Naphthalene type Superplasticizer(Sodium Napthalene Sulphonate) is still the most widely used admixture nowadays due to its high cost-performance rate, while the Na2SO4 in the napthalene superplasticizer plays an very important role. With certain amount of sodium sulfate, it can increase the cement paste fluidity, also improve the concrete initial strength. But in the winter seasons, crystallization of sodium sulfate limits the usage of naphthalene based superplasticizer badly.
1. When the outdoor temperature bellow 16℃, the sodium sulfate will crystallize, which will then cause chain extension, development and growth, causing blockage of pipes and valves. As a result, the water reducing agent cannot enter the mixer, and the mixing station cannot be produced normally.
2. The crystallization will inevitably adsorp some effective components of napthalene superplasticizer, affecting water reducing ability and concrete strength.
Main reasons of Crystallization
The marketing-sold naphthalene type water reducing agent generally has a sodium sulfate content of 18-20% or even higher. The solid content in the aqueous solution is approximately 33-40%, and sodium sulfate is approximately 6 to 8%. Since the solubility of sodium sulfate is particularly affected by temperature changes, the sodium sulfate has a sudden drop when temperature bellow 20 ℃,which dropped from 40.8g per 100g of water to only 19.4g; after testing, crystallization does not occur only when the temperature is above 16 ℃.
Solutions of Na2SO4 Crystallization
1. Warm/Heat the aqueous solution to make sure the temperature is above 16℃.
2. Mix with or change to low sodium sulfate content napthalene superplasticizer, such as 5% model. Several months ago, my Chile customer use this method solved their stock of high sodium sulfate content napthalene superplasticizer.
3. According to the characteristics of sodium sulfate dissolved in water, the reason why the admixture crystallizes at low temperature is because at this temperature, the unit mass of water cannot dissolve so much sodium sulfate, and the excess sodium sulfate can only crystallize and precipitate. So, we can lower the concentration(increase the water quantity) of the aqueous solution.
4. Use sodium sulfate crystallization inhibitors.
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