Q: What are the varieties of cement? And its characteristics?
A: According to the composition, cement can be divided into Portland cement, aluminate cement and sulphoaluminate cement, etc. Portland cement is the most consumable and most widely used category of cement in civil and architectural engineering, with Portland cement clinker as the major component. According to the variety and dosage of mixed materials, it can be divided into Portland cement, ordinary Portland cement, slag Portland cement, pozzolana Portland cement, fly-ash Portland cement and composite Portland cement.
Q: What are the characteristics of Portland cement?
A: Without or with a very small amount of mixed materials in Portland cement, the strength grade of cement is high, so Portland cement applies to the preparation of high-strength concrete and prestressed concrete, etc., but does not apply to the preparation of ordinary mortar. In the preparation of ordinary mortar, in order to meet the requirement for the working performance of mortar, there are usually minimum limits on the amount of cement, so the strength grade of mortar is relatively low; if Portland cement is used for the preparation of mortar, the strength of mortar prepared in this way is relatively high, which will inevitably result in a waste of cement and poor working performance of mortar.
Q: What are the characteristics of ordinary Portland cement?
A: Ordinary Portland cement is only added with a small amount of mixed materials, has moderate strength grade of cement, and is the most consumable kind of cement in the current construction projects. When ordinary Portland cement is used for the preparation of mortar, due to the high strength of cement, the prepared mortar strength is high, resulting in a waste of cement; but when the amount of cement is low, the water retention of mortar is poor and it is easy for bleeding. To solve this problem, active mineral admixtures, such as fly ash, are usually added into the mortar, so that it can not only reduce the amount of cement, but also can improve the workability of mortar.
Q: What are the characteristics of ordinary slag cement?
A: The content of cement clinker mineral in the slag cement is much less than that of Portland cement, and the hydration reaction of mixed materials at room temperature is relatively slow, so the setting and hardening is relatively slow. The early strength is relatively low, but in the later stage (after 28d), due to the increase of hydration products, the strength of cement stone is growing, and finally exceeds that of Portland cement.
Slag cement needs a longer period of time for wet curing, and the effect of outside temperature on the hardening speed is more sensitive than that of Portland cement. At low temperatures, the hardening speed is slow and the early strength is significantly reduced; while the use of steam curing and other heat-moisture treatment can effectively accelerate its hardening rate and the late strength will also be growing.
In the slag cement, the content of mixed materials is large; the water requirement is great; the water retention is poor, and bleeding is large. In the preparation of concrete or mortar, it is easy to precipitate the excess moisture and form the capillary channels or coarse pores inside the cement paste, reducing the uniformity. In addition, the dry shrinkage of slag cement is large. If cured improperly and dried before adequate hydration, it will be easy to produce cracks. Therefore, the frost resistance, impermeability and resistance to alternate wetting and drying cycle performance of slag cement are not as good as ordinary cement.
But slag cement has good chemical stability and strong resistance to freshwater, seawater and sulfate attack, which is because the content of free calcium hydroxide and aluminate in the slag cement paste is small, suitable for hydraulic engineering and harbor works. In addition, slag cement has low heat of hydration, good heat resistance, and can be used for mass concrete works or heat-resistant concrete works.
Q: What are the characteristics of pozzolana cement?
A: The strength development of pozzolana cement is similar to slag cement, developing slowly in the early stage but rapidly in the later period. The reason for the increase of late strength is that the active oxides in the mixed materials react with calcium hydroxide to produce more hydrated calcium silicate gels than Portland cement does. The environmental conditions have significant effects on its strength development. When the environmental temperature is low, the coagulation and hardening significantly get slow; in the dry environment, the strength stops growing, and it is prone to dry shrinkage cracks, so it should not be used for construction in winter.
Similar to slag cement, pozzolana cement has a low content of free calcium hydroxide, and also has strong resistance to sulfate attack. In acidic water, especially in carbonated water, the corrosion resistance of pozzolana cement is poor. Under the long-term reaction of CO2 in the atmosphere, the hydration products will get decomposed, making the structure of cement stone destroyed, so the resistance to atmospheric stability of cement of this kind is poorer.
The water requirement and bleeding of pozzolana cement are related to the varieties of admixtures added. When hard admixtures such as tuff are used, the water requirement is similar to Portland cement; while soft admixtures such as diatom ooze are used, the water requirement will be larger and bleeding will be smaller, but the shrinkage deformation will be larger.
Q: What are the characteristics of fly ash cement?
A: The surface of fly ash spherical vitreous particles is denser and the activity is lower, not easy for hydration, so the hydration and hardening of fly ash cement is slower and the early strength is lower, but the late strength can catch up with and even exceed that of ordinary cement.
Since the structure of fly ash particles is denser, the specific surface area inside is small. And it contains spherical vitreous particles. Its water requirement is small. The workability of mortar and concrete prepared is good. Therefore, the cement has small drying shrinkage and good crack resistance.
For fly ash cement, the heat of hydration heat is low, and the resistance to sulfate corrosion is stronger, but second to slag cement, applicable to hydraulic engineering and harbor works. Fly ash cement has poor carbonation resistance and poor frost resistance.
But the bleeding of fly ash cement is fast, easily leading to water loss and cracks. Therefore, during the setting period of concrete and mortar, the plastering number of times should be appropriately increased. In early hardening stage, conservation should also be strengthened in order to ensure the normal development of concrete and mortar strength.
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