**8. Mechanical properties of cement-based materials with RC**

Various studies have already been carried out concerning the mechanical characterization of pastes or mortars produced with incorporation of 100% RC. For different dehydration temperatures and a wide range of w/b ratios the reported compressive strength varied as much as between about 4 and 30 MPa (**Table 3**). Besides these factors, the compressive strength is also affected by other parameters, such as the characteristics of the precursor material, grinding fineness and agglomeration

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


#### **Table 3.**

*Maximum values for the 28-day compressive strength of 100% thermoactivated recycled cement pastes, mortars and concrete reported in the literature.*

level of RC. When available, the relative compressive strength of RC versus that of reference OPC of equal w/b (fc,Rc/fc,OPC) is also indicated in **Table 3**. In all cases, the 28 days compressive strength of RC tended to be lower than that of OPC, confirming the development of a less dense net of interparticle hydration products, as discussed in Section 7.

When RC was directly thermoactivated from recycled concrete fines (RCF), the 28 days compressive strength was under about only 10 MPa (**Table 3**). This is related to the lack of effective separation methods allowing the increase of rehydratable compounds in the precursor material. Therefore, the development of new methods for the efficient individualization of waste concrete constituents is a priority goal in waste cement recycling. Following the new patented method of Bogas et al. [40], Real et al. [2] could retrieve cement paste from waste cement with only up to 12% aggregate contamination by volume. In this case, concrete produced with up to 30% incorporation of RC from waste concrete showed similar behavior to RC directly obtained from pure laboratory waste cement paste of equal composition.

As mentioned, most authors have opted to conduct their works using lab-made well-hydrated paste as precursor materials, in order to avoid the complex stage of concrete separation. Moreover, the full potential of RC is better accessed through non-contaminated waste precursors.

Xuan and Shui [60] showed that the mechanical strength of RC pastes is also affected by the w/c of the waste material. It was found that higher strength would be obtained using waste cement pastes with lower w/c, of 0.3 instead of 0.5. This was attributed to the higher amount of unhydrated cement particles in less hydrated low w/c pastes, which may contribute to the subsequent development of more hydrated products. Another reason may also be related to the possible production of more interparticle hydration products in these systems with less porous RC obtained from low w/c pastes.

Different trends may be found in the literature regarding the evolution of the compressive strength as a function of the treatment temperature. In some studies, the maximum mechanical strength was attained for dehydration temperatures around 500°C [42, 45, 54]. For an optimal temperature of 450°C, Wang et al. [42] explained the obtained highest strength by the quick rehydration of partially dehydrated tobermorite and disordered jennite. For higher temperatures, the presence of wollastonite and crystalline larnite reduced the subsequent reactivity of RC. However, in most cases the optimal mechanical strength has been reported to be attained within the range of 600–800°C [19, 44, 49, 56, 58, 60].

A maximum compressive strength of as high as 32 MPa was achieved by Serpell and Zunino [56] for RC treated at 750°C. As discussed in Section 3, at this temperature range the authors identified the formation of a more reactive polymorph form of dicalcium silicate (α'H-C2S), which progressively turned into the less reactive α-C2S at higher temperatures. From various studies, including those exploring the use of 29Si NMR analysis to characterize the structure of C-S-H in RC [2, 52], it seems reasonable to conclude that below 600°C the phase dehydration is incomplete and at higher levels the reactivity of the anhydrous Q1 phases are highly dependent on their morphology and crystallinity. It also seems evident that over a maximum optimal treatment temperature the compressive strength is reduced [2, 42, 46, 49, 54, 56]. In fact, various studies in the literature suggest that above about 800°C the RC dehydrated phases react slowly and compressive strength is reduced [2, 42, 49, 55].

Lü et al. [52] first demonstrated that at 900°C the dehydrated SiO2 tetrahedrons of α -C2S were not significantly repolymerized upon water contact. Similar findings were obtained by Real et al. [2].

Bogas et al. [1] showed that the compressive strength of RC mortars might be increased when the maximum particle size of RC is reduced from 250 μm to 63 μm. Similar findings were obtained by Letelier et al. [36], comparing the performance of mortars with RC of 150 μm and 300 μm. However, taking into account RC with up to 75 μm or 150 μm, the authors did not find significant differences in the compressive strength. One reason for these differences is attributed to the agglomeration state of RC particles in the upper particle size range.

The agglomeration issue related to fine RC particles is documented by various authors [1, 2, 43]. From SEM analysis, Shui et al. [49] confirmed the poor dispersion of fine RC. Then, the same authors [53] demonstrated that the compressive strength could be almost doubled when RC was previously dispersed through sonication.

As discussed in Section 6, the increase of the treatment temperature increases the w/c ratio needed for a given workability. Naturally, this leads to a reduction of the compressive strength when compared to reference OPC of equal workability. In fact, Real et al. [41] reported a reduction of about 15% in the compressive strength when only 15% OPC was replaced with RC treated at 650°C. To compensate this, high dosages of superplasticizer have been considered in RC cement-based materials [2, 33, 61, 68]. According to Real et al. [41], the superplasticizers (SP) are also effective in reducing the mixing water in RC concrete, but the SP saturation point tends to be higher due to the porous nature and high surface area of RC. Nevertheless, the authors report that when SP is adopted, a better dispersion is

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

attained, and a high percentage of RC may be incorporated without significantly affecting the mechanical strength.

