What is Volumetric Stability?

Cement volume stability refers to the performance of cement in the process of setting and hardening whether the volume changes uniformly. If the cement produces uneven volume changes after hardening, it is poor volume stability, which will cause expansion cracks in cement products or concrete components, reduce the quality of the building, and even cause serious accidents.

There are three main causes of poor cement stability: too much free calcium oxide in the clinker, too much free magnesium oxide in the clinker or too much gypsum mixed in.

The free calcium oxide or magnesium oxide contained in the clinker is overburned, curing is very slow, and it is only cured after the cement has hardened, which is a chemical reaction of volume expansion, which will cause uneven volume changes and crack the cement stone. When the amount of gypsum is too much, after the cement hardens, it will continue to react with the solid calcium hydrated aluminate to form high-sulfur calcium hydrated calcium sulfoaluminate, which will increase the volume by about two times, which will also cause cracking of cement stone.

The national standard stipulates that the stability of cement must be qualified by boiling method inspection (CAO); The magnesium oxide (MGO) content in the cement shall not exceed, if the cement is qualified by the autoclaving stability test, the content of magnesium oxide in the cement is allowed to be relaxed to; The amount of sulfur trioxide (SO3) in cement should not be exceeded.

The cement with unqualified stability should be treated as waste and cannot be used in the project.

Cement stability is one of the main indicators to determine whether the cement quality is qualified, which has the greatest impact on the quality of the project, and the factory inspection must be qualified before it can be used in construction projects.

A simple method for determining the stability of cement.

There are several simple methods to determine whether the stability of cement used in concrete engineering is qualified:

1) The appearance of qualified cement poured concrete is hard and prickly, while the stability of unqualified cement poured concrete gives people a soft, frozen and thawed feeling;

2) Most of the concrete poured by qualified cement is blue-gray and shiny, while the concrete poured by unqualified cement is mostly white and dull;

3) The concrete and aggregate mixed by qualified cement have strong grip and binding force, and it is difficult for stones to be peeled off from the surface of the component, while the concrete and aggregate mixed with unqualified stability have poor grip and wrapping force and small cohesion, and the stones are easy to peel off from the surface of concrete.

In the 54 groups of cement specimens that failed to stabilize the test conducted above, the proportion of low-grade cement was much higher: 325 cement.

52 groups, accounting for %; 425 cement 2 groups, only %; For cement with a grade of 525 and above, the stability of unqualified cement is 0From the perspective of cement production technology and equipment, the cement produced by ordinary vertical kilns has a very high stability failure rate, reaching about 70%; Mechanized vertical kiln production accounts for about 30%, indicating that the stability of vertical kiln cement is particularly prominent, and should be strictly controlled and taken seriously. The cement produced by large-scale rotary kiln has stable quality and high stability pass rate, which is the direction of the development of cement industry in the future.

The volume stability of cement refers to the uniformity of cement volume change in the process of cement setting and hardening. If the volume change is uneven after cement solidification and hardening, cement concrete component will produce expansive cracks, seriously affect the engineering quality of the building, and even cause serious engineering accidents.

What are the hazards of using cement with unsatisfactory stability in concrete engineering? The stability of cement refers to the stability of the volume change of the cement slurry after hardening. The reason for the unqualified stability of cement is that it contains too much free calcium oxide or free magnesium oxide in its clinker mineral composition, and the excessive amount of gypsum mixed with cement grinding.

The free calcium oxide or free magnesium oxide contained in the clinker is generated at high temperature, belongs to superburned lime, their hydration rate is very slow, often after the cement solidifies and hardens, it slowly begins to hydrate, and the volume expansion occurs during hydration, which causes uneven volume changes and cracks the hardened cement stone. The national standard stipulates that the unqualified stability caused by free calcium oxide can be tested by the cake test method or the Ray's method, and the Ray's method shall prevail when there is a dispute, and the unqualified stability caused by the free magnesium oxide must be detected by the autoclaved method. Cement with unqualified stability cannot be used in the project and can only be treated as waste.

Because of the use of unqualified cement, when the cement is hardened, the free calcium oxide and magnesium oxide begin to slowly hydrate, so that the concrete parts such as cement products, concrete components, beams, slabs, columns and other concrete parts of the building will crack due to uneven expansion in the later stage, and in serious cases, the structure will fail and cause serious engineering accidents.

The volume stability of cement refers to the uniformity of the volume change of cement in the process of setting and hardening.