Experimental Study on Mechanical Properties and the Hydration Mechanism of Fly Ash-Doped High-Water Filling Materials

Taking fly ash-doped high-water filling materials with equal substitution as the research object, this paper systematically investigates the influence laws of different fly ash dosages on the initial setting time, density, and 28 day mechanical properties of the materials under standard curing and natural weathering conditions. Fly ash replacement within 0-15% barely disturbs hydration and maintains stable mechanical performance, defined as the optimal doping range. Combined with microscopic testing methods such as X-ray diffraction (XRD) and scanning electron microscopy (SEM), the hydration products and microstructure evolution are analyzed to reveal the hydration mechanisms. The results show that equal substitution of fly ash prolongs the initial setting time of the material approximately linearly, and the mechanical properties of the material decrease overall with the increase of the fly ash dosage. For the adopted ultrafine fly ash, mechanical indicators drop sharply when the replacement ratio exceeds the critical threshold of 30%. When the dosage exceeds 20%, the strength, elastic modulus, and deformation modulus drop sharply. The fly ash dosage below 15% has little effect on the hydration reaction, while an excessive dosage inhibits the growth of ettringite crystals. Under a natural weathering environment, high-content fly ash can reduce the carbonization of ettringite but results in a low degree of hydration reaction. At 30% fly ash replacement, the compressive strength and elastic modulus exhibit a cliff-like attenuation of over 40% compared with the pure matrix specimen. The effects of fly ash on high-water filling materials are mainly physical filling and dilution and the formation of cementitious hydrated calcium silicate and hydrated calcium aluminate gels in the later alkaline environment, which can improve the internal cohesion and deformability of the material.