# Evaluation of the Performance of Previous Concrete Inspired by CO\({}_{2}\)-Curing Technology
###### Abstract  
Urban runoff is acidic in nature and mainly consists of heavy metals and sediments. In this study, the pervious concrete samples were cured in a CO\({}_{2}\)-rich environment and their performance under runoff conditions was evaluated by passing different solutions containing clay particles, heavy metal ions, and acid species. The compressive strength of these samples was reduced by up to 14% when they were cured in water instead of a CO\({}_{2}\) environment. Heavy metal ions, including lead and zinc, in the simulated runoff were adsorbed in these pervious concrete samples by up to 96% and 80% at the end of the experiment, but the acid species in this runoff could leach calcium ions from the cement components during passage. Clay particles in the runoff were trapped in the flow channels of samples, which marginally reduced the percolation rate by up to 14%. Concrete carbonation reduced the release of calcium ions under runoff conditions, and zinc removal was relatively lower because of the nonavailability of hydroxyl sites in the interconnected pore structure. The weight and strength losses in the carbonated concrete samples were relatively lower at the end of the acid storage experiment, suggesting that CO\({}_{2}\) curing reduces cement degradation in aggressive chemicals. The SEM and tomography images revealed the degraded microstructure, while the XRD results provided data on the mineralogical changes. CO\({}_{2}\) curing improves the strength gain and service life of pervious concrete in runoff environments. pervious concrete; carbonation; urban runoff; pore clogging; heavy metal removal 2024 14 4202 14 202  
and storage process that involves the reaction of CO2 with calcium-containing compounds, such as portlandite (Ca(OH)2), calcium silicate hydrate (CSH), tricalcium silicate (C3S), and dicalcium silicate (C2S), and the formation of stable carbonates (i.e., CaCO3), as listed in Equations (1)-(4) [8]. \[\text{Ca(OH)}_{2}+\text{CO}_{2}=\text{CaCO}_{3}+\text{H}_{2}\text{O} \tag{1}\]  
\[\text{CSH}+\text{CO}_{2}=\text{CaCO}_{3}+\text{SiO}_{2}+\text{nH}_{2}\text{O} \tag{2}\]  
\[\text{C}_{3}\text{S}+(3-\text{x})\text{CO}_{2}+\text{yH}_{2}\text{O}=\text{C}_ {\text{x}}\text{SH}_{\text{y}}+(3-\text{x})\text{CaCO}_{3} \tag{3}\]  
\[\text{C}_{2}\text{S}+(2-\text{x})\text{CO}_{2}+\text{yH}_{2}\text{O}=\text{C}_ {\text{x}}\text{SH}_{\text{y}}+(2-\text{x})\text{CaCO}_{3} \tag{4}\]  
Carbonation during concrete curing accelerates cement hydration and leads to a higher calcite content and increased strength [9]. There are several merits in the curing of prefabricated pervious concrete in a CO2-rich environment [10]. The interconnected pore channels within the pervious concrete increase the contact area between CO2 and the cement matrix, accelerating the diffusion of CO2 into the cement matrix, and, subsequently, carbonation can occur homogeneously in the material [11]. Furthermore, pervious concrete is applied in a moist environment and flows through water, and leaching damage is a potential risk that cannot be ignored [12]. Concrete curing in a CO2 environment can fix calcium in insoluble carbonates rather than portlandite, preventing the occurrence of calcium leaching, which is a combined diffusion and dissolution process in which cement hydrates dissociate due to the diffusion of ions driven by the concentration gradient between the alkaline pore solution and external acid solution [13]. Reduced calcium ion concentration in the pore solution leads to the further dissolution of portlandite and CSH to supply calcium ions to maintain equilibrium [14]. The relationship between calcium ion concentrations in solid phases and the pore solution is well known [15]. Carbonation curing densities the cement microstructure in pervious concrete [11], inhibits the diffusion of ions outward into the pore solution [16], and delays the leaching process [17]. Typically, the runoff from urban city roads is acidic and contains mainly sediments, heavy metals, hydrocarbons, phosphorus, oil, and nitrogen [18].