Study on the influence of carbonation curing on the mechanical properties of fly ash-sand-based lime bricks: a case study of a coal-fired power plant in northwest China

Large-scale coal utilization in Northwest China generates substantial carbon dioxide emissions and coal-based solid waste. This study investigates a low-carbon utilization route in which bricks made primarily from fly ash and aeolian sand are cured using flue gas from thermal power plants. The effects of mass-based mixture proportions, gas pressure, and curing time on the mechanical properties of fly ash-aeolian sand-based green bricks (FAGB) under carbonation curing were examined. Changes in phase composition and microstructure before and after curing were analyzed using uniaxial compressive strength testing, X-ray diffraction, and scanning electron microscopy. On this basis, the carbonation-curing mechanism of FAGB and the associated mechanical-performance evolution were discussed. Macroscopic results show that, under normal temperature and pressure curing conditions, the strength of the optimum mixture increased from 1.30 MPa at 3 d to 3.0 MPa at 28 d, corresponding to a 130.8% increase. Under ambient curing conditions, the optimum mixture had a slurry concentration of 79% (solid mass fraction of the fresh slurry), a sand-ash mass ratio of 13:6, and an ash-auxiliary-material mass ratio of 14:4. Under carbonation curing conditions, the compressive strength was enhanced by 1.8-4.2 times relative to specimens cured under ambient conditions at the same age, and the maximum value reached 9.8 MPa at 0.6 MPa and 14 d. The curing process can be divided into stages of initial strength, enhancement, and weakening. From a microscopic perspective, the improved mechanical performance under carbonation curing is mainly associated with the formation and distribution of calcium carbonate within the brick pores, where it plays filling, supporting, and bridging roles. The original fly ash-aeolian sand-based green bricks contained calcium hydroxide, which was progressively replaced by calcium carbonate during carbonation curing. Under excessive curing pressure or prolonged curing time, the strength no longer increased and may slightly decrease; however, the specific products responsible for this reduction were not directly identified in the present study. The results of this study contribute to the utilization of coal-based solid waste as a resource and provide an effective approach to reducing carbon dioxide emissions in thermal power plant and mining areas of Northwest China.