This study investigated the chemical compatibility of recycled rubber powder ``RRP'' with Portland cement by hydration test. Four mixes were prepared: pure cement paste and three rubber -cement pastes included 10%, 20%, and 30% of RRP. The compatibility of rubber - cement paste was evaluated by their temperature-time curves. The results of aptness and inhibitory index have shown that RRP rates higher than 10% was strongly inhibited the hydration reaction of cement. In addition, the setting time results revealed that rubber mixes require a longer curing time than pure cement paste, so the RRP could be used with cement-based materials as a setting retarder admixture. The recycling of rubber tire waste with cement-based materials could be reduced their accumulation in landfills and protects the natural and environment facing their harmful effects.