Superabsorbent polymers (SAPs) are widely employed as an internal curing agent to enhance the durability and shrinkage–cracking resistance of concrete. However, while its macroscopic effects on concrete properties (e.g., strength reduction) have been documented, the nanoscale mechanisms governing the mechanical behavior of calcium silicate hydrate (C-S-H) gel in SAP-modified concrete remain poorly understood. This knowledge gap limits the optimization of SAP content for balancing durability and strength, a critical challenge in high-performance concrete design. In this paper, we address this scientific problem by combining experimental characterization and molecular dynamics (MD) simulations to systematically investigate how SAP-induced pore structure modifications dictate the mechanical performance of C-S-H gel. First, we analyzed the effects of SAP on concrete pore structure and compressive strength, revealing its role in refining capillary pores into gel pores. Next, MD simulations were employed to construct C-S-H gel models with controlled pore size distributions at three SAP contents (0.2%, 0.3%, and 0.5%), to establish a quantitative relationship between pore characteristics and material performance. The results reveal that pores of ~0.74 nm diameter, predominantly located in weak interfacial regions, critically govern the mechanical behavior of C-S-H gel. At 0.2% SAP content, the C-S-H gel exhibits the highest bulk modulus (10.61 GPa) and optimal mechanical properties, whereas 0.3% SAP leads to a dominant pore cluster at 1.12 nm, resulting in significant reductions in bulk modulus (30.8%), shear modulus (29%), and Young’s modulus (22.3%). These findings establish a quantitative pore-property relationship, providing a mechanistic basis for tailoring SAP content to enhance both durability and mechanical performance in concrete, ultimately advancing the design of longer-lasting infrastructure.

This study systematically investigated the alkali activation behavior of construction waste micro-powder (CWM) to develop a low-carbon, high-performance cementitious material. The activator formulation was optimized, the hydration thermodynamics were analyzed, and a kinetics model was constructed to reveal the reaction mechanism. The composite activator (sodium silicate and Portland cement) exhibited a significant synergistic effect, outperforming single activators. The optimal ratio was determined: 40% CWM, 60% Portland cement, and 8% water glass (modulus 1.0), which balances the system’s alkalinity and silicate modulus. Thermogravimetric analysis revealed a notable net weight gain at 3 days, indicating an ongoing secondary hydration reaction. By 7 days, the main hydration was complete, accompanied by microstructural densification, which confirmed the efficiency of the composite activator. A key contribution was the successful application of the Krstulović–Dabić (KD) model to quantify the hydration mechanism. The hydration process evolved sequentially through nucleation and growth (NG, dominant before 0.05~0.15 h), phase boundary reaction (I), and diffusion (D). The period of 0.21–50 h was governed by both I and D, after which D became the sole rate-limiting step. The model yielded the rate constants (K_NG, K_I, K_D), Avrami exponent (n), and transition points (α1, α2), providing a kinetic explanation for the ‘early strength and rapid hardening’ characteristic. In conclusion, this work establishes a material design framework guided by activator optimization, supported by thermodynamics, and explained by kinetics. The KD model proves to be a powerful tool for deciphering the hydration behavior of alkali-activated CWM, offering theoretical guidance for developing sustainable cementitious materials with controllable performance.