# Evaluation of the Performance of Previous Concrete Inspired by CO\({}_{2}\)-Curing Technology
## 3 Results and Discussion
### Runoff Characteristics
Calcium leaching gradually degrades the microstructure of these cement surfaces, whereas hydraulic abrasion provides a shearing action and erodes the toxic heavy metal ions of the runoff that have become immobilized on such surfaces. These ill effects reduce the service life of concrete, and detached heavy metal ions pollute the surrounding environment [49]. The leaching process is usually controlled by diffusion from within the concrete and ongoing surface dissolution mechanisms [50]. Alkalis in cement matrices are released by diffusion, but calcium release is mainly due to the dissolution of surface portlandite and, at a pH less than 12.4, decalcification of the CSH gel. Calcium release is lower in carbonated concrete because it is transferred from portlandite and CSH to less soluble calcite. A low concentration of alumina and silicates exists in the pores of ettringite and monosulfate, which is why their release is due to the dissolution of these compounds on concrete surfaces. The release of alumina from carbonated concrete is very low because the solubility control is transferred to aluminum hydroxide (i.e., gibbsite). In contrast, ettringite in carbonated concrete dissolves so that additional silicates are released by diffusion out of the carbonated layer [50]. Although percolated water has a pH that exceeds the maximum permissible limits of drinking water according to the WHO standards [51], several soils below the concrete pavement have a modest self-buffering capacity for the attenuation of high-pH water [52]. In this study, aqueous solutions containing lead and zinc were passed through the concrete samples for ten repeat cycles. The concentration of these metal ions in the solutions was limited to 10 mg/L in this study. Figure 8 shows the percentage of removal of these metal ions from concrete samples in each cycle and the subsequent change in the pH of the percolated solutions. The removal of lead and zinc in non-carbonated samples reached 96% and 80% at the end of the experiment. The removal of lead in carbonated samples was found to be almost the same, but the removal of zinc at the end of the experiment was 62%. Zinc removal is reduced when concrete samples are carbonated. This finding indicates that lead adsorption on the hydroxyl and carbonate sites is stronger, but zinc adsorption is relatively stronger on the hydroxyl sites. Zinc precipitation occurs at a pH greater than 8.4 and may have precipitated as Zn(OH)2 as a result of the high neutralizing capacity of the non-carbonated samples. Lead species adsorb strongly onto the surfaces of CSH and calcium silicates and precipitate as lead silicates [53]. Lead carbonate sulfate hydroxide (Pb4 SO4(CO3)2(OH)2) and lead carbonate hydroxide hydrate (3 PbCO3 2 Pb(OH)2-H2O) form by chemical adsorption of the redissolved lead species in the pore channels of the  
Figure 8: Change in characteristics due to the passage of aqueous solutions simulating urban runoff. previous concrete [54]. The precipitation of heavy metal hydroxides might be the dominant removal mechanism of pervious concrete samples. [PERSON] et al. [55] reported that the removal of lead, zinc, and cadmium in pervious concrete is primarily caused by the localized pH in the pore water due to the hydration of cement compounds. Calcium, hydroxides, sulfates, carbonates, and silicates are readily available in pore water during and after cement hydration in pervious concrete. These ions in the pore solution increase ionic strength and promote reactions that form calcium heavy-metal double hydroxides and heavy-metal carbonates. Secondary removal occurs via sorption, wherein heavy metal hydroxides, produced as a result of the high pH, favorably adsorby onto and then mainly diffuse into the cement matrix. In the case of cement-bound waste, CO\({}_{2}\) curing led to the formation of an improved calcite content and a significant reduction in the leaching of calcium, arsenic, nickel, zinc, copper, lead, chromium, and molybdenum [56].