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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 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]