Effects of slag and superplasticizers on alkali activated geopolymer paste

Abstract

Geopolymers are a type of amorphous alumino-silicate product, and can be synthesised by the polycondensation reaction of geopolymeric precursor and alkali polysilicates. The binder is prepared by the alkaline activation of industrial by-product materials like fly ash, slag, metakaolin, rice husk ash, etc. as a source for aluminosilicate. The most commonly used alkaline activator is the combination of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). Chemical admixtures are indispensable in alkali activated systems owing to its poor fresh properties such as workability, too quick setting etc. In this study, the chemical admixture - alkali activated binder interactions were carried out by using latest generation superplasticizers, such as Polycarboxylic ether admixture (PCE) and Sulphonated naphthalene formaldehyde (SNF). Mini slump test, initial and final setting time tests were conducted in alkali activated slag blended fly ash based geopolymer paste and the test results were analyzed with respect to parameters such as alkali to binder ratio (a/b), proportion of slag in blended geopolymer paste, type and dosage of superplasticizers. The test results indicated that the workability and setting time of alkali activated geopolymer paste enhanced due to the increase in alkali content and the reduction in slag content. Based on the effect of chemical admixtures in geopolymer mix, SNF performed better than PCE admixture with regards to mini slump test for all the mixes; whereas in the case of setting time test, SNF showed better results only for the mix with 100% slag. In the case of slag blended fly ash based geopolymer paste, both PCE and SNF showed similar effects, and also it was observed that, the flow of the paste got enhanced when the dosage of superplasticizers is increased from 0 to 3% by weight of binders.