**5. Kinetics of RC hydration**

The hydration mechanism of RC has yet to be fully understood. As discussed in Section 3, the dehydrated phase composition depends on the treatment.

The mechanism and kinetics behind the hydration process of RC are still barely understood and subject of current research. Based on isothermal calorimetry (IC) tests, Wang et al. [42] analyzed the rehydration kinetics of RC heated at 200–1010°C. A significant initial heat release rate immediately upon contact with water was observed, followed by an induction period of a few hours and an acceleration stage with a maximum peak at about 10 hours. After the acceleration stage, the heat release rate was lower in rehydrated RC than in Ref. OPC. Among all tested temperatures, RC treated at 450°C showed the highest peak of heat release, as well as the highest cumulative hydration heat. The high reactivity for 450°C was attributed to the faster rehydration of partly dehydrated tobermorite and jennite (Section 3). This seems to have been triggered by the high calcite content that may have also provided nucleation and filler effects. Similar findings were reported by Angulo et al. [54] for RC treated at 500°C, then presenting less reactivity at 650°C.

#### *Thermoactivated Recycled Cement DOI: http://dx.doi.org/10.5772/intechopen.98488*

As discussed in Section 4, other studies have shown higher RC reactivity for treatment temperatures above 600°C.

Carriço et al. [58] also found that similarly to OPC, the hydration mechanism of RC treated at 700°C involves four main stages corresponding to the initial hydrolysis, induction, acceleration and deceleration. However, RC shows a remarkably higher initial heat rate, especially up to 90 minutes, in which the heat release was nearly three times higher than that of reference OPC. This phenomenon was in line with the observation of a lower setting time in RC. However, the long-term heat release of RC tended to be lower than that of OPC, suggesting the formation of less hydrated products over time.

Real et al. [2] analyzed the hydration heat evolution of RC treated between 400 and 900°C through IC analysis. The maximum heat release and acceleration peak were attained for RC treated at 600°C. The authors also observed that RC required longer periods to initiate the acceleration stage and reach the maximum heat peak than OPC, especially those cements treated above 800°C. Moreover, the RC heat release during the acceleration stage was significantly lower than that of OPC, regardless the treatment temperature. RC treated at 400°C had no significant long-term reactivity, explained by the low depolymerization level of this product. The low reactivity of RC treated at 900°C was related to the low reactivity of the C2S polymorph obtained at this temperature. Overall, it was concluded that RC pastes tended to present longer induction periods and less intense acceleration stages.

In all these studies [2, 42, 54, 58] the IC analysis were carried out after external mixing, preventing the determination of the heat released during the first few minutes of hydration. This initial higher hydration heat release of RC was attributed by various authors to the exothermic reaction of CaO [36, 42, 54]. However, according to Baldusco et al. [57] the high initial hydration cannot be solely attributed to the hydration of CaO, because high hydration rates are observed even in RC treated at as low as 500°C, in which the amount of free lime is not as significant. The authors suggest that as stated by Shui et al. [49], this phenomenon is better explained by the high surface area and instable nature of dehydrated RC phases, which are ready to repolymerize, despite the high calcium aluminate content of RC.

Taking into account the thermoactivation at 500°C of a waste binder composed by OPC and GGBFS, Balsusco et al. [57] documented a different shape for the IC curve. In this case, the curve was characterized by a high initial peak of heat release followed by a continuous deceleration, without the identification of typical induction and acceleration stages. According to the authors, instead of a dissolutionprecipitation mechanism, the rehydration followed an instantaneous re-adsorption of water. These apparently contradictory results may be explained by the partly dehydrated state of RC anhydrous phases at 500°C. Indeed, the same behavior was found by Real et al. [2] for RC treated at 400°C. As the IC analyses were prepared with internal mixing, allowing to record the heat release upon contact with water, the cumulative heat was higher in RC than in OPC. Zhang et al. [43] also recorded the initial heat release of RC treated at 600°C, reporting that this was 10 times higher than in Ref. OPC paste. In this case, the addition of gypsum was not successful in reducing the initial reactivity of RC. In another study, Zhang et al. [68] tested other set retarders in order to control the hydration evolution of RC. For RC treated at 500°C the authors found that 1–1.5% of sodium borate was able to reduce the initial peak of heat release and extend the initial setting time.

In sum, although it is generally accepted that RC is responsible for a significant initial heat release upon contact with water, the kinetic mechanisms and involved rehydration reactions are still under debate.
