Metakaolin-Portland limestone cements: Evaluating the effects of chemical admixtures on early and late age behavior
Hosseinzadeh Zaribaf, Behnaz
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One of the most effective and applicable ways to mitigate the greenhouse gas emissions contributed by cement production is to partially replace a portion of the cement clinker with alternative materials such as mineral filler (e.g., limestone) and pozzolanic materials (e.g., calcined clay or “metakaolin”), that require much less energy-intensive production processes compared to that of portland cement. However, the use of finer materials at higher rates has been limited by reductions in concrete workability, incomplete understanding of water reducing admixture (WRA) compatibility, and potential for increased shrinkage. To evaluate the combined use of metakaolin at higher replacement levels (> 10% by mass) with portland limestone cement (PLC) and their interaction with chemical admixtures, this investigation examines (1) early age hydration and microstructure development, (2) mechanical properties, and (3) durability including propensity for shrinkage-induced cracking in the context of understanding the interactions of water-reducing and shrinkage-reducing admixtures (WRAs, SRAs) with PLC-MK blended cements and concrete. Among the four common WRA types, lignosulfonate (LS) was the least effective and polycarboxylate ether (PCE) the most effective admixture to improve the workability of PLC-MK blended cements. To improve predictions of heat of hydration in PLC-MK systems, a new model based on parameters obtained through isothermal calorimetry is proposed. Accelerated cement hydration in the presence of 30% MK lead to greater autogenous shrinkage in the first 3 days, but the 10% PLC-MK paste showed the greatest autogenous shrinkage at 28 days. The use of SRAs at 1% by mass of binder was effective in mitigating autogenous shrinkage, decreasing shrinkage in PLC-MK by 50-60%, but also moderately delaying the cement hydration. The use of MK in PLC concrete increased the compressive strength by more than 90% as early as one day and by 130% at 90 days of age. In comparing PCE with another WRA (i.e., polymelamine sulfonate, PMS), some retardation in strength development occurred and approximately 20% reductions in tensile strength and elastic modulus development were found; in contrast, improvements up to 43% in tensile strength and elastic modulus were found in the PCE PLC-MK compared to those of PLC concrete. This additional evidence points to the superiority of the PCE admixture in this system. With respect to durability, the impermeability of PLC-MK concrete was significantly improved with increasing MK use. Although autogenous shrinkage was greater in the MK-containing pastes, MK-concrete exhibited less drying shrinkage, likely because less evaporable water is available during this period due to pozzolanic activity of MK, formation of new phases (e.g., secondary calcium silicate hydrates, carboaluminate), and the resulting densification of the paste. Overall, chemical admixtures were found to facilitate the use of MK at up to 30% by mass of PLC, with mixtures achieving similar or improved early and late age properties relative to PLC alone.