Use of paper sludge ash as a calcium-based stabiliser for clay soils

Chemical ground improvement of soils of poor quality for construction has been increasingly used as a means of promoting sustainable construction practices. The production of conventional soil stabilisers such as cement or lime involves non-renewable natural resource and energy consumption and high carbon dioxide emissions; therefore, alternative stabilisers are sought. This study used waste paper sludge ash (PSA) to treat three different clays. The aim was to assess PSA effectiveness as an alternative to lime or cement for clay stabilisation based on plasticity characteristics, unconfined compressive strength (UCS), water retention and volumetric stability. PSA-treated soil specimens were shown to perform well compared to lime-treated or cement-treated ones: (a) PSA considerably lowered the plasticity indices of the two expansive clays, in a similar way as lime; (b) in most cases PSA dosages equal to or greater than the initial consumption of lime gave UCS at least twice as high compared to those obtained using commercial limes at equivalent dosages (> 1 MPa for the two expansive soils after 7 or 28 days of curing) and in the inspected cases also higher UCS than cement; and (c) consistently with the plasticity results PSA-treated specimens swelled less during wetting and had lower volumetric strains upon drying (better volumetric stability) compared to lime-treated or cement-treated soils. Overall the results give promise for a valorisation route of this waste material in the field of ground improvement.