Effect of nano silica on compressive strength and microstructures of high volume blast furnace slag and high volume blast furnace slag-fly ash blended pastes

This paper presents the effect of nano silica (NS)on compressive strength and microstructure of cement paste containing high volume blast furnace slag (BFS)and high volume BFS-fly ash (FA)blend as partial replacement of ordinary Portland cement (OPC). Results show that high volume BFS pastes containing 60% and 70% BFS exhibited 4% higher and comparable compressive strength, respectively than control cement paste. However, about 9% and 37% reduction in compressive strength in high volume BFS pastes is observed due to use of 80% and 90% BFS, respectively. The high volume BFS-fly ash pastes containing total BFS and FA content of 60% exhibited about 5%–16% higher compressive strength than control cement paste. However, significant reduction in compressive strength is observed in higher BFS-FA blends with increasing fly ash contents. Results also show that the addition of 1–4% NS improves the compressive strength of high volume BFS paste containing 70% slag by about 9–24%. However, at higher BFS contents of 80% and 90% this improvement is even higher e.g. 11–29% and 17–41%, respectively. The NS addition also significantly improves the compressive strength of high volume BFS-FA pastes. The thermogravimetric analysis (TGA)results confirm the reduction of calcium hydroxide (CH)in high volume BFS and combined BFS-FA pastes containing NS indicating the formation of additional calcium silicate hydrate (CSH)gels in the system. The addition of NS also significantly reduced the pore volume of high volume BFS and combined BFS-FA pastes. The X-ray diffraction (XRD)analysis also confirms the reduction of intensity of CH peaks indicating its consumption in pozzolanic reaction. The scanning electron microscopy (SEM)images also confirm denser microstructure of high volume BFS and combined BFS-FA pastes containing NS compared to those without NS. By combining slag, fly ash and NS in high volumes e.g. 70–80%, the carbon footprint of cement paste is reduced significantly while maintains the similar compressive strength of control cement paste.