*3.1. Characterization of Raw Materials*

Figure 2 shows the XRD patterns of NA, RMA, and cement. Quartz (SiO2) (05- 0490) [62] was the main phase for NA and RMA. Other minority phases were also found and were described in greater detail in other research [25,36]. The diffractogram of the cement was in agreement with the finding of other authors [65–69]. Table 4 shows the XRF results found for NA, RMA, and cement, which are in agreement with the phases found in XRD.

**Figure 2.** XRD patterns of NA, RMA, and cement.



#### *3.2. Compressive and Flexural Strength*

Figure 3 shows the CS results for the four mixes and two curing regimes at ages of 1, 3, and 7 days of curing. When comparing NA-H2O-CC with NA-CO2·H2O-CC at 1 day of age, CS decreased by 8.6%. According to Valdemir dos Santos et al. [20], this result is related to the reduced AFm formation in the microstructure during the early hydration period [20]. Similar results were reported by Lippiatt et al. [5] in a cement paste aged 1 d using carbonated water. In addition, the low pH value of carbonated water (4.8) can negatively affect the strength [70], delay the setting [71], and produce changes in the cement paste structure [72]. It is possible that a low pH leads to the reduction of hydrated calcium silicate and hydrated calcium aluminate because the reactions of equations 8–10 occur. Additionally, the amount of Portlandite present decreases. Therefore, the structure of the cement paste will be weaker. Nevertheless, at 3 and 7 d, an increment of 18% and 12.5% was obtained, respectively, when using carbonated water for the NA mixture and CC regime. When kneading cement and water, the pH increases rapidly, and this solution becomes saturated with Ca(OH)2 after 24 h [73,74]. Furthermore, the calcite phase found in NA (Figure 2) can act as a buffer when added to carbonated water, as observed by Lippiatt et al. [5] to achieve simultaneous hydration and carbonation in cement. This saturated solution of Ca(OH)2, together with CO2 in the carbonated water, favoured the carbonation reaction and increased CS at 3 and7d[22–24]. Equations (8)–(10) show the chemical reaction with carbonated water [20].

$$\rm CO\_{2(gasous)} + 2OH^- \text{ (gasous)} \leftrightarrow CO\_3^{2-} \text{ (augrous)} + H\_2O \text{ (liquid)}\tag{8}$$

$$\text{Ca(OH)}\_{2(aqucous)} \leftrightarrow \text{Ca}^{2+} \text{(quaeous)} + 2\text{OH}^- \text{(queous)} \tag{9}$$

$$\text{Ca}^{2+} \text{(aqucous)} + \text{CO}\_3^{2-} \text{(aqucous)} + \text{H}\_2\text{O}\_{\text{(liquid)}} \leftrightarrow \text{CaCO}\_{3(solid)} + \text{H}\_2\text{O}\_{\text{(liquid)}}\tag{10}$$

**Figure 3.** Compressive strength at different curing ages and hardening environments.

Compared to NA, RMA mixture under CC regime, using carbonated and normal water (RMA-H2O-CC and RMA-CO2·H2O-CC), had slightly lower CS. This loss of mechanical properties agreed with other studies when the percentage of substitution of NA for RA was 100% [14,75–79]. However, compared to normal water, the carbonated water was beneficial in this case for all ages of curing (19.3%, 12.1%, and 4.4% for 1, 3, and 7 d, respectively) due to the presence of CaCO3 and Ca(OH)2 in RMA (Figure 2). These phases act as a buffer of carbonated water [5], increase pH, avoid the loss of mechanical resistance, and delay hydration [70,71] that occur in the mixture with NA with 1 d of curing. Thus, carbonated water with RMA can improve the mechanical strength under the CC regime.

For NA and RMA mixtures, the increase in CS in samples cured with CO2·C (NA-H2O-CC vs. NA-H2O-CO2·C and RMA-H2O-CC vs. RMA-H2O-CO2·C) agree with the

results in [13,14,18,24,27,28,56,80–82]. For NA mixtures, with 1 d and under CO2·C, when using carbonated water in the kneading, compared with normal water, decreased the CS by 38.77% (NA-H2O-CO2·C vs. NA-CO2·H2O-CO2·C). The low pH value of carbonated water along with accelerated carbonation (CO2·C) results in a negative effect on strength [70] and delayed setting [71], which lowers the pH values of the mix [83–85]. For 3 d of curing, the effect of carbonated water on CS was still negative. For 7 d of curing, an increment of 13.3% was observed. This could indicate the regulation of pH [22–24] and the carbonation of the sample.

The same behaviour was observed in the samples with RMA (RMA-H2O-CO2·C vs. RMA-CO2·H2O-CO2·C), although with minor decreases for 1 and 3 d. This is again due to CaCO3 and Ca(OH)2 in RMA (Figure 2) acting as a buffer of carbonated water [5], maintaining a pH higher than that with NA. Thus, carbonated water under accelerated carbonation (with NA and RMA) is beneficial only after 7 d of curing.

The FS results for all the mixes under CC and CO2-C at the ages of 1, 3, and 7 d, in Figure 4, reveal the same trend as CS.

**Figure 4.** Flexural strength at different curing ages and hardening environments.
