Unveiling the impact of water-to-binder ratio on foaming behavior of foamed magnesium phosphate cement

Foamed magnesium phosphate cement (MPC) has recently garnered increasing research interest. However, tailoring its water-to-binder (w/b) ratio remains challenging due to the unclear foaming behavior. This study investigates the foaming behavior of foamed MPC across a wide w/b ratio range (0.2–0.4), focusing on the fresh properties of MPC pastes, slurry expansion kinetics, surface morphology, and bubble film rupture. The results show that a higher w/b ratio significantly enhances expansion speed by accelerating gas generation. The viscosity-induced confining pressure, estimated using the island model, was only a few pascals—insufficient to constrain expansion volume or speed. While open pores continuously form during foaming, a sufficiently high w/b ratio (≥0.25) with low yield stress can retrofit them and prevent the development of gas escape channels. Consequently, the final expansion volume increased significantly from 76.1 % (w/b = 0.2) to 93.4 % (w/b = 0.25) of the theoretical gas volume. An optimal w/b ratio (0.25–0.3) also facilitated bubble film rupture, reducing holes in the pore shells. Meanwhile, as the w/b ratio increased from 0.2 to 0.4, the 28-day compressive strength declined from 0.86 to 0.14 MPa, while thermal conductivity decreased from 0.131 to 0.082 W·m−1·K−1 due to looser hydrated structure in the interstitial MPC matrix. Despite the substantial loss in compressive strength—more pronounced than in cement—a sufficiently high w/b ratio is recommended to improve surface quality and minimize gas loss. © 2025 Elsevier Ltd