Concrete, this word is very interesting, even if you don't know you should be able to guess what it means, concrete is composed of three parts: people, work, and stone, which means artificial stone. The concrete character was coined in 1953 by Professor Cai Fangyin, a famous structural scientist, who is a shorthand for concrete. Concrete is a large artificial stone formed by the slow reaction of cement, sand and gravel mixed with water, which has similar properties to natural stone, and is strong and hard.
Cement is the most important artificial building material in modern times, and its origin is that in the 18th and 19th centuries, the British successively discovered that limestone containing clay, natural marl, etc., after high-temperature calcination, the powder will become very hard after being stained with water, and these high-temperature calcined powders are the prototype of cement. Its appearance has almost changed the composition of building materials, and has become the most popular in the world in one fell swoop, replacing natural materials such as clay, stone, bamboo and wood, as well as China's Sanhe clay and Roman mortar in Europe. So now no matter how you go to the top, cement accounts for the majority.
Cement raw meal is mainly calcareous CaCO3, clay (Al2O3·2SiO2·2H2O) and other Mg, Fe, Al and other compounds sintered to form, on the surface its main material is similar to China's traditional triple soil, but its ratio is very different, the soil-cement ratio is 1:3 1:7, in the cement firing process clay has become a proper supporting role. The production of cement is mainly divided into two grinding and one burning, that is, three processes: raw material grinding, high-temperature calcination, and clinker grinding. There are three main processes in the sintering process:
Stage 1: Drying and dehydration。When the temperature of the kiln material in this process rises to 100 150, all the free water in the raw meal is excluded; As the temperature rises further, the adsorbed water and the combined water in the clay are removed to form an amorphous metakaolin.
Stage 2: Carbonate decomposition。When the material temperature rises to 600, the calcium carbonate in the limestone and the magnesium carbonate included in the raw material are decomposed, and the violent decomposition temperature of magnesium carbonate and calcium carbonate is 750 and 900 respectively under the partial pressure of CO2 is one atmosphere. The decomposition process releases a large amount of CO2, which makes the product loose and porous, which is convenient for the subsequent reaction.
mgco3=mgo+co2 caco3=cao+co2
Stage 3: Solid-phase reaction and sintering. The active free calcium oxide and SiO2, Al2O3 and other solid-phase reactions in the material form calcium silicate and aluminate compounds, mainly including tricalcium silicate (3Cao·SiO2), dicalcium silicate (2Cao·SiO2), tricalcium aluminate (3Cao·Al2O3) and tetracalcium ferroaluminate (4Cao· Al2O3·Fe2O3) and so on.
When traditional ceramic tiles are used, they are first molded by water and molding, and then fired at high temperatures. Cement, on the other hand, is fired first, then mixed with water, and gradually shaped.
After sintering, the cement clinker is very stable in the dry state, but it reacts vigorously with water and slowly cross-links into a hard block similar to a stone, which is the so-called hydraulic hardness of cement. Two of the main reactions are:
3cao·sio2+h2o → xcao·sio2·yh2o+ca(oh)2
2cao·sio2+h2o → xcao·sio2·yh2o+ca(oh)2
Xcao·SiO2·YH2O Hydrated Calcium Silicate Gel, abbreviated as C-S-H, is an amorphous, amorphous substance, which usually grows into a ...... similar to a film or honeycomb, or needle-like, thread-likeThese products grow in close proximity to each other under the action of hydrogen bonds and intermolecular forces, forming long chain structures that are intertwined with each other.
These layered and intersecting structures lock water, sand, etc. into it tightly to form a complete body. Some articles compare calcium hydrated silicate to Xanthium, or a messy ball of wool, and this interlocking structure of calcium silicate gel hydrate is the basis and reason for the strength of cement-based materials.
Since the invention of cement cement is almost the greatest invention of modern times, its emergence has changed our traditional dependence on natural earth and wood materials, and shaped modern building structures, making buildings taller, bigger and more durable. Without cement, it is estimated that it is difficult to see skyscrapers, water dams, roads and bridges, etc., and we may still live in brick and wood houses, imagining what the heavenly palace should be like.
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