Cement, Energy and Environment

material processing conditions, solution composition and curing conditions. After achieving the stage of complete gelation the precipitating mass continues to rearrange and reorganize, resulting in the three dimensional aluminosilicate network commonly attributed to geopolymer formation. Growth is the stage during which the nuclei reach a critical size and crystals begin to develop. These processes of structural reorganization determine the microstructure and pore distribution of the material. The ultimate performance on hardening depends on various parameters such as availability of silica, concentration of calcium, pH and temperature during the initial stage of reaction . Curing Conditions on Geopolymer Formations: The synthesis of geopolymers by alkali activation of low calcium solid alumino-silicates generally needs mild thermal curing. The increase in the temperature accelerates the reaction and also the mechanical strength of the product. The geopolymers achieving compressive strength as high as 63 MPa on 24 hours initial curing at 85°C have been reported. Under similar conditions, initial curing at 650C the compressive strength was reportd to be 38.7 MPa. lnfact, there is a threshold value of temperature for mechanical strength development above which the rate of gain of strength is slow. In general, a temperature range of of -400 to -950C is considered necessary for establishing an adequately interconnected lattice of bonds during geopolymer formation. Effect of Alkali Activator on Geopolymerization: KOH solutions are reported to be more effective in dissolution of aluminosilicates. However, the sodium ions are easily accommodated in geopolymeric network owing to smaller ionic size and greater mobility. In case of activation by alkalisilicates, the availability of ractive silicate ions enhances the degree of polymerization at relatively lower alkali concentration. The geopolymers prepared by activation of fly ash by sodium hydroxide solution showed more stability in sulfate solutions compared to specimens synthesized using sodium silicate as in these geopolymers formation of ettringite may cause loss of strength. Effect of Silica and Alumina Availability on the Geopolymeric Reaction: As the geopolymerization reaction involves dissolution of AI and Si in highly alkaline solution,it produces vitreous component.Further as the reaction proceeds, first meta-stable/ intermediate AI- rich phase develops followed by a more stable three dimensional structure.The polymerization degree depends on the available Si0 2 /AI20 3 ratio,curing conditions and pH of the alkaline solution. In case of activating sodium silicate solutions, the molar ratio, Si0 2 /Na 2 0 , was found to have critical effect on ultimate strength development. The activating solution of Na or K hydroxide mixed with respective alkali silicate was found to increase the rate of reaction and the mechanical strength development of geopolymeric product compared to activation by alkali hydroxide on ly. High Temperature Performance of Metakaolin and Fly Ash Based Geopolymers It has been reported by different workers that geopolymers can sustain at higher temperatures and are fire resistant.The Si/AI ratio played important role in such mixes.A comparative study on geopolymers made with metakaolin and fly ash after exposure to elevated temperatures i.e. up to 800°C has been reported. The solid to liquid ratios in metakaolin and flyash based geopolymer pastes were kept 0.8 and 3.0 respectively as metakaolin had higher liquid demand compared to fly ash due to its finer particle sizes.The metakaolin based geopolymer suffered strength loss after exposure to high temperature, while fly ash based geopolymer gained strength under similar conditions. Geopolymerization of Mixture of Fly Ash and/or Metakaolin with Slag: It has been found that partial replacement of metakaolin/ fly ash by blast furnace slag improved the strength of the product.The formation of CSH gel within a geopolymeric binder could work as a micro-aggregate to fill voids and holes resulting in improvement in strength. Depending upon the alkalinity of the system, it is possible that as the calcium concentration increases, the formation of geopolymeric gel and CSH gel could compete against each other. Instead of having one phase acting as a micro-aggregate to fill voids and holes of the binder, the two reactions may compete for soluble silicates and available space for growth. Consequently, the resultant binder may be disordered and more residual holes are produced resulting in strength reduction. The participation of Ca(OHh or calcium silicate hydrates in optimum percentage may provide nucleation sites which then trigger rapid geopolymer gel formation. 10 -