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.