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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 ...

The early age ambient temperature hydration of a hybrid cement formulation containing very high volumes of coal fly ash (~80% by dry mass) and activated by Na2SO4 is presented. The Na2SO4 salt acts as a safe and convenient in situ source of alkali to activate fly ash glassy phases without undesirable effects on cement clinker hydration. Comparison to a reference paste with gypsum instead of sodium sulfate revealed that Na2SO4 reduced setting times, shortened the induction period, and increased early alite hydration and compressive strength development, but also restricted ettringite formation. When replacing the active fly ash component for milled sand of a similar particle size, the Na2SO4‐activated pastes set even quicker, no ettringite was observed, and early strengths were considerably reduced. Possible reaction mechanisms in the hybrid pastes are discussed.

Chemical extraction of phosphorus (P) from incinerated sewage sludge ash (ISSA) is adversely influenced by co-dissolution of metals and metalloids. This study investigated P recovery and leaching of Zn, Cu, Pb, As and Ni from ISSA using inorganic acids (sulphuric acid and nitric acid), organic acids (oxalic acid and citric acid), and chelating agents (ethylenediaminetetraacetic acid (EDTA) and ethylene diamine tetramethylene phosphonate (EDTMP)). The aim of this study was to optimize a leaching process to recover P-leachate with high purity for P fertilizer production. The results show that both organic and inorganic acids extract P-containing phases but organic acids leach more trace elements, particularly Cu, Zn, Pb and As. Sulphuric acid was the most efficient for P recovery and achieved 94% of total extraction under the optimal conditions, which were 2-h reaction with 0.2 mol/L HSO at a liquid-to-solid ratio of 20:1. EDTA extracted only 20% of the available P, but the leachates were contaminated with high levels of trace elements under optimum conditions (3-h reaction with EDTA at 0.02 mol/L, pH 2, and liquid-to-solid ratio of 20:1). Therefore, EDTA was considered an appropriate pre-treatment agent for reducing the total metal/metalloid content in ISSA, which produced negligible changes in the structure of ISSA and reduced contamination during subsequent P extraction using sulphuric acid.

Ambient temperature drying shrinkage in metakaolin-based geopolymer pastes exposed to low relative humidity environments has been investigated. The effect of varying the geopolymer composition (water content, Si:Al ratio, Na:Al ratio and Na + vs K + cations) on the sensitivity to ambient temperature drying shrinkage is reported. The definition of "structural" water as being the minimum water content required to prevent contractions in the gel structure, and thus drying shrinkage from occurring, is introduced. From the results presented, it is clear that the ionic charge density of cations, the total quantity of cations and the relative quantities and stabilities of cation: AlO 4 -pairs in the paste are the major factors affecting the sensitivity of pastes to ambient temperature drying shrinkage.Keywords: geopolymer, metakaolin, drying, shrinkage, cracking INTRODUCTIONGeopolymers, also often referred to as alkali activated cements, have been the subject of a great deal of research interest, particularly during the last decade. Aluminosilicate materials such as metakoalin (MK), coal fly ash and blast furnace slag, react to form a cementitious gel via a two stage reaction in which poorly ordered and XRD amorphous aluminosilicates present in the material are dissolved in a highly alkaline medium and the paste cured at temperatures ranging from 20-90°C. During setting and hardening, the dissolved aluminate and silicate groups poly-condense into short-range ordered and cross-linked chains to form a cementitious gel that is responsible for the binding properties of these materials [1,2].The general trend seen in the literature with these materials is that MK based binders are referred to as "geopolymers" and fly ash or slag based binders as "alkali activated cements".The latter two starting materials are industrial by products, whose effectiveness as alternative, Portland cement-free binders has been largely pioneered by the research groups at the Eduardo Torroja Institute in Madrid and the Glukhovsky Institute in Kiev [3][4][5][6][7]. Youngs modulus values but that the opposite trend was noted with K based samples [34,35].Although resistance to thermal shrinkage of MK geopolymers exposed at temperatures above 700 °C is in many cases far superior to Portland cements [36][37][38][39], very little attention has been given to the wider issue of ambient temperature drying shrinkage [40]. The image in Figure 1 gives a typical example of what can happen with many MK-pastes subject to ambient temperature drying at low relative humidity.Unlike Portland cement, water is not incorporated directly into the geopolymer gel product.Only a small percentage of the mixing water remains as interstitial water in the geopolymer gel [41]. This fact, combined with the high water requirement to mix MK-geopolymer pastes, means that there is a large excess of unbound or free water, which can evaporate from the hardened paste under low relative humidity conditions at ambient temperature [39]. Despite the lack of chemically bound water, ...