Effects of fly ash, activator ratio and steel fiber on freeze–thaw cycle, sulfuric acid, thermal conductivity and impact resistance of silica fume based AAHSMs

Abstract

This study is focused on the effects of Class C fly ash (FA), NaOH/Na2SiO3 (SH/SS) ratio and steel fiber (STF) on the freeze–thaw (F-T) cycle, sulfuric acid, thermal conductivity and impact resistance of silica fume (SF) based alkali activated high strength mortars (AAHSMs). In the AAHSM mixtures, FA in different proportions (10 %, 15 %, 20 % and 25 %) is used instead of SF by weight. All the mixtures are prepared with water-to-binder rate of 0.32. F-T cycle results exhibit that the flexural strength (ffs) and compressive strength (fc) values of SF based AAHSMs without and with STF are less affected than those of ordinary Portland cement (OPC) based high strength mortars (OPCHSMs) without and with STF. After 300 F-T cycles, the ffs and fc values of SF based AAHSMs without and with STF are still over 14 MPa and 95 MPa, respectively. After exposure to 20 % sulfuric acid solution for 90 and 180 days, the SF based AAHSMs without and with STF exhibit super performance (i.e., fc values between 78.95 MPa and 96.81 MPa), while the surfaces of OPCHSMs without and with STF are disintegrated. Additionally, the effects of F-T cycle and sulfuric acid solution on SF based AAHSM and OPCHSM are examined with the microstructure analyses. Thermal conductivity results show that the SF based AAHSMs without and with STF perform better than the OPCHSMs without and with STF. The thermal conductivity results of OPCHSMs without and with STF vary between 0.630 W/m.K and 0.697 W/m.K, while the thermal conductivity results of SF based AAHSMs without and with STF range from 0.507 W/m.K and 0.623 W/m.K. Thermal conductivity results are adversely influenced by increasing STF content in the SF based AAHSMs and OPCHSMs. Impact resistance results show that the SF based AAHSMs without and with STF perform better than the OPCHSMs without and with STF. Moreover, the impact resistance values increase, as the amount of STF used in the SF based AAHSMs and OPCHSMs increases.