*3.3. DBD and APW*

DBD and APW are shown in Figure 5 for 7 d of curing under CC and CO2·C. On the NA mixture, under the CC regime, the use of carbonated water as kneading water, compared with normal water, incremented the DBD by 3.3%. This result agrees with the increase in the mechanical properties in Figures 3 and 4. Carbonated water favours the carbonation reaction at 7 d of curing, increasing the DBD [22–24]. The APW also increased by 5.28% when using carbonated water during kneading. This result is in accordance with Valdemir et al. [20], who found that CO2 released by the carbonated water could generate additional porosity. The same behaviour was observed with RMA (RMA-H2O-CC and RMA-CO2·H2O-CC), in which DBD increased by 0.8% and APW by 19.06%.

**Figure 5.** DBD and APW for 7 d under CC and CO2·C.

For the RMA mixture under the CC regime, when using carbonated and normal water (RMA-H2O-CC and RMA-CO2·H2O-CC), DBD and APW were higher than those in the same mixtures with NA. This agrees with the lower particle dry density, and higher water absorption of RMA reported in [51,79,86].

For the NA and RMA mixtures, using normal water, an increase in DBD and a decrease in APW were observed for samples cured in CO2·C (NA-H2O-CC vs. NA-H2O-CO2·C and RMA-H2O-CC vs. RMA-H2O-CO2·C). These mechanical properties could be due to sample carbonation, as observed in [13,14,18,24,27,28,56,80]. Carbonated water for kneading water under accelerated carbonation, compared to normal water, increased the DBD and APW (NA-H2O-CO2·C vs. NA-CO2·H2O-CO2·C and RMA-H2O-CO2·C vs. RMA-CO2·H2O-CO2·C). These results agree with the mechanical properties observed in Figures 3 and 4.

#### *3.4. XRD*

XRD obtained for NA using normal and carbonated water as kneading water under CC are shown in Figure 6. For normal water at 1 d, the main phases found were quartz (05-0490) [62], calcite (05-0586) [62], dolomite (11-0078), albite (10-0393) [85], and microline (19-0926) [84], which agrees with the fundamental composition of NA in Figure 2. Hatrurite (86-0402) [62], larnite (33-0302) [62] from the cement used (Figure 2), portlandite (44-1481) [62], and ettringite (37-1479) [62] from the reaction products of Ordinary Portland cement (OPC) [87,88] were also observed. Comparing the phases found using normal or carbonated water as kneading water, a sharp decrease of the phases hatrurite and larnite were observed (Inset Figure 6 labelled "C3S and C2S", red colour "1 day normal water", purple colour "1 day carbonated water"). Furthermore, the formation of portlandite Ca(OH)2 was affected by the carbonated water as kneading water (Inset Figure 7 labelled "Portlandite", red colour "1 day normal water", purple colour "1 day carbonated water") and is in accordance with Equation (7). The loss of intensity of hatrurite and larnite peaks and delay in the formation of portlandite were also reported by Hou et al. [71] with acid water. The observed results can be because of the pH of the carbonated water (4.8) and decreased mechanical strength at 1 d of curing, as shown in Figures 4 and 5.

Comparing the diffractogram of 1 d with those obtained at the ages of 3 and 7 d for carbonated water, the same phases were identified but an increase in the intensity was observed in the calcite phase (Inset Figure 6 labelled "CaCO3 (3 days)" and "CaCO3 (7 days)"), suggesting that carbonated water as kneading water produced carbonation [22–24]. This also explains the increased mechanical strength in Figures 3 and 4 and DBD in Figure 5.

