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How much do you know about special cement --calcium sulphoaluminate cement?

01 History

In 1908, the patent for aluminate cement was published in France, and in 1908 it was first industrialized.  After decades of development, a series of aluminate cements including expanded cement, self-stressing cement and refractory cement have been formed. The cement of this series is characterized by its clinker mineral composition, which is mainly composed of CA, which makes it have special properties such as early strength and fire resistance. Aluminate cement is now mainly used for high temperature resistant casting materials.  It was no longer used on the building due to its late strength collapse.

In 1975, China Building Materials Institute successfully developed calcium sulphoaluminate cement. In 1982, based on calcium sulphoaluminate cement China Building Materials Institute successfully developed iron-aluminate cement.  If we say that we call the Portland cement series the first series of cement, the aluminate cement series is called the second series of cement. Then, we can refer to the calcium sulphoaluminate cement and the iron aluminate cement and other cement types derived from them as the third series of cement.  The mineral composition of this series of cements is characterized by the presence of large amounts of C₄A₃ minerals.  This is different from other series of cement.  And it constitutes the third series of cement's early strength, high strength, high impermeability, high frost resistance, corrosion resistance, low alkali and low energy consumption.  The third series of cement has been widely used in China.

The calcium sulphoaluminate cement was invented by the China Academy of Building Materials Science. In 2000, China's calcium sulphoaluminate cement production was only 672,500 tons.  By 2005, China's sulfur (iron) aluminate cement production reached 1.253 million tons.  At present, there are 30 enterprises producing calcium sulphoaluminate cement in China, and the output of calcium sulphoaluminate cement in the country is basically stable at about 1.25 million tons.

02 definition

Calcium sulphoaluminate cement is an important branch of China's special cement industry. It has fast hard calcium sulphoaluminate cement (mixed material is less than 15% of cement clinker) and low alkalinity calcium sulphoaluminate cement (mixed material content).  It accounts for 15% to 35% of cement clinker), self-stressing calcium sulphoaluminate cement, expanded calcium sulphoaluminate cement, etc., which make China have special cement varieties with independent intellectual property rights.
The main components are C₂S, anhydrous calcium calcium sulphoaluminate and mixed materials (stone powder, gypsum, etc.), which have a series of excellent hydraulic properties such as early strength, fast hardness and low alkalinity.

03 main performance

1 Early strength and high strength performance: The two fast hardening cements not only have higher early strength, but also have a growing late strength. At the same time, it has a setting time that meets the requirements for use.  The compressive strength of 12h~1d can reach 35~50 MPa; the flexural strength can reach 6.5~7.5 MPa.  3 days compressive strength can reach 50 ~ 70 MPa; flexural strength can reach 7.5 ~ 8.5 MPa.  The cement number is determined according to the compressive strength of the 3d cement mortar. The cement currently on the market are 425#, 525# and 625#. The two cements have a production history of 10 to 20 years.  From the long-term strength results of cement and concrete from 5 to 10 years, it can be seen that with the increase of curing age, the strength continues to increase, and the highest strength can reach 100 MPa.

2 high frost resistance: Both two fast hardening cements show excellent frost resistance.  It has the following characteristics: a. It is used at a low temperature of 0 °C ~ 10 °C, and the early strength is 5-8 times that of Portland cement.  b. If we use at a negative temperature of 0 ° C ~ -20 ° C, and add a small amount of antifreeze, and the concrete mold temperature is maintained above 5 ° C, then normal construction.  The strength of concrete 3 ~ 7d can reach 70 ~ 80% of the design number.  c. Construction under alternating positive and negative temperatures has little effect on late strength growth.  The laboratory has 200 freeze-thaw cycles, and the concrete strength loss is not obvious.  The antifreeze label can reach more than 200#.

3 Corrosion resistance: The two cements are excellent for seawater, chloride salts (NaCl, MgCl2), sulfates (Na2SO4, MgSO4, (NH4)2SO4), especially their composite salts (MgSO4+ NaCl).  Corrosion resistance.  The corrosion resistance of fast hard aluminosilicate cement is better than that of fast hard calcium sulphoaluminate cement.  It can be seen from the results of the corrosion resistance laboratory for 2 years that the corrosion resistance coefficient is greater than 1, which is obviously superior to the high sulfur-resistant Portland cement and high-alumina cement.

4 High impermeability: The cement stone structure of the two cements is relatively dense, so the concrete impermeability is 2 to 3 times that of the same grade Portland cement concrete.

