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Assessment of the pozzolanic activity of cement replacement materials is increasingly important because of the need for more sustainable cementitious products. The pozzolanic activity of metakaolin, silica fume, coal fly ash, incinerated sewage sludge ash and sand have been compared using the Frattini test, the saturated lime test and the strength activity index test. There was significant correlation between the strength activity index test and the Frattini test results, but the test results from these tests did not correlate with the saturated lime test results. The weight ratio of Ca(OH) 2 to test pozzolan is an important parameter. In the Frattini test and strength activity index test the ratio is approximately 1: 1, whereas in the saturated lime test the ratio is 0.15: 1. This explains why the saturated lime test shows higher removal of Ca(OH) 2 and why the results from this test do not correlate with the other test methods.Keywords: Pozzolan; Portland cement; Sustainable development; Sewage sludge ash; Metakaolin; Silica fume; Pulverised fuel ash; Frattini test INTRODUCTION A pozzolan is defined as (ASTM C125) "a siliceous and aluminous material which, in itself, possesses little or no cementitious value but which will, in finely divided form in the presence of moisture, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties".Pozzolans were the first cementitious materials used by early civilisations and some of the most important historical buildings rely on pozzolanic cement systems. Pozzolans are of increasing interest because their use reduces overall environmental impact and cost when mixed with Portland cement (CEM-I) in blended cement systems. The use of pozzolans reduces the carbon dioxide emitted per tonne of product and can also improve various physical properties of the resulting concrete [1].Pozzolans are obtained from various sources and can be naturally occurring minerals or industrial byproducts. They do not necessarily interact with Portland cement in the same way, and the mechanisms involved in hydration of coal fly ash (FA) and silica fume to form hydration products are different. A major benefit of blended cements is improved durability and this occurs because the reaction between the pozzolan and excess Ca(OH) 2 produced by CEM-I hydration forms calcium silicate hydrate (C-S-H) gel, which reduces the porosity of the binder.A wide range of test methods for assessing pozzolanic activity have been reported in the literature. These can be categorised as either direct or indirect methods. Direct methods monitor the presence of Ca(OH) 2 and its subsequent reduction in abundance with time as the pozzolanic reaction proceeds, using analytical methods such as X-ray diffraction (XRD), thermo-gravimetric analysis (TGA) or classical chemical titration. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 ...