3D printed geopolymer composites: A review S. Qaidi a, b, * , A. Yahia c , B.A. Tayeh d , H. Unis e , R. Faraj f , A. Mohammed e a Department of Civil Engineering, College of Engineering, University of Duhok, Duhok, KR, Iraq b Department of Civil Engineering, College of Engineering, Nawroz University, 42001 Duhok, Iraq c Department of Civil Engineering, Universit e de Sherbrooke, 2500 Blvd. de L'Universit e, Sherbrooke, QC J1K 2R1, Canada d Civil Engineering Department, Faculty of Engineering, Islamic University of Gaza, P.O. Box 108, Gaza Strip, Palestine e Civil Engineering Department, College of Engineering, University of Sulaimani, Sulaimaniyah, KR, Iraq f Civil Engineering Department, University of Halabja, Halabja, Kurdistan Region, Iraq article info Article history: Received 14 May 2022 Received in revised form 21 June 2022 Accepted 24 September 2022 Available online 10 October 2022 Keywords: 3D printed geopolymers Concrete Processing Performance requirements Mix design Rheology Mechanical properties Applications Challenges Economic Environmental footprints Sustainability abstract The application of three-dimensional printed (3DP) technology is expected to lead to the industrial revolution 4.0, disrupting the economy and providing design customization and adaptation. The con- struction sector is rapidly catching up to this modern technology with the production of a 3D printer for concrete to provide a healthy work environment, economic independence, and architectural freedom. Despite the fact that 3DP concrete technology has progressed significantly in recent decades, there is an urgent need to develop appropriate 3DP materials that improve performance while reducing material consumption, which is critical for reducing carbon dioxide emissions. Geopolymers (GPs) have been found to be a promising alternative to cement-based materials for 3DP in the construction industry, which could help make it more environmentally friendly. This article comprehensively reviews the printing process, performance requirements, advantages, disadvantages, and common 3DP concrete technologies. This article also provides in-depth studies on the behaviors and characteristics of GP composites utilized in 3DP production, such as mix design, rheology, and mechanical characteristics. Besides, study developments are moving towards a comprehensive understanding of the environmental footprints and economic benefits of 3DP concrete for building applications utilizing GPs as suitable concrete materials for the emerging environmentally friendly robust concrete compound for digital constructions today. This review article also highlights knowledge gaps or potential challenges that must be overcome to progress GP composites for 3DP, as well as future study opportunities based on prior research and existing challenges. © 2022 Elsevier Ltd. All rights reserved. 1. Introduction Three-dimensional printed (3DP) concrete is a type of additive manufacturing (AM) that is gaining traction in the construction sector due to its feasibility. The simplest and most commonly uti- lized type of 3DP concrete is extrusion-based concrete printing: a layer-by-layer concrete extrusion process that enables the fabrica- tion of medium-to-large-scale civil engineering structures like pedestrian bridges, office buildings, single-or multistorey houses, and similar structures. Usually, and at an accelerating rate, novel demonstrations of the technique are made public. Some examples of large-scale 3DP concrete structures are shown in Fig. 1 . The primary benefits and possibilities of 3DP concrete are its capacity to create structures with minimum human intervention and sufficiently short time. Although the procedure is still more costly than traditional structures, due to the additional structural stability concerns, 3DP concrete offers a clear cost-benefit when increasing automation or complexity is demanded. Several of the earliest research publications reporting on 3DP concrete were published about a decade ago [1 ,2]. Numerous evaluations of 3DP concrete have been written since then, covering a range of diverse issues in this sector, comprising the field's patterns [3e5], the primary opportunities and problems [6e10], the broader idea of digitized concrete [11 , 12], the methods and technologies employed [13, 14], and the basic physics of the extrusion of cementitious materials [15e18]. Although the accomplishments to date in achieving large-scale complex concrete buildings [19e25], the exponential growth in design complexity in constructing concrete * Corresponding author. E-mail address: shaker.abdal@uod.ac (S. Qaidi). Contents lists available at ScienceDirect Materials Today Sustainability journal homepage: https://www.journals.elsevier.com/ materials-today-sustainability https://doi.org/10.1016/j.mtsust.2022.100240 2589-2347/© 2022 Elsevier Ltd. All rights reserved. Materials Today Sustainability 20 (2022) 100240