**6. Fresh properties of recycled cement**

Two main obstacles that have been hindering the application of RC are its high water demand [43, 46, 48, 53] and fast setting time [49, 58, 67]. The high water demand is essentially attributed to the high surface area [57] and porous nature [2, 43, 48, 58] of RC, and in a second plan to the free lime content [49, 53, 55, 60] and particle agglomeration [53]. Therefore, for a given workability the RC pastes usually require a high water/binder (w/b) ratio.

Shui et al. [49] found that the amount of water required for normal consistency increased from 0.48 to 0.68 with increasing treatment temperature between 300°C and 900°C. These values were about 1.8 to 2.5 times higher than those obtained for OPC. The free lime and high surface area of RC were the main reasons attributed to this phenomenon. Both factors increased with increasing temperature. Xuan and Shui [60] studied the influence of the source paste composition (w/c ratio) on the water demand of RC treated at 200–900°C. The authors found that increasing the w/c of the source paste from 0.3 to 0.5 led to a 10–20% increase of the RC water demand, regardless of the treatment temperature. This may be explained by the higher porosity and higher amount of dehydrated products, with subsequent increase of the specific surface area, in RC treated from high w/c waste paste.

Yu and Shui [53] confirmed the great propensity for RC particles to agglomerate, which is encouraged by their high surface area. The particles are agglomerated by capillary action, trapping the mixing water between them. Therefore, part of the mixing water becomes unavailable and the water demand is increased for a given workability. After sonication, the authors found a slight increase of workability due to the partial dispersion of RC particles.

However, the authors found that the water demand was not significantly affected up to 50% replacement of OPC with RC treated at 700°C. The reported increase in water demand was 1.4 and 2.5 times higher for 50% and 100% RC incorporation, respectively. According to the authors, the lubrication effect provided by the fine OPC particles aided the reduction of the friction between coarser particles, partly compensating the adverse effect of RC. In a later study, Real et al. [2] found that RC treated between 400°C and 900°C may present 2 to 3 times higher water demand than OPC pastes of equal normal consistency, increasing with the treatment temperature. A more significant increase was found above 800°C, which was attributed to the higher free lime content, after decarbonation.

In order to reduce the effect of free lime on the water demand, Serpell and Lopez [55] suggested a two-stage mixing procedure. First, water is added in order to slake the free lime and the additional water is added to compensate the water loss by evaporation, which is measured by weight difference. Nevertheless, the reported increase for RC heated between 650 and 850°C was lower than 2%, which is not significant. Similar values were reported by Carriço et al. [58]. It was concluded that the surface roughness and porosity of RC were the most influential parameters.

Most studies report lower initial and final setting time in RC than in OPC [1, 35, 43, 48, 49, 55, 58, 60, 62, 68]. The initial setting time may be as low as less than 20 minutes, which makes it less viable for building applications [49, 58]. The low setting times in RC are associated to the fast rehydration of RC, owed to their higher surface area and higher reactivity [43, 49, 57].

The low setting time is also attributed to the free lime content [24, 46]. According to Vyšvaril et al. [46] this may explain the decrease of setting time with increasing dehydration temperature. In addition, Serpell and Lopez [55] suggested that the apparent rapid setting of RC is related to a "false setting" phenomenon that

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

may disappear after a while. A similar phenomenon is confirmed by Carriço et al. [58] for RC treated over 700°C.

The reduction of the setting time with the treatment temperature up to 800°C has been reported by various authors [43, 49, 57, 60]. Shui et al. [49] found that the initial setting time decreased from 42 to 17 minutes when the treatment temperature was increased from 300–800°C. This phenomenon was attributed to the quick repolymerization of new C-S-H, which does not involve the dissolution-precipitation process of OPC. Carriço et al. [58] found that the potential setting time of RC treated at 900°C was 2 times longer than that treated at 600°C, confirming the lower reactivity of RC for high dehydration temperatures. Similar findings were reported by Real et al. [2]. The authors also found that the potential replacement of OPC with up to 50% RC only slightly affected the setting time (less than 10%). However, Yu and Shui [53] reported a progressive reduction of the setting time with the incorporation of up to 30% RC. The same trend was documented by Yu et al. [61], for replacement percentages between 5 and 25%.

However, less expected high setting times in RC than in OPC were found by some authors [1, 2, 46]. The authors attributed this phenomenon to the possible agglomeration of RC, as well as the eventual pre-hydration of RC during cooling and storage. Setting times over 2 times higher in RC than in OPC are reported by Real et al. [2].

Taking into account source cement pastes of distinct w/c, Xuan and Shui [60] found lower setting times for high w/c cement pastes, attributed to the greater amount of dehydrated phases.

As well known, the setting time of OPC pastes is governed by the dissolution rate of C3A and its combination with added calcium sulphate [51, 70]. However, in RC the lack of knowledge regarding the morphology of the dehydrated aluminate phases at different temperatures and their reaction mechanism does not allow to consider the same reasoning. As mentioned in Section 5, Zhang et al. [68] explored the use of different set retarders in RC treated at 600°C. The authors found that the addition of 4% gypsum only slightly increased the setting time. It seems that the dissolution of gypsum in RC can be slower than the reaction involved in the rehydration of dehydrated compounds. Contrary to OPC, in which the gypsum dissolution prevents the formation of CAH from C3A, its addition showed to be ineffective in RC [15, 68]. Nevertheless, Sun et al. [62] documented a 6-fold increase of the setting time when 8% gypsum was added to RC thermoactivated at 550°C.

The above studies underline the challenging task of controlling the fresh behavior of RC pastes, still without an efficient solution to overcome their high water demand and non-standard setting time. Further research is needed in this domain, contributing to higher confidence in using RC.
