How was cement discovered? What are the raw materials of cement? What is the structure?

After a volcano erupts, it will produce a lot of volcanic ash. These pozzolans condense when they meet water and have good strength and water resistance. Volcanic ash is the earliest cement. 1756, when British engineer J. Smeaton was studying the hardening of some lime in water, he found that the most ideal components were hydraulic lime and pozzolan, which was the earliest cement theory. In 18 13, Gaby, a French civil engineer, found that the cement made by mixing lime and clay in a ratio of three to one has the best performance. 1824, British construction worker Joseph asp Ding invented cement and obtained the patent right of portland cement. He used limestone and clay as raw materials, mixed them in a certain proportion, calcined the mature materials in a vertical kiln similar to lime burning, and then made cement by grinding.

Limestone and clay are the main raw materials for producing portland cement, and the main chemical components are calcium oxide CaO, silicon dioxide SiO2, ferric oxide Fe2O3 and alumina Al2O3. There are two main methods to produce cement, wet method and dry method. There are mainly two kinds of equipment for calcining clinker: vertical kiln and rotary kiln. Vertical kilns are suitable for small factories, and rotary kilns are suitable for large and medium-sized factories. The specific gravity of standard cement is 3. 1, and the bulk density is usually 1300kg/m3. According to the strength grade, it is divided into four grades: 32.5, 42.5, 52.5 and 62.5. Grade 32.5 cement is mainly used as stabilizer for low-strength cement, grade 52.5 cement for high-strength concrete and grade 62.5 cement for ultra-high-strength concrete. At present, grade 42.5 cement is the most widely used cement. The production and inspection standard of cement is Ordinary Portland Cement (GB 175-2007).

Cement can't be kept for long. Usually it will be used within one month after leaving the factory. If it is not used for three months, it will degenerate. If it is not used for six months, it must be scrapped and cannot be used for engineering construction. After mixing cement and water, a series of chemical reactions will occur, and a lot of heat will be released during the chemical reaction, and this chemical reaction lasts for a long time. For mass concrete, low-calorie cement must be used to reduce the generation of hydration heat; Usually, fly ash is used to replace part of water conservancy for mass concrete to reduce hydration heat. The concrete gravity dam will also install cold water pipes inside the concrete to reduce the internal temperature of the concrete by circulating cold water. Mass concrete is prone to cracks, the main reason is that the temperature difference between inside and outside of concrete is large, and the temperature stress leads to concrete cracks.

Cement concrete has high compressive strength and low tensile strength, so some steel bars are usually added to the concrete structure to increase the tensile strength of concrete. Beam-slab structure bears large tensile stress, so both beams and slabs are reinforced; Columns are mainly compressed, but if the columns are too long, they will be subjected to eccentric tension, so steel bars will also be arranged in the columns. The main girder structure of the bridge will also exert prestress on the steel bars to resist the tensile stress generated by the beams. Cracks in reinforced concrete are usually caused by tensile stress. Once cracks appear, internal grouting and external reinforcement are needed to ensure structural safety. For underwater concrete, there is also special corrosion-resistant cement, that is, high sulfur-resistant cement; In order to increase the impermeability and frost resistance of concrete, additives such as air entraining agent and water reducing agent will be added to concrete.