Effect of Mechanically Activated Fly Ash, a Superplasticizer, and an Air-Entraining Admixture on the Freeze–Thaw Resistance of Fine-Grained Concrete

This study examines how mechanically activated fly ash (TF), a polycarboxylate superplasticizer (PC), and an air-entraining admixture (AE) affect the hydration, microstructure, pore structure, mechanical properties, and freeze–thaw resistance of fine-grained concrete. Mechanical activation increased the specific surface area of the untreated ash (UF) from 3710 to 6450 cm2/g and reduced its mean particle size to 6.06 μm. Replacing 5 wt.% of cement with TF accelerated the nucleation of hydration products and densified the microstructure, as confirmed by exothermic profiling, XRD, and SEM. PC delayed the exothermic peak yet produced the highest heat-release intensity, indicating more efficient wetting and dispersion of reactive surfaces. The TF–PC system reached 39.3 MPa at 28 days, while the ternary TF–PC–AE composition achieved 33.2 MPa with a refined pore-size distribution (mercury intrusion porosimetry). The combined use of PC and AE tripled the pore volume in the 0.01–0.10 μm range and increased it by 28.7% (0.1–1 μm) and 46.21% (1.0–10 μm). The calculated potential freeze–thaw resistance rose from 54.25 to 61.97 and 72.83, confirming the beneficial role of 3–10 μm pores. After 110 accelerated freeze–thaw cycles, the optimized ternary composition lost only 0.55% of its mass.