Preparation technology of concrete antifreeze agent

Antifreeze is an additive that can harden concrete at negative temperatures and achieve sufficient antifreeze strength within a specified time. Concrete antifreeze is a necessary admixture for winter construction and one of the main types of concrete admixtures.

The development of antifreeze has progressed through several stages in composition, including chloride salt type, chloride salt rust-inhibitor type, chlorine-free high-alkali type, and chlorine-free low-alkali type. Among them, chloride salt antifreeze is not permitted for use in reinforced and prestressed concrete structures and is now mostly used as a mortar antifreeze. From the perspective of antifreeze dosage in concrete, it has also evolved from high dosage (10%~15%) to low dosage (3%~5%).

Due to the extensive use of ready-mixed concrete, the form of antifreeze has gradually evolved from powder to liquid products. With advances in technology, it is encouraged to develop liquid antifreeze agents and organic components that are chlorine-free, low-alkali, low-dose, and high-efficiency.

The antifreeze produced by current additive companies generally contains water-reducing, early-strength, antifreeze, and air-entraining components, which aligns with the international trend in antifreeze development. Its principle is to reduce water consumption in concrete, improve early strength, and prevent concrete from freezing at low temperatures to achieve the concrete’s antifreeze performance. Whether by lowering the freezing point of water in the capillary pores of concrete or by changing the crystalline morphology of ice, the hydration rate of cement at negative temperatures is very slow. More importantly, concrete should have the highest possible strength when exposed to negative temperatures to improve its resistance to freezing damage.

Each country has set the critical strength for concrete to withstand freezing. China and the United States have set the critical strength for concrete at 3.5 MPa, meaning that once the concrete strength reaches 3.5 MPa, it is less susceptible to freezing damage. The antifreeze and early-strength components currently available that do not contain alkali, chloride salts, or ammonia are generally calcium nitrate and calcium nitrite. Calcium formate in organic matter also has a good early strength effect, but there are few manufacturers, and it is currently rarely used.

The problem with calcium nitrate and calcium nitrite is that they precipitate during the preparation of liquid antifreeze agents, and fresh concrete experiences rapid slump loss. The early strength and antifreeze effect are not as good as those of the corresponding sodium salts. Therefore, many manufacturers currently use a combination of sodium and calcium salts as a measure.

According to the Japan Concrete Admixtures Association, the main components of chlorine-free and low-alkali antifreeze products on the market are nitrates, nitrites, and certain surfactants. Their alkali content is generally less than 0.2%, and their chloride ion content is less than 0.1%. They are all liquid products with a density between 1.30 and 1.45. The recommended usage is 3-5 liters per 100 kilograms of cement, which is approximately 4% to 7% of the weight of cement
Among them, NMB company’s antifreeze uses polyethylene glycol ester and inorganic nitrate as antifreeze components. Some companies have also used ethylene glycol and even methanol as antifreeze components, but their effectiveness has not been reported. But methanol is a toxic substance, and its use in concrete is definitely much more harmful to the environment than urea.

So far, research on antifreeze has focused on selecting early-strength and antifreeze components, and no relevant studies have reported the effect of water-reducing components on antifreeze performance. Studies have shown that a modified melamine superplasticizer can be prepared by sulfonating melamine with N-sulfonic acid, introducing -NH2 groups into the water-reducing agent molecules, and polymerizing under appropriate acidity.

This high-efficiency water-reducing agent has a high water-reducing rate, high early strength, good frost resistance down to -10 ℃, no chloride salts, low alkali content, and no crystallization or precipitation. Adding a small amount of antifreeze components can meet the quality requirements of antifreeze (-10 ℃). It has introduced a new approach to the preparation of low-alkali liquid concrete antifreeze and is worth learning from and further in-depth research.

To investigate the effects of different water reducing components on the antifreeze performance of concrete, we designed experiments using the ordinary naphthalene-based high-efficiency water reducing agent (UNF-5), the amino sulfonate high-efficiency water-reducing agent (QJ-5), and the modified melamine high-efficiency water-reducing agent (QJ-5A) as water-reducing components to prepare antifreeze agents. The experiment was conducted using Jidong 42.5 ordinary Portland cement, in accordance with JC472-92 (96) “Concrete Antifreeze” and DBJ01-2002 “Technical Specification for Concrete Admixtures and Applications” (pumping antifreeze). During the experiment, the concrete is placed in a freezer at -10 ° C to -12 ° C after being molded for 3 hours. After 7 days of negative-temperature curing, it is removed.

The experimental results show that due to the unique molecular structure of QJ-5A, which contains a large number of amino groups, as well as hydrophilic functional groups such as carboxyl and sulfonic acid groups, although the gas content is low, the concrete strength can still continue to increase at negative temperatures without the addition of antifreeze components. R-7 reaches 21.5% of the benchmark concrete strength, meeting the standard requirements.

However, the strength of concrete mixed with amino sulfonic acid salt, high-efficiency water reducers and naphthalene-based high-efficiency water reducers increases slowly at negative temperatures, which does not meet the standard requirements. The modified melamine high-efficiency water reducing agent QJ-5A has improved its frost resistance after being compounded with an air-entraining agent. Its R-7+28 strength exceeds the standard 28-day concrete strength. Further compounding with an air entraining agent and a small amount of antifreeze components significantly improves the R-7 strength of concrete, but the increase in R-7+28 strength is not very significant.

The strength and water reduction rate of modified melamine-based high-efficiency water reducing agents are not significantly different from those of naphthalene-based and amino sulfonic acid-based high-efficiency water reducing agents at normal temperatures, but the strength development is significantly different at negative temperatures. In addition, whether used alone or in combination, modified melamine-based high-efficiency water-reducing agents exhibit good slump retention.

Given the transportation and storage of liquid antifreeze, liquid additive products must exhibit good low-temperature stability. Experiments were conducted on the stability of different high-efficiency water reducing agents at negative temperatures, and the results showed that the modified melamine-based high-efficiency water reducing agent did not exhibit crystallization or significant changes in appearance when stored at -10 ℃.

Modified melamine high-efficiency water-reducing agent contains numerous amino and other hydrophilic groups, which exhibit good water-reducing and dispersing effects, and the slump loss of fresh concrete is small. The modified melamine high-efficiency water reducer itself has good stability at low temperatures, without precipitation or crystallization. Concrete mixed with the modified melamine series high-efficiency water reducer QJ-5A shows rapid strength development at negative temperatures. By combining appropriate antifreeze components, a liquid antifreeze agent that meets standard requirements can be formulated, offering a new approach to the development of chlorine-free, low-alkali liquid antifreeze agents.