Rheological and Mechanical Properties of High-Performance Fiber-Reinforced Cement Composites with a Low Water–Cement Ratio

[Image: see text] High-performance fiber-reinforced cement composites (HPFRCCs) have been widely used in structural engineering due to their excellent performance. With the trend of lightweight construction, these materials, which can be used in prefabricated components, are becoming more and more important. This study investigated the influence of the water–cement (w/c) ratio, within the 0.19–0.28 range, on the rheological and mechanical properties of HPFRCCs; the pore structure and microstructure were observed to evaluate its effect. An elastic modulus test showed that a smaller w/c ratio would result in a higher rigidity of the material. Both the yield shear stress and plastic viscosity decreased to significantly different degrees with an increasing w/c ratio; a decrease in the yield shear stress and plastic viscosity was conducive to air discharge from the composite and, hence, reduced the air content. Most of the internal pores had a diameter of 20–100 nm or larger than 200 nm, while the proportion of those with a diameter of 100–200 nm was relatively low. When the w/c ratio was below 0.22, the flexural and compressive strengths barely increased due to an increment in the number of larger pores (i.e., diameter >200 nm). The results showed that the yield shear stress, plastic viscosity, pore uniformity, and the number of pores with a diameter above 200 nm are the dominant factors affecting the HPFRCC performance at a low w/c ratio.