Effect of enhanced pozzolanic activity in nickel slag modified with Al2O3 on mechanical properties of cemented fine tailings backfill

Abstract

Cemented tailings backfill has gained widespread acceptance in mining industries due to its stable performance and environmental advantages. However, Ordinary Portland cement (OPC), based on engineering experience, falls short in meeting the requirements of cost-effectiveness and environmental protection criteria for cemented fine tailings (median size d50 <20 µm) backfill. Nickel slag (NS), an industrial solid waste with potential reactivity derived from nickel manufacturing processes, is currently employed as a concrete additive at a low rate. In this work, a modification was conducted by melting-quenching the NS-Al2O3 mixtures, followed by a physicochemical analysis of the resulting granulated samples (GAN) using BET and X-ray fluorescence. The mineralogy and structural properties were examined for the GAN samples using X-ray diffraction and Fourier transform infrared spectroscopy. Each of the T-CGAN pastes was formulated by replacing 40 wt% of OPC with the GAN, and then blended with fine tailings at 1:4. The flowability and compressive strength were measured for the T-CGAN pastes, while the hydration products and microstructures of the paste samples were analyzed by derivative thermogravimetry and scanning electron microscope, respectively. The results indicate that the addition of Al2O3 not only provides abundant network formers to GAN glass, but also facilitates the conversion of [SiO4] tetrahedrons from Q4 with strong bond energy to Q3 and Q2 with lower ones, thereby significantly enhancing the pozzolanic activity of the original NS. The T-CGAN slurries exhibit slight decreases in slump values from 228.8 to 224.9 mm compared to T-OPC slurry (236.5 mm), attributed mainly to the strong water absorption of GAN. T-CGAN pastes with higher Al2O3 addition acquired greater compressive strengths during the whole curing ages. After 90 days of curing, T-CGAN20 paste achieves the maximum compressive strength (5.14 MPa), higher than that of the T-OPC by 1.03 MPa. The T-CGAN pastes richer in Al2O3 release more reactive Si and Al to participate in the pozzolanic reaction with calcium hydroxide, contributing to additional formations of C-S-H gel and C-A-H, with SEM micrographs showing a compact microstructure with elevated ratios of Ca/Si, Al/Si, and Fe/Si. This study suggests an effective and environmental way to enhance the pozzolanic activity of the original NS by incorporating Al2O3. The modified NS can then be utilized as a high-performance cement substitute for cemented fine tailings backfill, which also significantly enhances the sustainability for both mining and building industries.