Utilization of Copper Mine Tailings as Road Construction Materials through Geopolymerization

Abstract

This research studies the feasibility of using copper mine tailings (MT) as an alternative road construction material through geopolymerization. First, an extensive literature review on the use of MT as road construction material was performed. The utilization of MT in road construction can be divided into two general categories based on the stabilization method: using conventional stabilizers and through geopolymerization. Despite of the efforts of many researchers, utilization of MT in road construction is still very limited. The possible reasons are the potential contamination from the waste, the lack of relevant standards and the slow acceptance on utilization of waste materials in construction by public and industry. Further research is needed on such aspects as environmental, technical, economic, government policy and public education related to utilization of MT in road construction. Second, a systematic study on the utilization of copper mine tailings (MT) as road base construction material through geopolymerization was performed. Specifically, MT was mixed with different amount of sodium hydroxide (NaOH) solution at various concentrations from 0 to 11 M, compacted and then cured at 35°C. After 7 days’ curing, unconfined compression tests were performed on the specimens to determine their unconfined compressive strength (UCS). Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analyses were also performed to study the microstructure and chemical composition of the specimens at different conditions. The study has systematically investigated the effect of two main factors, NaOH concentration and moisture content, on the behavior of geopolymerized MT. The results show that the maximum dry unit weight of the compacted MT is influenced by the NaOH concentration, higher NaOH concentration leading to larger maximum dry unit weight. The behavior of the final geopolymerized MT depends strongly on the NaOH concentration and moisture content of the initial compacted MT. At a constant moisture content, the UCS of geopolymerized MT increases with higher NaOH concentration up to a certain level and then decreases. This behavior is simply related to the effect of NaOH content or Na/Al ratio on the geopolymerization. For specimens prepared at the same NaOH concentration, the highest UCS does not necessarily occur at the optimum water content or the maximum dry unit weight, emphasizing the contribution of geopolymerization to the UCS. Moreover, this study demonstrates that by selecting appropriate moisture content and NaOH concentration, the geopolymerized MT can meet the strength requirements for road base by different State DOTs and the FHWA in the United States. Third, the durability characteristics of the geopolymerized MT for road base were studied. To improve the mechanical properties of the geopolymerized MT, low-calcium slag (SG) was incorporated to the MT. MT/SG-based geopolymer specimens were produced using several SG contents, 0%, 5%, 10%, 30% and 50%, by total MT/SG solid weight. Sodium hydroxide (NaOH) solution at 7 M concentration was used as the alkaline activator. Mixtures were prepared at a moisture content of 14% and then compacted. After compaction, the specimens were cured in the oven at 35°C and 60°C for 7 and 14 days, respectively. The durability characteristics were determined by wet and dry (w-d) cycles and water immersion. Unconfined compression tests were performed on the specimens to determine the unconfined compressive strength (UCS). The UCS was obtained after curing, at dry and saturated conditions (0th cycle), after the 1st, 3rd, 7th and 12th w-d cycle, and after water immersion. The loss of mass and pH were also recorded after each cycle. Additionally, leaching tests based on the TCLP method were performed to investigate the release of heavy metals. Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analyses were also performed to study the microstructure and chemical composition of the specimens at different conditions. The results showed that the w-d cycles and water immersion significantly affect the UCS of the specimens. This effect is more evident on the specimens cured at 35°C. However, the compressive strength of the specimens improves with curing temperature and SG content. The SEM/EDS results showed that after the w-d cycles Na is still present in the geopolymer microstructure although at a lower content. Based on the results, the geopolymerized MT/SG can be used as road base construction material. Fourth, MT/SG-based geopolymer binder was experimentally studied. The geopolymer binder was produced with copper mine tailings (MT) and low-calcium slag (SG). The effect of water to solid (w/s) ratio, SG content (0, 25 and 50%), sodium hydroxide (NaOH) concentration (5, 10 and 15 M), and sodium silicate (SS) to sodium hydroxide ratio (0.0, 0.5, 1.0 and 1.5) on the unconfined compressive strength (UCS) were studied systematically. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) were also performed to characterize the microstructure and phase composition of the geopolymer specimens. . The results show that the inclusion of SG improves the UCS and reduces the initial water content required for achieving a certain workability of the geopolymer paste. The geopolymer binder specimens prepared at 50% SG, 10 M NaOH, SS/NaOH = 1.0 and cured at 60°C for 7 days reached the highest UCS of 23.5 MPa. The geopolymer paste prepared at 50% SG, 15 M NaOH concentration and SS/NaOH ratios of 0.5 and 1.0 showed flash setting which led to poorer quality specimens and lower UCS. The SEM, EDS and XRD analyses clearly show the participation of iron dissolved from SG in the formation of geopolymer gels. This research helps to promote the reuse of MT and SG through geopolymerization and contributes to the knowledge of geopolymer materials. Finally, the production of geopolymer concrete by using copper mine tailings (MT) and low-calcium slag (SG) as both the cementitious binder and aggregates was studied. 50% MT and 50% SG, both passing sieve No. 200, were used together as the binder material, and MT and SG, with a particle size larger than sieve No. 200 and at designed combination, were used as the aggregates. Combined sodium hydroxide (NaOH) and sodium silicate (SS) were used as the alkaline activator. A curing temperature of 60°C was used to prepare the geopolymer concrete specimens. The effect of different factors including water to binder (w/b) ratio, NaOH concentration, SS/NaOH ratio, cement/aggregate ratio, curing time, and curing conditions on the compressive strength of geopolymer concrete were experimentally studied. The results showed that w/b = 0.26, 10 M NaOH concentration, SS/NaOH = 1.0 and cement/aggregate = 0.19 are the optimum values. This research promotes the utilization of mining wastes in road construction and contributes to the reduction of CO2 emissions.