Under the same other conditions, the smaller the water demand, the higher the quality of cement paste.

Therefore, both production and use units attach great importance to the fineness of cement.

Free calcium oxide is the main factor affecting the stability of cement.
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Generally, there are two methods to indicate the water demand for cement: water consumption for standard consistency and fluidity at a certain water cement ratio.

The cause of false setting is generally believed to be due to dehydration of some dihydrate gypsum into hemihydrate gypsum due to high temperature in the grinding process during cement grinding.

All factors that affect the hydration rate of cement must affect the setting time of cement, such as ambient temperature and humidity, the content of free calcium oxide in the clinker, the content of potassium oxide and sodium oxide, the mineral composition of the clinker, the amount of admixture added, the fineness of grinding, the water consumption of cement, storage time, the form and dosage of gypsum, and additives.

The cement slurry with false setting can be restored to plasticity by mixing again without adding water, and can still be poured for construction, with a slight decrease in strength.

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If the coarse and fine particles in cement are properly graded, good flowability can be obtained.

Therefore μ The content of particles below m should be minimized as much as possible.

Under the same grinding conditions, the main factors affecting the fineness of cement grinding are the grindability of the clinker, the grindability of the mixture, and the amount added.

Cement produced by low-temperature calcination and slow cooling clinker may also produce rapid setting; Cement with a high C3A content in the clinker may also produce rapid setting.

Poor soundness of cement is generally caused by free calcium oxide, periclase, or the addition of excessive gypsum in the clinker.

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Less than 10 μ The particles of m mainly play an early strength role, and among them, 3 μ Particles below m only have an early strength effect.

4、 The uniformity of volume change after setting and hardening of volume stable cement is one of the important indicators for evaluating cement quality.

In addition, the burning (i.e., running away from raw material), over burning, and slow cooling of clinker can also easily cause false setting of cement.

The compatibility of cement and admixtures with high water demand is poor, and the slump loss of concrete is fast.

Rapid setting: When it occurs, the characteristic is that after mixing cement with water, the cement paste quickly condenses into a coarse, non plastic mixture, while emitting a large amount of heat, making construction difficult.

There are many factors that affect the water demand of cement, the most important of which are the fineness of grinding, mineral composition, and the variety and amount of admixture.

Among the mixed materials, pozzolanic materials and fly ash are easy to grind and slag is difficult to grind.

To allow sufficient time for mixing, transporting, pouring, or masonry of concrete and mortar, the initial setting of cement should not be too early; When the construction is completed, it is required to harden as soon as possible to produce strength, so the final setting time cannot be too late.

After setting and hardening, cement will expand locally due to internal chemical reactions, resulting in damage to the structure of the cement paste, which is called poor volume stability.

It should be noted that there are two abnormal condensation phenomena: false coagulation (viscous coagulation) and rapid coagulation (instantaneous coagulation).

The fineness of the other five types of general Portland cement is expressed as sieve residue, which is 80% μ The residue on the m square hole sieve shall not be greater than 10% or 45% μ The residue on the m square hole sieve shall not be greater than 30%.

There are many factors that affect the setting time of cement.

The greater the fineness of cement, the greater the content of fine particles, and the greater the water demand.

The shape of the cement particles obtained by the cement final grinding system varies depending on the mills used (ball mill, roller mill, vibration mill).

Especially when the clinker strength is low and the admixture content is high, it is often necessary to improve the grinding fineness to ensure the strength of the cement.

If forced construction is carried out, the concrete will lose its strength.

The fineness of cement directly affects a series of physical properties such as the setting and hardening speed, strength, water demand, water precipitation rate, dry shrinkage, and hydration heat of cement.

Therefore, in cement μ The particles of m usually account for more than 90%.

The fineness of cement can be expressed in terms of percentage of sieve residue, specific surface area, average particle diameter, and particle grading.

If the specific surface area is too small, the cement is prone to bleeding and loses its gelling effect; If the specific surface area is too large, the water demand for cement will significantly increase, which is easy to cause shrinkage of concrete pole pieces, causing cracks, and reducing the strength of cement pole pieces.

The fineness of ordinary Portland cement is expressed in terms of specific surface area, which is not less than 300m2/kg.

3、 Setting time The setting time of cement is divided into initial setting and final setting.

Therefore, water demand is also one of the important building properties of cement.

The main reason for rapid setting is due to the absence or lack of gypsum in the secondary cement.

Under the same fineness and particle composition, the greater the sphericity of cement particles, the smaller the water demand, and the better the compatibility with additives.

Water Demand: When preparing cement paste, mortar, or mixing concrete, a certain amount of water needs to be added.

It is generally believed that 3 to 30% of cement μ The particles of m mainly play a strength increasing role, while the particles larger than 60 μ Due to the low hydration degree of m particles, they have little contribution to the strength of cement.

Generally speaking, clinker with high C3S content is easy to grind, while clinker with high C2S content is difficult to grind.

1、 Fineness and Specific Surface Area Cement is generally composed of particles with different sizes from a few microns to tens of microns, and its fineness (particle size) is called cement fineness.

False setting: A phenomenon in which cement solidifies within a few minutes after adding water, but does not emit a certain amount of heat like rapid setting.

The former is mainly used for cement paste, while the latter is mainly used for cement mortar and concrete.

Generally, the fineness index of cement is determined based on its strength.

Initial setting refers to the time from the time of mixing cement with water until the cement slurry begins to lose its plasticity, and final setting refers to the time from the time of mixing cement with water until the cement slurry completely loses its plasticity and begins to generate strength.

ten μ The particles below m have a large specific surface area, a large water demand, and a fast hydration rate, resulting in an unfavorable flow performance of cement.

This moisture, on the one hand, acts as a hydration agent with the cement powder to set and harden it, and on the other hand, it provides a certain fluidity for the cement paste, mortar, and concrete to facilitate operation and construction.