## 3 Results and Discussion  
### Compressive Strength and Pore Distribution  
Figure 3 reveals the strength of the correlation between the specimen porosity ratio and the 28-day compressive strength. Pore structures were categorized into four classes based on pore size [47]: harmful pores (>1000 nm), mesopores (100-1000 nm), small pores (10-100 nm), and gel pores (<10 nm). As shown in Figure 3a, the harmful pore fraction exhibits a highly significant negative correlation with compressive strength (Pearson correlation coefficient r = \(-\)0.97, \(\left|\,\mathrm{r}\,\right|\) > 0.9). In contrast, the mesopore fraction demonstrates a weaker positive correlation with strength (r = 0.37, \(\left|\,\mathrm{r}\,\right|\) < 0.9) (Figure 3b). Neither small pores (r = 0.12, Figure 3c) nor gel pores (r = 0.29, Figure 3d) show statistically significant positive correlations with compressive strength, confirming that the volumetric fraction of harmful pores governs the 28-day compressive strength evolution in carbonation-cured  
specimens. The weak linear correlation between mesopores, small pores, and gel pores may be due to the stronger linear correlation between strength and other factors, such as the degree of alkali-activated reaction and the uniformity of pore size distribution. Figure 4a-f systematically characterize the regulatory effects of relative humidity on the 28-day compressive strength and pore size distribution of carbonation-cured specimens under three temperature gradients (10 \({}^{\circ}\)C, 20 \({}^{\circ}\)C, and 30 \({}^{\circ}\)C). As the relative humidity increased from 60% to 80%, all temperature groups exhibited an initial strength increase, followed by a decrease. The 20 \({}^{\circ}\)C--70% RH group achieved the maximum strength of 22.51 MPa, representing a 36.1% enhancement compared to the standard-cured baseline strength (16.54 MPa). In contrast, the 30 \({}^{\circ}\)C--60% RH group reached only 13.47 MPa, showing an 18.6% reduction relative to the baseline. Notably, the 28-day strength evolution trend demonstrated a significant inverse correlation with a harmful pore fraction (>1000 nm), confirming that optimizing pore structure is the critical factor governing strength enhancement. Figure 3: Relationship between 28-day compressive strength and pore distribution in specimens subjected to carbonation curing under varying temperature–humidity conditions: (**a**) harmful pores (>1000 nm), (**b**) mesopores (100–1000 nm), (**c**) small pores (10–100 nm), (**d**) gel pores (<10 nm). The experimental data indicate that moderate humidity elevation optimizes the carbonation reaction process. At 70% RH, a dynamic equilibrium between the CO\({}_{2}\) diffusion rate and the product deposition efficiency is achieved, enabling an effective filling of harmful pores (>1000 nm) by carbonation products. However, excessive humidity (>70% RH) increases pore water saturation, obstructs CO\({}_{2}\) transport pathways, and suppresses secondary hydration product formation, ultimately degrading the pore structure. On the contrary, low humidity conditions (60% RH) accelerate the diffusion of CO\({}_{2}\) but lead to excessive carbonation, generating expansive stresses that promote microcrack propagation, consequently leading to mechanical performance deterioration. Figure 4: Effects of humidity on 28-day compressive strength and pore distribution after carbonation curing under varying temperatures: (**a,b**) 10 \({}^{\circ}\)C, (**c,d**) 20 \({}^{\circ}\)C, (**e,f**) 30 \({}^{\circ}\)C.  
Figure 5a-f demonstrate the regulatory effects of temperature gradients on the 28-day compressive strength and the pore size distribution of carbonation-cured specimens under 60%, 70%, and 80% relative humidity (RH) conditions. As the temperature increased from 10 \({}^{\circ}\)C to 30 \({}^{\circ}\)C, all humidity groups exhibited a pronounced unimodal evolution pattern in both strength and harmful pore fraction (>1000 nm), which confirmed the threshold effect of temperature on the optimization of pore structure.