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CONCRETE PRODUCTS & CAST STONEwww.cpi-worldwide.com CPI %u2013 Concrete Plant International | India Edition %u2013 4 | 2025 25The initial scanning electron microscope (SEM) images (Fig. 2) were taken using the coarse material, clearly demonstrating the necessity of fine grinding to optimize it for potential use as a binder with pozzolanic properties. Furthermore, it was speculated that mechanochemical activation could enhance the pozzolanic reactivity of the material. The SEM analysis revealed the crystalline structure of the residual material, which is initially considered unfavourable in terms of reactivity.To assess the emission behaviour of potentially environmentally relevant contaminants in the tailings material, a percolation test was carried out in accordance with DIN 19528:2023 (Table 1). The leachable concentrations were compared to the assessment criteria for soil material in class BM-F3.The results show that most of the examined parameters are below the applicable limits. Of particular note is the arsenic content of 741 %u00b5g/l, which exceeds the permissible limit of 100 %u00b5g/l according to BM-F3 by a multiple. This exceedance indicates an environmental toxicological risk when utilizing the raw material. Therefore, there is a need for targeted removal or inertization of the arsenic, for example through mineralogical separation, chemical stabilization, or selective processing, before the use of the tailings material can be considered.Investigation of the pozzolanic reactivity of the tailings material to assess its potential as a cement substituteTo evaluate the potential pozzolanic reactivity, the tailings material was first subjected to mechanical fine grinding, achieving a specific surface area of 4,000 cm%u00b2/g (Blaine). Subsequently, a test based on the Frattini method in accordance with DIN EN 196-5:2011 was performed to determine the material%u2019s ability to react with calcium hydroxide (Ca(OH)2) in an aqueous environment.The analysis is based on quantifying hydroxide (OH%u2013) and calcium ion (Ca%u00b2+) concentrations after a storage period of 15 days. For the test, a cement-tailings mixture with 80 mass-% CEMI 52.5 N and 20 mass-% tailings was used.For comparison, additional materials with known reactivity were included: metakaolin and coal fly ash as established pozzolanic additives, as well as limestone powder as a typical inert filler. Another reference sample consisted of pure CEM I. Figure 3 shows that both metakaolin and coal fly ash lie below the saturation curve, indicating pronounced pozzolanic reactivity. In contrast, the tailings material and limestone powder lie above the saturation curve, confirming their non-reactive, inert nature.As part of the investigations, the influence of the tailings material on the mechanical properties of concrete mixes was analyzed after partially substituting the cement content. The mixes were prepared and tested in accordance with DIN EN 196-1:2016. The reference mix consisted of 450 kg/m%u00b3 CEMI 52.5 N with a water/cement ratio of 0.50. Figure 4 shows the results of the compressive and flexural tensile strength tests after 28 days.At low substitution levels between 5 and 15 wt.-% based on the cement content, an increase in strength was observed compared to the reference mix. This effect is attributed to an increased packing density in the fine particle range as well as the function of the tailings material as a crystallization nucleus during cement hydration.05101520253030 40 50 60 70 80 90 100CaO (mmol/l)OH- (mmol/l)Puzzolanic Non-puzzolanic Limestone powder Tailings MetakaolinCEM I Fly ash Saturation curveFig. 3: Frattini-Test according to EN 196-5.