Evaluation of Fly Ash Reactivity Potential Using a Lime Consumption Test

The reactivity of fly ash, for use in concrete, is normally evaluated in the UK/EU with the Activity Index test (on mortars at 28 and 90 days (BS EN 450-1)). The paper reports on the application of a lime consumption test, based on BS EN 196-5 (pozzolanic cement test), to determine this more rapidly. The method uses a Portland cement (PC) / fly ash slurry, stored for 8 or 15 days at 40°C, and measures OHand CaO concentrations of the filtrate. Seven fly ashes, including those produced using modern power station techniques (e.g. co-combustion, low NOx) and 3 PCs with different characteristics were tested. Early experiments established good repeatability for the test and that pozzolanic reactions mainly occur during the initial 8 to 15 days’ storage. Similar behaviour for fly ash was noted to that in thermogravimetric analysis and Activity Index tests from related studies. Strong correlations were obtained for fly ash fineness and CaO consumed. While there was general agreement for CaO consumed and Activity Index, correlations were poor. Similar type effects occur for mortar (Activity Index) and concrete. The lime consumption test can assess fly ash reactivity, but should be combined with measurements of the materials’ fineness.