1. Organic cementitious materials
Organic cementitious materials refer to a kind of cementitious materials with natural or synthetic polymer compounds as the basic components. The most commonly used are asphalt, resin, rubber, etc.
2. Inorganic cementitious materials
Inorganic cementitious materials refer to a kind of cementitious materials mainly composed of inorganic oxides or minerals. The most commonly used are lime, gypsum, water glass, magnesite and various cements. Sometimes it also includes active mixed materials such as zeolite powder, fly ash, slag and volcanic ash.
According to the setting and hardening conditions and application characteristics, inorganic cementitious materials are usually divided into two categories: gas-hardening and hydraulic.
Pneumatic cementing material refers to a material that can only cure and harden in air and maintain and develop its strength. Mainly lime, gypsum, water glass, magnesite and so on. This material does not condense when it meets water, and its strength is very low, even in a humid environment, so it is usually not suitable for use.
Hydraulic cementing material refers to a material that can not only coagulate and harden in air, but also maintain and develop its strength in water. There are all kinds of cement and some composite materials. This material condenses and hardens more easily in water than in air. Therefore, when they are used in the air, they should be watered or kept wet as much as possible in the initial stage of coagulation and hardening for curing.
The coagulation and hardening process of cementitious materials is usually accompanied by a series of complex physical and chemical reactions and volume changes. Many internal and external factors affect this process, which eventually makes the performance of products after coagulation and hardening very different. The difference between different cementitious materials is greater.
lime
Lime is a traditional air hardening cementitious material. It has a wide range of raw materials, simple production process, low cost and some excellent properties, and is still widely used in civil engineering.
First, the raw materials of lime
The main raw materials of lime are limestone, dolomite and chalk containing calcium carbonate (CaCO3). The types and sources of raw materials have great influence on the properties of lime. Generally, the content of clay impurities in raw materials is less than 8%.
Some industrial by-products can also be used as raw materials for lime production or directly used. For example, calcium carbide slag produced when calcium carbide (CaC2) is used to produce acetylene, and its main component is calcium hydroxide [Ca(OH)2], which can be used directly, but its performance is not satisfactory. Another example is the residue of ammonia-alkali process, the main component of which is calcium carbonate. This section mainly introduces the lime produced by limestone which is most commonly used in civil engineering.
Second, the production of lime
1. quicklime
The production of lime is actually calcining limestone at high temperature, which decomposes calcium carbonate into CaO and CO2, and CO2 escapes in the form of gas. The reaction formula is as follows:
The CaO produced is called quicklime, which is a white or gray block substance.
Characteristics of quicklime: when it meets water, it quickly produces hydration reaction, expands in volume and releases a lot of heat. Calcined quicklime can react with water in a few seconds, and its volume expands about twice.
Thirdly, the curing of lime.
(1) Curing and slaked lime
The process of adding water to quicklime CaO to generate Ca(OH)2 is called curing. The product Ca(OH)2 is called slaked lime. The reaction formula is as follows:
Characteristics of curing process:
1. Soon. When the calcined CaO comes into contact with water, the reaction is completed in a few seconds.
2. Volume expansion. When CaO reacts with water to produce Ca(OH)2, its volume increases by 1.5 ~ 2.0 times.
3. Give off a lot of heat. 1 mol CaO solidified to generate 1 mol Ca(OH)2, which generated about 64.9kJ of heat.
(2) Lime plaster
When a large amount of water is added in the curing process, lime slurry will be produced. The theoretical water requirement for CaO to generate Ca(OH)2 is only 32. 1%, and excessive water is added in the actual curing process. On the one hand, the evaporation loss of water caused by exotherm during curing is considered, on the other hand, CaO is fully cured. The production of lime paste is often carried out in the ash pool of the construction site, that is, the massive quicklime is washed with water, filtered through the screen to remove immature lime and impurities, and then flows into the ash pool for precipitation. Lime plaster surface must be stored and maintained, in order to isolate direct contact with air, prevent dry hardening and carbonation consolidation, so as not to affect normal use and effect.
(3) slaked lime powder
When an appropriate amount of water (60% ~ 80%) is added in the curing process, powdered hydrated lime is generated. This process is usually called digestion, and its product is called hydrated lime powder. It can be digested by manual spraying in layers on the construction site, but usually the slaked lime powder is produced centrally in the factory and sold as a product.
Lime "Chen Fu"
As mentioned above, too high calcination temperature or too long calcination time will produce calcined lime, which is very inevitable in lime calcination. Because the surface of calcined lime is covered with a layer of glass glaze, the curing is very slow. If the lime is cured after use and hardening, the volume expansion will cause local blistering, swelling and cracking. In order to eliminate the hazards of the above-mentioned quicklime, the lime paste should be stored in the ashing tank for more than 2 weeks before use, so that the quicklime can be fully solidified. This process is called "aging". The slaked lime powder produced on site generally needs to be "aged".
However, if the quicklime is used after grinding, there is no need to "age". This is because the surface area of quicklime is greatly increased in the grinding process, and the reaction speed with water is accelerated, which can be solidified almost at the same time and dispersed evenly in quicklime powder, without causing various hazards of quicklime.
plaster
First, the raw materials of gypsum
(1) gypsum
Gypsum usually refers to natural gypsum whose molecular formula is dihydrate, also known as soft gypsum. It is the most important raw material for producing building gypsum. Raw gypsum powder does not harden and cement when added with water.
(2) Chemical gypsum
Refers to the chemical by-product containing the mixture of calcium sulfate dihydrate () and CaSO4. For example, the waste in the production of phosphoric acid and phosphate fertilizer is called phosphogypsum; The waste in hydrofluoric acid production is called fluorgypsum. In addition, there are salt gypsum, mirabilite gypsum and titanium gypsum. It can also be used as a raw material for producing building gypsum, but its performance is not as good as that of raw gypsum.
(III) Anhydrite
Refers to the natural anhydrous gypsum with the molecular formula CaSO4. Does not contain crystal water, which is very different from gypsum. Usually used to produce building gypsum products or additives. I won't introduce it in detail here.
Second, the production of building gypsum
Gypsum hemihydrate is made by calcining raw gypsum at 107 ~ 170℃ and removing some crystal water. It is called building gypsum, also known as plaster of Paris, and its molecular formula is. The reaction formula is as follows:
During the heating process of gypsum, the properties of its products will also change with the difference of temperature and pressure. The hemihydrate gypsum produced under the above conditions is also the most commonly used building gypsum. If the raw gypsum is steamed in an autoclave at 125℃ and the pressure is 0. 13MPa, the generated hemihydrate gypsum is called high-strength gypsum because of its coarse particles and less water consumption when mixing gypsum slurry.
When the calcination temperature rises to 170 ~ 300℃, the hemihydrate gypsum will continue to dehydrate to produce soluble anhydrite (caso4-Ⅲ). The setting speed is faster than that of gypsum hemihydrate, but the water demand is high and the strength is low. When the temperature continues to rise to 400 ~ 1000℃, slowly dissolving anhydrite (caso4-Ⅱ) will be generated. This kind of gypsum is insoluble in water, and only by adding some activator can it have hydration and hardening ability, but its strength is higher and its wear resistance is better. The product of caso 4-Ⅱ mixed with activator is called anhydrite cement.