Development of sprayable ultra-high ductility magnesium phosphate cement-based composites based on the rheological properties

To develop a sprayable ultra-high ductility magnesium phosphate cement-based composite (UHDMC), this study explored the effects of starch ether content, water-solid ratio, powder-binder ratio, borax content (B/M), fly ash (FA) substitution and volume content of polyvinyl alcohol (PVA) fiber on the shootability and rheological properties of magnesium phosphate cement-based materials. The correlation between shootability factors (buildup thickness and rebound rate) and rheological parameters (yield stress and plastic viscosity) was established, and the optimum mixture proportion suitable for the sprayed UHDMC was selected and then analyzed by the mechanical properties and pore structure between the cast and sprayed specimens. The results showed that the optimum mixture proportion of the sprayed UHDMC was as follows: starch ether content of 0.15%, water-solid ratio of 0.14, powder-binder ratio of 0.3, borax content (B/M) of 8%, FA substitution of 40%, and the PVA fiber volume content of 2.0%. The yield stress and plastic viscosity were both positively correlated with the build-up thickness, and negatively correlated with rebound rate. To some extent, rheological parameters can be used to characterize the shootability factors, in which the best shootability was shown when the ranges of yield stress and plastic viscosity were 91.64-118.77 Pa and 4.85-7.17 Pa.s, respectively. The compressive strength and ultimate tensile stress of the sprayed UHDMC were lower than that of the cast UHDMC, while the ultimate tensile strain of the sprayed UHDMC could still reach above3.6%, presenting good performance to meet the requirement of above 3% strain of high ductility cement-based materials. Moreover, the analysis of pore structure verified that the capillary pores and macropores contents were mainly affected the strength and tensile strain, respectively.