Cement early strength agent is an additive that can improve the early strength of concrete, mostly used in winter or emergency repair. Cement early strength agent does not have a significant effect on the late strength of concrete. Cement early strength agent varieties are calcium chloride, sodium chloride, nitrate, gypsum, water glass, sodium aluminate, sodium carbonate, calcium carbonate, sodium fluoride, triethanolamine, and so on.
Cement early strength agent is one of the earliest varieties used in the development history of admixtures. Its main function is to accelerate the hydration rate of cement and it promotes the development of early strength of concrete. Cement early strength agents are in great demand in engineering as an admixture to accelerate the early strength development of concrete, especially in winter construction or in emergency repair works. At the same time, the performance of cement early strength agent and the standardization of its use are also directly related to the safety and longevity of the whole project.
The following three kinds of early strength agents are commonly used:
(1) Chloride early strength agent
CaCl2 in addition to improving the early strength of concrete also has the effect of promoting coagulation and anti-freezing. It is inexpensive and easy to use, and the general mixing amount is 1%~2%. However, it can lead to corrosion of reinforcing steel. When CaCl2 is used in concrete, its dosage should not exceed 1% of the amount of cement. It is usually used in combination with NaNO2, a rust inhibitor.
(2) Sulfate bath early strength agent
Sodium sulfate, also known as anhydrous sodium sulfate, is a white powder. Most of the composite use suitable dosages of 0.5% ~ 2%, such as NC, sodium sulfate, calcium sugar, and green sand mixed with a compound early strength agent.
(3) Organic bath early strength agent
Organic series early strength agents mainly include triethanolamine, triisopropanolamine, methanol, ethanol, etc. The most commonly used is triethanolamine. Triethanolamine is a colorless or light yellow transparent oily liquid, easily soluble in water. Generally, it is not mixed alone and the dosage is 0.02%~0.05%. It is often used in combination with other early strength agents to have a retarding effect.
1) Effect on concrete mixture
Early-strengthening agents can generally shorten the setting time of concrete. However, when the content of tricalcium aluminate in cement is low or the content of tricalcium aluminate is lower than that of gypsum sulfate will retard the setting time of cement. Early strength reducers generally do not increase the air content in concrete. The gas content of the early strength reducer is determined by the gas content of the water reducing agent, such as the gas content does not increase when compounded with sugar calcium water reducing agent, and the gas content increases significantly when compounded with wood calcium water reducing agent.
2) Effect on concrete:
Early strength agents can improve the early strength of concrete. The degree of improvement of the same type of early-strength agent depends on the amount of early-strength agent incorporated, ambient temperature, curing conditions, water-cement ratio, and cement variety. However, the effect on the long-term strength of concrete is not uniform, sometimes increasing and sometimes decreasing. Early-strengthening agents are more effective within a reasonable admixture range, but when the admixture is exceeded, the late strength and durability of the concrete will be adversely affected. Early-strength water-reducing additives also have a better early-strength effect, and their performance is better than that of early-strength agents, which can control the change of concrete strength in the late stage. Triethanolamine can stimulate the early-strengthening effect of cement and accelerate the hydration reaction of tricalcium aluminate but will retard the hydration reaction of tricalcium silicate and dicalcium silicate. If the dosage is exceeded, it will cause the strength of the concrete to decrease.
Sulfate-based early strengthening agent has no rusting effect on steel bars. However, the large amount of chloride ions in the chloride early strength agent will promote the corrosion of steel bars. When the dosage of chloride early strength agent is excessive, it will not only reduce the chemical erosion resistance, abrasion resistance, and freeze-thaw resistance of concrete, but also reduce the flexural strength of concrete and increase the early shrinkage of concrete. However, the later impact on concrete is not significant. According to the new national standards, the use of chloride-containing additives has been banned. To prevent the corrosion effect of chlorine salt on steel bars, rust inhibitors, and chlorine salts are usually used in combination.
It is also important to note when using sulfate early strength agents that they increase the alkalinity in the liquid phase of the concrete, and therefore when reactive silica is present in the aggregate, it can contribute to the occurrence of alkali reaction with the aggregate, causing problems with the destruction of the concrete due to alkaline expansion.
Triethanolamine increases the shrinkage of concrete, and at a dosage greater than 0.05%, it reduces the strength in the later stages, and the higher the dosage the greater the reduction. Early-strength water-reducing agents can make up for the defects of early-strength agents due to the role of water-reducing agents. So that the pore microstructure of concrete changes. Densification to improve the concrete resistance to freezing and thawing and resistance to mixing, compression, flexural, modulus of elasticity, and steel bonding are improved. In a reasonable range of admixture on the corrosion of steel reinforcement has no adverse effect, early contraction slightly increased, widely used in China.
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