Alshamsi et al.  reported that addition of micro-silica lengthened the setting time of pastes. This was expected since micro-silica replaces part of the OPC, reducing the early stiffening potential. While the addition of micro-silica (10%)
Table 2.5 Mix proportions, consistency of pastes made of cement and NS and silica fume  Sample Mix proportion in mass Consistency (mm)
had little effect on setting times, higher percentages produced significant influences. There was 6-20% increase in setting times when OPC was replaced with 20% micro-silica.
Lohtia and Joshi  concluded that the addition of silica fume to concrete in the absence of water-reducer or superplasticizer causes delay in setting time, compared to non-silica fume concrete of equal strength, especially when the silica fume content was high. The additions of 5-10% silica fume to either super- plasticized or non-superplasticized concrete with W/(C + SF) ratio of 0.40 did not exhibit any significant increase in setting time. However, when 15% silica fume was added with superplasticizer, both the initial and final setting times were delayed by approximately 1 and 2 h, respectively. The observed delay was attributed to the relatively high dose of superplasticizers needed for the high amount of silica fume added to concrete.
Rao  studied the influence of silica fume on the setting time of cement paste. Specific gravity and specific surface of the silica fume were 2.05 and 16,000 m2/kg, respectively. Figure 2.5 shows the variation of setting times with the addition of silica fume in cement pastes. It was observed that initial setting time decreased with the increase in silica fume content. At smaller contents, the setting time of cement paste did not affect much. However, at higher silica fume contents, the initial setting time was significantly decreased. At 30% silica fume, the initial setting time had been only 30 min. The final setting time seem to be not influenced by the silica fume. The pozzolanic action of silica fume seems to be very active at early hours of hydration. Therefore, he concluded that silica fume contents result in quick setting of cement.
Qing et al.  investigated the influence of nano-SiO2 (NS) on the setting time of cement paste incorporating NS or silica fume. The influence of NS or silica fume addition on consistency and setting time of fresh pastes is presented in
Table 2.6. It was observed that the setting of fresh pastes (sample A-series) was slightly accelerated but the difference between initial and final setting time decreased with increase in NS content. While the setting of fresh pastes (sample B- series) was obviously retarded and the difference was also decreased with increasing the silica fume content. They concluded that silica fume makes cement paste thinner and retards the cement setting process as compared with NS.