Limestone in cement

How is limestone used for making cement?

Limestone is a mineral mainly composed of calcite (CaCO3). It is rich in natural resources. As an industrial raw material, it has a wide range of applications, such as building materials, metallurgy, chemicals, and construction. It is also an irreplaceable main raw material in cement production. Why use limestone to make cement? This article will focus on the role and mechanism of limestone in cement, and answer it for you.

1. Role

Limestone, the raw material in cement, can be used in the two processes, playing different roles. One is to be raw materials for cement raw meal, providing the CaO required for the manufacture of cement clinker. Secondly, when grinding finished cement, according to the requirements of the type of cement, it is mixed into the cement as an admixture to improve and enhance the performance and quality of cement and concrete.

1.1 Used as raw material for preparing cement clinker

Cement, according to the main mineral composition of cement clinker can be divided into three series: Portland cement, aluminate cement, and sulphoaluminate cement. The chemical composition of various types of cement clinker is listed in Table 1, from which we can see that CaO is the main chemical composition of various types of cement clinker.

The essential component of limestone is calcite CaCO3. When the clinker is fired, the CaCO3 in the limestone is decomposed into CaO and CO2 when heated, and the decomposed CO2 is discharged into the atmosphere with the flue gas in a gaseous state. Cao is combined with other chemical components to synthesize basic mineral components required for various types of cement such as calcium silicate, calcium aluminate and calcium sulfoaluminate at high temperatures.

1.2 Used as cement admixture

The finished cement includes cement without mixed materials and cement with mixed materials. The main function of admixtures is to reduce the amount of cement used in cement-based materials, improve workability, reduce the heat of hydration and shrinkage, and improve strength and durability. Commonly used admixtures include fly ash, slag, silica fume, and natural pozzolanic materials. However, fly ash and mineral powder are non-renewable resources. With the over-exploitation of natural resources by humans, the number of upstream products coal and ore is decreasing, causing the supply of fly ash and mineral powder on the market to become increasingly tight.

Limestone, as an inert material, is rich in natural resources, widely distributed, easy to process, and is a cheap and easy-to-obtain material. Therefore, the use of limestone powder as a cement admixture has become a hot spot in recent years.

2 Mechanism of action:

The action mechanism of limestone powder in cement mainly includes: filling effect, active effect, and acceleration effect. In the early stage of hydration, the filling effect and accelerated reaction are the main ones, while in the later stage of hydration, the filling effect and the active effect are the main ones.2.1 Filling effect.

The filling effect is mainly manifested as the filling effect of limestone powder on the pores in the cement slurry matrix and the interface transition zone, making the slurry more compact, reducing the porosity and pore diameter, and improving the pore structure. After adding limestone powder, because the particle size of limestone powder is very small, it can form a good gradation with cement clinker and other mineral admixtures, so it has a good filling effect. Experiments show that limestone powder has a significant improvement effect on the pore structure of mortar and concrete. With the increase of limestone powder content, the pore structure larger than 200 nm is significantly reduced, and the pore structure below 50 and 20 nm is significantly increased. Limestone powder The refinement effect on mortar pores is obvious.

2. 1 Active effect.

In the early stage of hydration, limestone powder does not participate in the hydration reaction, and no new hydration products are formed. As the hydration reaction continues, hydrated carbonate aluminate is formed in the later stage of hydration. This shows that limestone powder has hydration activity in the later stage of hydration. In addition, the hydrated carbonate and other hydration products overlap each other to make the cement stone structure more compact, thereby improving the strength and durability of concrete.

2.2 Acceleration effect.

Limestone powder has an accelerating effect in the hardening process of concrete. The appropriate amount of limestone powder acts as the nucleation matrix of C-H-S, reduces the nucleation barrier, accelerates the hydration reaction, and can increase the compactness of concrete. Regardless of the cement, the addition of limestone powder accelerates its hydration. The greater the fineness of the limestone powder, the more obvious the increase in its early compressive strength.

Limestone powder can not only provide CaO, the key component of cement, but also improve cement performance, improve product quality and strength, and save resources and energy.

Want to know more about the cement production process, read the article Cement Grinding Station.

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