**4. Rehydration of RC**

The hydration mechanism of RC has yet to be fully understood. As discussed in Section 3, the dehydrated phase composition depends on the treatment temperature of RC, thus affecting the phase composition after rehydration. From XRD analysis, Xuan and Shui [60] characterized the crystalline phase composition of hydrated RC treated up to 800°C. The main phases identified in common hydrated OPC were also confirmed in RC, namely CH, CaCO3 and residual C2S and C4AF, but associated to broader and less intense peaks. From SEM analysis, the authors found that at 400°C the RC pastes presented a looser microstructure with fine bundles of C-S-H intermixed with visible Aft phases of ettringite. However, for high temperatures up to 800°C more rehydration products were developed, and the microstructure was densified, but also with a rough and irregular morphology.

In rehydrated pastes treated up to 600°C, Vyšvařil et al. [46] also reported the presence of CH, CaCO3, ettringite, C2S and C4AF. However, at 800°C ettringite peaks were absent and gismondine (CA2S2H4) was identified above 600°C. Wang et al. [42] found that CH was almost absent in rehydrated RC treated at 450°C, confirmed by TG and XRD analysis. This was compensated by the increase of CaCO3 content when compared to the source hydrated paste. From SEM/EDS analysis, RC presented a distinct morphology, characterized by massive nanosized clusters of C-S-H gel and CaCO3, also evidencing the presence of calcium carboaluminates resulting from the reaction between calcite and aluminates. Baldusco et al. [57], for paste thermoactivated at 500°C, also reported the development of carboaluminates. In a recent work of the authors of this chapter [67], it was also confirmed the possible development of carboaluminates in regions where the calcium/carbon ratio measured by SEM/EDS was nearly 4.

According to Zhang et al. [68] the rehydration mechanism of RC implies the C-S-H formation, attributed to the repolymerization of partly dehydrated C-S-H in the presence of Ca2+ and water, as well as the hydration of β-C2S. In addition, the formation of portlandite from CaO and calcium aluminates (possible C2AH8) from dehydrated aluminate phases was identified by the authors.

Real et al. [2] analyzed the phase composition of RC pastes after testing different temperatures, between 400°C and 900°C, through TG, XRD and 29Si NMR analysis.

In general, the authors found that the rehydration was effective for any treatment temperature, presenting a similar amount of binding water as reference OPC. However, incomplete depolymerized RC, treated up to 500°C, failed to provide significant cohesive bonding between anhydrous particles. Above 600°C, the weakly formed CH presented less binding energy than the original ones in source OPC pastes. The authors found slight differences between RC and reference OPC from TG analysis, namely related to the formation of higher amounts of carbonate/sulfate Afm phases, a lower amount of CH and a higher amount of CaCO3 in RC pastes. XRD analysis also confirmed the formation of similar crystalline phases in RC and OPC pastes, supporting the adequate rehydration ability of thermoactivated waste cement, especially above 600°C. The 29Si NMR analysis clearly evidenced the effective depolymerization and rehydration of pastes treated above 600°C. Rehydrated RC treated between 600 and 900°C presented mean silicate chain lengths (MCL) between 3 and 6, which indicates the formation of C-S-H with a C/S ratio over 1.2 [51, 69]. Moreover, the MCL and Q2 /Q1 ratio were of the same order in OPC and RC pastes treated at 700–800°C, suggesting the development of the same type of C-S-H in both materials. However, the estimated coefficient of hydration was higher in RC pastes, indicating the formation of a higher volume of C-S-H, at least up to 28 days. This was attributed to the higher surface area of RC and the development of interparticle products as discussed later in Section 7. Noteworthy was the slightly higher MCL and Q2 /Q1 ratio reported for RC treated at 600°C, indicating a higher reactivity of this product with α'-C2S in its constitution, which was able to form a greater amount of C-S-H of longer chain length. On the other hand, above 800°C an opposite trend was found in the MCL and Q2 /Q1 ratio, confirming the slower rehydration capacity of RC associated with the formation of less reactive C2S polymorph.

From the above studies, it is concluded that the rehydration of RC involves the generation of C-S-H, as found in OPC, and AFt or Afm phases associated with sulfoaluminate and/or carboaluminate compounds. The type and morphology of hydrated phases depend on the treatment temperature of RC. The carbonated products tend to be more abundant in RC pastes, due to residual carbonation products in source waste cement paste and the possible carbonation that occurs after thermoactivation.
