Evidence of scawtite and tilleyite formation at ambient conditions in hydrated Portland cement blended with freshly-precipitated nano-size calcium carbonate to reduce greenhouse gas emissions

Abstract

Activated calcium carbonate (a-CaCO3) is used partially to replace Portland cement. a-CaCO3 is comprised of nanoscale calcium carbonate, in amorphous and calcite forms, and its enhanced carbonate activity converts calcium carbonate from being an inert filler to a reactive component. Its reaction with the C–S–H phase alters the conventional hydrate mineralogy with spontaneous formation at ∼20 °C of scawtite, Ca7(Si6O18)CO3·2H2O and tilleyite, Ca5Si2O7(CO3)2. Compressive strength measurements show that up to 20 mass% cement replacement by calcium carbonate does not decrease 7- and 28-day compressive strengths compared to a Portland cement benchmark. a-CaCO3 also accelerates the hydration of silicate clinker minerals. Using activated calcium carbonate as a supplementary cementing material enables substantial reduction of CO2 emissions, firstly by capturing part of the CO2 from cement kilns to make nanoscale calcium carbonate and secondly, by using the a-CaCO3 capture product to replace part of the cement.