Effect and mechanism of recycled clay brick powder on compressive strength of different types of concretes

Abstract

To investigate the effects of recycled clay brick powder (CBP) partially replacing cement in different types of concretes, and reveal the combined influencing mechanisms of CBP with the water-to-binder (W/B) ratio, additives like water reducers, viscosity modifying agent, and fibers in concrete, the compressive strengths of eight types of concretes with different CBP contents were tested, and their hydration heat, pore structures and microscopic morphologies were examined. The results showed that for the concretes with a higher W/B ratio or with polycarboxylate superplasticizer (PCE) added, the internal curing effect of CBP was insignificant, and the compressive strength monotonically decreased with an increase in CBP replacement ratio; while the strength of concrete with a lower W/B ratio and without PCE did not always decrease with CBP content due to the significant internal curing effect of CBP, which made the reduction in strength at the same CBP replacement ratio less than that observed in concrete with a higher W/B ratio. For example, replacing cement with 30 % CBP caused the 28-day compressive strength of concrete to decrease by 25.2 % at a W/B ratio of 0.5, whereas at a W/B ratio of 0.33, the strength only decreased by 14.2 % compared to concrete without CBP. For concrete with PP fibers, the reduction in compressive strength induced by CBP replacement was mitigated, especially at a larger CBP replacement ratio, due to the bridging effect provided by PP fibers penetrated the micropores of CBP. For the concrete with hydroxypropyl methylcellulose (HPMC), replacing cement with 30 % CBP hardly reduced the concrete's compressive strength. The filling of CBP particles into the macropores introduced by HPMC reduced the pore sizes and offset the negative impact of the lessening in available reactive cementitious materials, and thus reducing the influence of CBP content on the strength. This study can serve as a reference for the utilization of CBP in cementitious materials, thereby facilitating the recycling of CBP in engineering applications.