Performance evaluation of silica fume and metakaolin with identical finenesses in self compacting and fiber reinforced concretes

Partial replacement of cement by metakaolin (MK) increases the strength of concrete, but since the properties of MK are important parameters it is not entirely clear whether MK is more effective than silica fume (SF) in enhancing the properties of concrete. In this study, in high-performance self-compacting concretes (SCC) with two different water/binder (w/b) ratios (0.28 and 0.35), cement was replaced by MK or SF at the weight fraction of 10%. In order to provide a meaningful comparison, the pozzolans with similar particle sizes were used. The effectiveness of MK and SF on the autogenous shrinkage, calcium hydroxide (CH) consumption, flowability, mechanical properties and fracture behavior of SCC was examined. In addition, the performances of these two types of mineral additives in fiber reinforced concrete under four-point bending were documented. It was shown that both MK and SF had an increasing effect on autogenous shrinkage of paste with 0.35 w/b ratios, while at 0.28 w/b ratio the mixtures with MK showed a lesser amount of autogenous shrinkage at 48 h. CH consumption was found to be slightly higher in the pastes containing MK compared to those with SF. The long-term autogenous shrinkage of SCCs with MK was lower than those with SF. The use of MK and SF as mineral additives caused a decrease in fracture energies of concretes, and this effect is more pronounced in SF blended concretes, while no significant change was observed in the compressive strength developments of the mixtures. Fiber reinforced concretes containing MK were found to have much higher fracture energy than that with SF, indicating that the MK addition improves the matrix/steel fiber bonding properties.