Effect of high MgO content on the performance of alkali-activated fine slag under water and air curing conditions Chao-Lung Hwang a,⇑ , Duy-Hai Vo a , Vu-An Tran b , Mitiku Damtie Yehualaw a a Department of Civil and Construction Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Taipei 10607, Taiwan, ROC b Department of Civil Engineering, College of Engineering Technology, Can Tho University, Campus II, 3/2 Street, Ninh Kieu District, Can Tho City 900000, Viet Nam highlights  A mixture of slag and MgO was activated using NaOH and Na 2 SiO 3 to produce alkali-activated pastes.  Using finer slag and increasing the content of MgO reduced the workability and the setting time of fresh pastes.  Adding MgO and using finer slag improved the early age strength of AASM.  Method of curing affected the performance of AASM significantly.  Adding MgO and using finer slag created cracks on water-cured samples. article info Article history: Received 22 February 2018 Received in revised form 11 June 2018 Accepted 18 July 2018 Keywords: Alkali-activated slag Magnesium oxide Hydrotalcite-like phase (Ht) Microstructure Compressive strength abstract The present study investigates the effect of magnesium oxide (MgO) on the respective performance of water- and air-cured alkali-activated fine slag (AAS). Ground granulated blast furnace slag (GGBFS) of two different fineness levels were used to produce two AAS mixtures. These mixtures were alkali- activated with sodium hydroxide and sodium silicate and prepared into samples by adding, respectively, 2.5%, 5%, 7.5%, 10% and 15% MgO by total binder weight. A series of tests, including slump flow, setting time, compressive strength, X-ray diffraction (XRD), scanning electronic microscopy (SEM) and thermo- gravimetric analysis were conducted in accordance with the relevant standards. In terms of findings, the addition of MgO promoted the hydration of AAS significantly at early curing ages. Optimum MgO content was 7.5% for slag-4000 specimens and 5% for slag-6000 specimens. Cracks were observed in the water- cured AAS samples, with greater cracking at higher levels of MgO content. Moreover, the results show that the main contribution of MgO is in the hydrotalcite-like phase (Ht), where it adds volume, which improves strength the AAS paste. The effect of MgO was significantly increased in the samples that used finer-particle GGBFS. Adding MgO contributed to the more formation of hydration products, which was observed in the present study using X-ray diffraction and SEM and derivative thermogravimetric analysis. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction Portland cement is a key component in construction materials due to its excellent performance and quality. Around 1500 million tons of Portland cement are produced annually worldwide [1]. However, the cement-manufacturing process is a major contribu- tor to global industrial CO 2 emissions, with Portland cement pro- duction generating an estimated 5–8% of total global industrial CO 2 emissions and about 17% of the total pollution output of the building and construction industry [2]. The continued rising demand for cement worldwide is hampering policy efforts to decrease pollution. Furthermore, producing Portland cement requires large inputs of natural resources, which exacerbates the depletion of these resources. As a result, technological progress and the use of alternative raw materials are considered critical to improving current cement production processes. Moreover, newly developed binders are currently under investigation for their potential to make cement production greener and more sustain- able while improving the properties of conventional Portland cement. Alkali-activated slag (AAS), a type of cement that incorporates industrial by-products such as fly ash and ground blast furnace slag, is currently produced at a cost that is significantly lower than https://doi.org/10.1016/j.conbuildmat.2018.07.129 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: mikehwang0102@gmail.com (C.-L. Hwang). Construction and Building Materials 186 (2018) 503–513 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat