Abstract
Low-lime fly ashes produced from modern coal-fired power technologies (developed to enhance efficiency/lower emissions), including nitrogen oxide (NOx) reduction, co-combustion, supercritical steam and oxy-fuel combustion, and their effects on chloride ingress and carbonation of concrete are investigated in this paper. Earlier work indicates that some of these technologies influence fly ash properties, but they mainly follow typical behaviour found for the material (consistence and compressive strength) in concrete. Both accelerated and normal-type exposure tests were carried out on a range of practical water/cement ratio concretes (also enabling interpolation for comparisons at equal 28 d strength). The test fly ash concretes were evaluated against (i) those containing three reference fly ashes covering a range of fineness and (ii) corresponding studies on fly ash concretes from the 1990s. The results show that there was an influence of fly ash fineness, reflected in reactivity/porosity (measured on mortar), and aspects of chemistry on chloride ingress, but there appeared to be minor material effects on carbonation. Comparison with the 1990s data indicated similar behaviour for the materials between studies for both properties. A relationship was also identified for the product of reactive alumina and sub-10 μm contents of the modern fly ashes and chloride resistance of concrete.
Highlights
Developments in electricity generation mean that some countries have seen a gradual change in how this is sourced (McCarthy et al, 2017a)
In order to investigate modern coal-fired power technology influences, fly ashes were obtained from various sources providing reference and test materials
The results indicate that reactivity and packing effects associated with finer material (Chindaprasirt et al, 2005) occur with the modern fly ashes
Summary
Developments in electricity generation mean that some countries have seen a gradual change in how this is sourced (McCarthy et al, 2017a). Given the effects noted for fly ash from modern coal-fired power technologies (McCarthy et al, 2018), it is possible that they may affect chloride ingress and carbonation of concrete, and a study was established to address this. In order to investigate modern coal-fired power technology influences, fly ashes were obtained from various sources (mainly from within the UK) providing reference and test materials Following characterisation of their properties, concrete mixes were developed, covering the practical range of water/cement (w/c) ratios relevant to the durability properties being studied (BSI, 2015). The normal-type exposure test conditions followed those given in prEN 12390-10 (CEN, 2007) – that is 0·035% carbon dioxide, 20°C temperature and 65% RH – using two 100 Â 100 Â 400 mm concrete prisms water-cured for 7 d. The mean of 40 measurements (four faces; 20 per prism) was taken as the depth
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