Today's concrete is a powerful building material, made from a combination of Portland cement, coarse and fine aggregates of stone and sand, and water. This paste binds together “aggregates” - pieces of rock and sand - to create a strong, durable material. Contrary to popular belief, concrete is not the same as cement; cement is an ingredient used to make concrete. Cement is made from limestone and clay, and when combined with other ingredients, it melts at dizzying temperatures.
Researchers led by geologist Marie Jackson of the University of Utah have been studying the crystalline structure of Roman concrete for years, and have now accurately mapped how this ancient material solidifies over time. Mix design and cement information are important basics to consider when working with concrete. Most of the concrete is poured with reinforcing materials (such as reinforcing bars) embedded to provide tensile strength, resulting in reinforced concrete. The hydration process - which increases strength - occurs during the first month of the concrete life cycle, but continues at a slower rate for many years.
To speed up this process, steam is often used to heat the poured concrete and keep it moist. Portland cement, the most commonly used type in concrete, is made from a combination of a calcareous material (usually limestone) and silica and alumina found in the form of clay or shale. Polymer concretes have advantages over cement-based concretes; they have significant tensile strength even without reinforcement and are largely impermeable to water. To ensure complete hydration during curing, concrete slabs are often sprayed with curing compounds that create a water-retaining film on the surface.
If the surface of the concrete pour is isolated from outside temperatures, the heat of hydration will prevent it from freezing. Concrete manufacturers mix these ingredients in the right quantities to make one of the strongest building materials in the world. One cubic meter of waste lightweight concrete contains 1.1-1.3 m3 of crushed waste and no other aggregates. Excess bleeding water can delay the finishing operation and cause serious problems with the concrete surface. The NIST Building Technology Center is also conducting research to improve concrete performance.