Drying Shrinkage Behavior and Micro-Mechanism of Concrete Based on a Thermodynamic Fractal Model

This research systematically evaluated the durability performance of low-carbon cement concrete prepared with industrial solid wastes under harsh service conditions. Measurements included mechanical properties and drying shrinkage. Microstructural characterization was carried out using SEM-EDS, MIP, and TG-DTG, revealing a synergistic relationship between microstructural changes and the resulting mechanical and durability behavior of the concrete. The experimental results indicated that adding 25% fly ash (FA) lowered the compressive strength of the CF-25 and BF-25 concrete by 11.30% and 11.39%, respectively, while reducing drying shrinkage by roughly 9.2–9.5%. In comparison, incorporating 5% silica fume (SF) had contrasting effects. It significantly improved the compressive strength of the CS-5 and BS-5 concrete by 18.92% and 9.94%, respectively, but at the cost of increasing drying shrinkage by 6.30% and 18.68%, respectively. Fractal dimension analysis based on thermodynamic relationships showed that the pore structure fractal dimension (Ds) ranged from 2.88 to 2.93. Group C exhibited a higher Ds (2.93) than Group B (2.90), indicating a more intricate pore network associated with greater C-S-H gel formation. With FA addition, Ds decreased to 2.91601 for CF-25 but rose to 2.93244 for BF-25. With SF addition, Ds fell to 2.91182 for CS-5 and 2.88102 for BS-5. Micro-mechanistic analysis revealed that the limited pozzolanic activity of FA at early ages resulted in insufficient hydration products and increased porosity. In contrast, SF contributed to a dense, highly polymerized gel structure and an optimized pore size distribution through its strong pozzolanic reactivity and nano-filling action. The distinct chemical properties of high-calcium and low-calcium cementitious systems further accentuated the differential influences of these supplementary cementitious materials.