Design and optimization of alkaline binders of limestone-metakaolin – A comparison of strength, microstructure and sustainability with portland cement and geopolymers

Abstract

Studies on alkali activated cements (AAC) of metakaolin (MK) with up to 50% replacement by limestone (LS) have recently emerged; although interesting, no reduction in the alkali content has generally been implemented, in detriment of the durability, safety, environmental impact and costs. This paper presents an innovative approach for designing sustainable AAC of composites precursors of MK with high contents of LS, activated by NaOH–Na2O·nSiO2. The method combines design of mixtures and a multi-response statistical optimization, in which the 28-day strength was maximized while the CO2-emissions, energy demand and production cost were simultaneously minimized. The approach resulted in a binder with high LS and reduced MK and alkali contents. The optimal formulation was 80%LS, Na2O/Al2O3 = 0.94, and SiO2/Al2O3 = 3.54, equivalent to only 4.7% Na2O relative to the mass of the precursors (MK + LS), which reduces the risk of deleterious carbonation and is lower than the requirements of AAC of 100%MK and similar to those of AAC of blast furnace slag. This optimal binder had predicted and measured strengths of 49.96 ± 13.1 and 51.9 ± 0.7 MPa, respectively, which confirmed the accuracy of the modelling; its microstructure had a dense matrix of cementitious products with partially reacted limestone particles distributed throughout, the reaction products were amorphous with a chemical composition that suggested a mixture of (C,N)-A-S-H with N-A-S-H, C-A-S-H and C–S–H. Compared to a reference portland cement, the optimal formulation emits 75.1% less CO2, consumes 41.1% less energy and is 42.6% less expensive; while compared to the AAC of 100%MK, the reductions were 51.6, 53.8 and 57.5%, respectively. The optimal formulation represents a competitive sustainable alternative cement.