Expansive Agent-Modified Geopolymer for Medium-to-Wide Concrete Crack Remediation: Workability, Mechanical Performance, and Durability

The inherent drying shrinkage of geopolymers restricts their widespread application in concrete crack repair, particularly for medium-to-wide cracks that demand stringent workability and penetrability. This study systematically investigates the effects of three single-component expansive agents (MgO, CaO, and CSA) on the fresh properties, mechanical performance, and microstructural evolution of a slag-fly ash-based geopolymer. The optimal modified formulation was subsequently evaluated for remediating preinduced concrete cracks (2.0, 2.5 and 3.0 mm apertures) and benchmarked against ordinary Portland cement and epoxy resin. The results indicate that while CaO and CSA severely compromise paste fluidity and induce rapid setting, MgO modification provides an exceptional operational window. An 8 wt.% MgO dosage (MG8) induces only a marginal 3.73% reduction in paste fluidity and maintains stable initial and final setting times, thereby preserving excellent workability retention and enabling precise construction scheduling. Microstructural analyses (XRD, SEM, and MIP) reveal that the precipitation of micro expansive Mg(OH)2 effectively suppresses the 28-day drying shrinkage to 0.23%, while facilitating the attainment of a robust compressive strength of 44.1 MPa and preserving a highly favorable strength development trajectory. In the structural repair phase, the MG8 demonstrated outstanding compressive strength recovery, peaking at 28.80 MPa for 2.0 mm cracks, which significantly outperformed both the cement and epoxy resin repaired groups. Conversely, the epoxy resin repaired specimens exhibited superior splitting tensile strength due to the inherent elongation properties of the flexible macromolecular polymer. Comprehensive durability assessments revealed that the MG8 repair system exhibits exceptional resistance against freeze–thaw cycles and sulfate/chloride attacks, ensuring long-term structural integrity that significantly outperforms conventional materials. Overall, this work presents a viable and durable geopolymer-based alternative to traditional materials, aiming to ensure timely and reliable remediation concrete cracks that do not cause structural damage.