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Geopolymers, silico-aluminate materials formed through mimicking natural rock forming process, are fast emerging as new class of greenbuilding construction materials. In the process of geo-synthesis,silicon (Si) and aluminium (Al) atoms react to form molecules that arechemically and structurally comparable to those binding natural rock andallows for novel products synthesis that exhibit the most ideal properties ofrock-forming elements, i.e., hardness, chemical stability and longevity. Flyash, blast furnace slag and red mud are the three major industrial wastes inIndia. Presently over 100 million tonnes of fly ash, 12 million tonnes ofblast furnace slag and nearly 4 million tonnes of red mud are generated. It isestimated that production of these wastes will double in foreseeable futuredue to rapid expansion coal based power generation, and increase in theproduction of iron & steel and aluminium through primary processing. Thesewaste materials contain  SiO2  and Al2O3  , along with  Fe2O3  ,  CaO ,  MgO, MnO ,etc, and have immense potential as man made raw materials forgeopolymers.

During geopolymerisation process, the alumino-silicate fraction reactswith alkaline media and transform into a solid geopolymer product, via a dissolution-polycondensation-structural reorganisation mechanism, to developstrength. Blast furnace slag behaves differently during geopolymerisationas compared to fly ash and clay. This is attributed to its higher reactivity dueto mostly glassy structure which, leads to faster dissolution of  Si  and Al duringgeopolymerisation. The  CaO  portion of the slag particles does not necessarilyparticipate in polycondensation, but reacts with water and may undergo hydrationreaction. It has also been reported that addition of blast furnaceslag in the conventional silico-aluminate geopolymer cement and concreteimproves setting characteristics. Use of red mud in geopolymers appearsto be an attractive proposition from the point of view of its high alkaline content. However, there have very limited attempts in this direction.

Geopolymerisation Of Waste

Most proposed mechanisms of geopolymerisation consist of dissolution of aluminosilicate phase, polymerisation and re-precipitation of gel phase, and transformation of the gel phase into geopolymer of varying crystallinity and structure. Depending upon experimental conditions, the different stages of geopolymer formation may overlap and even merge with each other. Isothermal conduction calorimetry was used to study the geopolymerisation of fly ash, mixture of (GBFS+fly ash), and the mixture containing (fly ash+GBFS+red mud).

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