Why does dry-mixed mortar have the problem of alkali return?

Cause analysis:

In the rush period (common in winter and spring), use Na2SO4\CaCl or their composite products as early strength agents to increase the soluble substances of cement-based materials.
The presence of a certain amount of alkali in the material itself is a prerequisite, and the reason for this is that cement-based materials are porous materials. There are a large number of capillary pores of different sizes inside, which become channels for soluble substances to migrate from the inside to the surface under the drive of water.
Cement-based materials are subjected to rainwater soaking during use, when moisture seeps into its interior. It brings out the soluble substances inside, reacts and precipitates on the surface. Acid rain penetrates into the interior of the substrate, combines with the alkaline substances in the substrate and crystallizes as the moisture migrates to the surface, which can also cause whitening.

Prevention and control measures:

1.There is no cure, only to reduce the probability of its occurrence as much as possible. Control the amount of water added during the mixing process of ready-mixed mortar. During construction, the floor material should not bleed, and the surface should not be exposed to rainwater before it is completely dry.
2.Try to use low-alkali cement and admixtures.
3.Optimize the mix ratio, increase the compactness of cement-based materials, and reduce capillary pores. For example, use other clinker and filler to replace part of the cement.
4.Use of reversion inhibitors. Such as ELOTEX ERA-100, but this situation cannot be completely improved after use, but can only alleviate the situation of alkali reversion to a certain extent.
5.Avoid construction under dry, windy and low temperature conditions.
6.The combined use of Portland cement and high-strength sulfoaluminate cement has a certain effect.

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