CHALLENGE JOURNAL OF CONCRETE RESEARCH LETTERS 13 (3) (2022) 101–106 * Corresponding author. Tel.: +90-318-357-4242/1263 ; E-mail address: ozersevim@kku.edu.tr (Ö. Sevim) ISSN: 2548-0928 / DOI: https://doi.org/10.20528/cjcrl.2022.03.003 Research Article Effect of waste concrete powder on slag-based sustainable geopolymer composite mortars Erdinç Halis Alakara a , Özer Sevim b, * , İlhami Demir b , Gazi Günel c a Department of Civil Engineering, Tokat Gaziosmanpasa University, 60150 Tokat, Turkey b Department of Civil Engineering, Kırıkkale University, 71451 Kırıkkale, Turkey c Graduate School of Natural Applied Sciences, Kırıkkale University, 71451 Kırıkkale, Turkey A B S T R A C T In this study, the effect of waste concrete powder (WCP) on slag-based geopolymer composite mortars was investigated. Blast furnace slag (BFS) and WCP were used as binders in geopolymer mortars. WCP was substituted into the geopolymer mortar composites at rates of 10%, 20%, 30%, and 40% by weight of slag. Sodium hydroxide (NaOH) solution was used as the alkali activator in the mixtures and the solution acti- vator concentration was chosen as 16 molars (M). After the prepared mortars were cured at 100°C for 24 hours, they were subjected to flexural strength (ffs), compressive strength (fcs), and ultrasonic pulse velocity (UPV) tests. Results showed that ffs, fcs, and UPV decreased with the increase in WCP replacement ratio. These decrements were seen clearly, especially after the 20% replacement ratio. However, despite these decre- ments, the compressive strengths of all groups were found to be above 50 MPa. In ad- dition, it is thought that environmental pollution can be reduced by using WCP in geo- polymer composite mortars. A R T I C L E I N F O Article history: Received 7 August 2022 Revised 22 August 2022 Accepted 14 September 2022 Keywords: Waste concrete powder Geopolymer composite mortars Alkali-activated slag Compressive strength Flexural strength Ultrasonic pulse velocity 1. Introduction The developments in concrete technology and the rapid increase in the global population have also in- creased housing needs. As a result of this situation, ordi- nary Portland cement (OPC) has become the world's most widely used construction material. A high number of raw materials and energy are consumed during the produc- tion of OPC (Li et al. 2019; Tahwia et al. 2022, Toklu 2021). It is also stated that OPC production is responsible for 7% of the world's greenhouse gas emissions (Meyer 2009; Se- vim and Sengul 2021; Toklu and Şimşek 2018). These rea- sons have led researchers to work on developing more en- vironmentally friendly and alternative binders to OPC (Sahmaran et al. 2013; Mehta and Siddique 2017; Demir et al. 2018; Sevim and Demir 2019). Developed as an al- ternative to OPC, geopolymer binders have been studied with increasing interest in recent years, as they consume less energy and emit less CO2 compared to OPC (Provis 2014; Geraldo et al. 2017; Alhawat et al. 2022). It has been stated that geopolymers have many ad- vantages such as superior mechanical properties (Mahmoodi et al. 2022), high resistance to elevated tem- perature (Çelikten et al. 2020), acid (Thokchom et al. 2009), and sulfate effects (Bhutta et al. 2013), as well as environmentally friendly properties. Geopolymers are produced as a result of the chemical reaction between aluminosilicate sources (precursors) such as me- takaolin, blast furnace slag (BFS), fly ash (FA), and alka- line activators such as NaOH, Na2SiO3 (Rakhimova 2020; Öztürk and Atabey 2022). These aluminosilicate sources, widely used in geopolymers' production, were formerly seen as industrial by-products or waste. However, these products are no longer considered waste due to their successful and widespread use for years (Ulugöl et al. 2021). This situation has led researchers to investigate the use of different waste materials in geopolymer com- posites (Shoaei et al. 2019; Mahmoodi et al. 2021a). Construction and demolition waste (CDW), which is re- sponsible for most of the global solid waste production,