Response surface methodology-based modelling and optimization of sustainable UHPC containing ultrafine fly ash and metakaolin

•Response surface methodology is utilized to determine the optimal composition of UHPC.•Quadratic polynomial model is established and the results show good fitting and high predication accuracy for strength.•Experimental verification confirmed the results of response surface methodology optimization.•UFFA and MK reduces the overall cost and carbon emissions.•Incorporation of UFFA improves compressive strength of concrete significantly. The current work studies the applicability of using a statistical model supported by response surface methodology (RSM) in the formulation and optimization of sustainable UHPC mixtures incorporating industrial waste materials. The effects of the three main variable components of UHPC, ultra-fine fly ash (UFFA), metakaolin (MK), and steel fibre content, were investigated to understand the behaviour of sustainable UHPC. Twenty UHPC mixtures were designed and generated experimentally using the central composite design concept in RSM. Workability, density, compressive, and splitting tensile strengths were the most important performance requirements and responses of these UHPC mixtures. The predicted models showed that all inputs and outputs are perfectly correlated. Moreover, a multi-objective optimization technique shows that the properties of hardened UHPC improved as the content of UFFA and steel fibres increased. As a result, a sustainable UHPC with a compressive strength of 164.10 MPa, a splitting strength of 16.1 MPa, and a slump value of 139 mm was produced using 675.43 kg/m3 of cement content, 135 kg/m3 of silica fume, 236 kg/m3 of steel fiber, and the optimum content of industrial wastes (UFFA = 163.74 kg/m3).To investigate the microstructure and composition of the hydration products, optimally proportioned and control mix samples were examined by SEM and XRD. Macroscopic results revealed that the UHPC with optimal UFFA content has a denser microstructure, which confirms the effectiveness of RSM in determining the optimal mix parameters of the UHPC design. The results of economic and environmental evaluations demonstrate that the incorporation of UFFA and MK significantly reduces the cost and carbon emissions of UHPC, and the effect of UFFA is even more significant.