Effect Of Alkali Content On The Geopolymer Activated By Sodium Silicate

Geopolymer is an inorganic gel material with three-dimensional structure,which is generally made of aluminosilicate raw materials and alkaline activators.In general,the reaction of fly ash activated by sodium silicate is one of the important methods for preparing geopolymer.The modulus（molar ratio of SiO₂ to Na₂O）is usually used to express the structural properties of water glass,but it is not clear how the changes in the effects on the activation of fly ash by different contents of SiO₂ and Na₂O respectively.This paper is aimed to study the effects on the activation of fly ash by different contents of SiO₂ and Na₂O respectively and the activation of fly ash mixed with clay minerals by different contents of Na₂O.Unconfined compressive strength,Scanning Electron Microscope（SEM）,Fourier Transform Infrared（FTIR）,X-ray Diffraction（XRD）,and Brunner Emmet Teller（BET）tests were performed to illustrate the effects on the strength,chemical composition,mineral composition,and microstructure of geopolymer by the sodium silicate with different content of SiO₂and Na₂O,meanwhile,to study the activation effects and the reaction mechanism of fly ash mixed with clay minerals by the sodium silicate with different contents of Na₂O.The main conclusions and innovations achieved are as follows:（1）When the SiO₂ content is constant,the strength increases with the increase of Na₂O content,but the strength decreases when it exceeds 7%.When the Na₂O content is less than 3%,the strength first increases and then decreases with the increase of SiO₂ content;when the Na₂O content is more than 3%,the strength increases non-linearly with the increase of SiO₂ content.（2）Changes in the content of SiO₂ and Na₂O will not affect the material structure and mineral composition of alkali-activated geopolymer.The content of SiO₂ and Na₂O will have a greater impact on the microstructure and pore structure,and thus will affect the strength of alkali-activated geopolymer.（3）Clay mineral particles interact with geopolymer to transform from loose particles to massive structures.With the increase of Na₂O content,the structure becomes denser,but the electronic binding energies of Al,Si,and Ca elements in the samples decrease.The characteristic peaks of montmorillonite at about 2θ=5°（001）tends to shift to the right region with the increase of Na₂O content,so that the corresponding 2θincreases and the interlayer spacing between the crystals decreases.The content of adsorbed water decreases with increasing Na₂O content,and then the hydrophilic capacity decreases.（4）Silicate from the activator and aluminosilicate generated from the alkali-activation of fly ash can adsorb on the surface of mineral particles,so that the ends of crystals are filled,and provide an alkaline environment for the polycondensation reaction of geopolymer.More amorphous gel products are filled between the particles,contributing to form a denser block structure and then limit the activity of minerals.