B. Low density cement

(1) The density range of cement slurry is 1.20 ~ 1.40g/cm.

(2) The lightening agents used include fly ash hollow beads, hollow ceramic beads, etc.

Fly ash hollow beads are the dried materials of fine particles floating on the water surface after the fly ash blown into the water after the combustion of coal in thermal power plant. When magnified under the microscope, it is a small hollow sphere filled with ammonia or carbon dioxide. Its composition is close to that of aluminum clay and has certain activity. The beads have the characteristics of light weight, fine particle and thin wall, and form cementitious compounds with Ca (OH) З or CaSO4.

(3) The density of hollow bead low density cement decreases with the increase of bead dosage.

A characteristic of hollow bead low density cement is that its density is affected by shear mixing and pressure. The density increases with the increase of mixing strength and mixing time. The thickening time can be adjusted by adding admixtures to meet the requirements of construction, and the control of water loss can also be adjusted by adding fluid loss additive. Experiments show that the increase of temperature has an effect on the water loss, and the water loss increases with the increase of temperature.

The compressive strength of low-density cement stone decreases with the decrease of density, and so does the strength of hollow micro bead cement stone. The factors affecting the strength of cement paste include cement grade, weight of admixture, water cement ratio and reaction conditions. Due to the low water cement ratio of hollow bead cement, the strength of cement stone is high, especially at high temperature. Because the glass shell containing aluminum silicate of hollow bead has certain activity, it can react chemically to produce compounds with cementitious properties and improve the strength. Early strength agent should be used at low temperature.

In addition, special attention should be paid to the stability of hollow bead cement slurry. Due to the large density difference between hollow beads and cement particles, special attention should be paid to the stability of cement slurry in use. If the stability is not good, it will cause the hollow beads to float up and the cement particles to sink. In order to avoid this phenomenon, try to select micro beads with small particle diameter and appropriately reduce the water cement ratio, which can reduce the floating space and increase the viscosity of cement slurry. Admixtures should be used to ensure the stability of the cement slurry. It is necessary to disperse the cement particles and make the cement slurry viscous. High molecular thickener or flocculant can be added to improve the viscosity and increase the shear force of the cement slurry to prevent the hollow beads from floating up and ensure that the cement slurry will not separate before setting.