We’re all familiar with concrete, even if we don’t know what it is. It’s the gray stuff that our roads and sidewalks and buildings are made of. But what exactly is concrete? And how does it work?
Concrete is a mixture of cement paste and aggregates, such as sand and gravel. When mixed with water, these ingredients combine to form a slurry that hardens over time. Concrete can be molded into any shape, so it can be used for a broad range of applications from roads to skyscrapers to sidewalks. Its wide availability and low cost make concrete one of the most commonly used materials in the world.
The cement paste is the key ingredient that binds everything together. It is composed primarily of two chemicals: calcium oxide and aluminum oxide, which are also known as lime and silica, respectively. However, these chemicals do not exist in their pure forms so they must first undergo a chemical reaction that converts them into clinker – the main ingredient in cement – through a process called calcination (1). The clinker then reacts with gypsum to form cement (2).
Hydraulic cement is a material which sets and hardens to adhere to other materials and resist water. Hydraulic cements (e.g., Portland cement) are usually made of a mixture of calcium silicates and other hydraulic materials.
The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water-soluble. These hydrates are called hydraulic because they are hardened or set by hydration, forming a rocklike mass, and will bond well with other materials.
Most hydraulic cements derive their strength from the chemical reaction between the cement powder and water, known as hydration. Hydration products cause the hardening of concrete through a chain of chemical and physical processes that occur within the mineral assemblage of the cement paste matrix. These processes include crystallization, precipitation, hydration reactions, diffusion, dissolution and precipitation of dissolved ions and gases, mechanical straining and restructuring, consolidation by capillary action and/or by pressure from within or without, shrinkage and swelling due to temperature changes or moisture variations, carbonation by atmospheric gases, silicate polymerization reactions at high temperatures in fire or furnace environments, or oxidation at elevated temperatures in air.
Hydraulic cement is a type of cement which hardens when water is added. Portland cement is a well-known example, and it is the most common type of cement used in construction around the world.
There are four types of hydraulic cements: portland cement, slag cement, fly ash cement, and silica fume cement (which have varying advantages over portland cement).
The chemical reaction that creates hydraulic cement begins when you add water to dry powdered materials. The water causes the powdered materials to chemically react and become a paste. The paste then hardens over time – this is what we generally refer to as concrete or mortar.
The advantage of hydraulic cement is that it can set underwater, or when water is added to the dry material in wet conditions. Another property of hydraulic cements is that they harden by reacting with carbon dioxide in the air – this causes them to cure rather than just dry out like other adhesives.
Portland cement is the most common cement used in concrete. It is made from a combination of calcium, silicon, aluminum and iron oxides. When mixed with water, these materials combine chemically to form hydrated cement paste that hardens over time through a process called hydration. The water reacts with the cement and other ingredients to form a hard matrix that binds the materials together into a durable stone-like material that has many uses.
Hydraulic cements (e.g., Portland cement) are made of a mixture of silicates and oxides, the four main components being:
1) calcium oxide (CaO), commonly known as lime;
2) silicon dioxide (SiO2), commonly known as silica;
3) alumina (Al2O3); and
4) iron oxide (Fe2O3).
In the manufacture of Portland cement, clinker occurs as lumps or nodules, usually 3 millimetres (0.12 in) to 25 millimetres (0.98 in) in diameter, produced by sintering limestone and alumina aluminosilicate materials such as clay during the cement kiln stage.
Hydraulic cement (non-hydraulic cement will not set under water) is made of materials which are found in common alluvial soils: limestone and clay. The raw materials are readily available worldwide, the clinker is ground into a fine powder and mixed with gypsum to control the setting time.
Portland cement is an artificial mixture of lime and silica made by heating limestone and clay minerals in a kiln to form clinker, grinding the clinker, and mixing with small amounts of other materials. The greenish gray powder is composed primarily of calcium silicates, calcium aluminates, and calcium ferrites. When mixed with water, portland cement reacts chemically, binds together the aggregate particles, and eventually hardens into a rocklike mass.
The word “portland” refers to the resemblance of its color to Portland stone when first made. Natural cements were used in Europe for more than 2000 years before portland cements were developed in 1824.
Hydraulic cement was developed in 1824 by Joseph Aspdin of Leeds, England.
Aspdin named the cement Portland Cement because it resembled natural stone from the Isle of Portland.
Portland cement is a finely ground powder that binds sand and gravel together to make concrete. It will harden when mixed with water and resist heat, chemicals, and the weather. It is the predominant building material used worldwide for its strength and durability.
Joseph Aspdin was not the first to produce cement, but he was the first to develop an improved manufacturing process for producing a stronger product than his competitors. Aspdin’s innovation was to use powdered limestone mixed with clay and heated in a furnace until it formed clinker. The clinker was then ground into a fine powder to make hydraulic cement.
The patent documents indicate that Joseph Aspdin learned how to make hydraulic cement while working as a bricklayer in Wakefield, Yorkshire. In the early 1800s he worked at least three years in Wakefield as a “bricklayer and plasterer.” He then moved to Leeds where he set up his own business as a builder and plasterer about 1810 or 1811.[4] The move from Wakefield to Leeds was made easier because Asp
Hydraulic cement, on the other hand, depends upon a chemical reaction between water and calcium silicates to form a substance that is insoluble in water. When hydraulic cement is mixed with sand and gravel, the resulting product is called concrete. The chemical reaction between the cement and water is known as hydration. The best known of the hydraulic cements are portland cement, natural cement, and slag-lime cements.
Portland Cement
The most common of all hydraulic cements is Portland cement. It was first manufactured in England in 1824 by Joseph Aspdin of Leeds; he named it Portland cement because it resembled stone quarried on the Isle of Portland in Dorset County, England. The most important ingredient in Portland cement is lime. In 1843 Isaac Johnson patented a process for improving lime by burning it in kilns where the fuel was fed separately from the limestone, thus producing a more uniform product and increasing its strength. This process enabled manufacturers to produce Portland cements that were stronger than natural cements. By 1915 about 90 percent of the portland cements made in the United States were manufactured using Johnson’s process.