What is the volumetric stability of cement? What is the cause of poor volume stability of cement

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

There are many reasons for poor cement stability, and there are three main ones:

1. There is too much free calcium oxide in the clinker, too much free magnesium oxide in the clinker or too much gypsum incorporated.

2. The free calcium oxide or magnesium oxide contained in the clinker is overburned, and the curing is very slow, and the curing is carried out only after the cement is hardened, which is a chemical reaction of volume expansion, which will cause uneven volume changes and crack the cement stone.

3. When the amount of gypsum is too much, after the cement is hardened, it will continue to react with the solid calcium hydrated aluminate to form high-sulfur hydrated calcium sulfoaluminate, which will increase the volume by about two times, and will also cause cracking of cement stone.

Cement stability is simply the ability to keep the shape unchanged, and the phenomenon of cracking and expansion after the use of cement indicates that the stability of cement is poor, and vice versa means that the stability is good. The stability of cement mainly depends on the free calcium oxide and free magnesium oxide in the cement, and too much free calcium oxide will react with water to form calcium hydroxide, resulting in poor stability of cement.

Cement volume stability refers to the performance of whether the volume change of cement is uniform in the process of setting and hardening, if the cement produces uneven volume change after hardening, it is poor volume stability;

The stability is tested using the cake test method and the Raynell clamp method.

There are many reasons for poor cement stability, mainly too much free calcium oxide in the clinker, too much free magnesium oxide in the clinker or too much gypsum incorporated. The free calcium oxide or magnesium oxide contained in the clinker is overburned, matures very slowly, and is only cured after the cement has hardened.

Local volume expansion, magnesium oxide and free calcium oxide in cement will be the main cause of volume expansion. The main hazard is that it will cause the local expansion of cement, resulting in the decline of cement strength and affecting the stability of the building.

After the cement is solidified and hardened, the cement stone structure is damaged due to the local expansion caused by the internal chemical reaction, which is the so-called poor volume stability. Generally, it is caused by high free calcium oxide, chlorite or excessive gypsum mixed into the clinker. High free calcium oxide is a major factor causing poor stability.

Stability refers to the uniformity of the volume change of the cement slurry after hardening. If the volume change of cement after hardening is unstable and uniform, that is, the so-called poor stability, it will lead to the expansion and destruction of concrete, resulting in serious engineering quality accidents. Therefore, the national standards of the country stipulate that:

If the stability of cement is unqualified, it should be disposed of as waste products and shall not be used in any project.

In cement, due to the incomplete calcination of clinker, there are free CAO and MGO (F-CAO, F-MGO), because it is generated at high temperature, the hydration activity is small, and the hydration after hardening of cement produces volume expansion; Too much gypsum is added during the production of cement, and after the cement hardens, it will continue to react with the solid calcium hydrate aluminate hydrate to form calcium sulfoaluminate hydrate, resulting in volume expansion. The expansion caused by these three substances can lead to poor cement stability, even if the hardened cement stone can cause bending, cracks and even crushing damage.

Cement volume stability refers to the uniformity of the volume change of cement after setting and hardening. The poor stability of cement is generally caused by the free calcium oxide, chlorite contained in the clinker or the incorporation of excessive gypsum, and the free calcium oxide is the main factor affecting the stability of cement. The methods for testing the stability of cement include the cake test method and the Ray's method, and when there is a dispute, the Ray's method shall prevail.

Cement should have good stability of body and rolling, and the FCAO in clinker is the main factor for the poor stability of cement. There are both quantitative and qualitative reasons, and some clinker does not have high FCAO content, but the safety period is very long; Although the FCAO content of some clinker is higher, the safety period is shorter.