RESEARCH PAPER Carbonation Versus Efflorescence in Alkali-Activated Blast- Furnace Slag in Relation with Chemical Composition of Activator Ali Allahverdi 1,3 • Ebrahim Najafi Kani 2 • Babak Shaverdi 3 Received: 15 December 2014 / Revised: 7 November 2015 / Accepted: 14 December 2015 Ó Iran University of Science and Technology 2017 Abstract The present work is devoted to a better insight into the identification of carbonation versus efflorescence formation in alkali-activated blast-furnace slag and inves- tigates the relation between the chemical composition of the alkali-activator and the extent of the occurrence of these two phenomena. Obtained results showed that mixes of relatively lower alkali contents suffer not only from weak compressive strength due microstructural defects, but also from carbonation during the first few days. On the other hand, mixes containing relatively higher alkali con- tents strongly suffer from efflorescence formation in spite of their interestingly high compressive strengths. Carbon- ation during the first few days can partially neutralize the alkali content of the surface layers of the material which in turn significantly affects the activation mechanism leading to the formation of binding compounds of different molecular structure. Keywords Carbonation Á Efflorescence Á Alkali activator Á Blast furnace slag 1 Introduction Efflorescence formation in construction materials could be due to the precipitation of the dissolved soluble salts on the surface of the material along with deposition of a white salt by evaporation and/or chemical reaction of the salt solu- tion. Soluble salts usually originate from the used material itself or the surrounding environment. For concrete struc- tures, the appearance of efflorescence may be an indication of attack by aggressive chemicals originating from sur- rounding environment. Efflorescence, however, is distinct from the atmospheric carbonation. Atmospheric carbonation refers to gradual pH reduction of the pore solution due to ingress and chemical reaction of atmospheric carbon dioxide. This phenomenon includes the deposition of carbonate products in the bulk of material. It should be noted that continued carbonation over long periods of time is usually harmful to cement mortar and concrete and can result in material degradation. Efflorescence, however, forms visible deposits on the sur- face of the material, which are usually visible to the naked eyes. Severe efflorescence could results in gradual degra- dation of the binder. Some of the alkali-activated and geopolymer cements can be considered as the most suitable alternatives to Portland cement [1, 2]. These binders, however, are reported to be more prone to carbonation and efflorescence formation. These binders can be produced by reacting alkaline solutions and solid aluminosilicate precursors [3, 4]. There is a simplified model presented by Provis et al. [5] for the mechanism of alkali-activation and gel forma- tion of a solid aluminosilicate material into a synthetic binder. According to this model, the activation process starts with dissolution of aluminosilicate source material in alkaline solution and oligomerization of silicate and & Ali Allahverdi ali.allahverdi@iust.ac.ir 1 Research Laboratory of Inorganic Chemical Process Technologies, School of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran 1684613114, Iran 2 Faculty of Chemical, Petroleum, and Gas Engineering, Semnan University, Semnan 35195-363, Iran 3 Cement Research Center, Iran University of Science and Technology, Narmak, Tehran 1684613114, Iran 123 Int J Civ Eng DOI 10.1007/s40999-017-0225-4