End-of-life tire (ELT) rubber has been widely researched to replace fine or coarse aggregates in cementitious composites. While most studies paid attention to its effect on the engineering properties, very few considered chemical reactions with pore solution and the potential for environmental leachate. Recently the authors developed a methodology to remove zinc from the ELT rubber, since zinc can be toxic if it is leached into the environment. In this study, the authors utilized ELT rubber before and after the zinc extraction process to partially replace fine aggregate in a mortar. Flowability, compressive strength, flexural strength, and ultrasonic pulse velocity were measured for the engineering properties of rubberized mortars. Simultaneously, isothermal calorimetry was also employed to investigate the effects of ELT rubber on the hydration process of the rubberized mortars. In addition, the pore solution and leaching solutions were taken at different curing ages and then analyzed for elemental and total organic carbon (TOC) contents. The results showed a remarkable loss in engineering properties of rubberized mortar when ELT rubber was utilized, and the decrease in performance was more pronounced in the samples with zinc-extracted ELT rubber. The pore solution was found to contain significant quantities of zinc and TOC. However, the authors also found that using silica fume to partially replace cement could effectively recover the loss in strength and could reduce the leachability of zinc and TOC.