Shrinkage-Mitigation Mechanism and Prediction Model of Slag/Fly Ash-Based Alkali-Activated Concrete Internally Cured with Superabsorbent Polymers

Alkali-activated concrete (AAC) offers the advantages of high mechanical strength and excellent corrosion resistance, making it a promising low-carbon material. However, its widespread application in practical engineering is severely limited by the serious issues of high shrinkage and susceptibility to cracking. To address the challenges, this work proposes incorporating superabsorbent polymers (SAPs) into AAC to mitigate its shrinkage problems. A comprehensive investigation is conducted on fresh, mechanical and shrinkage properties of SAP-modified AAC. The underlying shrinkage-mitigating mechanism is revealed through various tests including hydration heat, X-ray diffraction, scanning electron microscope (SEM), and nuclear magnetic resonance (NMR) analysis. Results indicate that SAP prolongs concrete setting times. With SAP addition, the overall porosity and weak interfacial transition zones are increased, leading to decreases in the compressive strength by approximately 6.9–20.2% at 28 d. Both splitting tensile and flexural strengths show varying degrees of improvement. Adding 0.1–0.3% SAP reduces the 3 d autogenous shrinkage by about 33.08–41.14%, and 56 d drying shrinkage by 12.4–32.7%. SAPs act as internal water reservoirs, regulating humidity and reducing AAC’s sensitivity to internal relative humidity. Furthermore, SAPs optimize AAC pore structure distributions, as revealed by SEM and NMR analyses. Given the impact of SAP dosage (0–0.3%), autogenous and drying shrinkage prediction models are respectively established based on CEB-FIP 90 and GL 2000 models, showing superior agreement between predicted results and test data, with all the fitting coefficients over 0.9.