Effects of fly ash content on the macro-performance and microstructure of self-compacting concrete using tuff powder under freeze-thaw cycles

Fly ash (FA) and tuff powder (TP) were incorporated to improve the macro-performance and microstructure of self-compacting concrete (SCC) under freeze-thaw cycles (FTCs). Three SCC mixtures with FA replacement levels of 0%, 10%, and 20%, along with 15% TP, were tested for rheological properties (slump flow and V-funnel), mechanical performance (compressive and splitting tensile strength), freeze-thaw resistance, and microstructural characteristics. Freeze-thaw resistance was evaluated using mass loss rate, relative dynamic elastic modulus (RDEM), and relative compressive strength. The results indicate that FA improves the SCC flowability but reduces segregation resistance. While FA decreases compressive and splitting tensile strengths, it enhances compressive strength development over time. Additionally, FA lowers mass loss rates, mitigates RDEM deterioration, and increases relative compressive strength during FTC exposure. Microstructural analysis reveals that FA decreases overall porosity and significantly reduces the volume of harmful and multi-harmful pores. A decline in harmful and multi-harmful pores corresponds with lower mass loss rates and higher RDEM values. Finally, damage models based on mass loss and dynamic elastic modulus are developed to predict SCC service life under FTC conditions, providing a valuable reference for SCC production in the Xizang Autonomous Region.