Strength and Microscopic Characteristics of Slag-Based Geopolymer-Solidified Sludge after Dry–Wet Cycling with Chloride Salt Solutions

Abstract

As a new type of green soil-curing agent, geopolymers have the advantages of high strength, good durability, and low carbon emissions, and slag is widely used as a common geopolymer precursor in geopolymer production. Therefore, it is important to investigate the strength and durability of slag-based polymer-solidified sludge under the action of dry–wet cycling with chloride salt solutions. After curing the slag-based polymer-solidified sludge with NaOH (NO), Na2SiO3 (NS), and calcium carbide slag (CS) as alkali excitants to different curing ages (7, 14, 28 days), experiments involving different numbers of dry–wet cycles (0, 5, 10, 20, 30) with NaCl solutions of different concentrations (0%, 5%, 10%) were performed to investigate the unconfined compressive strength and micromorphology of the solidified sludge after undergoing dry–wet cycling. The test results showed that the strength of sludge cured with NO for 7 and 14 days showed a trend of increasing and then decreasing with an increasing number of dry–wet cycles, and the strength of sludge cured with NO for 28 days showed a trend of gradually decreasing with an increasing number of dry–wet cycles. The order of the resistance of sludge cured with the three excitants to the effect of the dry–wet cycles of chlorine salt was CS > NO > NS. After the dry–wet cycling, small cracks appeared on the surface of the NS-cured soil, the surface of the NO-cured soil was intact, and a small amount of surface peeling was observed for the CS-cured soil. Scanning electron microscopy test results showed that the chloride salt dry–wet cycles caused the formation of pores and cracks in the soil, and NaCl crystallization and Friedel’s salt production were observed in the soil, thus reducing the strength of the solidified sludge.