19The improvement of geotechnical properties is often achieved by the addition of traditional 20 binders, such as cement or lime. However, the use of such binders implies a considerable 21 financial and environmental cost that needs to be mitigated. An unconventional solution, similar 22 to cement in terms of performance but more environmentally friendly, consists in the use of 23 binders made from alkaline activated industrial residues. The technique consists on the 24 activation of raw materials (such as fly ash or blast furnace slag) rich in Si, Al, or even Ca, with 25 high pH alkaline solutions. The present work was developed aiming the possible stabilization, 26 using different fly ash contents, of a clayey soil with sand. The activator solution was composed 27 of sodium hydroxide and sodium silicate. The extended experimental campaign included 28 unconfined compressive strength (UCS), California Bearing Ratio (CBR), pulse velocity tests 29 and triaxial tests to assess the geomechanical improvement induced by the new binder. As a 30 mean of comparison, the experimental campaign included also the stabilization of the same soil 31 with either cement or lime. The obtained data indicates that the use of alkaline activation as a 32 soil stabilization technique provides competitive geomechanical results, when compared with 33 those obtained with traditional binders.34 35

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The influence of suction on the mechanical behaviour of unsaturated chemically stabilised soils is still mostly unknown and unquantified. This is also motivated by the difficulties associated with the experimental procedure required to fully characterise the unsaturated response of the soil, including its direct influence on traditional strength tests. The present paper presents the soil water retention curves obtained for a Portuguese soil before and after being stabilised with Portland cement (OPC) and an alkali-activated cement (AAC). Saturated undrained triaxial tests were also performed for the same curing conditions (0, 28, and 90 days). Previous attempts to characterise the retention curve of soils stabilised with AAC are unknown, and the results showed that the pore volume structure is already formed after 28 days, prior to the full development of the gel matrix responsible for the strength increase between 28 and 90 days. The curve changed after stabilisation, and with each binder, as the OPC presented a higher air-entry value and a narrower suction range compared to the AAC solution. The significant differences between the curves obtained from each binder suggest the future development of a methodology to assess the quality of the AAC stabilisation.

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