Chemical and microstructural development of cements with metakaolin
Metakaolin (MK) is a nearly-anhydrous solid obtained by heating kaolin in the temperature range 450-800° C. Metakaolin shows pozzolanic properties and reacts with sodium, potassium and calcium hydroxides, with gypsum and with Portland cement or mixtures thereof. Since its reaction with calcium hydroxide yields mainly a product similar to C-S-H, obtained from Portland cement, MK can potentially be used as a major component in energy-saving binders, especially in tropical or subtropical countries where kaolin precursor is widely available. This project was aimed in achieving further progress on the use of metakaolin as a pozzolanic material. In doing so the hydration behaviour of metakaolin has been investigated. Calcium hydroxide was used as the main activator and, the effects of other secondary activators such as NaOH, Na2SO4, CaSO4 or a NaOH/CaSO4 combination have also been studied. The extent and kinetics of various reactions occurring during early hydration were investigated by calorimetry. The hydration behaviour of metakaolin cements has been found to manifest pronounced similarities to OPC hydration, especially in terms of heat evolution and set characteristics. The quantitative treatment of calorimetric data has unravelled an important feature of hydration process namely, the occurrence of both endothermic and exothermic reactions. This was mainly a feature of sulphate-containing systems, especially associated with the well known ettringite -+ monosul-phate transformation. The quantitative treatment of calorimetric data is found to be a tool in understanding mechanism of hydration reactions. The solid hydrates were characterized by XRD, DTA and aem techniques. The stability relations of the hydrates formed with respect to initial mix compositions are discussed in the temperature range 20-55°C. An attempt has been made to interpret the phase relations of CaO-Al2O3-SiO2-H2O system with regard to the products of metakaolin/Ca(OH)2 hydration. In lime rich formulations silicon-containing hydrogamet was found to be stable with C-S-H. When the amount of lime is less, metakaolin tends to form calcium aluminium silicate hydrate gels which are in equilibrium with C2ASHp. At higher temperatures (85°Q) die zeolite, gismondine, has been formed from the above gel. This finding is important as the formation of zeolites in cement mixtures has an ability to change the immobilization potential of cement. The continuous development of chemical environment -both solid and aqueous phases- of metakaolin/Ca(OH)2 system with secondary activators has also been examined. One of the important discoveries in the present study is the effect of mica-like impurities on the hydration of metakaolin. Solution phase studies disclose that muscovite, present as an impurity in kaolin, is activated during the calcination process. As a consequence, potassium is released to the pore solution with subsequent enhancement of the reactivity of the metakaolin, most notably with Ca(OH)2 and CaSO4. Thus the performance and activation of kaolins may depend partly on their alkali content. The effects of w/s ratio, curing temperature, initial MK/CH ratio and secondary activators on the mechanical properties of metakaolin cements have been examined. The consequence of solid phase volume changes associated with phase transformations on the compressive strength of metakaolin cements is also discussed. It has been found that the activators affect the hydration of metakaolin in a similar way as Portland or blended cements. The calcium hydroxide-CaSO4 combination was found to give the best strengths. The best binder phase for metakaolin cements is an aluminous C-S-H phase. Compressive strength development with time and temperature of MK cements is also found to be broadly similar to those of Portland cement systems.