Influence of nano-CaCO₃ addition on the compressive strength and microstructure of high volume slag and high volume slag-fly ash blended pastes

This paper presents the effects of various nano-CaCO₃ (NC) contents on 28 days compressive strength and microstructure of high volume slag (HVS) pastes and high volume slag-fly ash (HVSFA) blended pastes. The results show that addition of 1–4% NC improved the compressive strengths of HVS pastes containing 70%, 80% and 90% slag, respectively by 8–24%, 1–16% and 2–20% compared to their reference pastes. The compressive strength of HVSFA paste containing combined slag and fly ash content of 70% is also increased significantly by 16–24% than the reference control paste due to the addition of 1–4% NC and maintained higher compressive strength than OPC paste after 28 days of curing. The addition of NC reduced the large capillary pores and cumulative pore volume of HVS pastes and HVSFA paste. Significant reduction of calcium hydroxide (CH) in HVSFA paste containing combined slag and fly ash content of 70% is found due to the addition of NC by thermogravimetric analysis (TGA) and a slight increase of CH is noticed in all HVS paste. A denser microstructure with fewer internal micro-cracks and more hydrated products are revealed in scanning electron microscopic (SEM) images and EDS trace analysis in all HVS and HVSFA pastes due to the addition of NC. HVS and HVSFA paste with significantly lower carbon footprint and higher compressive strength than control cement paste can be achieved with the inclusion of 1% NC.