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Anti-Freeze Additives for Concrete
Anti-Freeze Additives for Concrete
If concrete freezes, ice crystals, because of their expansion, destroy (tear apart) the structure of the concrete, and the resulting product afterward has extremely low strength and worsened other characteristics.
To solve this problem, you can use:
– heating the structure until the mix has set;
– adding special additives that prevent water from freezing.
Such anti-freeze additives allow work to be carried out down to minus 15 degrees.
However, the additives introduced change the properties of the final product, both positively and negatively.
Mechanism of Action of the Additives:
They lower the freezing point of the water in the concrete mix, which prevents it from freezing during delivery and before the mix actually sets. This allows the reaction between the water and the cement powder to proceed completely. After the additive is introduced, the concrete becomes frost-resistant.
GOST 24211-91 recommends the use of anti-freeze additives in the temperature range of minus 10-15°C. In practice, the additives make it possible to use concrete even at lower temperatures, but the setting process will take longer.
Anti-Freeze Additives Can Be Divided Into Groups by Degree of Effect:
– those that lower the freezing point of water. These also have a minor effect on the setting speed of the mix.
– those that promote rapid formation of a dense cement structure (stone). Heat is released during the reaction with the mix components. Does not lower the freezing point of water. (mainly sulfates).
– accelerating anti-freeze additive. Increases the solubility of the mix components. Some also lower the freezing point of water.
Complex formulations combining several groups of additives are often used.
Let’s Look at the Main Chemical Substances That Are Anti-Freeze Additives:
Potash — an anti-freeze hardening accelerator. To prevent a reduction in the strength of the final product, it is often used together with setting retarders. Content not more than 30%.
Sodium formate is an anti-freeze additive that accelerates hardening. It is used with naphthalene lignosulfonate to increase the plasticizing properties of the mix and increases water reduction (decreasing the amount of water in the mix). Content 2-6% of the volume of cement.
Sodium nitrite — accelerates hardening. It is forbidden to use it mixed with lignosulfonates because of the release of a poisonous gas during the reaction. Content in the mix up to 10% of the mass of cement. Used down to minus 25°C.
Ammonia water (aqueous ammonia) — an anti-freeze additive. Does not corrode reinforcing elements, and is a corrosion inhibitor with respect to the other components of the mix. It slows down the hardening of the mix. Compared to potash and calcium chloride, it has a lower degree of expansion.
The Concentration Is Calculated Based on the Ambient Temperature:
down to minus 10°C — 5% aqueous ammonia solution;
in the range of minus 10-20°C — 10% aqueous ammonia solution;
in the range of minus 20-35°C — 15% aqueous ammonia solution;
below minus 35°C — 20% aqueous ammonia solution.
Calcium and Sodium chloride — reduce setting time. The main drawbacks are corrosion of reinforcing elements and the appearance of efflorescence. They are introduced together with Sodium nitrite to reduce corrosion of reinforcing elements caused by calcium chloride. Ammonia water is also used for this purpose (it was tested during the construction of the Kremenchuk Hydroelectric Power Station).
Carbamide (Urea) — not recommended for use as a standalone additive. As an additional additive, it acts as a plasticizer, converts salts (nitrites and nitrates) into poorly soluble forms, and lowers the freezing point. It also increases closed porosity. It is often used together with calcium nitrate. When used together with Calcium nitrate (1 to 4), despite a small decrease in freezing point, the mix hardens well at reduced temperatures.
Sodium thiocyanate — a setting accelerator. It is a component of many ready-made complex anti-freeze additives produced by domestic manufacturers.
Borax (Sodium tetraborate) — an anti-freeze additive. It helps preserve the integrity of the product after thawing, and also prevents crack formation in the monolithic structure. It reduces water permeability and increases strength by up to 30%.
It is recommended as an additional additive when using calcium carbonate (Potash) to increase strength after thawing.
Soda ash (Sodium carbonate) — a hardening accelerator.
Dependence of setting speed on the amount of soda in the mix:
0% Initial set: 100 min Final set: 305 min
2% Initial set: 5 min Final set: 45 min
5% Initial set: 3 min Final set: 17 min
However, such rapid hardening significantly reduces strength.
Trisodium phosphate — accelerates the hardening of concrete. It can be used in small amounts. Adding up to 1.5% is sufficient. It is used on its own only at positive temperatures.
Ammonium chloride — a mix setting accelerator and anti-freeze additive.
Sodium or potassium silicate (liquid glass) — a strong hardening accelerator. The final product becomes more moisture-resistant and gains increased strength.
Hydrochloric acid together with Lime (quicklime) — a hardening accelerator. It can reduce work time by up to 4 times. Heat is released when the lime is slaked.
Potassium sulfate — increases frost resistance, improves water resistance. It also reduces shrinkage.
Particular Features:
During the hardening of the mix, additives can redistribute throughout the volume and accumulate in corners and other locations. This can affect the uniformity of property distribution.
Also, upon setting, additives may change volume during crystallization, which can lead to internal stresses and defects.
The use of anti-freeze additives should be carried out under the guidance of a specialist, who will select the necessary additives to use and calculate the required amount.
A large number of ready-made formulations have now been developed, as well as various special additives, for example polycarboxylate superplasticizers.
Article about the production of porous concrete using hydrogen peroxide.
