Research shows that the main cause of sanding is concrete bleeding, which causes the water-cement ratio of the concrete surface to be too large and the hydration is relatively sufficient. The main reasons affecting concrete bleeding are raw materials, mix ratio, construction and maintenance.

Raw material reasons

1) The cement strength grade is low, and the required cement strength grade is not used, resulting in the concrete strength after pouring not meeting the standard.

Or cement that has exceeded the shelf life is used. After the cement exceeds the shelf life, it absorbs water by itself, causing the cement to become damp and agglomerated. Which reduces the strength grade of the cement.
Under the action of external forces, the concrete surface peels off and sanding is formed. Concrete sanding is caused by the choice of cement variety. Different cement varieties have large differences in workability. For example, silicate cement should be used in roads, but composite silicate cement is used for concrete mixing, which causes concrete sanding.
The reason is the amount of admixture in composite silicate cement is much higher than that of silicate cement. When composite silicate cement is used for concrete preparation, during the pouring and tamping process, fly ash and other light-density admixtures tend to float to the surface of the concrete. This reduces the strength of the surface concrete. And in severe cases, it will cause the surface of the concrete to fail to condense and harden normally.

2) Fine aggregates such as sand have a high fineness modulus, large particle size, and large porosity.

Under the same cement dosage, the density of concrete is relatively low, and therefore the strength is also low. And the coarser the fine aggregates such as sand, the easier it is for the concrete to segregate and bleed, thereby reducing the strength of the concrete surface and causing the concrete to sand.
On the contrary, the finer the fine aggregates such as sand, the more cement paste is needed. Under the same cement dosage. If the fine aggregates such as sand are too fine, segregation and other phenomena will occur, which will reduce the strength of the concrete and form sand.

3) The particle grading of sand is also an important reason for the sanding of concrete.

The grading of sand is poor and the voids are large. When the amount of cement remains unchanged, the voids need to be filled with a large amount of cement paste. This causes some aggregates to fail to be wrapped by the paste, resulting in low concrete strength and sand on the concrete surface.

4) The water-cement ratio is too large and the amount of cement is too little.

Water seepage occurs on the surface, causing the strength of the mortar or fine stone concrete to decrease and easy to sand.

5) Excessive admixture in the concrete.
This will cause a large amount of water seepage in the freshly mixed concrete. A large amount of free water seeps out of the concrete surface. This affects the normal coagulation and hardening of cement. This causes sand.

6) The mud content of coarse and fine aggregates is large, which seriously affects the early hydration of cement.

This causes water seepage in the concrete and causes sand. The reason is: the mud in the aggregate wraps the cement particles. And the water and cement particles cannot be fully in contact, which delays the hydration of cement.
Analysis of the causes of concrete sanding(Ⅱ)

HPMC FOR DIR MIX MORTAR