Compositional design and slow-release regulation of calcium-source microcapsules

Abstract

To address the critical bottlenecks wherein free calcium ions trigger flash setting in alkali-activated geopolymer pastes—thereby deteriorating workability and construction fluidity—and high concentrations of free calcium induce high osmotic pressure toxicity in self-healing microorganisms, latent-disintegrating C@EH microcapsules were fabricated via a double emulsion-solvent evaporation method. The orthogonal experimental design is used to optimise the processing parameters. The results show that the proportion of the shell material and the core-shell ratio dominate the mass rate and the shear quality maintenance rate respectively. The optimized formulation (shell material ratio 6:1, core-to-shell ratio 2:1, crosslinking temperature 40°C, sodium silicate 8 mL, and stirring speed 1000 rpm) yielded a Quality rate of 84.2% and a Quality retention rate of 90.1%. Dynamic conductivity monitoring confirmed a distinct S-shaped delayed controlled-release profile in alkaline media. This delayed response stems from the synergistic barrier of the organic blended film and the outer inorganic brittle shell, where alkaline penetration triggers HPMC swelling and hydrogel formation, leading to scaffold rupture. Consequently, the microcapsules exhibited negligible leakage during the initial 2 h (conductivity increase of only 0.118 mS/cm), preventing flash setting and osmotic defects. Within the 2-8 h window, the proportion of calcium sources released reached an incremental value of 85.82%, achieving scheduled, targeted, and on-demand release. This provides an abundant material foundation for the early-age strength enhancement of alkali-activated materials and late-stage microbial mineralized deposition.