Key Engineering Materials Vol. 1060

Abstract: Mine tailings are solid residues from mineral processing that exhibit physical characteristics comparable to conventional mineral fillers used in asphalt mastics. However, mine tailings contain toxic elements that raise concerns about their environmental safety. This study evaluates the chemical and physical characterization of mine tailings to assess their feasibility as fillers in modified asphalts. Chemical digestion using HNO₃, HCl, and HF was performed to quantify toxic elements, while scanning electron microscopy (SEM) was used to examine their interaction with asphalt. Results indicate that the concentrations of toxic elements are below the permissible limits established by Mexican soil standards. The tailings present particle sizes under 75 µm, meeting the requirements for filler applications. SEM analysis revealed effective physical interaction between the asphalt matrix and tailings particles rich in Ca, Si, and Fe, confirming their potential to form a stable modified asphalt. Finally, leaching tests corroborated the immobilization of toxic elements in mastic samples showing that concentration of toxic elements dropping significatively and down to zero for Cd and As.

Abstract: Ultra-High Performance Concrete (UHPC) exhibits exceptional mechanical properties but suffers from significant strength degradation and microstructural deterioration when exposed to elevated temperatures. This study investigates the effectiveness of calcined bauxite as a sustainable material for enhancing the thermal resistance of UHPC. The research focuses on characterizing the microstructural changes in calcined bauxite-enhanced UHPC specimens subjected to thermal treatment at temperatures ranging from 200°C to 600°C. Through comprehensive analyses using scanning electron microscopy (SEM) and Dispersive X-ray Spectroscopy (EDS), the study reveals that the incorporation of calcined bauxite significantly modifies the microstructural evolution of UHPC under thermal exposure. The results demonstrate improved stability of the cementitious matrix, reduced crack formation, and enhanced phase preservation in calcined bauxite-modified UHPC compared to conventional UHPC. The findings provide valuable insights into the microstructural mechanisms responsible for the improved thermal performance and suggest promising applications for calcined bauxite-enhanced UHPC in fire-resistant construction.

Abstract: A previous macroscopic study demonstrated that the incorporation of allophane as a nanoadditive increase the resistant of asphalt mixtures by (28.92 + 2%) under aging conditions, prolonging the useful life of the mixture. Based on these results, analyses were performed using scanning electron microscopy (SEM/EDS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) to characterize and explain the microstructural interaction between the aggregates, the asphalt binder, and the nanomodifying additive (allophane) responsible for this improvement. The SEM-EDS analysis showed that allophane improves the compaction and continuity of the asphalt matrix, reducing voids by ≈7.5% and mass loss due to aging from 11.07% to 1.74%. Likewise, the modified mixtures showed higher C and O contents and lower Fe, evidencing greater binder retention and a mineral coating effect that increases the stability and durability of the system. TEM microscopy visually revealed areas with greater homogeneity in electron density in long-term modified samples as a compact monolithe, unlike conventional samples that show areas of low contrast. In terms of X-ray diffraction characterization, the crystallinity index of the long-term aged conventional mixture was 3.75%, while the modified mixture had a minimum variation of 0.55%. Likewise, the average crystallite size of the conventional mixture was 32.42 [nm], while the mixture with nanoadditive was 30.40 [nm], which translates into lower structural rigidity and greater flexibility of the material. These findings demonstrate that allophane acts as an effective nanomodifier by stabilizing the amorphous fraction of asphalt mixtures, limiting microstructural degradation during aging, and providing a mechanistic explanation for the enhanced macroscopic performance observed.

Abstract: This research studies the manufacture of a composite material based on gold mine tailings and metakaolin as a novel construction material through a geopolymer process. Geopolymers, derived from aluminosilicate-rich mining waste, offer a promising alternative to traditional construction materials by mitigating environmental pollution and reducing production costs. Different combinations and solution contents were used, with sodium hydroxide as the alkaline activator at different molarities and temperatures of 70°C and 110°C. To determine the optimal mixture, an indirect tensile test was performed to determine the optimization factor (Fopt), which resulted in a mixture of 60% tailings and 40% metakaolin in a 24% solution at a concentration of 6 molar and 110°C. Using the optimal mixture, brick tiles were manufactured, which were tested under simple compression, achieving a resistance of 6MPa; the specimens were also tested using the flexural strength test, obtaining a result of 11MPa, thus complying with the Ecuadorian standard for their use as a construction material.