# Study of Eco-Friendly Belite-Calcium Sulfoaluminate Cements Obtained from Special Wastes
## 3 Results and Discussion
### Hydration of BCSA Cements
The reason for this behavior is most likely related to a change in the resulting solid phase equilibria, which may have caused changes in the pore solution chemistry. The porosimetric characteristics of OPC pastes have been widely studied and extensively reported in the scientific and technical literature [56; 57; 58]; it is well known that total porosity, threshold pore width and cumulative pore volume decrease as curing times increase [18]. At early ages, the differential curves show an initial sharply defined peak, indicating a unimodal pore size distribution, centered on the lowest width of pore necks connecting a continuous system. At longer curing times, a second peak appears, corresponding to the pressure required to break through the blockages formed by the hydration products which isolate the interior pore space. Figure 6 reports both cumulative and derivative plots for intruded Hg volume vs. pore radius for CEM_R at various curing times. For periods between 7 and 90 days, the pore size distribution was unimodal, with a threshold pore width ranging from about 300 to 15 nm. During the same period, the cumulative pore volume significantly reduces (from about 215 to 130 mm\({}^{3}\)/g). Figure 4: XRD patterns for CEM_R (**top left**) and CEM_T (**top right**); CEM_W (**bottom left**) and CEM_TW (**bottom right**) cured at various ages. (A, anhydrite; B, belite; E, ettrinigite; M, monosulfate). Figure 5: Cumulative (**left**) and specific (**right**) heat release of BCSA cement samples. Figure 6: Cumulative (**left**) and derivative (**right**) Hg volume vs. pore radius for CEM_R (**a**), CEM_T (**b**), CEM_W (**c**) and CEM_TW (**d**) cement pastes cured at various ages. Compared to CEM_R, CEM_T always showed a bimodal pore size distribution. The peak oriented toward higher radii values varied in a small range, from 200 at 7 days to about 180 nm at 90 days; however, the second peak ranged from about 100 at 7 days to 35 nm at 90 days. The cumulative Hg volume for CEM_T comprised the small range of 180-165 mm\({}^{3}\)/g. Figure 6 displays both cumulative and derivative curves for CEM_W. At 7 days of aging, the pore size distribution was trimodal, with a pore radius equal to about 650, 50 and 4 nm. After longer curing periods, the pore size distribution became bimodal, with pore radii equal to about 550 and 45 nm at 28 days as well as 150 and 20 nm at 90 days; this was due to the shift of the smallest peak beyond the minimum detectable pore value by the porosimeter (3.6 nm). During the same period, the cumulative pore volume decreased from about 180 to 140 mm\({}^{3}\)/g. At 7 days of curing, the differential curve for CEM_TW exhibited two quite wide peaks centered at about 500 and 25 nm, respectively, indicating a bimodal pore size distribution. A unimodal pore size distribution was established over longer aging periods (peaks centered at about 25 and 20 nm, respectively, at 28 and 90 days). The cumulative pore volume appreciably reduced between 7 and 90 days (about 175 to 100 mm\({}^{3}\)/g).