Mechanical performance and micro-structure of bentonite-fly ash and bentonite-sand mixes for landfill liner application

Abstract Low permeability (10-9 m/s) and adequate unconfined compressive strength (UCS = 200 kPa) forms the basis of using compacted bentonite (B) clay in landfill liner. However, B induces cracks or swells upon excessive drying and wetting events, respectively. To overcome failure of liner layer, B is amended with cohesionless sand (S), despite S being costly and ecologically unsustainable. In the recent past, fly ash (FA) a by-product from thermal power plants, has been explored as a suitable additive material in landfill liner, in line with the adoption of cleaner usage of waste material in construction. The objective of this study was to investigate the mechanical performance of both B-FA and B-S mixes along with the related changes in microstructure and physio-chemistry. 180 sets of UCS tests were conducted on 4 B-FA mixes (including class C and F) and B-S mixes at different compaction state and additive rate. Unlike B-S mixes, all B-FA mixes showed an increase in UCS with respect to B regardless of compaction state and additive rate. Detailed microstructure and physio-chemical characterization for B-FAs revealed the relative abundance in the formation of calcium alumino silicate hydrate, calcium aluminosilicate hydrate, and ettringite minerals after curing as compared to B-S mixes. In fact, Class C FA exhibited the largest UCS even at high moisture content, which can be attributed to higher formation of minerals upon curing. All B-FA mixes showed UCS greater than the minimum limit for liner application regardless of the compaction state. All B-S mixes did not adhere to the minimum UCS criterion at higher moisture content. FA additive at 50% showcased the highest increase in strength improvement factor (10 out of 12 cases), while for B-S mixes, this factor decreased with an increase in additive rate. Based on the measured data, two new linear relationships of UCS with pH and electrical conductivity (both indicative of reaction) of the mixes were observed. The relationship with respect to pH was more pronounced at higher moisture content as pH indicates the existence of CaO minerals.