Antifreeze is an admixture that can harden commercial concrete at negative temperatures and achieve sufficient antifreeze strength within a specified time. Our country stipulates that when the temperature is lower than 5℃ for 5 consecutive days, the winter construction phase will be entered. The vast northern regions of our country have relatively cold temperatures in winter, which last for 4 to 6 months every year. Beijing belongs to the North China region. Under normal circumstances, winter construction begins on November 15th every year and ends on March 15th of the following year. The winter construction time is about 4 months. Commercial concrete antifreeze is an admixture that must be used in winter construction in northern areas. It is also one of the main types of commercial concrete admixtures.
The development of antifreeze in my country has generally gone through several types in terms of composition: chlorine-containing salt type, chlorine salt rust-inhibiting type, chlorine-free high-alkali type and chlorine-free low-alkali type. At this stage, antifreeze containing chlorine salts was not allowed to be used in reinforced commercial concrete and prestressed commercial concrete structures. Now most of them are used as mortar antifreeze. Judging from the dosage of antifreeze in commercial concrete, antifreeze has also experienced a development process from high dosage (10% to 15%) to low dosage (3% to 5%). Due to the extensive use of ready-mixed commercial concrete, the form of antifreeze has gradually evolved from powder to mainly liquid products. In the next few years, Beijing will promote the application of chlorine-free, low-alkali, low-dosage, high-efficiency, liquid antifreeze, and encourage the development of liquid antifreeze with organic components and organic-inorganic component composites. Liquid antifreeze, and actively develop air-entraining antifreeze. At present, our country has not yet made a major breakthrough in the research and development of raw materials for new antifreeze, and has yet to produce new antifreeze products with high cost performance and high quality.
The antifreeze produced by admixture companies in Beijing now generally contains water-reducing components, early-strength components, antifreeze components and air-entraining components. It is also in line with the internationally popular antifreeze development trend. Its principle is to obtain the antifreeze performance of commercial concrete by reducing the water consumption of commercial concrete, improving early strength as early as possible and preventing commercial concrete from being frozen at negative temperatures. Whether it is by lowering the freezing point of water in the capillary pores of commercial concrete or changing the crystalline morphology of ice, the hydration rate of cement at negative temperatures is very slow. What is more important is that commercial concrete should have the highest possible strength when entering a negative temperature environment to improve its ability to resist frost damage. Various countries have stipulated the critical freezing strength of commercial concrete. my country and the United States stipulate that the critical freezing strength of commercial concrete is 3.5MPa, which means that commercial concrete will not be susceptible to frost damage after its strength reaches 3.5MPa. The antifreeze and early-strength components that can be selected now that do not contain alkali, chlorine salts, and ammonia are generally calcium nitrate and calcium nitrite. Calcium formate in organic matter also has a good early-strength effect, but there are fewer manufacturers. Currently rarely used. The problems with calcium nitrate and calcium nitrite are that there is precipitation when preparing liquid antifreeze, the slump loss of fresh commercial concrete is fast, and the early strength and antifreeze effect are not as good as the corresponding sodium salts. Therefore, many manufacturers currently use sodium salts and calcium salts. Measures for the use of salt compounds.
According to statistics from the Japan Commercial Concrete Admixture Association, the main ingredients of chlorine-free and low-alkali antifreeze products on the market are nitrates, nitrites and some special surfactants. , its alkali content is generally less than 0.2%, and its 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 to 5 liters/100 kg of cement, which is approximately equivalent to 4% of the cement weight. ~7%. Among them, NMB Company’s antifreeze uses polyethylene glycol ester and inorganic nitrate as antifreeze components. Some companies in our country have used ethylene glycol as an antifreeze component, and even methanol as an antifreeze component, but there are no reports on its effectiveness. However, methanol is a toxic substance, and its use in commercial concrete is certainly much more harmful to the environment than urea.
Research on antifreeze so far has focused on the selection of early strength components and antifreeze components, while the impact of water-reducing components on antifreeze performance is not relevant Research reports. Beijing Kefeng Building Materials Factory has conducted a large number of experimental studies on water-reducing components for antifreeze. Melamine is sulfonated with N-sulfonic acid, and -NH2 groups are introduced into the water-reducing agent molecules and polymerized under appropriate acidity. The modified melamine high-efficiency water-reducing agent was prepared, and its performance indicators are shown in Table 1. This high-efficiency water-reducing agent has high water-reducing rate, high early-stage strength, good anti-freeze performance within -10°C, no chlorine salt, low alkali content, and no crystal precipitation. Adding a small amount of anti-freeze components can meet the requirements of anti-freeze ( -10℃), it has opened up a new idea in the preparation of liquid low-alkali commercial concrete antifreeze, which is worthy of reference and further in-depth research.
In order to explore the impact of different water-reducing components on the antifreeze performance of commercial concrete, we designed a method using ordinary naphthalene series high-efficiency water-reducing agent (UNF-5), sulfamate Test on the performance of acid salt superplasticizer (QJ-5) and modified melamine superplasticizer (QJ-5A) as water-reducing components to prepare antifreeze. The test was conducted using Jidong 42.5 ordinary Portland cement and was conducted with reference to JC472-92 (96) "Commercial Concrete Antifreeze" and DBJ01-2002 "Commercial Concrete Admixtures and Application Technical Regulations" (Pumping Antifreeze). During the test, the commercial concrete was put into the mold for 3 hours and moved into a freezer at -10℃~-12℃, and was taken out after negative temperature curing for 7 days.
The experimental results show that because the special molecular structure of QJ-5A itself contains more amino groups, as well as hydrophilic functional groups such as carboxyl groups and sulfonic acid groups, although the gas content is low, it will not dope at negative temperatures. Adding antifreeze components can still ensure that the strength of commercial concrete continues to increase. R-7 reaches 21.5% of the strength of benchmark commercial concrete, meeting the standard requirements. Products mixed with sulfamate-based high-efficiency water-reducing agents and naphthalene-based high-efficiency water-reducing agents The strength of concrete increases slowly at negative temperatures and cannot meet the standard requirements. After the modified melamine high-efficiency water-reducing agent QJ-5A is compounded with an air-entraining agent, its frost resistance has improved, and its R-7+28 strength exceeds the benchmark The standard strength of commercial concrete is maintained for 28 days. After further compounding with air-entraining agent and a small amount of anti-freeze ingredients, the strength of commercial concrete R-7 is significantly improved, but the strength increase of R-7+28 is not obvious. Modified melamine-based high-efficiency water-reducing agent is used in At normal temperatures, its strength and water-reducing rate are not much different from naphthalene-based and sulfamate-based high-efficiency water-reducing agents, but the strength development is significantly different at negative temperatures. In addition, whether it is compounded or used alone, modified melamine-based superplasticizers High-efficiency water reducing agents all have good slump retention properties.
Considering the transportation and storage of liquid antifreeze, liquid admixture products are required to have good low temperature Stability performance. The stability of different high-efficiency water-reducing agents at negative temperatures was tested. The results are shown in Table 2. The modified melamine-based high-efficiency water-reducing agent does not produce crystallization when stored at -10°C, and there is no obvious change in appearance.
Modified melamine high-efficiency water-reducing agent contains a large number of amino groups and other hydrophilic groups, which has a good water-reducing and dispersing effect and reduces the slump of fresh commercial concrete. The degree of loss is small. The modified melamine superplasticizer itself has good stability under negative temperature conditions and does not produce precipitation and crystallization. The commercial concrete mixed with the modified melamine superplasticizer QJ-5A has rapid strength development under negative temperatures. , compounding an appropriate amount of antifreeze components can prepare a liquid antifreeze that meets the standard requirements, providing a new idea for the preparation of chlorine-free, low-alkali liquid antifreeze.
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