Effect of phosphogypsum on the stability upon firing treatment of alkali-activated slag D. Vaic ˇiukyniene ˙ a,⇑ , D. Nizevic ˇiene ˙ b , A. Kiele ˙ a , E. Janavic ˇius a , D. Pupeikis a a Faculty of Architecture and Civil Engineering, Kaunas University of Technology, Lithuania b Faculty of Electrical and Electronics Engineering, Kaunas University of Technology, Lithuania highlights  PG shortens the setting times of alkali activated slag cured at room temperature.  During hydration process sodium sulfate forms in alkali activated specimens.  The highest compressive strength of specimens is with 5% of PG.  The specimens with 5% of PG exhibit greater residual strength. article info Article history: Received 21 March 2018 Received in revised form 5 June 2018 Accepted 26 June 2018 Keywords: Alkali-activated slag Phosphogypsum additive Elevated temperatures abstract The paper analyses the preparation of alkali-activated slag with phosphogypsum additive. The specimens of alkali-activated slag were tested both at room and at elevated temperatures. It was determined that phosphogypsum shortened the setting times of alkali activated slag mixture cured at room temperature. During hydration process sodium sulfate formed in alkali activated specimens. Compressive strength depends on phosphogypsum amount too. The highest compressive strength MPa of specimens was with 5% of phosphogypsum. Similar situation was after exposure elevated temperatures. The inclusion of 5% phosphogypsum showed the optimum content in which higher residual strength was obtained after being treated at 400, 600, 800 and 1000 °C temperatures. These type specimens exhibited approximately an average of 1.2 times greater residual strength than specimens without phosphogypsum. This means that alkali-activated slag blended with phosphogypsum have great potential applications for fire resis- tance. It is possible to recycle phosphogypsum in alkali activated slag blends. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction Alkali-activated materials are a rapidly emerging sustainable alternative to Portland cement binder because of their high strength and durability and low environmental impact. There is a growing interest in the development of new cementitious binders, which enhance optimal utilization of industrial by-products, such as phosphogypsum and slag. Great interest in alkali-activated materials resulted in a large amount of waste recycling. This type of binder has good fire resistance properties and low strength loss in elevated temperature, as well as spalling resistance. Alkali-activated binders are known for superior thermal stability and fire resistance. Several studies were devoted to investigating the durability of alkali-activated binders at elevated temperatures. Rashad et al. [1] investigated the possibility of using granulated blast-furnace slag as partial or full natural silica sand replacement in alkali-activated slag mortar. The results indicated that the com- pressive strength of the mortar specimens before and after thermal treatment increased with increasing granulated blast-furnace slag sand content. Park et al. [2] studied alkali-activated fly ash/slag exposed to high temperatures. The strength increase below 400 °C was attributed to the binder gel, which formed after the expo- sure to heat, thus decreasing the porosity. The crystallization of the binder gel resulted in an increase in the porosity, thereby inducing a decrease in the strength above 400 °C. Lee et al. [3] investigated the influence of binder composition on the gel struc- ture in alkali-activated fly ash/slag paste exposed to elevated tem- peratures. The amount of crystalline phase formed after exposure to 800 °C is highly dependent on the Ca/Al ratio of the precursor, slag and fly ash. The pore structure of the fly ash/slag paste with a lower slag content after exposure to 800 °C became more porous. Another study [4] focused on developing thermally-stable materials based on alkali-activation of slag, fly ash, and metakaolin https://doi.org/10.1016/j.conbuildmat.2018.06.213 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: danute.vaiciukyniene@ktu.lt (D. Vaic ˇiukyniene ˙ ). Construction and Building Materials 184 (2018) 485–491 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat