## 3 Results and Discussion
### Testing the Carbonation Products Formed in the Pastes
The reaction of portlandite with CO\({}_{2}\) after hydration is given in Equation (1) and results in the formation of CaCO\({}_{3}\), whereas that for alite carbonates is given in Equation (2) and that for belite is given in Equation (3) or Equation (4). \[\mathrm{Ca^{2+}(aq)}+2\mathrm{OH^{-}(aq)}+\mathrm{H_{2}CO_{3}(aq)}\ \rightarrow\ \mathrm{CaCO_{3}(s)}+2\mathrm{H_{2}O} \tag{1}\]  
\[(2\mathrm{CaO\cdot SiO_{2})}+2\mathrm{CO_{2}}\ +\mathrm{nH_{2}O}\ \rightarrow\ ( \mathrm{SiO_{2}\cdot nH_{2}O})+2\mathrm{CaCO_{3}(s)} \tag{2}\]  
\[(3\mathrm{CaO\cdot 2 SiO_{2}\cdot 3H_{2}O})+3\mathrm{CO_{2}}\ \rightarrow\ 3 \mathrm{CaCO_{3}\cdot 2 SiO_{2}\cdot 3H_{2}O} \tag{3}\]  
\[(3\mathrm{CaO\cdot SiO_{2})}+3\mathrm{CO_{2}}+\mathrm{nH_{2}O}\ \rightarrow\ ( \mathrm{SiO_{2}\cdot nH_{2}O})+3\mathrm{CaCO_{3}(s)} \tag{4}\]  
However, the high intensity of the CaCO\({}_{3}\) observed at 20 = 29.39\({}^{\circ}\) indicates that another compound (or phase) other than portlandite, alite, or belite must have contributed to this high intensity. The only other source of CaCO\({}_{3}\) apart from the portlandite, belite, and alite was \(\gamma\)-C\({}_{2}\)S; the highest intensity for CaCO\({}_{3}\) at 20 = 29.39\({}^{\circ}\) occurred when the specimen under test was that with the highest ratio of replacement used, 25 wt.% \(\gamma\)-C\({}_{2}\)S, thereby proving that \(\gamma\)-C\({}_{2}\)S was responsible for the generation of the additional CaCO\({}_{3}\) in the carbonated regions. Similar observations have been reported in the literature [9; 11]. The CaCO\({}_{3}\) formed from all four sources was rendered in the solid state owing to its low solubility. The decreasing XRD intensity for portlandite in the specimen from C\({}_{2}\)S0 to C\({}_{2}\)S15 to C\({}_{2}\)S25 at 24 h shown in Figure 3a shows that most of it was sourced from OPC, and this observation was given credence because its content was lowest in C\({}_{2}\)S25. Additionally, the low intensity of calcite after 24 h of carbonation for the wholly OPC-constituted specimens (C\({}_{2}\)S0) compared to the C\({}_{2}\)S15 and C\({}_{2}\)S25 specimens indicated that the formation of CaCO\({}_{3}\) occurred more in these specimens than in the C\({}_{2}\)S0 specimens at 24 h. Figure 4b shows that the highest amount of calcite was formed at 72 h, and that this was in the C\({}_{2}\)S25 specimen with the highest substitution amount of \(\gamma\)-C\({}_{2}\)S for OPC. Therefore, most of the CaCO\({}_{3}\) was not formed from \(\gamma\)-C\({}_{2}\)S and not portlandite, indicating that portlandite is the main source of CaCO\({}_{3}\) during the carbonation of OPC mortars. In mortars whose OPC has been substituted with \(\gamma\)-C\({}_{2}\)S, additional CaCO\({}_{3}\) is formed, indicating that when \(\gamma\)-C\({}_{2}\)S is exposed to CO\({}_{2}\) in moist conditions, CaCO\({}_{3}\) is formed [12; 13]. Although supplementary proof for the formation of CaCO\({}_{3}\) from \(\gamma\)-C\({}_{2}\)S is needed in the form of porosity measurements of both the uncarbonated and carbonated regions of the specimens, it was not available.