Hydration reaction and compressive strength of small amount of silica fume on cement-fly ash matrix

In 2021, the price of silica fume had fallen to 400 RMB/ton in some areas, while that of cement had risen to 650 RMB/ton, which provided a realistic basis for applying silica fume in low-and medium-strength concrete. In this paper, the effects of adding a small amount of silica fume on the water demand, setting time, exothermic heat of hydration, fluidity, mechanical properties, and water absorption of mortar with 0.5%, 1.0%, 1.5%, 2.0%, 2.5% and 3.0% of silica fume were investigated respectively. XRD and TG-DTG were used for microscopic analysis. The results showed that silica fume improved the fluidity of the cement fly ash system and reduced the water demand of the cement fly ash system, and that the water demand of the paste was reduced by about 4.5%. Therefore, water or superplasticizers could be reduced while maintaining the same fluidity. The exotherm peak during the hydration reaction are reduced. The silica fume improved the later strength of the specimen, accompanied by a slight decrease in the early strength. The water absorption was significantly reduced. Silica fume consumed CH in the cement-fly ash system and promoted the production of C-S-H. Therefore, the cost of cement consumption and improve performance could be lowered by replacing cement with silica fume.