Freeze-thaw Resistance of Concrete Based on Fly Ash and Municipal Sludge Under Alkali Excitation

In this study, municipal sludge modified with quicklime is employed to replace 10% of the fine aggregate, while fly ash replaced 10%, 20%, and 30% of the cement to mitigate the adverse effects of municipal sludge on the freeze–thaw resistance of concrete. This is assessed under the combined excitation of sodium hydroxide and sodium silicate. The research investigates the changes in apparent phenomena, relative mass, relative dynamic elastic modulus, relative compressive strength, and concrete microstructure with varying fly ash content and alkali equivalent under different freeze–thaw cycles. Models of freeze–thaw damage are established, using relative dynamic elastic modulus and relative compressive strength as damage variables. The results indicated that with an increase in freeze–thaw cycles, the concrete sustains more severe damage, exhibits enlargement in internal pores, and gradually reduces relative dynamic elastic modulus and compressive strength. Incorporating fly ash and an alkali activator mitigates the mass, dynamic elastic modulus, and compressive strength losses in concrete during the freeze–thaw cycles, reducing the deteriorative impact of municipal sludge on the concrete’s durability compared to the control group. Optimal results are shown when the fly ash content is 10% and the alkali equivalent is 4%; under these conditions, the fly ash is fully activated by the combined action of the alkali initiator and the residual alkali in the modified sludge. This combination absorbs the residual alkali, diminishing the negative effect of municipal sludge on the concrete’s freeze–thaw resistance. At this point, the concrete achieves its highest hydration degree, exhibits the lowest losses in mass, dynamic elastic modulus, and compressive strength during freeze–thaw cycles, and achieves the longest service life, exceeding 16 years. A strong correlation is found between the relative dynamic elastic modulus and the relative compressive strength, and the freeze–thaw damage models based on these variables closely match the experimental findings. © 2024 Sichuan University. All rights reserved.