Alkali activated metakaolin with high limestone contents – Statistical modeling of strength and environmental and cost analyses

Alkaline binders of metakaolin (MK) and limestone (LS) are promising sustainable cementitious new alternatives; however, systematic studies are required to understand them to take them to industrial level. This study presents the modeling and optimization of strength using the surface response method of such binders with up to 80%LS and molar ratios of Na/Al = 0.5–1.86 and Si/Al = 1.81–3.13. A quadratic model (R-squared = 94.31%) indicated an optimal formulation of 32.32%LS, Na/Al = 1.34 and Si/Al = 2.96 with a predicted, and experimentally corroborated, 28-day strength of 70 ± 3.9 MPa, which was higher than references of 100%MK and blended Portland cement (BPC). The microstructural features were in agreement with the high strength. The analyses of cost, CO2-emissions and energy demand indicated pastes of 100%MK are better than BPC only regarding CO2-emissions, while blends with %LS>32.12% were better in all indicators relative to both of the former. As limestone is abundant, cheap, reduces alkali demand and do not require calcination, these binders are actually sustainable.