Class C fly ashproduced from the burning of younger lignite or sub-bituminous coal, in additionto having pozzolanic properties. Class C fly ash also has some self-cementingproperties. In the presence of H2O, it hardens and gets strongerover time. Class C fly ash generally encompasses more than 20 % of lime (CaO).Disparate Class F, self-cementing Class C fly ash does not require anactivator.
Alkali and sulfate (SO4) contents are normally higher inClass C fly ashes 21.Class F fly ashis obtained from burning of harder, older anthracite, and bituminous coal.Class F fly ash is pozzolanic in nature and contains less than 7 % of lime(CaO). Possessing pozzolanic properties, the glassy silica and alumina of ClassF fly ash require a cementing agent, such as Portland cement, hydrated lime orquicklime, which is react with water and form cementitious compounds.Otherwise, adding a chemical activator like sodium silicate (water glass) to aClass F ash can form a geopolymer 21.Class F fly ashbased geopolymers have been mainly proposed as new insulating materials whichpossess several properties such as low cost and simple processing conditions,high maximum application temperature, non-flammability, etc. than other widelyused traditional insulating materials like polystyrene, polyurethane, melamineand glass wool, glass foams, and perlite. Thermal insulating materials whichhave low thermal conductivity, as well as acceptable mechanical strength, areimportant properties of energy efficient building blocks 1,22.
Geopolymermaterial has an amorphous microstructure and resembles like high-performancezeolite, such as very good acid and fire resistance and also produces highcompressive strength. The earlier literature showed that adding some mineralsto the geopolymer can increase the performance. The first geopolymerapplications were building product developed in 1973-1976 23.Mostly,silicon 24 and aluminum 25,26 power and hydrogen peroxide solution19,27–29 is utilized as a blowing agent to generate porous components fromgeopolymeric slurries, however the redox reactions of metallic silicon (Si) andaluminum (Al) are very strong, due to utilization of high alkaline system10,19.
In the preparation of fly ash-based geopolymer, the feedstock fly ashis usually mixed with an alkali activator, such as sodium hydroxide (NaOH) orpotassium hydroxide (KOH) solution for the geopolymerization process. Alkalisolution used to activate the precursor by dissolving them into Si(OH)4and Al(OH)4 monomers. The most common used alkaline activators ingeopolymerization are a combination of sodium silicate and sodium hydroxide.
Sodium hydroxide is used to react with the silicate and aluminum by adding Na+ion. Further Na+ ion is onlyused during the polymerization 30. Theexact mechanism through geopolymer hardening and setting occurs is not yet understood.However, the utmost proposed mechanisms for the geo-polymerisation includes thefollowing stages that proceed in parallel and thus, it is difficult to bedistinguished: (i) dissolution of Si and Al from the solid alumino-silicatematerials in the strong alkaline aqueous solution, (ii) formation ofgeopolymers precursors (oligomers species) consisting of polymeric bonds ofSi-O-Al and/or Si-O-Si type, (iii) poly-condensation of the oligomers to form athree-dimensional alumino-silicate framework and (iv) bonding of the unreactedsolid particles and filler materials into the Geopolymeric structure andhardening of the entire system into a final solid polymeric structure. Fly ash, which is rich in silica and alumina, has fullpotential to use as one of the source material for Geopolymer binder. Itis the main solid waste generated from the coal combustion in the powerstations. Since the Worldwide electric power industry depends on heavily use ofcoal as a primary energy source; enormous quantities of fly ash are generatedevery year.
According to 2000 estimation, the annual worldwide fly ashproduction was more than 600 million tons. In India, the annual production offly ash is nearly 110 million ton and its generation is likely to reach 170million tons. Presently, as per the Indian Ministry of Environment and Forestfigures, only 20 % to 30 % of fly ash is utilized in manufacturing of cement,concrete, construction, block and tiles and some disposed off in landfills andembankments, but a huge amount is unutilized which causes several environmentalissues related to air, soils and surface and ground-water pollution. The use offly ash in the preparation of geopolymeric materials for construction purposeshas been and continues to be subject of many research studies 31.Inrecent years, class F fly ash based geopolymer foam has attracted attentionfrom the scientific community due to its high potential as a green alternativeto conventional foams based Portland cement, and as a precursor for the productionof foams by appropriate pore forming agents 32,33. In previous studies, thecommonly used pore-forming agents include aluminum powder which liberateshydrogen gas 34-37, metallic silica which produces silica fumes 38,39,hydrogen peroxide which liberates oxygen gas 1,40,. Recent literature alsoreported few other pore forming agents such as sodium hypochlorite 41,perborates 42, and limestone 43.
All the reported pore forming agents havedrawback either of metallic toxicity, harsh reaction condition or use of thehazardous reagent.In this paper, weprepared class F fly ash geopolymer at temperature 100 °C using sodiumcarbonate as mild alkali activator as well as a pore-forming agent. Thisgeopolymer is a Portland cement-free, non-autoclaved, foamed, composed ofalkali-activated class F fly ash, wherein, the CO2 liberation duringthe activation process leads to make a pore network.