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


#### **Table 2.**

*Dehydrated phases of RC identified through XRD according to various authors.*

partial carbonation of dehydrated CH at intermediate temperatures. In addition, a higher content of carbonation products in RC is expected, because old concrete is prone to carbonate [2, 58].

Naturally, the free lime content tends to increase with the treatment temperature, especially after the decarbonation stage [25, 66]. The free lime must be taken into account during RC rehydration, since it contributes to the increase of heat release, consumption of mixing water and false setting [2, 55, 67]. Due to the high hydration susceptibility of free lime, its previous partial hydration has been reported during the stages of cooling or storage [2, 58]. It was also suggested that these newly formed CH presented lower bonding energy than the original CH in waste cement.

Wang et al. [42] reported the presence of tobermorite and jennite within the temperature range 120–450°C. The former gradually dehydrated with increasing temperature, reducing the C-S-H layer spacing from 1.2 nm to 0.96 nm up to 450°C. Above 450°C, the diffraction peaks of tobermorite and jennite disappeared, suggesting the full depolymerization of C-S-H. At 750°C poorly crystalized peaks of wollastonite (CS) and larnite (C2S) were identified, both presenting lower reactivity than partly dehydrated C-S-H phases [42].

In another study, Lü et al. [52] documented the full dehydration of C-S-H at 400°C. This was confirmed through 29Si NMR analysis, in which incipient Q0 peaks started to replace the disilicates and chain silicates (Q1 , Q2 ) of the C-S-H structures. Over 650°C, Q0 peaks were predominant, indicating a relevant decomposition of C-S-H into what was termed as poorly crystallized β-C2S. Similar findings were obtained by Alonso and Fernandez [24] in RC treated at 750°C, but the Q0 peaks were attributed to a new nesosilicate of higher reactivity than β-C2S. Finally, at 900°C, only sharp Q0 peaks were identified by Lü et al. [52], suggesting the formation C2S of low reactivity. The identification of β-C2S peaks from XRD at 900°C was also reported by Serpell and Lopez [55], which become progressively sharper up to 940°C. However, at lower temperatures in the range 660–800°C, the presence of C2S was attributed to the high temperature polymorph α'-C2S, which is usually unstable at room temperature. The formation of this polymorph of higher reactivity than β-C2S allowed to explain the better performance of RC treated at this temperature range than over 900°C, as also found by Shui et al. [49]. Based on XRD Rietveld analysis, Serpell and Zunino [56] later explored the development of different C2S polymorphs in thermoactivated RC, for the range 600–800°C. According to the authors, the fraction of β-C2S increased with the treatment temperature, substituting the polymorphs α'H-C2S and, with less relevance, γ-C2S. The low temperature formation of α'H-C2S was attributed to the direct decomposition of C-S-H into this polymorph. These new polymorphs of higher surface area and lower crystallite size presented higher reactivity [56].

Considering a wide range of RC treated between 400°C and 900°C, Real et al. [2] confirmed the partial dehydration of C-S-H to tobermorite 9 Å up to 500°C, as well as the absence of ettringite. Up to this temperature, only incipient C2S peaks were detected by XRD, indicating the onset of C-S-H transformation into C2S crystalline polymorphs. After 600°C, the intensity of these peaks was increased and the C-S-H depolymerization was confirmed through 29Si NMR analysis. The new nesosilicate form was identified as possible α'L-C2S or α'H-C2S. Finally, above 800°C, the C2S peaks were more intense and sharper, indicating a higher crystallinity and β-C2S was progressively formed, suggesting the generation of a less reactive product as also reported by other authors [52, 55].

The influence of the residence time and cooling rate in the RC thermoactivation has barely been studied. Based on a surface response model, the influence of these parameters on the formation of C2S polymorphs was analyzed by Serpell and Zunino [56]. For cooling rates ranging 187°C/h to 1925°C/h only a slight reduction of strength development was found at higher rates. The formation of dehydrated phases was significantly affected by varying the residence time between 40 and 130 minutes, increasing the fraction of α'H-C2S for RC treated at nearly 700°C. However, for higher temperatures, this factor assumed less relevance.

The phase composition of dehydrated RC is also affected by the composition of the precursor source material. From XRD analysis, Vyšvařil et al. [46] analyzed

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

the relative amount of crystalline phases in different cement pastes (only OPC; 20% ground granulated blast furnace slag (GGBFS); 20% fly ash (FA)) subjected to dehydration temperatures between 200°C and 1200°C. In all samples C2S was identified and mayenite appeared in GGBFS and FA pastes treated over 800°C. Calcium sulfoaluminate AFt and AFm phases were not identified at any temperature and treated material. However, these compounds are expected to be present in an amorphous form. This was observed by Baldusco et al. [57] from SEM images.

In sum, these studies suggest the development of a more reactive calcium silicate polymorph at intermediate temperatures, which becomes less reactive for temperatures higher than about 800°C. Overall, optimal thermoactivation temperatures are defined for the range 600–800°C. The final complex composition of RC depends on the thermal activation curve and the precursor source waste material [46, 49]. Other factors, such as the cooling rate and residence time may also affect the phase composition of RC, but further research is needed [56]. There is still some uncertainty in the characterization of the phase composition of anhydrous RC, which is affected by the different thermoactivation procedures and the limitation of the available techniques in identifying and quantifying the formed compounds (**Table 2**).
