SCMT4 Las Vegas, USA, August 7-11, 2016 Drying Shrinkage of Alkali Activated Fly Ash: Effect of Activator Composition and Ambient Relative Humidity Maryam Hojati 1a , Farshad Rajabipour 1b , and Aleksandra Radlińska 1c 1 Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, USA. 1a Email: <hojati.maryam@gmail.com>; 1b Email: <Farshad@psu.edu>; 1c Email: <ara@engr.psu.edu>. ABSTRACT Alkali activated fly ash (AAFA) is an alternative concrete binder that could serve as a substitute for ordinary Portland cement (OPC) for certain engineering applications. While this material has significant environmental and durability benefits, its potential volume instability and propensity to shrinkage and cracking could be of great concern. The subject of the present paper is evaluating the magnitude of drying shrinkage in AAFA binders of different compositions and at various ambient relative humidities (RH). Four AAFA paste mixtures were prepared using class F fly ash and liquid sodium silicate activators with various alkalinity (pH) and silicate modulus (n=(SiO2/Na2O)molar). The results were also compared with an OPC paste with a similar initial porosity. All AAFA pastes were steam cured at 60°C for 24 hours, followed by 6 days of moist curing at 23 o C, while the OPC specimens were moist cured at 23 o C for 7 days. The shrinkage specimens were dried under nitrogen purge at different relative humidities. The results show that activators with intermediate pH and modulus led to AAFA binders with higher compressive strength, denser pore structure, and larger drying shrinkage. Regardless of the activating solution, AAFA pastes dried quicker, and reached equilibrium faster than OPC paste. AAFA pastes also lost more moisture, but generally exhibited a lower drying shrinkage than OPC at similar RH. Steam curing of AAFA for a longer period (7-day vs. 1 day) stabilized the structure of AAFA binders and lowered the drying shrinkage. INTRODUCTION Alkali activated fly ash (AAFA) could serve as an alternative to ordinary Portland cement, OPC, binder for concrete and provide significant environmental and durability benefits [Fernández-Jiménez et al. 2006 and 2008, Provis and Van Deventer 2009]. In order to manufacture AAFA, pulverized coal fly ash is mixed with a concentrated alkaline solution (instead of water), and often heat cured under steam or dry conditions. This results in dissolution of fly ash and formation of a three dimensional sodium alumino- silicate binder (referred to as N–A–S–H or geopolymer gel). Despite its numerous benefits, one factor that has mitigated a widespread commercialization and industry acceptance of alkali activated concrete is a potential volumetric instability and propensity to shrinkage and cracking in these materials. Large Fourth International Conference on Sustainable Construction Materials and Technologies http://www.claisse.info/Proceedings.htm