Evaluation of fly ash reactivity potential using a lime consumption test

Abstract

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.