Effect of recycled concrete powder on the evolution of the static yield stress of cement paste

Abstract

Current research on recycled concrete powder (RCP) influencing the cement-based material's rheology primarily focuses on the initial rheology, with limited attention given to long-term rheology (rest time over 60 min). This work investigates RCP influencing the paste's static yield stress evolving with time (10–120 min). Results show that RCP significantly influences the cement paste's static yield stress, increasing it by about 89 times when the incorporation ratio of RCP reaches 25 % and rest time extends to 120 min. According to existing theoretical foundations and models, it is believed that the underlying influencing mechanisms can be elaborated from three aspects: the cementitious particle system's colloidal and contact interactions and the cementitious paste's structural build-up rate. RCP improves the particle system's capacity to adsorb polycarboxylate ether (PCE), lowering the average coverage ratio of the particle surface coated by PCE and the separation distance between particles, thus increasing the initial colloidal interactions. This promotes flocculation and strengthens the colloidal network, raising the paste's static yield stress; RCP increases the particle system's average size, volume fraction, packing density, and hydration degree and decreases particle number and hydrated volume, thus affecting the contact interactions evolution over time. However, the highest incorporation ratio does not result in the highest contact interactions; RCP promotes the paste's early hydration, generating more stable C-S-H. This accelerates the connections between the particles transition from flexible colloidal to rigid bridging, increasing the paste's structural build-up rate, thus raising the linear growth rate of the paste's static yield stress.