Sustainable recycling of phosphogypsum and recycled concrete aggregate into base course for pavement: Mix design and durability evaluation

Phosphogypsum (PG) and recycled concrete aggregates (RCA) are commonly used in base course materials as substitutes for natural aggregates. However, the use of PG and RCA alone may lead to issues such as poor water stability and insufficient durability. Therefore, this study considered using sodium metasilicate nonahydrate (SMN), calcium stearate emulsion (CSE), and emulsified styrene-butadiene rubber (ESBR) as additives to improve their water stability and durability for preparing base course materials. Dry-wet cycling tests, freeze-thaw cycling tests, shrinkage experiments, and fatigue tests were conducted to evaluate the effects of the adopted additives on the comprehensive performance of calcined PG (CPG)/RCA base course material. The results indicated that with the addition of SMN and CSE, the 28-day strength of CPG/RCA-SC increases to 8.29 MPa. After five freeze–thaw cycles, the residual compressive strength ratio of CPG/RCA-SC reaches 72%, which is substantially higher than the 36.7% observed for PG/RCA and the 50.6% for CPG/RCA. Furthermore, the incorporation of SMN, CSE, and ESBR reduces the 180-day drying shrinkage strain from 1172.5 × 10^-6 for RCA to 570 × 10^-6 for CPG/RCA-SCE. In terms of fatigue performance, the k-value is markedly increased from 1.083 for PG/RCA to 418.287 for CPG/RCA-SCE. Overall, the adopted additives can help largely consume PG and RCA into base course for more sustainable and durable pavement application.