Felix Okonta

Particle crushing changes both particle size distribution and particle shape, which significantly affects the mechanical behavior of sand. This study uses a multi-scale discrete element method to investigate the direct-shear response of sand under prescribed post-crushing scenarios. Instead of modeling dynamic particle splitting during shearing, assemblies with predefined fractal particle size distributions and size-dependent angular shapes were assigned prior to testing. The performance of multi-shape assemblies was systematically compared against single-shape circular baselines. The numerical model was calibrated against plane-strain direct-shear laboratory experiments using granular analogs. Macro-scale findings reveal that shear strength and volumetric dilation exhibit non-monotonic trends, reaching a peak response at intermediate fractal dimensions. Angular fragments act as structural wedges that sustain interlocking. Micro-scale analysis demonstrates that peak shear strength is consistent with the evolution of contact force anisotropy. Conversely, particle rotations show an inverse relationship with shear strength and angular fragments impose anti-rotational constraints along the shear band.

The effect of ratio of water to solids on undrained shear strength (τ _u ) by fall cone apparatus as well as the effect of volumetric cement content and porosity on the Unconfined compression strength (UCS) and modulus ratio (E ₅₀ /UCS) of Harmony and DRD tailings sands were investigated. 4% cemented fresh paste of 3:7 and 2:8 water to solid ratio were tested for τ _u and samples blended with cement content of 2–7% and compacted to 80–95% of the maximum dry density were fabricated for UCS tests. The result revealed that for increase in water/solid of 13.60 to 49.20, τ _u increased from 52 to 66 kPa and 32 kPa to 46 kPa for Harmony and DRD tailings respectively. For Harmony and DRD tailings stabilized with 2 to 7% cement content and compacted to 95% MDD, the UCS ranged from 0,81 MPa to 2.445 MPa and 0.954 MPa to 2.603 MPa respectively. The exponential relationships between the UCS, and the ratio of porosity to volumetric cement content (η/Ccv) were strongly correlated with a high coefficient of determination (r ² ). The effect of threefold difference in non-plastic fines are most indicated by τ _u , E ₅₀ and CaO – pozzolan ratio. Tailing sands that were blended with 35–% cement content and compacted to porosity < 0.35 met the local geotechnical specifications for mine backfills, flowable fills, and low traffic volume road layers.

In this study, the radiation-shielding, mechanical, microstructural, and thermal properties of gypsum-based composites modified with thermally treated colemanite were investigated. Composite samples containing 1, 2.5, 5, and 10 wt.% colemanite were prepared, and the additive was pre-treated at 650 °C to improve its stability and compatibility with the matrix. Gamma-ray attenuation was evaluated at different photon energies, and neutron attenuation was determined using a ²⁴¹Am–Be source. The results showed that colemanite addition improves gamma-ray attenuation, particularly at low and medium energies, with the best performance observed at 2.5–5 wt.%. Neutron attenuation increased with an increasing colemanite content due to the presence of boron. Compressive strength exhibited a non-linear trend, reaching its maximum at 2.5 wt.% and decreasing at higher contents due to microstructural changes. Thermal conductivity also showed non-linear behavior, with the lowest value obtained at 5 wt.% colemanite. SEM and FTIR analyses confirmed the incorporation of colemanite and its influence on the microstructure. Overall, the results indicate that colemanite-modified gypsum composites provide a balanced combination of radiation-shielding, thermal insulation, and mechanical performance.

Fly ash (FA) from Eskom coal-fired power stations in South Africa is readily available. It is produced in more than 30 million tonnes annually. It presents a huge waste handling challenge and an opportunity for South Africa to offset it carbon emissions by developing and using Ultra High-Volume Fly Ash binders with FA content > 70%. The current study analyses peer-reviewed research articles on HVFA binders’ strength and durability using cooccurrence and bibliographic coupling of authors, institutions, countries, journals, and keywords using Covidence, Bibliometrix, and VOS viewer. Findings reveal that research efforts toward increasing fly ash dosages in HVFA binders remain significantly limited below 70%. HVFA binder strength and durability research depends on a nation’s coal consumption level and fly ash availability. Leading global coal consumers are also leading nations in HVFA binders’ research. Although South Africa is among the leading coal consumers and carbon emitters, it is ranked the least among nations contributing to developing high-strength and durability HVFA binders. The lack of research on HVFA binder strength and durability in South Africa calls for urgent scholarly efforts, given that findings from other nations and power plants cannot be generalised due to wide variations in fly ash characteristics within the same classes. The variations within same fly ash classes calls for further research to develop classification systems for HVFA binders’ development. Compressive strength and shrinkage were identified as the predominant properties of interest. The findings suggest that higher dosages of fly ash (FA) tend to reduce shrinkage. However, there are exceptions challenging this trend, underscoring the need for further research to address inconsistencies.

South Africa is endowed with a wealth of coal-fired power stations that can produce extremely high volumes of fly ash per year exceeding 34 million tonnes. The use of high-volume fly ash (HVFA) binders in the construction sector has the capacity to significantly reduce greenhouse gas emissions associated with traditional cement production and offset the carbon footprint of Eskom. The excessive production of fly ash by Eskom warrants the need for developing ultra-high-volume fly ash binders (UHVFA, fly ash/binder > 60 wt%). Nonetheless, fly ash (FA) replacement of cement is still largely limited to 35% regardless of more ambitious research indicating the potential to surpass 60%. In view of the urgent need for South Africa to offset and reduce its carbon footprint, this work reviews and summarises the literature on the performance of HVFA binders with a focus on two specific areas: (i) strength and (ii) durability. On HVFA binder strength, the focus is drawn on work that analysed the compressive strength, flexural strength, and split tensile strength. This review focuses on the extant literature analysing the durability of HVFA binders using various tests, including sorptivity, resistivity, permeability, tortuosity, rapid chloride penetration tests, resistance to sulphate attack, and microstructural analysis. As the FA content increases towards optima, i.e., 50–80%, the most indicative composite characteristics of the strength and durability properties are UCS (30–90 MPa) and permeability (low). This review reveals the leading methodologies, instrumentation, findings, challenges, and contradictions.