Effect of Sodium Chloride and Fibre-Reinforcement on the Durability of Sand–Coal Fly Ash–Lime Mixes Subjected to Freeze–Thaw Cycles

The use of industrial residues such as coal fly ash in earthworks is of great interest for geotechnical engineers, since it reduces the consumption of natural resources. In that sense, the use of sand–coal fly ash-hydrated lime has been vastly reported in the last years. The potential for increasing mechanical properties of such blends seems promising. Therefore, the addition of small quantities of NaCl and polypropylene fibres to sand–coal fly ash-hydrated lime mixtures is assessed in the present research in terms of unconfined compressive strength and durability to freeze–thaw exposure. Unconfined compressive tests showed that addition of NaCl seems more effective in enhancing strength of sand–fly ash–lime blends in comparison to fibre inclusions. On the other hand, the inclusion of fibres to the same blends provided a greater gain in durability respect to those specimens with NaCl. Both fibres and NaCl added separately, though, provided increase in strength and durability respect to mixtures without any fibres or NaCl. The improvement in engineering properties of sand–coal fly ash–lime blends provided by NaCl was found to be related to the formation of the phase thomsonite [NaCa₂Al₅Si₄O₂₀(6H₂O)] and to the NaCl action as a catalyser. The enhancement caused by fibre inclusions, in turn, can be attributed to the fact that fibres inhibit water from expanding under low temperatures, therefore preventing cementitious bonds breakage.