Felix Ndubisi Okonta

Potassium Aluminate Geopolymerisation of Acidic Gold Mine Tailings

Acidic gold mine tailings were alkaline activated using KOH. The effect of potassium aluminate (KA) on the strength and durability of the geopolymers was investigated. A 2.8 KA:KOH geopolymer had a UCS of 18.10 MPa after curing for 5 days at 100 °C. There was an increase in UCS with an increase in loading of KA up to a ratio of 2.8. Beyond the KA:KOH ratio of 2.8, there was a 48% drop in UCS due to excess K+ ions in the system which resulted in the loss of charge balance of the system leading to reduction of UCS. It is worth mentioning that the KA:KOH ratio of 2.8 represented a Si/Al ratio of 1.02. This study showed that KA activation of acidic gold mine tailings is an attractive route to stabilise/solidify hazardous tailing material. Though there is use of elevated temperature to achieve high strength for the KA based geopolymer, this pales in comparison to energy requirements of cement manufacturing and clay brick firing.

Geotechnical Properties and Microstructure of Lime-Fly Ash-Phosphogypsum-Stabilized Soil

The use of industrial waste as a potential stabilizer of marginal construction materials is cost effective. Phosphogypsum and fly ash are industrial wastes generated in very large quantities and readily available in South Africa. In order to explore the potential stabilization of vastly abundant expansive soil using larger quantity phosphogypsum waste as a potential modifier, composites with a mixture of lime-fly ash-phosphogypsum-basic oxygen furnace slag were developed. However because of the presence of radionuclide, it was necessary to treat the phosphogypsum waste with mild citric acid. The effect of the acid treatment on the geotechnical properties and microstructure of expansive soil stabilized with phosphogypsum-lime-fly ash-basic oxygen furnace slag (PG-LFA-BOF) paste was evaluated, in comparison with the untreated phosphogypsum. Expansive soil stabilized with acid-treated PG-LFA-BOF paste exhibited better geotechnical properties; in particular, the high strength mobilized was associated primarily with the formation of various calcium magnesium silicide and coating by calcium silicate hydrate and calcium aluminate hydrate. The soil microstructure was improved due to the formation of hydration products. The stabilized expansive soil met the specification for road subgrades and subbase. Stabilization of expansive soils with phosphogypsum, fly ash, and basic oxygen fly ash does not only improve engineering properties of soil but also provides a solution in relation to disposal and environmental pollution challenges.

The Strength of Lightly Cemented Power Plant Ash

Coal ash from most of Eskom power plants consists of 70–85% fly ash and 15–30% bottom ash. A total of 25 million tons of ash is produced from approximately 109 million tons of coal per annum. Small percentage of the ash were used in cement production and other construction applications and almost 80% of the ash were disposed into ash dams. The need for high volume utilization is important because of the cost of disposal and associated environmental impact. The mechanical properties of Eskom ash that were stabilized with cement was investigated. Specimens of ash were stabilized with 2% to 10% of rapid hardening Portland cement (52.5R), and compacted at two different moulding water content; (a) the optimum moisture contents of stabilized specimens (15%–19%) and (b) moisture content wet of the OMC (30%). The unconfined compressive strength (UCS), soaked UCS, secant modulus and microstructure of the stabilized specimens were evaluated. The result indicated that specimens that were compacted at 30% moisture content mobilized greater UCS than those that were compacted at OMC. For specimens that there stabilized with high cement content of 8%–10% and compacted at OMC, soaking for 24 h only indicated a marginal reduction in UCS. The increase in secant modulus with cement content was nonlinear and indicated a decreasing rate with increase in cement content. The XRD and SEM results revealed that strength development was associated with the predominance of calcium silicate hydrate (CSH) and needle shaped ettringite in cement stabilized ash. Based on limited test data, only specimens that were stabilized at 30% moisture content and with greater than 4% cement met the SANS (2007) criteria for masonry and TRH (2010) criteria for pavement backfill.