Stabilizing reactive cement phase through targeted doping: the case of β-Belite and boric acid

Abstract

This study investigates the stabilization of the metastable β polymorph of dicalcium silicate (belite) through doping with boric acid (H₃BO₃). Belite, a major constituent of Portland cement clinker, exhibits varying degrees of stability under different conditions. The β polymorph is desirable for its hydraulic activity, but it tends to transform to the less reactive γ form upon cooling. Belite (phase-purity > 95%) was synthesized with H₃BO₃ dopant and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques and ImageJ-based grain size analysis. The results show that boric acid doping effectively stabilizes the β-belite polymorph at room temperature. In particular, a 0.35 wt% H₃BO₃ addition was sufficient to stabilize the β-belite phase, resulting in 97.71% β-C₂S content. The stabilization mechanism is attributed to the substitution of BO₃ groups for SiO₄ tetrahedra in the crystal structure. Increased sintering time (3.5 h) resulted in higher β-C₂S content and decreased free lime. The synthesis method yields approximately 800 g of β-belite phase (phase-purity > 95%) in a single 7-h run. The results provide valuable insights into the effects of boric acid doping on the stability and hydration behavior of β-belite, which could inform the development of novel cement-based materials with lower energy requirements and CO₂ emissions.