Cement mortar containing crumb rubber coated with geopolymer: From microstructural properties to compressive strength

Coating rubber particles with cement paste has been widely adopted as an effective method to improve the mechanical strengths of rubberised concrete. However, this method introduces an additional amount of cement into the mixture system, thus unnecessarily further increasing the overall cement usage. This study introduced a new method of coating rubber surfaces with cement-free geopolymer paste. The geopolymer-coated crumb rubber was incorporated into cast cement mortar at three different replacement levels of river sand, including 7.5%, 15% and 22.5% by weight. The 28-day compressive strengths of different mixtures with geopolymer-coated (GP series) or untreated crumb rubber (U series) were measured, and the strength recovery was evaluated. Although the compressive strengths of GP series were constantly higher than those of U series, the effectiveness of strength recovery due to geopolymer coating on the rubber surface became less prominent with the increase in the crumb rubber content. Microstructural investigations included nanoindentation and scanning electron microscopy (SEM). The interfacial transitional zone (ITZ) between the cement matrix and geopolymer paste was observed to have porous microstructures with relatively low indentation modulus. Meanwhile, X-ray micro-computed tomography (μCT) analysis in coated rubber particles showed air voids existed within large rubber particles due to the agglomeration of crumb rubber. The microstructure in the ITZ region and internal voids in large rubber particles, together with the increased stiffness of rubber particles due to coating, jointly affected the effectiveness of strength recovery in cement mortar containing different proportions of crumb rubber.