Hydration products, internal relative humidity and drying shrinkage of alkali activated slag mortar with expansion agents Zijian Jia, Yuanyuan Yang, Lingyan Yang, Yamei Zhang ⇑ , Zhengming Sun Jiangsu Key Laboratory of Construction Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, PR China highlights  Effects of different admixtures on drying shrinkage and IRH of AAS were evaluated.  CE with more CaO was found to have better effect than CSAE with more sulphoaluminate.  Linear relationship between drying shrinkage and IRH was found. article info Article history: Received 1 April 2017 Received in revised form 22 September 2017 Accepted 24 September 2017 Keywords: Alkali activated slag Expansion agent Internal relative humidity Shrinkage abstract In this paper, two types of composite expansion agents, CSAE (calcium sulphoaluminate type expansion agent) with more calcium sulphoaluminate than CaO, and CE (CaO type expansion agent) with more CaO than calcium sulphoaluminate, were used to improve the drying shrinkage of AAS (alkali activated slag) system, separately. For comparison, SRA (shrinkage reducing agent) was used for the same purpose as well. Results showed that the addition of CSAE, CE and SRA in AAS mortar reduced the drying shrinkage by about 41–45%, 54–56% and 35–44% at 56 d, respectively. At late ages, the IRHs in the three groups with admixtures were all higher than the reference group, though the addition of CSAE caused a significant decline of IRH (internal relative humidity) in AAS mortar at the first three days. Compared to CSAE, CE showed a better effect in drying shrinkage control due to the comprehensive effect of both low water con- sumption of CaO hydration and the high elastic modulus of CH in hydration products. It is found that dry- ing shrinkage was linearly related with IRH in AAS, and the incorporation of the admixtures reduced the sensitivity of drying shrinkage to IRH in AAS mortar. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Alkali activated slag (AAS) is a kind of cementitious material which has attracted many researchers for its lower energy con- sumption and lower CO 2 emission in the manufacturing process when compared to Portland cement. Previous studies indicate that AAS concrete possesses high early strength and good resistance to chemical attack, frost-thaw cycles and high temperature [1–7], which are superior to the performance of Portland cement con- crete. However, the high drying shrinkage of AAS [8–11] is a critical factor that restricts its application. The drying shrinkage of NaOH- activated AAS paste is about 4 times higher than that of Portland cement paste at 60 days [12]. The AAS mortar generally shows a drying shrinkage 2–4 times higher [11,13,14] than that of PC mortar and the shrinkage value can reach to about 2200 le. The investigation about AAS concrete [8] also found that the drying shrinkage of AAS concrete is 3 times higher than that of Portland cement concrete. Compared to Portland cement, more refined pores can be found in AAS and can result in higher capillary stress [10,12,15–18], which can considerably increase the shrinkage at drying condition. In addition to the pore structure, the hydration product of AAS is also one of the key factors that contributes to the high drying shrinkage of AAS [11,12,19]. The main hydration product in AAS is C-A-S-H, which is more amorphous than the C-S-H in Portland cement system [19]. The investigations from Ye [11,12] have explained the high drying shrinkage of AAS by understanding the behavior of hydration products at drying process. According to their research, the structural incorporation of alkali cations in C-A-S-H reduces the stacking regularity of C-A-S-H layers and make the C-A-S-H easier to collapse and redistribute upon drying, https://doi.org/10.1016/j.conbuildmat.2017.09.162 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: ymzhang@seu.edu.cn (Y. Zhang). Construction and Building Materials 158 (2018) 198–207 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat