Development and design of ultra-high ductile magnesium phosphate cement-based composite using fly ash and silica fume

Magnesium phosphate cement (MPC) is one of the most promising binders to partially replace Portland cement to reduce carbon emissions. This study attempted to develop and design an ultra-high ductile MPC-based composite (UHDMC) using fly ash (FA) and silica fume (SF). Firstly, the macro-mechanical analysis of the UHDMC indicated that both the ultimate tensile stress and the ultimate tensile strain improved greatly when the additions of FA and SF replacing MPC were 30% and 20%, respectively. Compared to the addition of SF, the addition of FA resulted in a lower ultimate tensile stress but a higher ultimate tensile strain of UHDMC. Secondly, the micro-mechanical properties of the UHDMC were examined, and the stress-based and energy-based pseudo strain hardening indices (PSHσ and PSHJ, respectively) were calculated. It was found that both PSHσ and PSHJ of the UHDMC increased as the addition of SF or FA contents replacing MPC increased. Compared to the addition of SF, the addition of FA resulted in a similar PSHσ but a higher PSHJ. A quantitative relationship between pseudo strain hardening indices and ultimate tensile strain was established to guide the design of UHDMC with a desired ultimate tensile strain. It was found that PSHσ and PSHJ should reach above 2.2 and 26.7 concurrently for the UHDMC with ultimate tensile strains up to and above 3%. Lastly, the microanalysis was conducted to reveal the influence mechanism of FA and SF on the UHDMC. This study provides some technical guidance on the development and optimal design of UHDMC.