Comparison of the effects of different expansion agents on alkali-activated rapid repair mortars: workability, mechanical properties, drying shrinkage

Alkali-activated materials (AAMs) are promising for rapid concrete repair but are hindered by excessive drying shrinkage. This study evaluates the efficacy of MgO-based (MEA), CaO-based (CSEA), and calcium sulfoaluminate-based (SEA) expansive agents in mitigating shrinkage and enhancing the performance of slag-fly ash-metakaolin repair mortars. The mechanism of the expansive agents’ impact on reaction products and microstructure was analyzed using X-ray diffraction (XRD), thermogravimetric analysis (TG), and scanning electron microscopy (SEM). The results indicate that increasing the amount of any of the three expansive agents reduces the setting time of the repair mortar. When the CSEA content is 8%, the initial and final setting times of the mortar are 18 min and 32 min, representing reductions of 55% and 46.7%, respectively. This acceleration facilitated superior early-age performance, achieving a 6-hour compressive strength of 25.0 MPa. While thermal stress from rapid hydration slightly compromised long-term compressive strength, CSEA increased the 28-day bond strength to 7.5 MPa by elevating the Ca/Si ratio and densifying the interfacial transition zone. However, the rapid hydration reaction leads to swift gel formation and dehydration within the specimens, releasing substantial heat and resulting in microcracks that affect strength development. MEA exhibit low reactivity in highly alkaline environments, failing to produce sufficient brucite to compensate for shrinkage, thus having a minimal impact on both mechanical strength and drying shrinkage of the specimens. The incorporation of SEA can generate ettringite (AFt) in the initial reaction stages, which improves early strength and reduces drying shrinkage. However, as the reaction progresses, AFt dehydrates and transforms into layered AFm, significantly diminishing the ability to resist shrinkage. This excessive shrinkage results in microcracks, causing a reduction in the later strength of the specimens.