**Figure 6.** XRD for NA with normal and carbonated water under CC.

**Figure 7.** XRD for RMA with normal and carbonated water under CC.

XRD obtained for NA using normal and carbonated water as kneading water under CC are shown in Figure 7. With carbonated water (Figure 7 inset labelled "C3S and C2S"), we observed a decrease in the peaks of the phases hatrurite and larnite. In addition, the formation of portlandite Ca(OH)2 was not significantly delayed when using carbonated water (Figure 8 inset labelled "Portlandite"). Both processes were due to the presence of CaCO3 and Ca(OH)2 in RMA (Figure 2). These phases acted as a buffer [5]. Hence, carbonated water can increase the mechanical properties at 1 d of age with RMA than with NA (Figures 3 and 4). These results highlight that RMA, acting as a buffer for carbonated water during kneading, avoids a decrease in pH without adding CaCO3 or Ca(OH)2, as previously proposed in [5,89]. Owing to its mineralogical composition, RMA has a similar effect as CaCO3 and Ca(OH)2.

**Figure 8.** XRD for NA with normal and carbonated water under CO2·C.

At 3 and 7 d with normal water, the same phases were identified as that of 1 d. Comparing these diffractograms with that obtained for 3 and 7 d using carbonated water, a higher intensity was observed in the calcite peaks (Figure 7 inset labelled "CaCO3 (3 days)" and "CaCO3 (7 days)"). This behaviour was already observed in the samples with NA (Figure 7) and indicates that the carbonated water produced carbonation [22–24]. This supports the increase in mechanical strength with carbonated water (in Figures 3 and 4) and DBD (in Figure 5).

XRD obtained for NA using normal and carbonated water as kneading water under CO2·C are shown in Figure 8. For 1 d, with normal water, the same phases as in CC were found. For 3 and 7 d, the disappearance of the portlandite phase was observed (Figure 8 inset labelled "Effect CO2"), which shows the consumption portlandite when it comes into contact with CO2 (Equation (1)). This concurs with an increase in the mechanical strength in samples cured in CO2·C (Figures 3 and 4). This was due to samples carbonation, as reported in [13,14,18,24,27,28,56,80]. The same phases were found for 1, 3, and 7 d with carbonated water. The portlandite also disappeared at the age of 3 and 7 d.

The effect of carbonated water at 1 d, under the CO2·C regime, is almost the same as that under CC (Figure 6). However, a decrease in the peaks of the hatrurite and larnite phases were observed (Figure 8 inset labelled "C3S and C2S"). For 3 d, the effect of carbonated water (Figure 6) is negligible on the calcite formed with respect to the effect produced by the carbonation chamber (Figure 8), because the amount of CO2 contributed by the CO2·C regime is greater than that of the carbonated water (Figure 8 inset labelled "CaCO3 (3 days)", similar intensity found for CaCO3 peaks). These results agree with the delay in setting [71] and strength development [70] due to the initial decrease in pH produced by combining carbonated water and CO2-C regimes. At 7 d, a greater intensity was observed in the calcite phase, more with carbonated water than with normal water (Figure 8 inset labelled "CaCO3 (7 days)"). This indicates that pH had been regulated [22–24] and that carbonation of the sample is better than in with normal water and agrees with the results of the mechanical properties in Figures 4 and 5 and DBD in Figure 6.

XRD obtained for RMA using normal and carbonated water as kneading water under CO2·C are shown in Figure 9. At 1 d, with normal water, the phases found were the same as those found in the CC regime (Figure 7). For 3 and 7 d, the disappearance of the portlandite phase was observed (Figure 9 inset labelled "Effect CO2"), indicating carbonation (Equation (1)) [13–17].

**Figure 9.** XRD for RMA with normal and carbonated water under CO2·C.

With carbonated water, the same phases were observed as that with normal water. For 1 d, a light decrease of hatrurite and larnite were observed (Figure 9 inset labelled "C3S and C2S", red colour "1 day normal water", purple colour "1 day carbonated water"), indicating that carbonated water has a retarding effect on the development of mechanical properties at a young age. As with NA (Figure 8), the same behaviour, including calcite peak intensities, was observed at the age of 3 d (Figure 9 inset labelled "CaCO3 (3)"). The low pH value of carbonated water along with accelerated carbonation (CO2·C) which also lowers the pH, negatively affects the strength, although less in the case of NA (Figures 3 and 4). At 7 d of curing, the calcite peaks were similar with carbonated and normal water (Figure 9 inset labelled "CaCO3 (7)") and agree with the mechanical properties in Figures 3 and 4 and DBD in Figure 5.

