Stabilised mineral–biomass mixtures in groundwork: fundamentals

This paper considers the fundamentals of mechanical structures and fluid flow characteristics in lime-stabilised mineral—biomass mixtures for potential novel applications in low-value bulk industrial products or wastes. As contained organic matter is expected to be essentially stable indefinitely under anaerobic conditions, the composites represent a form of solid carbon sequestration. Laboratory specimens containing designed combinations of coarse mineral (4 or 10 mm quartz) and fines (wet digested sewage sludge, pulverised-fuel ash and quicklime) were evaluated as possible precursors to composite emplacements in engineering groundwork. The volume proportion of coarse mineral to mixed fines was nominally 3:2, a value that allowed complete sludge infilling between coarse particles while ensuring particles remained in point contact to form a supporting matrix. The results showed that compositions of digested sewage (25% solids) and fuel ash in the approximate mass ratio 2:1, together with 3% quicklime, underwent pozzolanic strengthening to give an unconfined compressive strength of typically 250 ± 30 kPa after several months' ageing. This is similar in strength to stiff soil or soft rock and, in principle, indicates sufficient mechanical integrity for use as foundations beneath light building development or as a bulk fill underlying open amenities. Porosity (4–10%) and hydraulic conductivity (10⁻⁵–10⁻⁷ m/s) were moderate to low. Interstitial water contained significant concentrations of ammonia and heavy metals, suggesting that effective isolation from the outside environment would be required in field applications.