Effect of <i>n</i>-CaCO₃ and metakaolin on hydrated Portland cement

Abstract

The effect of calcium carbonate nanoparticles (nCaCO 3 ) on the hydration of ordinary Portland cement (OPC) blended with metakaolin was studied through comparison with OPC, OPC plus metakaolin and OPC plus nCaCO 3 control samples. Isothermal conduction calorimetry measurements showed that the hydration of the ternary OPC–metakaolin–nCaCO 3 blend had pronounced early secondary phase hydration activity compared to the controls. X-ray diffraction and thermogravimetric analysis identified the phases as calcium aluminosulfates, with both ettringite and monosulfate forming more quickly in the ternary blend than in the other samples. The presence of the nCaCO 3 also enhanced the formation of C 3 A·CaCO 3 ·12H 2 O in both blends that contained it. The presence of the secondary phases or the nCaCO 3 itself was associated with a significant reduction in strength in the ternary blend as compared to the OPC–metakaolin binary blend. The results suggest that caution is needed in the development of concrete with nanoparticle additions in order to avoid detrimental combinations of nanoparticles, OPC and supplementary cementing materials.