Investigation of Strength, Hydration, and Microstructure of Alkali-Activated Phosphogypsum-Based Road Base Materials

The utilization of phosphogypsum (PG) in road construction at high concentrations remains limited. This study employs an alkali-activation method, incorporating PG at a high substitution rate of up to 60%, along with slag, Type II anhydrite (AH), and cement to prepare the alkali-activated PG-based base materials (APBMs). Sodium silicate was employed as an activator. An array of tests, encompassing compaction, mechanical, and water stability evaluations, was executed to assess the performance of APBM. Furthermore, X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), and thermogravimetric (TG) analyses were utilized to scrutinize the phase composition, elemental distribution, and thermal stability. Results indicated that the addition of sodium silicate to APBM increased unconfined compressive strength (UCS) and split tensile strength. Compared to the control mixture (S0), 28 d compressive strengths increased significantly, while the mixture with a modulus of 1.0 (M1.0S2) showed substantial improvements in splitting strength over 7 and 28 d. The inclusion of 2% sodium silicate improved water stability, with a decrease in stability coefficients observed after 28 d, possibly due to complete hydration and water absorption by PG, leading to mechanical performance decline. TG analysis revealed decompositions of substances like ettringite (AFt), calcium (alumino)silicate hydrate [C─ (A)─ S─ H], PG, and CaCO3, with the mixture M1.0S2 showing significant weight loss indicating a higher degree of hydration. Alkaline conditions facilitated the dissolution of AH and the formation of new gypsum crystal. Moreover, the activator with a modulus of 1.0 (M1.0S2) catalyzes the formation of AFt, C─ (A)─ S─ H, and CaSO4·2H2O, resulting in a dense meso-structure.