Study on Low-Temperature Fracture-Bearing Capacity of Fly Ash Cement Paste Based on Acoustic Emission and Microscopic Characterization

This study investigates the damage evolution mechanism affecting the fracture-bearing performance of fly ash cement paste under low-temperature curing conditions. Pre-cut cement paste specimens with fly ash contents of 0%, 15%, and 25% were prepared and subjected to both standard curing and low-temperature curing at 5 °C for 28 days. Three-point bending tests combined with acoustic emission (AE) monitoring were conducted to analyze peak flexural load, AE ring count, cumulative energy, RA-AF crack classification, and b-value evolution. Additionally, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) were employed to characterize micromorphology and relative changes in hydration product content. The results indicate that both fly ash incorporation and low-temperature curing significantly reduce the flexural bearing capacity of pre-notched specimens. Under low-temperature curing, the peak loads of LF15 and LF25 decrease by 34.83% and 47.19%, respectively, compared to LF0. At the same fly ash replacement level, all low-temperature-cured specimens exhibited lower peak loads than those cured under standard conditions. Overall AE activity was reduced in low-temperature-cured specimens, with crack propagation instability occurring at lower load levels. The addition of fly ash shifted the fracture mode toward a tensile-dominated type, whereas low-temperature curing increased the proportion of shear-type AE events. Fly ash incorporation increased the relative content of calcium silicate hydrate (C-S-H) gel and decreased that of calcium hydroxide (CH); however, this did not result in improved peak flexural load. This outcome is attributed to the insufficient reactivity of fly ash at low temperatures, leading to residual unreacted spherical particles, dilution of clinker, and inadequate interfacial bonding, which collectively weaken the continuous load-bearing skeleton of the matrix. This paper establishes a multi-scale interpretation of the damage mechanisms affecting the low-temperature fracture-bearing performance of fly ash cement paste by correlating macroscopic bearing response, AE damage evolution, crack types, and hydration product composition.