Mechanical behaviour of soil waste-derived geopolymer mixtures for construction

Geopolymers are inorganic synthetic materials produced through alkali-activation of aluminosilicates. Hardened geopolymers exhibit ceramic-like high-strength properties, whose production generates produce up to 60% less CO2 emissions compared with Portland cement manufacture. Geopolymer pastes are prepared using alkali activators and cured for 24 hours at 50°C. A raw material, which has recently been identified as possessing great potential for producing geopolymers, is municipal soil waste from construction sites. The disposal cost of soil waste is one of the most significant in-ground costs for brownfield developments due to high landfill tax rates. Soils delivered to waste management companies can be washed ex-situ to remove contaminants and extract valuable aggregates for reuse in construction. However, whilst there is an immediate use for the washed sands and gravels, the soil washing plant also produces large quantities of highly saturated silt-clay based filter cake. To date, this filter cake has not been recycled for use as a construction product due to its weak mechanical strength behaviour.
The aim of the present work was to investigate whether silt-clay filter cake waste, derived from soil washing, could be reused as an alternative active component for geopolymer production. In this study, the filter cake has been produced in Teesside (UK). The mechanical properties of various hardened geopolymer mixtures were tested by standard unconfined compressive strength (UCS) testing. The microstructure, chemical composition and mineralogy of the materials were studied by direct physical methods including X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX).
The filter cake was modified to optimise moisture content and alkali constituents. An experimental design matrix was established for determining the mechanical characteristics of the geopolymer mixtures. Regression models were developed from the UCS tests. These were used to evaluate the impacts of the variable soil chemistry, water content and alkali components on the overall mechanical performance.
The maximum UCS recorded across all geopolymer mixtures was 25MPa, mechanical performance is show to be based on the ratio between alkali addition and soil silica and alumina composition.