"Optimization of Silica Fume, Fly Ash and Amorphous Nano-Silica in Superplasticized High-Performance Concrete"

Synopsis: The influence of some pozzolanic additions – such as silica fume, fly ash and ultra-fine amorphous colloidal silica (UFACS) – on the performance of superplasticized concrete was studied. Superplasticized mixtures in form of flowing (slump of 230 mm) or self-compacting concretes (slump flow of 735 mm) were manufactured all with a water-cement ratio as low as 0.44, in order to produce high-performance concretes (HPC). They were cured at room temperature (20°C) or steam-cured at 65°C in order to simulate the manufacturing of pre-cast members. Concretes with ternary combinations of silica fume (15-20 kg/m), fly ash (30-40 kg/m) and UFACS (5-8 kg/m) perform better – in terms of strength and durability – than those with fly ash alone (60 kg/m) and approximately as those with silica fume alone (60 kg/m). Due to the reduced availability of silica fume on the market, these ternary combinations can reduce by 60-70% the needed amount of silica fume for each pre-cast HPC element at a given performance level. Moreover, at later ages the strength reduction in steam-cured concretes with respect to the corresponding concretes cured at room temperature, is negligible or much lower in mixtures with the ternary combinations of pozzolanic additions.