The utilization of alkali-activated materials represents an effective approach to energy conservation and emission reduction in the construction sector, having a strategic importance for supporting the achievement of “dual-carbon” goals. However, alkali-activated mortar is prone to substantial volume deformation at early stage due to the coupling effect of chemical shrinkage and autogenous shrinkage. In the long term, its insufficient durability adversely affects the service life of repair structures, thereby restricting broader engineering applications. Furthermore, conventional internal curing methods using polyacrylic superabsorbent polymer (SAP) exhibit a poor durability in high-alkali and salt-rich environments. Therefore, this study was to synthesize a modified polyacrylic-based SAP (MSAP) through starch grafting and diatomite incorporation, with optimized design parameters. The influence of MSAP dosage on the rheological properties, strength development, shrinkage behavior, and durability of alkali-activated fly ash-slag mortar was investigated. The findings could provide new methodologies and technical support for controlling shrinkage and enhancing the durability of alkali-activated mortar, thereby facilitating its large-scale green application.

The MSAP was synthesized via aqueous solution polymerization. The key synthesis parameters (i.e., the acrylic acid-to-starch mass ratio, acrylic acid-to-diatomite mass ratio, amount of initiator ammonium persulfate, and amount of crosslinker N,N'-methylene bisacrylamide) were systematically optimized. The water retention and release performance of the MSAP were evaluated by a tea-bag method in different solutions. Multiple batches of mortar samples were casted. Mix proportions with favorable mechanical properties and bond strength were screened, and the optimal internal curing water dosage was determined for the mortar system. Furthermore, the influence of MSAP content (i.e., 0–0.5%) on the properties of alkali-activated mortar was investigated.

The workability (i.e., flowability and setting time) and mechanical properties (i.e., compressive strength) of the mortar were tested in accordance with the JGJ/T 70-2009. The chemical shrinkage was measured by an expansion method based on the standard ASTM C1608-12, while the autogenous volume deformation was determined via a non-contact corrugated tube tester based on the standard ASTM C1698-00. Internal relative humidity development within the mortar for 0–28 d of curing was monitored by HKT-XY humidity sensors. The capillary pressure was recorded by a capillary pressure sensing system. The microstructural and pore structure characteristics were examined by field emission scanning electron microscopy and mercury intrusion porosimetry.

The MSAP samples synthesized under different parameters all exhibit a rapid liquid absorption within the first 30 min in deionized water, followed by a gradual decrease in absorption rate until reaching equilibrium after 120 min. Compared to deionized water, the absorption capacity of MSAP in 0.9% NaCl solution is significantly reduced. A commercial SAP shows an absorption ratio of only 23 g in NaCl solution, indicating its inadequacy for high-salinity environments. The modified MSAP developed in this study achieves a markedly improved absorption ratio of up to 110 g/g. In the dry state, MSAP particles display a dense network structure with a low light transmittance. Microstructurally, the particles exhibit irregular morphologies with abundant pores, wrinkles, and fine granular protrusions on the surface.

The MSAP demonstrates significant liquid absorption and release behaviors in the pore solution of alkali-activated mortar, helping to maintain a high flowability (> 60 mm) even after 60 min. It delays the hydration reaction rate and inhibits the early formation of gel frameworks, resulting in prolonged setting time across mixes with different MSAP contents. For longer curing periods, however, the hydration degree improves and the microstructure becomes denser. The incorporation of MSAP at appropriate dosages (i.e., < 0.3%) does not significantly compromise the compressive strength. Sample S01 with 0.1% MSAP reaches a 28 d compressive strength of 52.4 MPa, which is slightly higher than that of the reference group.

The chemical shrinkage of alkali-activated mortar shows a multi-stage evolution, i.e., initial expansion, accelerated shrinkage, release-compensation, and steady phases. A greater MSAP content suppresses an early-age chemical shrinkage. However, at excessive dosages (e.g., 0.5%), notable negative volumetric effects emerge at later stages, with compensation rates varying widely from –77.4% to 73.6%.

A non-linear positive correlation occurs between MSAP content and autogenous shrinkage compensation, exhibiting a threshold effect. At 0.2% MSAP, the development of capillary pressure is significantly delayed, the autogenous shrinkage compensation rate reaches 52.2%–63.1%. Beyond 0.3% MSAP, the sensitivity of compensation efficiency to further dosage increases diminished, stabilizing within 65%–85%.

The physical filling effect and hydration promotion by MSAP increase a fluid flow resistance, thereby reducing the relative permeability coefficient and slowing moisture diffusion. At 0.2% MSAP, the relative permeability coefficient decreases to 4.29×10^–12 m/s, which is approximately 40.0% lower than that of the control group. Similarly, the chloride diffusion coefficient of sample S02 decreases to 6.0×10^–12 m²/s. Conversely, a greater MSAP content leads to interconnected pore networks and localized macro-cracks and voids, adversely affecting the durability of the mortar.

The optimal preparation parameters for starch-grafted/diatomite-modified polyacrylic acid MSAP involved the mass ratio of acrylic acid to starch of 6, the mass ratio of acrylic acid to diatomite of 10, the amount of initiator APS of 0.15%, and the amount of crosslinking agent MBA of 0.05%. An appropriate dosage of SAP particles effectively promoted the hydration process of alkali-activated gel products through the slow release of internally stored water, thereby improving the pore structure and enhancing both mechanical properties and durability. Meanwhile, MSAP could supply the consumption of free water in the system via releasing absorbed moisture, thus mitigating the development of capillary pressure within the alkali-activated mortar and reducing its autogenous shrinkage.