Effect of Synthesis Parameters on Compressive Strength of Fly ash-GGBS Based Geopolymer

Abstract

Cement is a conventional binding agent for concrete and it is also used as a stabilizing agent to soil and aggregates. This causes enormous increase of carbon dioxide emission all over the world causing fast degradation of environment. One ton carbon dioxide gas is released in to the atmosphere for each one ton production of OPC. Global cement production is expected to reach 5000 million tonnes in the year 2050. Furthermore researchers reported that the stabilization of soil using cement and lime is very costly. Due to the high costs of cement, effort is being intensified by engineers/researchers to examine the possibilities of utilizing cost-effective locally available industrial wastes for replacing cement. Reduction of CO2 emission has become a priority and it has become a necessity to move forward with environmental friendly alternatives for cement.

Ordinary Portland Cement contributes 5-7% of the overall green house emissions which is a threat to the global environment. Overall cement production all over the world is increasing rapidly. Due to these reasons the need for an environment friendly alternative for cement has increased. Studies reported that cement produced from ground grannulated blast furnace slag (GGBS) showed 80% less emission of green house gases and 80-90% in case of fly ash. These industrial by products has excellent binding property which make them a perfect substitute for OPC.
Geopolymer is a new group of materials produced by the alkaline activation of aluminosilicate compounds. Earlier researches around the world proposed that concrete from geopolymer binder exhibited superior engineering, thermal and durability properties than ordinary Portland cement (OPC) concrete, such as higher mechanical strengths, higher resistivity to sulphate and acid attacks and higher thermal resistivity. Geopolymers are generally made from activation of aluminosilicate powders by highly concentrated sodium hydroxide and or sodium silicate solutions, known as liquid-activated geopolymer.

The interest of this paper is to investigate the influences of synthesis parameters on compressive strength of a geopolymer. Geopolymer samples are prepared using industrial by- products such as fly ash and ground granulated blast furnace slag (GGBS) with KOH as alkaline activator. With sol/solid=0.2, 0.3, 0.4, 0.5 and KOH molarity of 4M, 8M, 12 geopolymer samples are prepared and cured at 7 and 28 days under ambient temperature of 270C. Compressive strength of these samples are found out and the optimum mix proportion is found out. Geopolymer samples made of GGBS with 8M KOH and sol/solid ratio 0.3 cured at 28 days in ambient tempeature showed maximum compressive strength of 24.37 MPa.

Further, the effects of curting temperature on compressive strength has been investigated. Temperature curing study of samples prepared at this mix proportion was carried out by curing at 600C and 900C in hot air oven and in water, each for a curing period of 7 and 28 days. Variation of compressive strength for different curing condition is analysed. Positive results were obtained for samples cured at 600C and those cured at 900C showed redution in strength as compared to sample cured at ambient temperature(270C ) and 600C.


Finally, the geopolymer paste was added with a weak soil and its improvement in strength properties was observed. UCS and compcation characteristics of the soil sample improved. 20% geopolymer addition gave compressive strength of 2.03 MPa for a curing period of 14 days. This was about 8 times the compressive strength of the virgin soil.
From this study it could be concluded that variation of molar concentration of alkali activator, alkali to source material ratio, and percentage addition of the source material. It was also observed that the slag content is the most dominating factor affecting the compressive strength of the geopolymer.