**3. Dehydrated thermoactivated recycled cement**

After thermoactivation, the dehydrated RC particles are characterized by a porous structure and rough surface with high surface area, which is up to 15 times higher than that of OPC particles [21, 57]. In opposition, common OPC particles are non-porous and with smooth surface [42]. The helium particle density of RC tends to be slightly lower than that of OPC, ranging between about 2400–3200 kg/m3 depending on the treatment temperature [2, 21, 33].

Dehydrated waste cement is usually characterized by the absence of tricalcium silicate (C3S), as well as the significant presence of free lime (CaO) or calcite (**Figure 2**) [1]. In fact, considering the main hydration reaction of calcium silicate

#### **Figure 2.**

*XRD of anhydrous OPC, hydrated cement paste (RCP, 20°C) and RC treated at 350 and 650°C (RCP, 350°C or RCP, 650°C) [1].*

compounds of OPC Eqs. (1) and (2), the obtained C-S-H may be assumed to present an average C/S ratio (CaO/SiO2) of about 1.7 [64]. Considering the reverse dehydration of the obtained hydration products, a C/S ratio lower than 2 would be expected for the new calcium silicate as well as the presence of free clime (CaO). Based on chemical and 29Si NMR analysis of RC treated at 750°C, Alonso and Fernandez [24] estimated the value of 1.78 for the C/S ratio of the new nesosilicate, concluding that the dehydrated phase coefficients pertained to a structure close to C2S.

$$\text{C}\_3\text{S} + 2.66\text{H} \rightarrow \text{C}\_{1.7}\text{SH}\_{1.36} + \text{1.3}\text{CH} \tag{1}$$

$$\text{C}\_2\text{S} + \text{1.66H} \rightarrow \text{C}\_{1.7}\text{SH}\_{1.36} + \text{0.3CH} \tag{2}$$

$$\text{(H}-\text{H}\_2\text{O; C}-\text{CaO; S}-\text{SiO}\_2\text{)}$$

The RC is also composed by other phases, such as dehydrated calcium aluminates and carbonated compounds, besides unreacted original anhydrous OPC grains and eventual residual hydration products [42, 49, 54, 58]. **Table 2** summarizes the main dehydrated phases identified by XRD in the literature.

Shui et al. [35] analyzed the dehydrated phases developed in RC treated at 200°C, 500°C and 800°C, by means of thermogravimetry (TG) and X-ray diffraction (XRD) analysis. Up to 500°C, RC was essentially composed of partially dehydrated C-S-H and CH, as well as CaCO3 and C2S, besides other amorphous dehydrated phases. Over 500°C free lime was progressively formed from the CH dehydroxylation and at 800°C the CaCO3 was also decomposed into free lime. Similar conclusions regarding these major transformations with increasing temperature were also documented by other authors [1, 2, 42]. However, Wang et al. [42] reported a higher content of CaCO3 after treatment at 450°C than in the source cement paste. The increase of carbonated products after thermoactivation was also documented by other authors [24, 46, 52, 65], attributing this phenomenon to the
