulted in less energy being produced. The most substantial impact was found to be the increased duration between the main hydration peak and the later sulfate depletion peaks. A shorter interval typically corresponds to higher early strength and is, therefore, a good indicator for early strength development. Additionally, the elevated heat generation rate of the Type IL(15) sulfate peak indicates that more sulfate would not be detrimental. Setting Time of Mortar. Penetration resistance was used to determine the initial and final setting times of mortar as defined in ASTM C403 (2008). Since this standard test method does not provide a specified mixture and the specific gravity of the blended cement varies, it was determined that a volumetric mixture would be used to maintain a consistent paste volume with a water-cement ratio. The absorption of the fine aggregate was also accounted for when proportioning the material. Figure 6 indicates the results of the setting time test. The setting time of the blended cements was delayed as a consequence of incorporating larger quantities of mineral admixtures. Only a slight increase in setting time was measured for Type IL(25) when compared to Type IL(15). However, the fly ash substantially hindered the setting time of the Type IT(L15)(P25) International Concrete Sustainability Conference 7

8 Heat Generation Rate (W) Type I Type IL(15) Type IL(25) Type IT(L15)(P25) Hydration Time (hr) Figure 5. Isothermal Calorimetry of Mortar. Penetration Resistance (MPa) Type I Type IL(15) Type IL(25) Type IT(L15)(P25) Final Set (27.6 MPa) Initial Set (3.4 MPa) Hydration Time (hr) Figure 6. Setting Time of Mortar. Flow of Mortar. The flow of each mixture used for setting time was measured with a flow table as per ASTM C1437 (2013). With a constant paste volume and water-cement ratio, the measurements in Table 5 signify no considerable variation in mortar flow International Concrete Sustainability Conference 8

9 Table 5. Flow of Mortar. Cement Type Mortar Flow (%) I 104 IL(15) 109 IL(25) 106 IT(L15)(P25) 109 Mechanical Behavior The compressive strength of mortar is commonly considered to be its most important characteristic, although in some cases, other mechanical properties may be more critical. Strength is not typically reduced when incorporating 5% to 10% limestone. If larger quantities of limestone are used, however, then the strength decreases compared to ordinary portland cement (Sprung & Siebel, 1991). Compressive Strength of Mortar. The compressive strength of mortar based on portland limestone cement was measured in accordance with ASTM C109 (2013). As displayed in Figure 7, the compressive strength of the Type IL(15) was comparable to Type I at 1 and 3 days. Slightly lower strength was measured at 7, 28 and 90 days. Mortar based on Type IL(25) exhibited further decreases in strength throughout the measurement period due to its increased limestone content. Although similar strength was measured for the Type IL(25) and Type IT(L15)(P25) at 28 days, the ternary blend had substantially lower early strength and higher late strength as a result of the delayed reactivity of fly ash. 40 Compressive Strength (MPa) Type I Type IL(15) Type IL(25) Type IT(L15)(P25) Age (days) Figure 7. Compressive Strength of Mortar International Concrete Sustainability Conference 9

10 Volumetric Stability Prior to implementing portland limestone cement in the construction industry, it is crucial to evaluate its ability to resist volumetric changes while maintaining desired engineering properties (TxDOT, 2013). The drying shrinkage of cement is a phenomenon that can cause decreases in volume due to influential factors, such as temperature, humidity or evaporation rate (CCAA, 2002). Previous studies on drying shrinkage have found no effect on volume change due to the addition of small amounts of limestone (Detwiler, 1996). Drying Shrinkage of Mortar. The drying shrinkage of mortar was determined as per ASTM C596 (2009), and the test results are shown in Figure 8. Mortar prisms for all cement types shrank approximately the same amount during the first 25 days of air storage. However, the divergence in the 90 day measurements indicates that long term drying shrinkage decreased for cement containing higher volumes of interground limestone and fly ash Drying Shrinkage (%) Type I Type IL(15) Type IL(25) Type IT(L15)(P25) Air Storage (days) Figure 8. Drying Shrinkage of Mortar. Conclusions The following conclusions were drawn from the results of the study: 1. The heat generated during cement hydration decreased with higher volumes of interground limestone. 2. Portland limestone cement exhibited a delayed setting time when compared to Type I cement, and the addition of fly ash further postponed hardening. 3. No substantial variation in mortar flow was measured with mixtures containing a constant paste volume and water-cement ratio International Concrete Sustainability Conference 10

