The properties of hardened concrete containing fly ash are improved in comparison to pure cement. Fly ash is a by-product of electric power generating plants, and is composed principally of the oxides of silicon, iron, calcium and aluminum. When finely divided it is carried by flue gases and collected in electrostatic precipitators or mechanical filters. It has been used in concrete since the 1930’s.
Fly ash reacts with the hydrated lime at the cement-water interface to form additional cementitious products. The required amount of water for hydration remains unaffected; however, the water demand for workability increases due to the fineness of fly ash. The increase in water demand can be compensated by using Superplasticizers.
The chemical composition and morphology determines the effectiveness of fly ash as a pozzolanic material. Higher contents of CaO, SiO2 and Al2O3 results in better performance as pozzolanic material. The overall fineness also directly influences its reactivity and thus the strength development at early ages.
The use of fly ash in concrete dates back to the 1930s, and its first use as a substitute for Portland cement was in California in 1935. By then, the properties of concrete containing fly ash had been well documented. For example, in 1936 the U.S. Bureau of Reclamation reported that “the addition of fly ash to concrete tends to increase the workability of plastic mixes without increasing the water content and reduces shrinkage, segregation, permeability, bleeding and cracking.”
The benefits of using fly ash were further enhanced when it was discovered that concrete made with fly ash required less water than concrete made with Portland cement alone. This is because fly ash particles are much finer than portland cement particles and can fill voids in a mixture more completely. In addition, the pozzolanic reaction between fly ash and calcium hydroxide results in denser microstructure and less permeability.
Although there are many advantages to using fly ash in concrete mixtures, some challenges must be addressed before fly ash can be used successfully. The most significant of these is variability. Fly ash from different coal sources or from different combustion conditions within the same plant will have different properties and can affect the performance of concrete differently.
Concrete is the most widely used man-made product in existence. It is second only to water as the most-consumed resource on the planet. But, unlike other common materials such as steel, wood and aluminum, concrete has a limited natural lifespan and will degrade over time.
Despite this fact, billions of tons are produced every year. Why? Because we have become very good at making it. In recent years, a growing body of scientific research has helped us learn how concrete becomes stronger and more durable. This knowledge has led to significant improvements in the quality of concrete used in construction around the world. Understanding how concrete works also allows us to make it better using a variety of performance enhancing additives known as admixtures.
One of those admixtures is fly ash, a byproduct from coal-fired power plants that has been used for decades in concrete to improve its final properties. In fact, fly ash is currently used in about 50 percent of all ready mixed concrete produced in the U.S., making it the second most commonly used supplementary cementitious material (SCM). While many people know what fly ash is and that it’s used in concrete, there still remains a great deal of confusion about how it actually works and how its benefits can be maximized during
Concrete is a mixture of water, aggregates (natural or artificial), cement, and sometimes admixtures. The word “cement” is often used interchangeably with the word “concrete,” but there is a difference. Cement is the paste that binds together the aggregates to form concrete. Concrete can be thought of as a man-made rock.
The cement, which is one of the constituents of concrete, is a hydraulic binder, i.e., it sets and hardens when water is added. The most common hydraulic binder in use today is Portland cement, named for its resemblance to Portland stone (a limestone found on the Isle of Portland in southern England). Portland cement consists primarily of finely ground clinker—a mixture of calcium silicates and aluminates—that may contain specified amounts of other ingredients such as gypsum and fly ash.
The cement industry has for some time been seeking alternative raw material for the Portland cement clinker production. The aim of this research was to investigate the possibility of utilizing iron ore tailings (IOT) to replace clay as aluminous material in the production of Portland cement clinker. For this purpose, two kinds of clinkers were prepared: one was prepared by IOT; the other was prepared by natural clay as aluminous material. The chemical compositions, mineralogical phases, and particle size distributions of raw materials were characterised by X-ray fluorescence, X-ray diffraction and laser diffraction techniques. The grindability indices (Bond work index) of raw materials and clinkers were determined using a Bond ball mill. The mineralogical phases and hydration properties of clinkers were evaluated using X-ray diffraction and isothermal conduction calorimetry methods, respectively. According to the results, IOT can be used as an alternative replacement for clay in order to produce Portland cement clinker. Moreover, IOT can be used in low percentages (e.g., 10%) to partially replace clay for producing Portland cement clinker with lower energy consumption due to its higher porosity and thus higher grindability index values obtained from Bond
Reinforced Concrete
Reinforced concrete is a composite material, and the average density is considered to be 150 lb/ft3. It has the properties desired for a construction material: strength, stiffness, and durability. These are provided by the use of steel rebar and concrete. The concrete resists compression well, but it has very little tensile strength. The steel rebar provides tensile strength to the overall structure. The steel and concrete work together in a bond that allows each material to work at its strengths.
Structural engineers can design reinforced concrete structures that can withstand loads imposed by earthquakes, high winds, fires and other forces. Reinforced concrete has been used in bridges and buildings since the beginning of the 20th century, but it was not until after World War II that widespread acceptance of reinforced concrete came into use in building construction.
In the early 20th century, reinforced concrete was widely used in large buildings, bridges and water tanks. Due to the expansion of mass production in the construction industry, it has become one of the most widely used building materials. Its usage in buildings began with the use of reinforced concrete frames and slabs, then later to the floor slab and beam structure. The application of concrete structures is not limited to civil engineering; it is also widely used in prestressed concrete structures and long-span space structures. With the development of science and technology, more advanced reinforced cement concrete structures have been built.
The basic idea of reinforced cement concretes is based on a combination of tensile strength (steel) and compressive strength (concrete). The steel bars are placed within the concrete to resist tension, while the concrete holds back compression. Reinforced cement concretes are always referred to as composite materials because they combine two different materials (reinforced concrete). Concrete is an excellent material to resist compression, but it is weak in tension. Steel can easily resist tension, but it will rust when exposed to high humidity for a long time. Therefore, steel requires a protective layer of paint or plastic to prevent corrosion from occurring. Steel bars are placed inside the concrete so that they