Effect of nano-silica on mechanical properties and microstructure of high-volume ground granulated blast furnace slag cement paste

Incorporating ground granulated blast furnace slag (GBS) as a supplementary cementitious material in cementitious materials can reduce the cement content and improve the performance of cement-based materials. However, adding GBS especially in large quantities, tends to decrease the early-age performance of cement-based materials, such as reducing early strength, increasing porosity, etc. This study introduced nanosilica (NS) into cementitious materials with a high volume GBS to address the deficiencies in early-age performance. This study aims to investigate and discuss the influence of NS on the mechanical properties and microstructure of cementitious materials with a high-volume GBS. The compressive strength, composition of hydration products, distribution of pores, composition of pore structure and microscopic morphology were tested and analyzed. Results showed a significant strength reduction in high-volume GBS (60 wt% GBS) cement paste when compared with pure cement paste samples. NS demonstrated a remarkable enhancement in the strength of high-volume GBS-contained samples in 3 and 7 days. The positive impact of NS on the strength of high-volume GBS-contained samples remained evident at 28 days of age. NS facilitated the hydration of C₃S and C₂S in high-volume GBS-contained samples. The enhancement in the hydration percentage of C₂S was particularly pronounced. In the high-volume GBS-containing cementitious materials, NS continued to consume CH and participate in pozzolanic reactions at a later age. NS decreased the pore volume in the ranges of >100 μm and 100 nm–1 μm in high-volume GBS-contained samples, reduced the quantity of unhydrated GBS and resulted in a more compact structure. The enhancement of strength and the promotion of C₃S and C₂S hydration in increased with the higher addition of NS at 1 wt% and 2 wt%. However, this enhancement and promotion effect diminished when the NS dosage increased to 3 wt%.