**Abstract**

This paper presents a review on fly ash as prime materials used for geopolymer. Due to its advantages of abundant resources, less in cost, great workability and high physical properties, fly ash leads to achieving high mechanical properties. Fly ash is considered as one of the largest generated industrial solid wastes or so-called industrial by-products, around the world particularly in China, India, and USA. The characteristics of fly ash allow it to be a geotechnical material to produce geopolymer cement or concrete as an alternative of ordinary Portland cement. Many efforts are made in this direction to formulate a suitable mix design of fly ashbased geopolymer by focusing on fly ash as the main prime material. The physical properties, chemical compositions, and chemical activation of fly ash are analyzed and evaluated in this review paper. Reference has been made to different ASTM, ACI standards, and other researches work in geopolymer area.

**Keywords:** fly ash, physical characteristics, specific gravity, particle shape, chemical activation, workability

### **1. Introduction**

The production of ordinary Portland cement (OPC) contributes approximately 7% to the total of global greenhouse gas emissions; this is considered as a serious problem for the environment [1, 2]. Recently, much research has been done to explore an alternative product which could replace OPC. Geopolymer was firstly proposed by Davidovits [3] as an alternative binder to OPC. Geopolymer is defined as a chemical reaction of aluminosilicate compounds, which have geological origins, such as clay and metakaolin or from industrial by-products, for example, fly ash (FA) and ground-granulated blast furnace slag (GGBFS).

The collection of FA from the coal-fired power plants is based on different equipment and filtration processes. The source and the structure of burned coal significantly impacted on FA compounds. But, generally, all types of FA contain a huge amount of silicon dioxide (SiO2) (both amorphous and crystalline), aluminum oxide (Al2O3), and calcium oxide (CaO) [4]. The typical chemical composition of FA which is obtained from Manjung power plant at Perak in Malaysia is presented in **Table 1**. It is classified as class F fly ash, which will be detailed in the next section. FA with a high percentage of silica (SiO2) and alumina (Al2O3) (more than 80%)


#### **Table 1.**

*Chemical compositions of FA [5].*

could be appropriate in the production of geopolymer as a raw material. According to Davidovits [3], geopolymers are classified as binder materials, which could be formed by the activation of aluminosilicate with alkaline solutions. The term "geopolymer" was first introduced by Davidovits and also well known as inorganic polymers or alkaline-activated binder material [4]. This review paper evaluates the significant characteristics of FA and its advantages as raw materials in geopolymer cement and concrete.