#### *3.5. SEM*

Figure 10 shows a general SEM and elemental composition mapping of the NA mixture with normal and carbonated water as kneading water under CC at low magnification (NA-H2O-CC vs. NA-CO2·H2O-CC). Two main zones were detected: siliceous aggregate and cement paste. The main element in the aggregates is Si and agrees with the chemical composition (Table 4), XRD (Figure 2) results. The main elements contained in the cement paste were Ca, Al, K, and Mg. At low magnification, no differences were observed using carbonated water.

**Figure 10.** SEM images and elemental composition mapping of NA with normal and carbonated water under normal curing regime CC (NA-H2O-CC vs. NA-CO2·H2O-CC) at low magnification.

However, by increasing the magnification over the cement paste zone, significant differences were found when using carbonated water (Figure 11). First, it seems that the structure of the cement paste with carbonated water was more porous than that obtained with normal water. The qualitative analysis by SEM agrees with the highest APW found with carbonated water (Figure 5). With normal water, it can be seen that the grains with rounded faces and edges were formed around the cement particles. Nevertheless, with carbonated water, large amounts of well-developed and intertwined needles particles, with very high surface areas are observed. Considering the morphological similarities with ettringite Ca6[Al(OH)6]2 (SO4)3·26H2O, it can be speculated that the needle-like structure is a carbonated ettringite with the chemical formula Ca6[Al(OH)6]2 (CO)3·26H2O [37]. Because of the high CO2 content of carbonated water, ion exchange occurs; that is, SO4 2− is fully or partially replaced by CO3 <sup>2</sup>−. A similar result was found by Pingping et al. [27] with water curing with CO2.

**Figure 11.** SEM images and elemental composition mapping of NA (zone cement paste) with normal and carbonated water under normal curing regime CC (NA-H2O-CC vs. NA-CO2·H2O-CC) at medium magnification.

SEM images with higher magnification were taken to confirm the above results (Figure 12). With normal water, grains with rounded faces and edges were observed. However, with carbonated water, hexagonal- or orthorhombic-shaped (1) and needleshaped particles (2) were observed. EDS analysis of the hexagonal particle revealed the presence of Ca, C, and O, indicating the possibility of CaCO3 [2,27]. This agrees with the greater intensity of calcite observed in XRD with carbonated water for NA (Figure 6 vs. Figure 7). For needle-shaped particles, EDS revealed a high concentration of C and O, indicating that SO4 <sup>2</sup><sup>−</sup> was fully or partially replaced by CO3 <sup>2</sup><sup>−</sup> to form carbonate ettringite [37]. Boumaza et al. [19] formed carbonated crystals having hexagonal or orthorhombic shapes between the needles of ettringite under a CO2 environment. The interlaced shape of the carbonate ettringite and greater presence of calcite (due to the carbonation produced by CO2 in the carbonated water) improved the mechanical properties (Figures 3 and 4) compared to normal water.

**Figure 12.** SEM images and EDS of NA (zone cement paste) with normal and carbonated water under normal curing regime CC (NA-H2O-CC vs. NA-CO2·H2O-CC) at high magnification. Elemental composition mapping.

Figure 13 shows a general SEM and elemental composition mapping of the RMA mixture with normal and carbonated water under CC at low magnification (RMA-H2O-CC vs. RMA-CO2·H2O-CC). In this case, two zones were observed: a siliceous aggregate or piece of brick, which is in accordance with the nature of the RMA (Figure 2), and cement paste containing Ca, Al, K, and Mg as the main elements. Furthermore, microcracks and a possible interfacial transition zone (ITZ), which is the area between the old and the new cement paste and is the weakest region in MRA mortar [3,90,91], were observed. These could explain the decrease in mechanical properties (Figures 3 and 4) with the replacement of NA by RMA (with normal and carbonated water under CC) and the higher porosity found with RMA (Figure 5). At this magnification, no differences were found between carbonated and normal water with RMA. The same areas as with normal water are also found. This is contrary to what is observed with NA (Figure 10).

