Hydration Process and Compressive Strength of Slag-CFBC Fly Ash Materials without Portland Cement Nguyen Tien Dung 1 ; Ta-Peng Chang 2 ; and Chun-Tao Chen 3 Abstract: This study mixed ground granulated blast-furnace slag (S) and circulating fluidized bed combustion (CFBC) fly ash (CA) without any portland cement or alkaline activator to produce an eco-binder, abbreviated as SCA binder . The hydration process, microstructure, and compressive strength of hydrated SCA materials were investigated. Although both the slag and CA had poor hydration with water, the SCA binder produced satisfactory hydration products with sufficient cementitious properties. These hydration products detected by Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) were ettringite (AFt), calcium silicate hydrate (C-S-H), and calcium aluminosilicate hydrate (C-A-S-H). The scanning electron microscope (SEM) micrograph showed these hydration products formed dense microstructure for SCA pastes. As a result, the SCA materials had sufficient compressive strength for practical applications in building ma- terials and civil engineering structures. The compressive strengths of the SCA paste and mortar reached 75 MPa at 28 days. Moreover, the SCA concretes met the requirements for structural concrete. An equation to predict the compressive strengths of the SCA concrete was proposed and agreed well with the experimental results. DOI: 10.1061/(ASCE)MT.1943-5533.0001177. © 2014 American Society of Civil Engineers. Author keywords: Hydration; Slag; Fly ash; Microstructure; Compressive strength. Introduction Circulating fluidized bed combustion (CFBC) technology is an advanced clean combustion technology in thermal power plants and has many advantages, such as high combustion efficiency, low combustion temperatures, and low NOx emissions (Armesto and Merino 1999; Guo Li et al. 2012a, b). Such a combustion pro- cess has two major byproducts, CFBC fly ash collected by either fabric filters or electrostatic precipitators, and CFBC bottom ash obtained from the bottom of boiler (Ghafoori and Mora 1998; Glinicki and Zielinski 2009). The CFBC ashes are very different from the conventional siliceous fly ashes from pulverized coals in terms of mineralogical composition, chemical composition, and morphology, because they are produced at the lower combustion temperature and a higher amount of limestone addition is used for sulfur removal (Sheng et al. 2007). The implementation of CFBC technology continuously increases due to its excellent envi- ronmental conservation and cost effectiveness so that the global amount of CFBC ashes will increase accordingly. Therefore, how to properly utilize and dispose the CFBC ashes has become a big challenge (Chi and Huang 2014). Several attempts to use CFBC fly ash (CA) in construction were reported. The dominant CA particles are coarse and irregular with a broad particle size range (Lecuyer et al. 1996) so that it has high demand of water and admixture (Fu et al. 2008). In addition, the high content of anhydrite in CA can result in excessive expansion in cement-based materials due to abundant ettringite formations (Bernardo et al. 2004; Sheng et al. 2007). Moreover, CA has less pozzolanic activity than the conventional fly ashes (Iribarne et al. 2001) and does not meet the requirements for ASTM C 618 (ASTM 2012) to be classified as either Class C or Class F fly ash (Glinicki and Zielinski 2009; Marks et al. 2012). As little as 5% CA is recommended to replace fine aggregates in roller- compacted concrete (Chi and Huang 2014). Thus, the utilization of CA is restricted in many applications where the pulverized fly ash often can be used (Sheng et al. 2012). As a result, it is mostly remained in landfills (Shon et al. 2010) which, however, also poses environmental problems, such as serious concerns over contamination of groundwater and surface waters, and the decreas- ing availability of landfill site (Wang et al. 2005; Fu et al. 2008). In addition, its highly exothermic reactions with water (Montagnaro et al. 2008) and high pH leachates and excessive expansion of solidified materials (Guo Li et al. 2012a, b) often induce high cost on the treatments and disposal. Several papers reported that CA had a self-cementing property (Sheng et al. 2007, 2012) so that it could set and harden in the presence of water. However, its self-cementitious strength was very weak even when the hydration of CA was enhanced by increased fineness (Sheng et al. 2007). Shon et al. (2010) also pointed out that the mixture of CA and pulverized fly ash could produce controlled low-strength material for backfills. Dung et al. (2014) reported that no-cement paste made by pure CA and pure ground granulated blast-furnace (GGBF) slag could produce compressive strength from about 40 to 75 MPa at the age of 28 days. This binder appa- rently had more advantages than other alternative binders to replace ordinary portland cement (OPC), such as geopolymers and super- sulfated cements with low cost and environmental impact. It used raw industrial wastes, while the geopolymers used an alkaline ac- tivator to initiate the reaction, and the supersulfated cements gen- erally used around 5% portland cement to dissolve and react with 80–85% GGBF slag and 10–15% anhydrite (hard burnt gypsum) 1 Ph.D. Candidate, Dept. of Civil and Construction Engineering, National Taiwan Univ. of Science and Technology (NTUST) (Taiwan Tech), Taipei 106, Taiwan (corresponding author). E-mail: tiendung568@ gmail.com 2 Professor, Dept. of Civil and Construction Engineering, National Taiwan Univ. of Science and Technology (NTUST) (Taiwan Tech), Taipei 106, Taiwan. E-mail: tpchang@mail.ntust.edu.tw 3 Assistant Professor, Dept. of Civil and Construction Engineering, National Taiwan Univ. of Science and Technology (NTUST) (Taiwan Tech), Taipei 106, Taiwan. E-mail: chuntaoc@gmail.com Note. This manuscript was submitted on March 17, 2014; approved on August 7, 2014; published online on September 19, 2014. Discussion per- iod open until February 19, 2015; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, © ASCE, ISSN 0899-1561/04014213(9)/$25.00. © ASCE 04014213-1 J. Mater. Civ. Eng. J. Mater. Civ. Eng. Downloaded from ascelibrary.org by on 10/27/14. Copyright ASCE. For personal use only; all rights reserved.