Acid–Base Cement Proportioned With Fly Ash and Oxalic Acid: Mechanisms, Performance, and Carbon Sequestration Potential

Abstract

Conventional Portland cement production is associated with substantial carbon emissions, while the utilization of off-specification fly ash (FA), particularly high-CaO FA, remains challenging due to its variable composition and limited reactivity. Developing alternative binders that can simultaneously valorize FA and enable carbon sequestration is therefore of significant interest. This study investigates the feasibility of an acid–base cement based on FA and oxalic acid (OA) and examines how key mix parameters govern its reaction, microstructure, and mechanical performance. FA-OA binders with varying FA/OA, water-to-cement (W/C), and borax-to-cement (B/C) ratios were systematically evaluated using calorimetry, phase characterization, microstructural analysis, and mechanical testing. The FA-OA system hardened through a dissolution-precipitation mechanism, forming predominantly calcium oxalate hydrates (CaC₂O₄·H₂O and CaC₂O₄·2H₂O), together with minor ferrous oxalate (FeC₂O₄·2H₂O) and Al/Si-rich gel phases. The FA/OA ratio governed the balance between oxalate formation and residual FA, thereby controlling mechanical performance. Increasing W/C reduced strength by increasing porosity, whereas increasing B/C adversely affected strength by inhibiting reactions and reducing the stability of reaction products. Depending on the FA/OA ratio, the FA-OA system could achieve carbon sequestration capacities of 140.4–350.9 kg CO₂ per ton of FA.