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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, ...

The hydration of reactive periclase (MgO) in the presence of hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O) was investigated by a variety of physical and chemical techniques. Hydration of pure MgO-water mixtures gave very weak pastes of brucite (Mg(OH) 2 ), but hydration of MgO-hydromagnesite blends gave pastes which set quickly and gave compressive strengths of potential interest for construction applications. The strengths of the blends increased with hydration time at least up to 28days, and were not significantly decreased by increasing the hydromagnesite content up to 30%. Raman spectroscopy suggests that an amorphous phase, of composition between that of brucite, hydromagnesite and water, may form. Small amounts of calcite also form due to CaO in the MgO source. Thermodynamic calculations imply that the crystalline phase artinite (MgCO 3 ·Mg(OH) 2 ·3H 2 O) should be the stable product in this system, but it is not observed by either XRD or FTIR techniques, which suggests that its growth may be kinetically hindered

This research has investigated the mechanical properties and microstructure of metakaolin derived geopolymer mortars containing 50% by weight of silica sand, after exposure to temperatures up to 1200 °C. The compressive strength, porosity and microstructure of the geopolymer mortar samples were not significantly affected by temperatures up to 800 °C.Nepheline (NaAlSiO 4 ) and carnegieite (NaAlSiO 4 ) form at 900 ºC in the geopolymer phase and after exposure to 1000 °C the mortar samples were transformed into polycrystalline nepheline/quartz ceramics with relatively high compressive strength (~275 MPa) and high Vickers hardness (~350 HV). Between 1000 and 1200 °C the samples soften with gas evolution causing the formation of closed porosity that reduced sample density and limited the mechanical properties.2