With slightly higher magnification, microcracks were more visible (Figure 14). There were fewer microcracks when using carbonated water as the carbonatation products (CaCO3 particles) can gradually fill pores and micropores [22–24,35]. This agrees with the improvement in the mechanical properties observed with carbonated water in RMA under the CC regime (RMA-H2O-CC vs. RMA-CO2·H2O-CC). Furthermore, this increase in carbonation products was also observed in the XRD analysis (Figure 7). Notably, when using RMA and carbonated water, the presence of carbonated ettringite was not observed, unlike when using NA (Figures 11 and 12) due to the existence of calcite and portlandite in RMA (Figure 2). Calcite and portlandite act as buffers for carbonated water [5], consuming CO2 from carbonated water, especially portlandite (Equation (1)), thereby avoiding the full or partial replacement of SO4 <sup>2</sup><sup>−</sup> by CO3 <sup>2</sup>−.

**Figure 13.** SEM images and elemental composition mapping of RMA with normal and carbonated water under normal curing regime CC (RMA-H2O-CC vs. RMA-CO2·H2O-CC) at low magnification.

**Figure 14.** SEM images and elemental composition mapping of RMA with normal and carbonated water under normal curing regime CC (RMA-H2O-CC vs. RMA-CO2·H2O-CC) at medium magnification.

At higher magnification (Figure 15), no microcracks were observed due to the filling of microcracks by the effect of carbonated water in the RMA. In addition, carbonate ettringite (needle-shaped particles) is not observed. The same behaviour was observed at very high magnification (Figure 16). Therefore, carbonated water on the microstructure of RMA serves the purpose of filling the microcracks. Studies on the influence of carbonated water with RMA have not been found in the literature.

**Figure 16.** SEM images and elemental composition mapping of RMA with normal and carbonated water under normal curing regime CC (RMA-H2O-CC vs. RMA-CO2·H2O-CC) at very high magnification.

#### *3.6. TGA-DTA*

To determine whether carbonated water produces a greater amount of CaCO3 in the mixes with NA and RMA in CC regime, TGA/DTA was performed (Figure 17). Five stages were observed for all the mixes with normal and carbonated water. In the stage from 480 to 1000 ◦C, CaCO3 decomposition occurred [2,56], attributed to the loss of mass resulting from calcium carbonate decomposition. A high loss of mass in this range indicates high calcium carbonate in the mix.

**Figure 17.** TGA (solid lines) and DTA (dotted lines) curves for (**A**) mix with NA and (**B**) mix with RMA. Use of normal and carbonated water.

For the mix with NA (Figure 17A), a mass loss of 3.8% and 9.19% were observed for normal and carbonated water, respectively, (NA-H2O-CC vs. NA-CO2·H2O-CC) in the range of 480–1000 ◦C, indicating a greater amount of CaCO3 (product of carbonation) formation with carbonated water. This is in agreement with the mechanical properties (Figures 3 and 4), DBD (Figure 6), XRD results (Figure 6), and SEM (Figures 10–12). The temperature of the decomposition peak of CaCO3 is different between NA-H2O-CC and NA-CO2·H2O-CC. This was due to the different "nature" of CaCO3. In the case of NA-H2O-CC, this CaCO3 is the result of the hardening process of the cement [36,92]. In the case of NA-CO2·H2O-CC, the calcium carbonate is the result of the carbonation produced in the sample and by them exist a delayed in the decomposition temperature. In contrast, for the mix with RMA (Figure 17B), the mass loss is 5.5 and 6.01 for normal and carbonated water, respectively (RMA-H2O-CC vs. RMA-CO2·H2O-CC), between 480 and 1000 ◦C. In this case, the difference in calcium carbonate formation was not as important as in NA (although it is still greater with carbonated water than with normal water). This was already described in the analysis of the intensity for calcite peaks in XRD. The difference between the intensity of the peaks was greater in the NA mixture than in the RMA mixture, at the age of 7 d (see Figure 6 inset labelled "CaCO3 (7 days)" vs. Figure 7 inset labelled "CaCO3 (7 days)").
