Limestone was the raw material for making cement in ancient times. Limestone is a stone formed by marine life millions of years ago, which is heated to about 1,000 degrees Celsius, separating carbon dioxide and leaving behind calcium oxide, or quicklime. Quicklime is ground into powder and then watered, and the quicklime undergoes an exothermic reaction to produce calcium hydroxide, which is then poured into a mold that can slowly absorb carbon dioxide from the air. As the water evaporates, it becomes the original carbonated ash.
However, the disadvantage of this cement was that it was very afraid of water, so the ancient Romans improved cement. They added volcanic ash to the limestone. The Romans used this cement to build the first unreinforced concrete dome, a pantheon that stood for more than 2,000 years. The point is that Roman cement can be hardened and solidified underwater, and docks can be built directly underwater. Due to the addition of volcanic ash, the performance of Roman cement is very good.
Later, it was discovered that volcanic ash could be replaced with clay and shale, as these raw materials contained silica, which changed the chemical properties of the cement. This is the recipe for modern cement, known as Portland cement, which was invented in the 40s of the 20th century.
Limestone is crushed, shale or clay is added, ground into powder, and then placed in a kiln to heat at high temperatures, and fired into this hard clinker block at high temperature. The clinker is ground into powder for cement, which is not used separately and is the main raw material of concrete.
In the concrete production process, a mixture of aggregate, gravel and sand is prepared and finally put into cement. After adding water, the cement powder begins to dissolve in the water, and a large amount of calcium hydroxide turns the water into a strong alkaline solution. At this point, the main component in the solution is tricalcium silicate, which will continue to react with water and then slowly condense into a crystal shape. As the crystals are staggered and denser, they eventually lead to hardening of the concrete.
In this process, water is essential for the formation of these crystals. In fact, the water does not evaporate, it is the water molecules that become part of the solid concrete after the chemical reaction, so it can be poured directly under the water, this chemical reaction is called cement hydration, and the hydration reaction accelerates the static, which is why the concrete car needs to be constantly rolling.
Both aggregate and crushed concrete require specific dimensions, and high-quality river sand is key to improving the strength of concrete. The concrete used on viaducts and floors must be made of lighter aggregates. The aggregates of the concrete plant must be screened and weighed, and proportionally blended according to different uses, and the blending ratio of each raw material is controlled by a computer.
The pressure test proved that the strength of concrete after adding aggregate and sand is much higher than that of pure cement. The strength of concrete varies according to the proportion of raw materials, one ton of cement can mix 5 tons of concrete, which can be made into any shape, which is not only durable, low cost, but also easy to produce, consuming 24 million tons of barrels per year.
Cement is the most used material in the world each year, and its consumption is far greater than all other materials combined.