SINERGI Vol. 28, No. 1, February 2024: 23-30 http://publikasi.mercubuana.ac.id/index.php/sinergi http://doi.org/10.22441/sinergi.2024.1.003 R. S. Edwin et al., Effect of mixing procedure and steam curing on the properties of … 23 Effect of mixing procedure and steam curing on the properties of Class C fly ash-cement based Geopolymer Romy Suryaningrat Edwin, Sulha Sulha, Fitriah Masud, Wayan Mustika Department of Civil Engineering, Universitas Halu Oleo, Indonesia Abstract This work investigated the performance of class C fly ash-cement- based geopolymer paste with different mixing procedures and under steam curing. The geopolymer paste containing class C fly ash was mixed with cement up to 20% at 10% increments. The alkaline activator was prepared 24 hours before mixing by blending Na2SiO3 and NaOH 14 M with a ratio of 2:1. Two methods of mixing procedure were used in this study to mix the geopolymer pastes. Steam curing at 90 ± 5  C for six hours was applied. Setting time, compressive strength and strength activity index were used in this research for performance analysis. According to the results, the setting time of geopolymer paste containing 100% C type flash ash is delayed by the use of method 2 in the mixing process. While, adding cement content speeds up both the initial and final setting times of fresh geopolymer pastes. Additional water at the early stage in the mixing process (method 2) generated excellent polymerization at longer curing periods. In contrast, the potentially agglomeration of fly ash and cement in the polymer matrix was observed after applying method 1, which is the reason for the lower strength of geopolymer paste. The strength activity index (SAI) of geopolymer paste containing cement increased at early curing periods for all mixing methods. The early-stage benefits of steam curing are achieved in geopolymer pastes containing 0% cement when mixed using techniques 1 and 2. This is an open access article under the CC BY-SA license Keywords: Class C Fly Ash; Geopolymer; Microstructures; Steam Curing; Strength Activity Index; Article History: Received: April 27, 2023 Revised: July, 6, 2023 Accepted: July 7, 2023 Published: February 2, 2024 Corresponding Author: Romy Suryaningrat Edwin Department of Civil Engineering, Universitas Halu Oleo, Indonesia Email: romy.edwin@uho.ac.id INTRODUCTION In recent decades, the tremendous production of Ordinary Portland Cement is facing a massive shortage of raw materials and depletion of fossil fuels, which will need a million years to renew [1]. Simultaneously, releasing an enormous amount of CO2 emission into the atmosphere from clinker calcination of cement production will soon lead to faster global warming. In the meantime, the production of fly ash from coal-fired power plants still increases, which needs a large stockpile area for storing the fly ash. Annually, fly ash production in Indonesia reaches almost 9 million tons, and only 10-12% is used for construction projects. These numbers imply that about 90% of fly ash has not been used, possibly resulting in ecological deterioration caused by noxious components present in fly ash. Therefore, utilizing fly ash on a large scale for construction projects is a key to preventing environmental degradation and saving energy shortly [2][3]. Most researchers reported that the use of class F fly ash in geopolymer concrete obtained a higher compressive strength than class C fly ash due to the higher content of SiO2 in class F fly ash. The most difficult problem will be the endeavor to improve class C fly ash-based geopolymer performance. Therefore, several researchers used cement and slag to improve the strength of geopolymer as reported by [4][5]. In literatures, most of the researchers study the mix design of geopolymer including water-to-geopolymer solid ratios and alkaline-to-