2623 Proceedings of the XVI ECSMGE Geotechnical Engineering for Infrastructure and Development ISBN 978-0-7277-6067-8 © The authors and ICE Publishing: All rights reserved, 2015 doi:10.1680/ecsmge.60678 Alkali activated soil-ash mixtures Activation alcaline de mélanges sol-cendre S. Rios *1 and A. Viana da Fonseca 1 1 Department of Civil Engineering (FEUP)/University of Porto, Porto, Portugal * Corresponding Author ABSTRACT Mixtures of soil, fly ash and an alkaline solution made from sodium hydroxide and sodium silicate were artificially moulded using different sodium hydroxide concentrations and water contents. The chemical reactions involved in the alkaline activation of fly ash produce a geopolymeric gel that links the soil particles with cementitious bonds. Two different soils were tested, and the effect of the acti- vator and water content on workability, stiffness and strength was carefully evaluated by means of unconfined compression strength tests, indirect tensile strength tests and seismic wave measurements. The results indicate that the addition of water has a negative effect on stiff- ness, in both soils, and in strength, for the coarse soil. RÉSUMÉ Mélanges de sol, cendres et une solution alcaline composée par hydroxyde et silicate de sodium ont été moulés artificiellement en utilisant différentes concentrations d'hydroxyde de sodium et teneurs en eau. Les réactions chimiques impliquées dans l'activation alca- line des cendres volantes produisent un gel géopolymèrique qui lie les particules de sol comme en ciment. Deux sols différents ont été es- sayés et l'effet de la teneur en liquide et de l'eau sur la faisabilité, la rigidité et la résistance ont été soigneusement évalués par essais de compression simple, essais de traction indirecte et des mesures d'ondes sismiques. Les résultats indiquent que l'ajout d'eau a un effet négatif sur la rigidité des deux sols et aussi dans la résistance du sol grossier. 1 INTRODUCTION The production of cement has severe environmental impacts, using vast amounts of fossil fuels and being responsible for the emission of around 5% of all the carbon dioxide worldwide (Worrell et al. 2005). In this paper an alternative to cement for soil improve- ment is proposed, based on the reuse of an industrial waste – coal burning fly ash. The alkaline activation of fly ash to create an alternative binder to Portland cement has been successfully applied to replace tradi- tional Portland cement concrete (Palomo & Fernan- dez-Jimenez 2004, Turner & Collins 2013). It consists in the reaction of a solid aluminosilicate (like fly ash) with a highly concentrated aqueous al- kali hydroxide or silicate solution producing a syn- thetic alkali aluminosilicate material with comparable performance to traditional cementitious binders, but with significantly reduced greenhouse emissions (Duxson et al. 2007). However, few studies have been carried out in soil improved by the alkaline activation of fly ash (Criste- lo et al. 2013). In fact, adding this new binder to a soil changes the conventional soil-cement paradigm in terms of moulding parameters. Void ratio, com- paction degree and water content, among others, which have a known effect on soil-cement mixtures (Consoli et al 2011, 2012), need to be validated for this new material. In previous work Rios & Viana da Fonseca (2014) highlighted the potential of this material since strength and stiffness are highly improved when the alkaline solution and fly ash are added. In this paper, the influence of water is more deeply analysed dis- tinguishing between water content and liquid content. While the water content is the ratio between the mass