Influence Factors of Rapid-hardening Controlled Low Strength Materials Made of Recycled Fine Aggregate Based on Red Brick Construction and Demolition Waste

To solve problems of low recycling level of urban construction and demolition (C&D) waste and long period of backfill construction for road trench, recycled fine aggregate from C&D waste was used in rapid-hardening controlled low strength materials (CLSM). The influence of four factors (including accelerator type, accelerator-to-binder, water-to-solid and binder-to-recycled fine aggregate ratios) on workability, mechanical properties and resistance to dry shrinkage of CLSM was investigated. Results show that the accelerator has no obvious effect on the flowability of CLSM, however, reduces its bleeding rate and dry shrinkage significantly. There is existing certain difference in the early strength characteristics of two types of accelerator. The optimal accelerator-to-binder value for best early strength development is 40% and 20%, respectively, with the 4-hour strength all reaching 0.7 MPa. Additionally, with the accelerator-to-binder ratio fixed, the early hour strength of CLSM is affected by the amount of cementitious materials and the actual content of free water. Increasing the water-to-solid ratio will increase the flowability, bleeding rate and resistance to dry shrinkage of CLSM, and reduce its mechanical properties; while increasing the binder-to-recycled fine aggregate ratio will increase the flowability and mechanical properties, and reduce its bleeding rate and resistance to dry shrinkage.