11 4. A high volume of interground limestone caused a decrease in compressive strength. The inclusion of fly ash also resulted in low early strength but higher long term strength evolution. 5. Drying shrinkage reduced with the inclusion of larger amounts of limestone and fly ash, making the mixtures more volumetrically stable. Future Research Based on the conclusions, there is a need for further research in the following areas: 1. Blended cements with high volume interground limestone should be investigated for use in concrete mixtures. 2. It is critical to evaluate the durability of portland limestone cement to determine if it is susceptible to sulfate attack or freeze-thaw damage. 3. A market impact analysis is needed to determine the commercial viability of portland limestone cement. References ASTM C1012. (2013). Standard test method for length change of hydraulic cement mortars exposed to a sulfate solution. West Conshohocken, Pennsylvania: ASTM International. ASTM C109. (2013). Standard test method for compressive strength of hydraulic cement mortars. West Conshohocken, Pennsylvania: ASTM International. ASTM C128. (2012). Standard test method for density, relative density (specific gravity) and absorption of fine aggregate. West Conshohocken, Pennsylvania: ASTM International. ASTM C136. (2006). Standard test method for sieve analysis of fine and coarse aggregates. West Conshohocken, Pennsylvania: ASTM International. ASTM C1437. (2013). Standard test method for flow of hydraulic cement mortar. West Conshohocken, Pennsylvania: ASTM International. ASTM C204. (2011). Standard test methods for fineness of hydraulic cement by air permeability apparatus. West Conshohocken, Pennsylvania: ASTM International. ASTM C403. (2008). Standard test method for time of setting of concrete mixtures by penetration resistance. West Conshohocken, Pennsylvania: ASTM International. ASTM C595. (2013). Standard specification for blended hydraulic cements. West Conshohocken, Pennsylvania: ASTM International. ASTM C596. (2009). Standard test method for drying shrinkage of mortar containing hydraulic cement. West Conshohocken, Pennsylvania: ASTM International. ASTM C778. (2013). Standard specification for standard sand. West Conshohocken, Pennsylvania: ASTM International. Atakan, V., Jain, J., Ravikumar, D., McCandlish, L., & DeCristofaro, N. (2014). Water savings in concrete made from solidia cement. Piscataway, New Jersey: Solidia Technologies. CCAA. (2002). Drying shrinkage of cement and concrete. Sydney, Australia: Cement Concrete & Aggregates Australia International Concrete Sustainability Conference 11

12 Detwiler, R. J. (1996). Properties of concretes made with fly ash and cements containing limestone. Skokie, Illinois: Portland Cement Association. Kosmatka, S. H., Kerkhoff, B., & Panarese, W. C. (2003). Design and control of concrete mixtures. Skokie, IL: Portland Cement Association. Livesey, P. (1991). Performance of limestone-filled cements. Blended Cements in Construction, Lothenbach, B., LeSaout, G., Gallucci, E., & Scrivener, K. (2008). Influence of limestone on the hydration of portland cements. Cement and Concrete Research, 38, Matschei, T., Lothenbach, B., & Glasser, F. P. (2007). The role of calcium carbonate in cement hydration. Cement and Concrete Research, 37, Neto, C. S., & Campiteli, V. C. (1990). The influence of limestone additions on the rheological properties and water retention value of portland cement slurries. Carbonate Additions to Cement, Prusinski, J. R. (2014). Texas cement demand presents challenges, opportunities. TACA Conveyor, Winter, Schiller, B., & Ellerbrock, H. G. (1992). The grinding and properties of cement with several main constituents. Zement-Kalk-Gips, 45 (7), Schmidt, M. (1992). Cement with interground additives - Capabilities and environmental relief. Zement-Kalk-Gips, 45 (2), Seto, K. C., Guneralp, B., & Hutyra, L. R. (2012). Global forecasts of urban expansion to 2030 and direct impacts on biodiversity and carbon pools. In B. L. Turner (Ed.), Proceedings of the National Academy of Sciences of the United States of America. 109, pp Boston, Massachusetts: Yale University. Sprung, S., & Siebel, E. (1991). Assessment of the suitability of limestone for producing portland limestone cement. Zement-Kalk-Gips, 44 (1), Tennis, P. D., Thomas, M. D., & Weiss, W. J. (2011). State-of-the-art report on use of limestone in cements at levels of up to 15%. Skokie, Illinois: Portland Cement Association. Tezuka, Y., Gomes, D., Martins, J. M., & Djanikian, J. G. (1992). Durability aspects of cements with high limestone filler content. 9th International Congress of the Chemistry of Cement, (pp ). New Delhi, India. TxDOT. (2013). Evaluating limestone cements containing greater than 15% limestone. Austin, Texas: Texas Department of Transportation. WBCSD. (2012). Cement sustainability initiative. Geneva, Switzerland: World Business Council for Sustainable Development. Worrell, E., Kermeli, K., & Galitsky, C. (2013). Energy efficiency improvement and cost saving opportunities for cement making. Washington, D.C.: United States Environmental Protection Agency International Concrete Sustainability Conference 12