Using rice husk ash in alkali-activated ultra-high-performance concrete: Flowability, early age strength and elasticity modulus

The growth of rice husk waste is eco-unfriendly all over the world. Some literatures focused on the consuming large quantities of rice husk in ultra-high-performance concrete (UHPC). But Portland cement UHPC has higher carbon emissions. So rice husk ash (RHA) was used to replace slag and fly ash, producing an environment friendly alkali-activated UHPC (AAUHPC) herein. The effect of steel fiber volume and RHA weight fraction were studied. The increase of steel fiber and RHA content both decreased the flowability of AAUHPC. At the comparable strength, AAUHPC has better fluidity than Portland cement UHPC, due to its higher water/cement ratio. The growth of steel fiber and RHA content both increased the compressive, splitting tensile strength and modulus of elasticity of AAUHPC. The reinforcement effect of steel fibers on long-age compressive strength is significantly better than that on early-age compressive strength. Less than 10 % RHA is favorable for early age compressive strength. Power function formulas were developed to predict the elastic modulus and splitting tensile strength of AAUHPC. With the ideal AAUHPC mix using 2 vol% steel fiber and 15 wt% RHA, the compressive, splitting tensile strength and modulus of elasticity were improved to 159 MPa, 15.4 MPa, and 32.5 GPa, respectively, while the assessed CO2 emission, embodied energy, and cost were decreased to 450.3 kg/m3, 6509.8 MJ/m3, and 1050.9 USD/m3, respectively. The SEM and MIP indicated that AAUHPC presented reduced porosity, pore size and improved interface bonding, with an increase of RHA content from 5 % to 15 %, improving the strength and modulus of elasticity of AAUHPC. Compared with other UHPC in literatures, the steel fiber content could be reduced without sacrificing the mechanical properties of AAUHPC if an appropriate amount of RHA is added to reduce the cost, CO2 emission and embodied energy.