Sustainable sodium hydroxide concentration for the synthesis of alkali-activated clay–fly ash in road subgrades

Highly concentrated alkaline solutions used for alkaline activation are of concern due to carbon dioxide emissions. The use of a reduced alkali concentration could thus be promising, especially for road subgrade stabilisation, where long-term serviceability (controlled by plasticity index (PI) and unconfined compressive strength (UCS)) is not overly demanding. The aim of this research was thus to evaluate the possible usage of low alkali solutions to fulfil the minimum requirements of UCS > 1033 kPa and PI < 30% for stabilised road subgrade at room temperature. For a constant fly ash (FA)/solid (soil + FA) ratio, 1, 4 and 8 M sodium hydroxide solutions and a residue cleaning solution (RCS) containing sodium hydroxide (≤ 1 M) were employed to treat a natural clay with UCS = 188 kPa and PI = 32.1%. Mechanical, characterisation and microstructural tests were conducted to assess how the various alkali concentrations promoted alkaline activation and affected the inherent properties of the clay. The activators containing ≤1 M sodium hydroxide successfully satisfied the design criteria, although they required a longer curing time than the 4 M and 8 M sodium hydroxide activators. Furthermore, compared with the 1 M sodium hydroxide activator, the RCS was found to perform better in terms of mechanical strength gain and particle connections due to its initial aluminium content.