Performance and interfacial strengthening mechanism of polypropylene fiber-reinforced foamed concrete based on surface modification with nano-SiO2 and silica fume

The reinforcement efficiency of polypropylene fibers in foamed concrete is fundamentally constrained by their inherent hydrophobicity, resulting in poor dispersion and weak interfacial adhesion. This study develops two surface modification strategies by constructing a continuous nano-SiO 2 coating (PFNS) and discrete microscale silica fume anchoring sites (PFSF) on the fiber surface via chemical oxidation and layer-by-layer self-assembly, respectively. These modifications transform the fiber surface from hydrophobic to hydrophilic. Experimental results demonstrate that both modifications improve composite performance relative to unmodified fibers. At a fiber content of 1.5 vol%, the flexural strength of specimens reinforced with PFNS and PFSF increases by 33.3% and 23.8%, respectively, corresponding to 3.5 and 2.5 times the increase achieved by the unmodified fibers. The 800 h drying shrinkage strain is reduced by 30.7% and 23.3%, respectively, while the cracking area of large slabs decreases by 71.6% and 41.5%, compared with the unmodified fiber group. The superior performance of PFNS is attributed to the continuous coating that promotes the growth of calcium silicate hydrate gels at the interfacial transition zone and densifies the matrix by filling micropores. In contrast PFSF strengthens the fiber-matrix interface primarily through the mechanical anchoring effect of discrete surface particles. The improved fiber dispersion and refined pore structure contribute to lower water absorption and reduced drying shrinkage. This work clarifies the regulatory role of fiber surface topography in governing the macroscopic performance of foamed concrete and offers a theoretical foundation for the interfacial design of high performance cement-based composites.