Ultra-low clinker cementitious binders with enhanced early strength via Ca(OH)₂ and calcium formate activation

The practical application of ultra-low clinker cement systems (≤10 % clinker) is often hindered by insufficient early strength. This study aims to overcome this limitation by incorporating calcium hydroxide (CH) as a partial substitute for cement clinker and calcium formate (CF) as a synergistic accelerator. Advanced microstructural analyses (XRD, FTIR NMR, SEM) reveal that CH acts as a dual-function additive: (1) elevating alkalinity and Ca^{2 +} availability to accelerate clinker hydration, forming primary C-S-H gels, and (2) activating dissolution of fly ash/slag aluminosilicates, generating secondary C-A-S-H networks. Use of CF further amplifies early strength via dual mechanisms: (1) formate ions (HCOO⁻) penetrate hydration barriers on clinker surfaces, catalyzing C₃S/C₂S dissolution, and (2) the decarboxylation of formate ions under high pH generates carbonate ions, which react with residual Ca²⁺ to form calcite, densifying the matrix. At optimal dosages (2 % CH, 1 % CF), the composite system achieves a remarkable 1-day compressive strength of 15.7 MPa as compared to 1.7 MPa of control, while maintaing 28-day strength. NMR analysis confirms enhanced silicate polymerization, while SEM images reveal pore refinement and reduced unreacted fly ash. This work demonstrates a scalable strategy and offers a feasible solution for the widespread application of green building materials with ultra-low clinker content.