Long-term sulfate resistance of cementitious composites containing fine crumb rubber

This paper presents the results of a study on the effects of fine crumb rubber on the sulfate resistance of mortar mixtures. Mixtures with and without 10% silica fume (SF), containing 0%, 10% and 15% fine crumb rubber as a fine aggregate substitute material were subjected to sodium sulfate wet-dry cycles for two years. Changes in compressive strength, mass and longitudinal expansion of mortar bars were monitored. Micro computed tomography scan (CT-Scan) and scanning electron microscopy - energy dispersive spectroscopy (SEM - EDS) were also utilized in investigating specimen microstructure and degradation. Results indicate that while the SF was effective in resisting the expansion of mortar bars, the presence of crumb rubber in mixtures prepared without the SF led to a 52-57% reductions in longitudinal expansion relative to that of the plain reference mortar. Although SEM microstructural analyses suggest a higher preponderance of calcium silicate hydrate (C-S-H) intermixed with sulfate reaction phases and entrapped air voids in the outer surface layer of rubberized specimens, matrix cracking was significantly lower compared to the plain reference specimen. CT-Scan results also confirmed the severe degradation of the plain reference mortar specimen and the tendency of crumb rubber to increase the air void content of mortar mixtures. The enhanced resistance of rubberized mortar specimens prepared without the SF is traced to the selective growth of sulfate crystals in entrapped micro-sized air voids, and the proclivity of rubber particles to not only blunt cracks, but also predispose specimens to minimal cracking during deformation.