(1) effect of cement composition
The mineral composition of cement and their proportions are the main factors affecting the setting and hardening of cement. As mentioned above, different mineral components will manifest different characteristics when they react with water. for example, C3A. increase of can do Accelerate the setting and hardening rate of cement, And this the heat of hydration is high at the same time.
Generally speaking, if the composite material is added to the cement clinker, the anti-erosion will increase, and the heat of hydration and starting strength will be reduced.
(2) gypsum mix volume
Gypsum is called the retarding agent of cement which is mainly used to regulate the setting time of cement and is an essential component. Without gypsum, cement clinker mixed with water can condense immediately and release heat. The main reason is that the C3A in the clinker can quickly dissolve in water to produce a kind of calcium aluminate hydrate, a clotting agent, which will destroy the normal use of cement. The retardation mechanism of gypsum is: when the cement is hydrated, the gypsum reacts quickly with C3A to generate calcium sulfoaluminate hydrate which acts as a catalyst for the cement particles to hinder the hydration of C3A and delay the setting time of the cement. Creates a protection film on
If the amount of gypsum is too small, the retarding effect will not be obvious. Too much gypsum will accelerate the setting of the cement because the gypsum itself can produce a clotting agent. The appropriate amount of gypsum depends on the content of C3A in the cement and the content of SO3 in the gypsum, and is also related to the fineness of the cement and the content of SO3 in the clinker. The amount of gypsum should be accounted for 3%-5% by mass of cement. If the amount of gypsum exceeds the limit, it will reduce the strength of the cement and may also lead to poor dimensional stability, which will lead to extended destruction of the cement paste. Thus, the national standard requires that the content of SO3 should not exceed 3.5%.
(3) Effect of fineness of cement
The size of the cement particles directly affects the hydration, setting and hardening, strength and heat of hydration.
The finer the cement particles, the larger the total surface area and the larger the water contact area. Thus, hydration will be accelerated, setting and hardening will be accelerated, and the initial strength will be high.
However, if the cement particles are too small, it is easy for them to react with water and calcium dioxide in the air to destroy the cement storage. If the cement is too fine, its shrinkage is large during the hardening process. Thus, the finer the cement, the greater the energy loss and the higher the cost. The grain size of cement particles is usually within 7–200 hrs (0.007–0.2 mm).
(4) effect of the condition being treated
The curing environment has sufficient temperature and moisture to conducive to the hydration and setting and hardening process of the cement and to benefit the development of starting strength. If the humidity of the environment is too dry, the water in the cement will evaporate, leaving insufficient hydration and hardening. Sometimes serious cracks will occur.
Typically, the temperature rises at the time of curing, and the hydration and early strength development of the cement is accelerated. If the hardening process takes place at a lower temperature, the final strength will not be affected, although the development of strength is slow. But if the temperature is 0. is less than0C, the hydration of cement will stop and the strength will not only stop increasing but also will destroy the structure of cement paste due to condensation of water.
In actual projects, the setting and hardening process of cement products is accelerated by steam curing and autoclave curing.
(5) effect of age of treatment
The hydration and hardening of cement is a long-term process. With an increase in the hydrating degree of various clinker minerals in the cement particles, the gel will increase and the capillary pores will shrink, which can lead to an increase in strength with aging. It has been proven that cement develops rapidly within 28d and slowly after 28d.
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(6) Effect of mixed water content
If cement consumption remains unchanged, Increasing the amount of mixed water will increase the amount of capillary pores, reduce the strength of the cement paste, and extend the setting time., Therefore, in practical projects, when the fluidity of cement concrete is adjusted, the amount of water and cement will be changed without modifying the water-cement ratio (the minimum amount of cement is regulated to ensure the durability of concrete).
(7) effect of mixing
The hydration, setting and hardening of Portland cement is constrained by C3S, C3A. And all admixtures that affect the hydration of C3S, C3A can alter the hydration, setting and hardening performance of Portland cement. For example, accelerating agents (such as CaC12, Na2S04) can accelerate the hydration and hardening of cement and improve its strength. Conversely, retarding agents (eg calcium lignosulfonate) can delay the hydration and hardening of the cement and affect the development of the starting strength.
(8) effect of storage conditions
Improper storage will expose the cement to moisture. Due to hydration the particles get deposited on the surface which seriously reduces the intensity. The effect of C02 in water and air will cause slow hydration and carbonization, even if the storage is good.
Strength reduces by 10%-20% after 3 months, 15%-30% after 6 months, 25%-40% after 1 year, so the effective storage period of cement is 3 months and cement should not be stored should for a long time.