5 Steel corrosion: the two cements are not exactly the same due to the different alkalinity of the hydration liquid. Fast hardening calcium sulphoaluminate cement has a low alkalinity (pH<12), and the surface of the steel bar does not form a passivation film, so it is not good for protecting the steel. In the early mixed concrete, due to the presence of more air and moisture, the concrete reinforcement was slightly rusted in the early stage. As the age increases, air and moisture gradually decrease and disappear. Due to the compact structure of the concrete, there was no obvious development in the later stage of corrosion. This is evidenced by the long-term observation of laboratory concrete reinforcements and cement products (10 years). If a small amount of alkaline admixture (NaNO2, etc.) and high-strength calcium sulphoaluminate cement are added to the concrete, there is no rust at the beginning. For fast-hardening iron aluminate cement, due to the high alkalinity of the cement hydration liquid (pH>12), the surface of the steel bar forms a passivation film similar to Portland cement concrete.  Both the rapid determination of the polarization curve and the routine laboratory tests showed that the fast-hardened iron aluminate cement did not rust the steel bars.  In summary, the two fast-hardening cements have good durability.

04 mixed materials

Plaster. At present, most of the domestic use of anhydrite as a mixture of fast hard and low alkali calcium sulphoaluminate cement, and in the early development of calcium sulphoaluminate cement, dihydrate gypsum is generally used as a mixed material.  The use of dihydrate gypsum has a strength of 3-5 MPa.  However, excessive amount of gypsum will convert the strength component of the cement into an expansion component, causing the expansion of the cement to be too large to cause cracks in the cement stone and causing damage in severe cases. Since the dissolution rate of anhydrite is lower than that of dihydrate gypsum, its late expansion risk is greater.  Therefore, it is necessary to limit the gypsum component in the cement. In the case of fast hard and low alkali calcium sulphoaluminate cement, the gypsum should be added in an amount less than 15%.

limestone. The initial purpose of adding limestone to calcium sulphoaluminate cement is to reduce the early hydration heat of the cement and overcome the resulting collapse of the flexural strength. The magnitude of the decrease in strength is small compared to other mixed materials.  Later, it was found that the addition of a certain amount of limestone reduced the alkalinity of the cement liquid phase and thus developed into today's low alkalinity calcium sulphoaluminate cement.

05 application

1 For GRC products70% of China's calcium sulphoaluminate cement is used in GRC products.  Low alkali aluminate cement hydration slurry PH is less than 10.5, especially suitable for glass fiber products

2 For quick repair materials.It has the characteristics of early strength and high strength. Its 3-day strength is equivalent to the 28-day strength of the same grade Portland cement. It is suitable for emergency repair and construction; in some special engineering fields, it requires high early strength, such as 1h strength, 2h. Due to the special performance requirements such as strength, calcium sulphoaluminate cement is usually used in quick repair materials due to its fast and hard early strength.

3 For special mortar materials.At present, the cement-based self-leveling uses a large amount of the silicon-sulfur aluminum-gypsum system, fully exerting the advantages of the ternary system, ensuring the early strength and ensuring the volume stability in the later stage.

4 It has good anti-freezing performance and is widely used in winter low temperature construction projects;

5 Impermeability and seawater corrosion resistance are much better than Portland cement, being suitable for marine construction projects;

6 Consolidation of hazardous waste has superior performance and special performance, and is suitable for curing of harmful and toxic waste.

06 Development prospects

In the process of special cement promotion, the contradiction between continuity, mass production and interstitial, and small batch use is often encountered, resulting in many varieties of cement researched in the laboratory, but few industrialized long-term production.  Therefore, in industrialized countries, in order to make cement concrete have a certain special performance, it is mainly to use a method of adding an additive in a mass-produced Portland cement. However, this method has certain limitations.  In many cases, the performance required by the user cannot be achieved, and the technical road of firing a specific mineral group mature material must be taken.

In China, the resources are widely distributed and the power is cheap. The cement industry still has economic value in the production of small scale, so it has good social conditions for changing the composition of clinker minerals to develop special cement.

The third series of cement has been successfully applied in various construction projects (especially in winter construction projects), seaport projects, underground works, various cement products and prefabricated components, such as cement pressure pipes, high-strength piles, large beams and columns, and kind of GRC products. The wide range of use provides conditions for cement to produce balanced production all the year round, which solves the contradiction between production and use.

At present, we are further developing its use. With the development of economic construction, the market for the third series of cement will become larger and larger. Now it has started mass export and has strong competitiveness in the international market. Specific social conditions have made China's third series of cement have good development prospects.

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