AbstractConcrete can be made without cement by using fly ash as an alternative to cement. Concrete is the common material in construction due to its properties in stress and ammonia and it is readily available. Cement is an excellent material, but its production requires a huge amount of energy that causes pollution and global warming. The process of cement production begins with the extraction of raw materials by mining, grinding, mixing and exposing them to the high temperature of up to 15000C and finally the production of cement from those materials. All cement manufacturing processes require a huge amount of energy that requires high cost and massive emissions of carbon dioxide and other greenhouse gases. Cement production contributes to the emission of 7% of greenhouse gases.As a result of the increasing infrastructure, demand for cement is increasing, so greenhouse gases cannot be controlled to prevent global warming.A clean alternative to cement is the use of fly ash, which has the same properties as physical and chemical cement. Fly ash is produced from thermal power plants, where it is the waste of these plants and has no use. The use of fly ash reduces energy demand for cement plants, minimizing the environmental impact of dumping of waste from cement plants and thermal power plants.The objective of this paper is to discuss the characteristics of Geopolymer concrete, how it differ from Ordinary Portland cement properties, and the properties of fly ash strength based on Geopolymer concrete and its ability of being used in Saudi Arabia as a sustainable construction material.Key words: Flyash; strength, Geopolymer, durability.1.0 Introduction:Concrete is one of the most widely used products in the world, and comes second only to water. The greater the demand for concrete as a building material, the greater the demand for Portland cement. Portland cement production generates high levels of carbon dioxide, one of the main causes of global warming. Actual production of Portland cement is estimated to generate at least 13.5 billion tons of carbon dioxide per year, equivalent to 7% of the total global CO2 emissions in the atmosphere. Researchers in King Abdul Aziz City have found a more environmentally friendly alternative material; The geopolymer. Geopolymer is an amorphous aluminum silicate. It can be manufactured from the multiple condensation reaction of the primary material of polymers with alkaline polycrystallines. For example, it can be made from waste materials such as fly ash. Because it is made from waste materials, it has no independent industry, and therefore does not directly contribute to the emission of carbon dioxide. Its production requires much less energy than the process required for the cement industry. The advantages of the geopolymer are not limited to the suitability for the environment and the low production capacity compared to cement, but it’s characterized with relatively higher strength, excellent stability in volume, and more durability, thus generally having a better performance than cement as a building material.’ We will seek to safeguard our environment by increasing the efficiency of waste management, establishing comprehensive recycling projects, reducing all types of pollution and fighting desertification.’1 One of the goals of the 2030 Saudi government vision to is establish more environmentally friendly projects and minimize the pollution resources. This research is studying the ability of using Geopolymer Concrete, which Fly ash is one of its sustainable materials, as an alternative material to Portland cement Concrete.2.0 Methodology Searching and collecting information from previous experiments and researches on fly ash .Analyzing a case study for projects that have similar weather and economical conditions of Saudi Arabia.3.0 Cement production:One of the major problems the world is facing is global warming, which is a result of harmful actions towards the environment. Some of these actions include depletion of energy and emission of large amounts of carbon dioxide. Theses things affect the OZONE layer in the atmosphere negatively, which push people to begin to search for solutions to conserve energy and solve the problem of global warming.Concrete is a material that is used widely in construction works. One of the main contents of concrete is cement. Production of cement requires large amount of energy. It also emits great quantities of carbon dioxide in the atmosphere during its production process. Cement consists of two main materials: the marl and the limestone.There are two materials added to it depending on the nature of the facility: the sand and the iron. While chemical composition of cement consists of four oxides: silicon dioxide ( SiO2 ), Aluminum Oxide ( Al2O3 ), Calcium oxide ( CaO ), iron oxide ( Fe2O3 ). When grinding the raw material of cement no chemical reactions occur, but the milling process is used to increase the area of the particle surface, so that it is ready to enter the chemical reactions. After the raw material is removed from the mill it is entered into the storage and the most important step in this stage is flipping the raw materials. This stage is followed by a chemical reaction phase, which is done in a huge building called the tower, which consists of two huge tanks one of them with a flame, and these two tanks are connected to each other. The raw material are entered in the kiln, which is composed of a large, slightly tilted cylinder that revolves around itself. At the end of the cylinder there is a huge flame. The four oxides coalesce into the kiln, forming the so-called clinker phases, which are raw materials in the cement industry. This stage need a huge amount of energy reached nearly to 1500 ° C and produce a huge amount of chemical reactions.So,one of the clean solutions alternative to cement is using the fly ash, which has the same properties of cement. The use of fly ash reduces the use of energy, minimizing the environmental impact of dumping the waste of cement plants and thermal power plants. 4.0 HypothesisAssuming that the Fly ash material based on geopolymer concrete is a sustainable construction material could be used in Saudi Arabia, and using it instead of the cement in the concrete mix to increase the concrete strength without hurting the environment .5.0 FlyashFly ash is one of the most widely used pozzolan materials in the world. They are silica, or silica and alumina together, which do not have concrete properties in themselves, but in the presence of water react with calcium hydroxide at normal temperature to give compounds possessing cement properties.As a result of the burning of coal in the energy production furnaces, carbon is consumed, and soft metal grains rich in silica, alumina and calcium remain. These grains harden in the form of very fine glass balls, collected from the exhaust pipe of the oven before Fly away so this product is called fly ash.5.1 Mechanical properties of fly ash:Fly ash granules are very smooth so they are very effective in filling concrete voids, improving the operation of concrete mix.Fly ash granules are solid and round, making it a spherical behavior that allows the production of concrete by using less water, which improves the operation of the concrete mixture. (Fly ash can reduce 1-3 gallons of water per cubic meter of concrete.5.2 Chemical properties of fly ash:When hydrating the cement, calcium hydroxide is formed. About 20% of the weight of the cement is converted to calcium hydroxide, which does not gain concrete the resistance property.When the fly ash is added, it reacts with calcium hydroxide (lime) to form a cement compound (calcium silicate hydrate), which is one of the most important components of concrete, which gain it strength and durability over time.5.3 Physical Properties of Flyash: Color: Flyash derives its color mainly from Carbon and iron. High carbon content changes the Color to grey or black and high iron content gives tin colored ash (Figure 1). Fineness: Fineness of flyash is expressed in terms of specific surface area, which varies from 2×107 to 5×107 N/mm2. Greater fineness of flyash increases the air entraining admixture demand. The large amount of surface energy is utilized to combine with lime. The fineness of the flyash used as a percentage by weight retained on 90 micron sieve is 9%.Shape: Flyash is basically spherical in shape and its size ranges from about 150 Micron to less than 1mm.Figure (1). Typical ash colors Portland cement(PC) + Water(H2O) = Calcium Silicate Hydrate(CSH)(Durable Binder)+ Free Lime(CaOH)(Non-Durable Binder)Portland cement(PC) +Fly Ash(FA)+ Water(H2O)= Calcium Silicate Hydrate(CSH)(Durable Binder) Through the above equations we find that fly ash is reacting with lime to give the same concrete compound as a cement emulsion.