# Effect of Curing Conditions on Microstructure and Pore-Structure of Brown Coal Fly Ash Geopolymers
## 1 Introduction  
Conventional concrete produced by using Portland Cement (PC) as the main binder releases between 0.7 to 1.0 kg of CO\({}_{2}\) per 1 kg of cement production [1; 2], and as a result, is responsible for between 5 to 7% of anthropogenic CO\({}_{2}\) emissions [1; 2] which contributes to the global warming. Thus, cement usage for concrete production is not sustainable as the production process is energy intensive and expensive. The Asia-Pacific population has grown to reach nearly 4.5 billion, while urbanization in most of the region is increasing. In response to this growth, the number of power plants across the region has increased significantly, bringing power to growing populations and industrial centers. In addition, the Asia-Pacific region's electricity is largely derived from coal, which represents over half of the power mix [3]. Hence, the current emphasis is on developing novel low-CO\({}_{2}\) binders, such as geopolymers, to meet the demand for concrete. Geopolymer concrete can be manufactured using a wide range of aluminate-silicate source materials, including different types of fly ash, blast furnace slag and metakaolin [4; 5; 6; 7; 8; 9]. There are two types of fly ash readily available, high-calcium (class C) and low-calcium (class F) fly ash [10]. The class F fly ash is produced from burning anthracite and bituminous coals, whereas class C fly ash isproduced from subbituminous and lignite coal. Due to the high aluminosilicates and low calcium, Class F fly ash is the preferred material for the production of geopolymer concrete. In Australia, lignite coal is referred to as brown coal, whereas in Europe, both subbituminous coal and lignite coal are called brown coal [11]. The state of Victoria in Australia utilizes approximately 70 million tonnes of brown coal deposits mined from the Latrobe Valley in order to satisfy nearly 90% of the annual electricity consumption. The annual worldwide production of brown coal was estimated to be approximately 950 million tonnes in 2008, and Australia's brown coal production is responsible for 9% of the total production [11]. Further, Australia's recoverable brown coal of 44.2 billion tonnes is approximately 19% of the world's recoverable resources, with 93% deposited in the Latrobe Valley, Victoria [12]. At present in Australia, brown coal fly ash is not used as a binder in concrete due to the composition of the coal. As brown coal fly ash has a high sulphur content (above 5%), this means it cannot be classified as either Class F or Class C according to ASTM C618 [10], resulting in large quantities of brown coal fly ash deposited in storage lagoons with subsequent environmental costs [13; 14]. The suitability of brown coal fly ash for geopolymer production has been investigated by [PERSON] et al. [15] who reported that the maximum 28-day compressive strength of the geopolymer mortar obtained was 57 MPa, which is comparable to a low-calcium (Class F) fly ash-based geopolymer. The leaching of heavy metals in blended brown coal fly ash-metakaoin geopolymer binders was examined by [PERSON] et al. [16], however, they did not expand this investigation to assess the engineering properties. The results showed that the geopolymer was effective at reducing the leaching rates of calcium, arsenic, selenium, strontium and barium. [PERSON] et al. [6] developed a blended brown coal fly ash geopolymer with low calcium fly ash and blast furnace slag. The study showed that brown coal fly ash with higher aluminium content improved the rate of reaction of the geopolymer. [PERSON] [17] reported that the geopolymers are ceramic materials that are produced by alkali activation of aluminosilicate raw materials, which are transformed into reaction product by polymerisation in a high pH environment and hydrothermal conditions at relatively low temperatures.