The effects of calcium carbonate on sodium metasilicate-activated metakaolin-based geopolymer pastes

The influence of pure calcium carbonate (CaCO₃) on the alkali-activation kinetics, microstructure, and compressive strength of sodium metasilicate-activated metakaolin (MK) geopolymer cements was investigated herein. Experimental results showed that replacing 10–50 % of MK with ground CaCO₃ particles delayed the acceleration stage of alkali-activation regardless of replacement amount. However, 10 % CaCO₃ replacement increased the overall degree of reaction and yielded pastes with similar 14- and 28-day compressive strengths compared to the experimental control with no CaCO₃ addition. In comparison, 50 % CaCO₃ replacement led to a lower overall degree of reaction and lower compressive strengths through 28 days of curing. Experimental evidence substantiates that N-A-S-H gel forms in samples containing 0 % and 10 % CaCO₃, while N-(C)-A-S-H gel and/or C-(N)-A-S-H are also present in the binding matrix near CaCO₃ particles in samples with higher CaCO₃ contents (i.e., 30 % and 50 %). While the CaCO₃ particles were primarily composed of calcite, vaterite, a less stable form of CaCO₃, was also evident in the samples containing 30 % and 50 %, suggesting that the CaCO₃ particles were moderately reactive during the alkali-activation. This study isolates the effects of pure CaCO₃ in sodium metasilicate-activated geopolymer systems, providing novel insights into its impact on geopolymerization kinetics and microstructural development for optimizing sustainable geopolymer cement formulations.