Arab J Sci Eng DOI 10.1007/s13369-015-1647-4 RESEARCH ARTICLE - CIVIL ENGINEERING Strength and Physico-chemical Characteristics of Fly Ash–Bottom Ash Mixture Nima Latifi 1 · Aminaton Marto 1 · Ahmad Safuan A. Rashid 1 · Jacky Ling Jia Yii 1 Received: 11 October 2014 / Accepted: 26 March 2015 © King Fahd University of Petroleum & Minerals 2015 Abstract The quantity of coal combustion products, partic- ularly fly ash (FA) and bottom ash (BA), has been increasing from coal power plants around the world. The major problem of a coal combustion-based power plant is that it produces huge quantities of solid waste. Recently, there have been efforts to use FA and BA together as a mixture in construc- tion works. This paper investigates morphology and chemical and strength characteristics of an FA–BA mixture for various curing periods. Scanning electron microscopy (SEM), X-ray fluorescence (XRF), and consolidated undrained triaxial tests were used to determine the physico-chemical characteristics of the mixture. Based on SEM results, it was found that, with an increasing ratio of BA to FA, the number of irregular par- ticles in the mixture increased. The results of XRF indicated noticeable changes in the surface composition of both FA and BA particles after mixing. The physico-chemical test results indicate the formation of a new gel form product in the mix- ture, which has been identified as calcium silicate hydrate (C-S-H). From an engineering point of view, the results indi- cated that the value of modulus of elasticity decreases with increasing BA content, from 30 to 70%, in the ash mixture. However, the increase in BA from 30 to 70% did not have any significant effect on the shear strength of the FA–BA mixture. Keywords Fly ash (FA) · Bottom ash (BA) · Morphology · SEM · Modulus of elasticity B Nima Latifi En_Latifi@yahoo.com 1 Geotechnic and Transportation Department, Faculty of Civil Engineering, Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia 1 Introduction The major problem of coal combustion products (CCPs) is the production of large quantities of solid waste. The CCPs include silica (61 %), alumina (22 %) and iron oxide (7 %), which make up to 90 % of the ash. The other elements are calcium (Ca), magnesium (Mg), sodium (Na), potassium (K) and sulphur (S) [1]. The quantity of CCP, particularly fly ash and bottom ash, has been increasing from the coal power plants around the world. The ash collected at the bottom of the boiler—the so-called bottom ash—is of coarse grain size, unlike the fine-sized fly ash [2]. Fly ash and bottom ash being industrial wastes, if not utilized suitably, have to be disposed off in landfill, ponds or rivers. However, since they are produced in large volumes as a residual matter, their disposal may result in serious environmental problems. For example, India produces about 100 million tons of Portland cement annually. Nevertheless, it also generates 100 million tons of fly ash annually [3]. In recent years, the cost of disposing of CCPs has been a cause for concern. Hence, engineers are forced to seek alternatives to take advantage of these coal solid wastes. Coal-burning power plants produce a large vol- ume of coal ash and bottom ash [4]. Most of the CCPs are disposed off as slurry into ash ponds, owing to low operation costs and easy operation. The solid CCPs are collected and mixed with water, and the slurry is pumped through a series of pressurized pipes into ash ponds [5]. In recent years, some studies have been conducted to iden- tify the CCPs’ physical and chemical properties [6, 7]. The chemical composition in fly ash depends mainly on the chem- ical properties of the coal burned. In addition, it is also affected by the grinding equipment, furnace, combustion process, and nitrogen oxide control equipment used. The key factor in determining its potential use in manufactur- 123