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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. [ABSTRACT FROM AUTHOR]

The work reported in this paper investigated the properties and use of fly ash (FA) produced from technologies developed to reduce the environmental impact and improve the efficiency of the coal-fired power generation process. These include nitrogen oxides (NOx) reduction, co-combustion, supercritical steam technology and oxy-fuel combustion. The nine FA samples examined from these technologies were characterised physically, chemically and in terms of their reactivity. Tests were also carried out to determine the consistence (slump) and compressive strength of FA concretes. Comparisons were made with a selection of reference FAs, the requirements of BS EN 450-1 and reported FA studies from the 1980s and 1990s. The results indicated that, for some technologies, the FA tended to be coarser and of higher loss on ignition (co-combustion and in-combustion NOx reduction) while, for others lower carbon contents were found (supercritical steam) or there was little obvious effect. FA chemistry was slightly affected in some cases, but mainly as expected for the coal being used. There was general agreement between water requirement and activity index with FA fineness for the materials tested. For the FA concretes, similar effects were noted in terms of the dosage of superplasticising admixture needed for a target slump and compressive strength. The behaviour of the materials from the new technologies was found to be similar to that reported in earlier studies on FA and suggests suitability for their use in concrete construction. [ABSTRACT FROM AUTHOR]

The reactivity of fly ash (FA), for use in concrete, is normally evaluated in the UK/EU by means of the activity index test (on mortars at 28 and 90 d (BS EN 450-1)). This paper reports on the application of a lime consumption test, based on BS EN 196-5 (pozzolanic cement test), to determine FA reactivity more rapidly. The method uses a Portland cement (PC)/FA slurry, stored for 8 or 15 d at 40°C, and measures hydroxyl ion (OH−) and calcium oxide (CaO) concentrations of the filtrate. Seven FAs, including those produced using modern power station techniques (e.g. co-combustion, low nitrogen oxide emissions), and three PCs with different characteristics were tested. Early experiments gave acceptable repeatability for the test and revealed that pozzolanic reactions mainly occur during the initial 8 to 15 d storage. Similar behaviour for FA to that in thermogravimetric analysis and activity index tests from related studies was noted. Strong correlations were obtained for FA fineness and calcium oxide consumed (difference in calcium oxide content between PC and PC/FA with respect to that in PC). While there was general agreement for calcium oxide consumed and activity index, the correlations were poor. Similar types of effect were found to occur for mortar (activity index) and concrete. The lime consumption test can assess FA reactivity, but should be combined with measurements of the material's fineness. [ABSTRACT FROM AUTHOR]