Regarding the strength evolution, there is an almost general consensus that the hardened properties are developed faster in RC than in OPC [41, 43, 57, 67]. Shui et al. [49] found that pastes with RC treated between 400°C and 800°C showed hydration degrees at 1, 3 and 28 days of 70, 80 and 90%, respectively. Moreover, the compressive strength of RC pastes at 3 days could be higher than that of reference OPC pastes. After this point, the relative strength of RC progressively decreased to 60% of that of OPC, at 28 days. Similar findings were obtained by Bogas et al. [67] and Real et al. [2]. As mentioned in Section 7, the compressive strength at 3 days was similar to higher in RC pastes than in OPC pastes but was about 30% lower at 28 days.

On the other hand, Baldusco et al. [45] reported that the 3 days compressive strength of RC paste could be about twice that of OPC. This was explained by the hydration of residual unhydrated calcium silicate grains, as well as the fast rehydration properties of RC. After 3 days, the strength evolution was not significant, being only 6% higher at 28 days. As discussed in Section 7, the authors suggested the formation of a long-term looser microstructure in RC pastes than in OPC pastes. According to Zhang et al. [43] in RC pastes, the compressive strength at 28 days is limited by the weaker RC particles of porous nature and low hardness. However, Bogas et al. [1] suggested that the strength evolution may be severely affected by the size and agglomeration state of RC particles, explaining the delayed strength evolution beyond 7 days found in RC mortars produced by the authors.

In order to access the potential use of RC as cement addition, various authors have explored various partial substitution percentages of OPC with RC. In a first study, Bogas et al. [1] analyzed the mechanical strength of mortars produced with 20–100% RC treated at 650°C. A progressive reduction of the compressive strength was observed, but up to 20% replacement the strength was similar to that of reference OPC mortars.

In mortar with 10 to 30% replacement of OPC with RCF treated at 500°C and 800°C, Florea et al. [33] confirmed the decrease of the compressive strength with increasing RCF content. Up to 10% replacement of OPC with RCF treated at 800°C, strength was little affected, but for 30% replacement, the reduction was as high as 35%. However, Xinwei et al. [48] reported more optimistic results for pastes with 40% replacement of OPC with RC treated at 750°C, leading to only a slight reduction of 12% in the compressive strength compared to reference OPC pastes. However, for 60% replacement, the compressive strength was 36% lower. Similar replacement ratios were studied by Araújo et al. [71] for RC treated at 700°C. The authors confirmed a slight reduction of the 28 days compressive strength for 40% replacement (of about 14%) but a significant decrease for 60% replacement. In another study involving pastes produced with 5–15% RC treated at 650°C, Yu and Shui [53] found maximum compressive strengths for 5% replacement, being 30% higher than that of OPC paste. The strength was further increased by about 15%, when the RC dispersion was improved by the addition of ethanol and a sodiumbased dispersant followed by sonication.

So far, only a few studies have been published regarding the mechanical strength of concrete with RC. Letelier et al. [36] analyzed the mechanical strength of concrete produced with 5–15% of RCF treated at 400°C, 500°C and 900°C and 20 to 40% recycled aggregates. No significant compressive strength reduction, below 1%, was observed when OPC was replaced with up to 15% RCF. The strength reduction was more affected by the substitution of natural aggregates with recycled aggregates than by the OPC replacement with RC.

Carriço et al. [58] studied the influence of the incorporation percentage and dehydration temperature on the physical and mechanical behavior of RC mortars. For replacement percentages with up to 20% RC the workability and compressive strength were not significantly affected. Mortars with RC treated at 600–800°C showed the best mechanical performance. For up to 50% incorporation, the mechanical strength was only 12 to 23% lower than that of reference OPC mortars of equal w/b. Moreover, even for a high w/b of about 0.6, mortars with 100% RC were able to attain as high as 27 MPa at 28 days. It was found that RC can be comparable to the low grade OPC of class 32.5.

Qian et al. [72] analyzed the production of ultra-high performance concrete produced with lime powder, silica fume and different replacement percentages of OPC with RC treated at 650°C. The authors found that up to 25% OPC replacement the concrete workability was only minorly affected. Regarding the compressive strength, it generally decreased with the incorporation of RC. The increased air content caused by the loss of workability was the main reason attributed to this reduction, especially for incorporation percentages over 25%. Nevertheless, at 7 days the compressive strength of concrete with up to 12.5% RC was higher than that of OPC concrete.

In a more recent study, Real et al. [41] investigated the mechanical strength behavior of concrete produced with RC treated at 650°C, obtained from waste cement paste or waste concrete. Concretes were produced with total or up to 40% RC incorporation. Up to 15% RC, workability was not significantly affected. Over this level, SP had to be incorporated in RC concrete production. Overall, the mechanical strength was not significantly affected or even increased by the incorporation of up to 40% RC. Even for 100% RC, the strength reduction was only 17%. It was thus concluded that the eco-efficient RC might have great potential as a supplementary cementitious material. Moreover, the RC concrete showed higher 3 days compressive strength than OPC concrete, regardless of the RC content. As expected, the modulus of elasticity decreased with the RC content due to its lower hardness and stiffness than OPC.
