Optimization of one-part geopolymer based on fly ash and volcanic ash for soil stabilization

Soil stabilization is a technique used to alter the natural properties of soil. Enhancing the strength and increasing the stability of in-situ soil can either be done mechanically or chemically using conventional materials such as gravel, crushed stone, lime, and cement. These materials are usually extracted through mining and quarrying which have been known to pose serious environmental impacts. In addition, the production of these materials, specifically cement, have a significant contribution to the global carbon dioxide emissions. Several approaches have been proposed to reduce the cement consumption in the construction industry by using pozzolanic materials such as coal fly ash, and volcanic ash. These materials can be utilized as substitute for chemical soil stabilization through a process called geopolymerization. Pertinent experimentations have already been undertaken with the hopes of promoting geopolymer in replacing cement as construction material. However, less attention has been given to its application as geopolymer stabilized soil. One reason could be attributed to the applicability of the traditional geopolymer synthesis in soil stabilization. The traditional geopolymer synthesis which uses alkali activator solution, and solid precursors is called the two-part geopolymer synthesis. This kind of synthesis is an impediment to geopolymer stabilized soil commercial viability due to the nature of alkali activator solution. Thus, a novel approach one-part geopolymer or just add water like in the case of ordinary Portland cement was employed in this study for possible application in structural fill. The proportion mix of precursors such as fly ash and volcanic, and ratio of solid alkali activators Sodium Silicate (SS):Sodium Hydroxide (SH):Sodium Aluminate (SA) were investigated. The optimum mix proportion that yields high value of unconfined compressive strength after 28 days of curing and after 28 days of immersion in 5% sulfuric acid solution was found to be the mixture having 25% fly ash/50% soil and 50% SS/50% SH. The developed optimum mix proportion showed good results in terms of strength as it gives a value of unconfined compressive strength (UCS) greater than 75 kPa, which is the minimum requirement of National Structural Code of the Philippines (NSCP) recommended for structural fill application. The resistance to acid of geopolymer stabilized soil was further supported with the increase in strength and significant difference in structural bonding, mineralogy, and morphological analysis observed on controlled specimen (Day 0) and soaked sample (Day 28). The results of leachability showed high concentrations of Al, Na and Si and low concentrations of Fe and Ca. Finally, the high concentration of As at 1st cycle may indicate that it is more mobile in basic solution compared with other heavy metals