Mechanical properties of ambient cured one-part hybrid OPC-geopolymer concrete Mahya Askarian ⇑ , Zhong Tao, Georgius Adam, Bijan Samali Centre for Infrastructure Engineering, Western Sydney University, Penrith, NSW 2751, Australia highlights  An ambient cured one-part hybrid OPC-geopolymer concrete is developed.  Slump, setting time, compressive strength and microstructure are affected by OPC content of binder.  Novel non-hygroscopic solid potassium carbonate activator is used.  The type/amount of activators has some influence on the strength of hybrid OPC-geopolymer mixes. article info Article history: Received 4 January 2018 Received in revised form 15 July 2018 Accepted 22 July 2018 Keywords: Ambient curing Hybrid one-part OPC-geopolymer concrete Potassium carbonate Calcium hydroxide Sodium silicate Microstructure abstract To produce geopolymers, a large amount of highly corrosive and hygroscopic alkaline activators, i.e, sodium hydroxide and potassium hydroxide, are commonly used. In this study, one-part hybrid ordinary Portland cement (OPC)-geopolymer concrete mixes were developed, in which solid potassium carbonate (7.5 wt% of the total geopolymeric raw materials) was used as the main activator and the OPC as a source of silicate and poly silicate was blended with geopolymeric raw materials (fly ash and ground granulated blast furnace slag) in different proportions. The influence of OPC content on the workability, setting time, compressive strength development and microstructure of the concrete mixes was investigated. It was found that the inclusion of OPC in geopolymer mixes reduced the workability and setting time but sig- nificantly increased the early age and ultimate compressive strength due to the quick reaction of OPC with alkali activators. By the inclusion of just 10% OPC in the binder system, the compressive strength at 28 days increased from 11.4 MPa to 33.4 MPa. Furthermore, control OPC concrete mixes were also pre- pared and tested. It was found that the percentage of strength increase at 28 days due to alkali activation decreased from 82.5% to 24.4% as the OPC content increased from 10% to 60%. Microstructural analysis was also conducted, which confirmed the coexistence of geopolymeric and CSH gels in the one-part hybrid OPC-geopolymer binder system. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction Geopolymeric materials have emerged as sustainable alterna- tives to Portland cement for a few decades. The term ‘‘geopolymer” was initially introduced by Davidovits [1]. Conventional forms of geopolymers are synthesised from a three-part mix, including high alkaline solutions of sodium hydroxide and sodium silicate with suitable concentration and geological origin materials such as metakaolin or industrial by-products such as fly ash and ground granulated blast furnace slag as a source of aluminosilicate [2]. To obtain suitable mechanical properties, heat treatment is often required during the geopolymerisation process to accelerate the reaction [3]. The outstanding properties of geopolymers such as high resistance to elevated temperatures and fire, high resistance to acids, salts and other corrosive materials, along with their envi- ronmentally friendly nature have resulted in considerable research and practical applications [4–6]. However, there are also two major drawbacks associated with the production of the conventional geopolymer concrete. Firstly, dealing with high concentration of corrosive and viscous alkaline solutions represents one of the main barriers for commercial appli- cations. Secondly, the need of heat curing prevents the wide appli- cation of fly ash based geopolymer concrete since this type of concrete cannot be cured at ambient condition [7]. The geopolymer systems are usually subjected to heat curing for hours or even days at a temperature higher than 50 °C [8]. https://doi.org/10.1016/j.conbuildmat.2018.07.160 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: m.askarian@westernsydney.edu.au (M. Askarian). Construction and Building Materials 186 (2018) 330–337 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat