Current Knowledge and Pending Research on Sulfate Resistance of Recycled Aggregate Concrete

Few studies have evaluated the performance of RAC under exposure conditions that promote PSA. Figure 3 shows the results collected from the literature.

Most of the cited works compare conventional concretes to concretes with 100% RA [55,63,77,84,89,91]. Most results show worse performance for RAC than for NAC under this type of exposure, with the degradation ratio ranging from 1.04 to 1.39. The only case where better results were obtained was in Boudali et al. [91], where self-compacting concretes with 100% RA and different cementitious compositions (replacement by SCM) were studied. In this case, significantly lower degradation ratios were observed in the RAC than in the standard, with the SCM having a significant effect.

Only three papers evaluate the performance of concretes with partial RA replacement, namely, the studies by Zega et al. [83], Qi et al. [65], and Al-Baghdadi [68]. As described for ESA, the results do not show a consistent trend in the relationship between RA content and deterioration, which can be attributed to the different setups of each campaign. In addition, better performance of RAC is sometimes observed with respect to the reference concrete, again showing the possible positive effect of RA in some cases.

In Zega et al. [83], two series of concretes were evaluated: Series 1 with w/c ratio = 0.50 and Series 2 with w/c ratio = 0.38. For the first series, after ten years of exposure to saline soil (semi-buried), the decrease in the modulus of elasticity of the concretes with 25 and 75% was higher than that of the reference concrete. On the other hand, for the second series, practically no differences were observed between the NAC and the RAC, showing that the matrix quality can control the possible adverse effect of the RA.

RAC tested by Qi et al. [65] was subjected to wetting and drying cycles in a sodium sulfate solution. In this case, smaller decreases in the modulus of elasticity of RAC than those of the reference concretes were observed for those made with 30 and 50% RA and larger decreases for concretes with 70 and 100% RA. These results are confirmed by the sulfate ingress profiles calculated by analytical chemistry. The authors attribute this result to a slight change in the effective w/c ratio due to the moisture state of the aggregates.

Finally, Al-Baghdadi [68] used concretes made with RA pretreated by saturation in a polyvinyl alcohol (PVA) solution and subjected to saturation and drying cycles with a magnesium sulfate solution. In this case, the concretes obtained better results as the inclusion of RA increased, and the authors attribute these results to an internal curing effect generated by the RA and promoted by the addition of PVA.

As mentioned above, the ISA issue becomes relevant for RA due to the potential for contamination with sulfate-rich building materials, mainly gypsum. Most regulations prescribe the limiting content of sulfates in aggregates as a total percentage of their weight, with values between 0.2 and 1.0% [9,11]. However, several studies have investigated concretes and mortars with recycled aggregates contaminated with sulfate-based materials, and the results show that these limits may be conservative.

A summary of the reviewed papers is presented in Table 1. Several studies evaluate the effect of the sulfate content in RA [46,48,73,80,81]. For example, Tovar-Rodriguez et al. [73] show that mortars with 4.3% SO₃ content present three to four times greater expansion than mortars with 2.9% SO₃ content, even when an SRPC is used. The authors also demonstrate that 100% FRA mortar performs adequately in terms of durability, even with a 2.9% SO₃ content, which exceeds regulatory limitations. They base this conclusion on a mathematical prediction of service-life expansion. Agrela et al. [46] investigate samples of cement-treated granular material (low cement content and mechanical compaction) with 100% RA, different cement types, and different levels of gypsum contamination. They show that by using an SRPC, the SO₃ content of the aggregate can be increased up to 1.3% without any risk of ISA failure.

Colman et al. [48] evaluate mortars with contaminated FRA and different set-up parameters, such as temperature, alkalinity, and gypsum source. Mortars with higher SO₃ content showed higher expansion, but other parameters showed a considerable effect, such as temperature, sulfate resistance of cement, and alkalinity (due to CH lixiviation of RAs). Colman et al. [47] evaluate RAMs using several commercial FRAs with sulfate contents ranging from 0.08% to 0.62% and mixtures contaminated with gypsum (up to 3.08% SO₃). Mortars with commercial FRAs showed similar behavior to control mortar with NA, while mortar with 3.08% SO₃ showed considerably greater expansion. The authors suggest that some regulations may have conservative prescriptions (0.2% in the authors’ location).

Other studies have shown the effects of RA on the development of ISA. Yammine et al. [81] evaluate mortars with and without RA from two different sources contaminated with sodium sulfate. These mortars were subjected to temperature curing, which is common in the precast industry and may promote the subsequent development of DEF (delayed ettringite formation). In this case, slightly greater expansion was found in the uncontaminated samples for the FRA mortars. At the same time, a strong positive effect of using FRA was observed in the contaminated samples. Microstructural analyses showed that the mortars with FRA had greater incorporation of air and, consequently, a higher number of small air bubbles (20 to 200 µm) acting as crystallization points without associated confinement. Once again, these results highlight the ambiguous effect of porosity on deterioration processes involving the formation of expansion products [100,142,143]. For example, Colman et al. [47] show greater expansion for mortar with a lower w/c ratio and, hence, lower porosity. Yammine et al. [80] evaluate the expansion of NAC and RAC cured at elevated temperatures. They report lower expansion for RAC than for NAC, which is attributed to the lower internal constraint that can be achieved using recycled aggregates. Moreover, the results of some papers [47,80] show that higher alkalinity for RA can improve the performance of mixes against the ISA, and it should be considered in further experimental setups. In the case of Abid et al. [45], concretes with contaminated RAC are evaluated for ISA. The results show an increased loss of mechanical strength with increasing RA content. However, only one level of sulfate contamination was used (not reported well), and so it cannot be confirmed whether the increased deterioration is an effect of the RA or the associated increased gypsum contamination.