Hybrid Environmentally Friendly Method for RCA Concrete Quality Improvement

In these experiments, jaw-crushed concrete waste from the origin was sieved between 5 and 10 mm. The initial natural coarse aggregates were also from the same origin with a water absorption value of 0.68% and a surface dry density of 2.67 g.cm⁻³. The recycled aggregates samples are categorized into 6 groups to be treated differently: untreated aggregates (Untreated), dry-milled aggregates (Dry), wet-milled aggregates (Wet), acid-milled aggregates with 5 min rotation phases (Acid 5), acid-milled aggregates with 10-min rotation phases (Acid 10) and acid soaking only with no agitation or milling (Soaking). For the samples using acid treatment, acetic acid of 99.9% mass concentration is used as a raw reagent before dilution, Portland cement from Taiheiyo company with a density of 3.16 g.cm⁻³, and river sand with particle size between 0.08 and 2 mm. All experiments were conducted at 20 °C. The particle size distribution of raw materials is shown in Fig. 1.

Six sample groups of RCAs were selected to study the effect of each phenomenon separately, following the classification of Fig. 2. The “Untreated” group serves as a reference for the recycled aggregates without treatment. The “Dry” group evaluates the efficiency of dry milling itself, while the “Wet” group evaluates the milling in wet conditions with water. Acetic acid with 3% concentration was used for pretreatment of RCA. The choice of acetic acid over others is motivated by three advantages: safer use and transport of the chemical substance, more cost-effective compared with strong acid treatments, and cleaner method overall as it does not introduce undesired ions in the aggregates or concrete. The “Acid soaking” group estimates the efficiency of the acid soaking itself without agitation or milling following (Wang, 2017), while the “Acid 5” and “Acid 10” groups combine acid soaking and milling together with different milling/soaking times. The expectation is that the longer the soaking time, the easier the residual mortar gets removed from the aggregates. Besides, the “NA” group consists of the same natural aggregates as the original ones to scale the improvement of each treatment. For the “Wet”, “Soaking”, “Acid 5”, and “Acid 10” groups, the soaking or milling solution is kept and filtered at the end to get used as a mixing solution to make concrete (Table 1).

After the treatment stage, RCA samples are used to make concrete. Before casting, as the aggregates are recycled, and the residual mortar can still absorb water, the aggregates were soaked in water for 24 h to be completely saturated so that the proportion of water in the mix design remains the same. The mix design of the concrete specimens was elaborated according to JAS5 industrial Japanese standards, adapted to the aggregates parameters of each group, as shown in Table 2, and set to produce 1 m³ of concrete. The water/cement ratio for all samples was 50% to achieve medium-strength concrete. In each group that uses wet conditions, the residual solution after aggregate treatment was used as mixing water to verify the effect of solution pH in the final concrete performance. During the treatment phase, the pH of each solution was measured at the beginning and the end after sieving and filtering. The aggregate after the treatment process was used in the concrete mixing procedure, following the JIS standard. Initially, cement was mixed with sand for approximately 15 s until a homogeneous mixture was achieved. Following this step, water with a chemical admixture (specifically, a water-reducing agent from BASF) was added to the mixer and mixed for about 30 s. Next, all material was scraped off from the wall of mixer and then mixed again for 60 s. Subsequently, coarse aggregate was added and mixed for 90 s before being poured into the mold. The mold size of concrete samples is 40 × 40 × 160 mm, with three samples for each group. After casting, all specimens were cured in an air-conditioned curing room at 20 °C for 28 days.

After the aggregate treatment phase, the water absorption, density, and mortar removal of each group are measured to verify the effect of each technique. In the next step, the concrete samples were prepared in 40 × 40 × 160 mm molds and cured under water for 28 days. The compressive strength was measured by the universal testing machine with 250 kN axial load capacity according to JIS A 1108, and the flexural strength of each sample was measured following JIS A 1106. SEM images on the surface of aggregate after the treatment process were observed using Hitachi TM-4000.

The pH was monitored during the treatment process, and the final pH is indicated in the bar graph in Fig. 3, while the pH of the original solution is indicated as 3.03. In the samples which contained an initially acidic solution with acetic acid, air bubbles were produced due to the same chemical reaction and decreased over time. The final pH in the Soaking group is 5.27, which shows that the solution is still acidic at the end and is expected to cause lower strength in the final concrete (Arunakanthi et al., 2012).

Although the starting solution and its pH are the same between the Soaking, Acid 5, and Acid 10 groups, the final value varies greatly depending on the rotation conditions. This suggests that both the soaking duration and the number of rotations influence the amount of mortar reacting with and dissolving in the solution.

The compression results of concrete made with treated and untreated RCA are shown in Fig. 8. The results showed that the mechanical properties of recycled aggregate concrete significantly increased after RCA was treated with the combination method, as shown in sample Acid 5 and Acid 10. These compressive strengths increased by about 10% compared to the untreated RAC. The combined method significantly improved the physical properties and microstructure surface of aggregate, which led to increasing the compressive strength of RAC. In addition, the solution after the combined treatment process with a pH value of around 11, which agrees with the optimal value reported by Bekir Çomak (Çomak, 2016), was used in the concrete mixture. While for the wet and acid milling samples, the improvement in compressive strength is negligible because of the mixing solution, the solution from the treatment process was used in the concrete mixture instead of industrial water for concrete. As these solutions have different compositions and pH compared with industrial water, the concrete quality is expected to vary according to these parameters as well (Çomak, 2016). To decouple the influence of the pH from that of the acetic acid, the concrete made from the wet milling sample uses only the water used during the milling, whose pH value (12.4) is only due to the reaction of the residual mortar with water. The final compressive strength for this concrete is 52.3 MPa, a value lower than with untreated RCA and water. In accordance with Bekir Çomak (Çomak, 2016), the very alkaline pH is responsible for the drop in strength observed here. Because it is mainly due to the high alkali hydroxide concentration in the initial solution does not accelerate OPC hydration, which adversely impacts the nucleation and precipitation of the C–S–H phase (Zajac et al., 2022). This observation aligns with prior research findings that when a solution with a pH of 12 is employed, the resulting compressive strength is lower compared to the use of regular water. Furthermore, the compressive strength exhibits minimal improvement even after a 21 day curing period (Utepov et al., 2022). In addition, within the wet milling method, the surplus of water present in the ITZ also contributes to the decreased strength of the resulting new concrete. Similarly, the pH of the solution from the acid-soaking sample group has a pH of 5.27, a value known to reduce the quality of the final concrete (Arunakanthi et al., 2012).

The same phenomena also were seen in flexural strength of RAC which was indicated in Fig. 9. The combined method improves about 11% compressive strength of RAC compared to that of untreated RCA concrete.