Concrete early strength admixture is a kind of admixture which can improve the early strength of concrete and has no significant effect on the later strength.The main function of strength enhancer is to accelerate the hydration rate of cement and promote the development of strength enhancer of concrete.It not only has early strong function, but also has certain water reduction and enhancement function.Concrete strength enhancer is one of the earliest admixtures in the history of development.So far, people have developed many kinds of strength enhancer in addition to chlorine salt and sulfate, such as nitrite, chromate, and organic strength enhancer, such as triethanolamine, calcium formate, urea and so on.
1.Chlorine salt early strength enhancer
The main components of chlorine salts are sodium chloride, calcium chloride, potassium chloride, aluminum chloride and so on.The mechanism that can produce early strength is mainly the effect of chloride and C3Al in cement, which can form hydrated chloraluminate which is insoluble in water and accelerate the hydration of C3Al in cement.The calcium hydroxide formed by the hydration of chloride and cement generates calcium chloride that is not easily soluble in water, which reduces the concentration of calcium hydroxide in liquid phase and accelerates the hydration rate of C3S. Moreover, the compound salt generated increases the volume of solid phase in cement slurry and forms the internal framework system, which is conducive to the formation of cement stone structure.At the same time, because the chloride is more soluble salt, has the effect of salt, can promote the dissolution rate of Portland cement clinker mineral, accelerate the hydration reaction process, so as to accelerate the hardening rate of concrete mixture, improve the early strength of concrete.
Precautions when using chlorine salts as strength enhancer:
In terms of early strength effect alone, the chloride system is the best material for strength enhancer and the earliest application of strength enhancer in construction engineering.However, due to the increase of chloride ion concentration, a larger electrode potential will be formed between steel bars and chloride ions in concrete, which will easily cause corrosion of steel bars, so it cannot be applied in reinforced concrete structural engineering.Therefore, the application of this kind of strength enhancer has great limitations. It is not allowed to be used in concrete structures with steel reinforcement.
2.Sulfate strength enhancer
Sulfate series strength enhancers are mainly sodium sulfate, calcium sulfate, aluminum sulfate and potassium sulfate and other materials.The most widely used is sodium sulfate strength enhancer, it is sulfate chemical plant production of mirth ore downstream products.Sodium sulfate strength enhancer under lower ambient temperature and early strength development is slow cement use effect is obvious, and the late strength has been greatly improved.It can not only accelerate the growth of strength in normal and low temperature environment, but also improve the strength of concrete under steam curing condition.The strength enhancer mechanism of sodium sulfate is mainly as follows:
1) Sodium sulfate is a strong electrolyte, which can increase the ionic strength of cement liquid phase and generate compression effect on the diffusion of double electric layer.Thus, the absolute value of F potential decreases and the setting and hardening process of cement is accelerated.
2) gypsum and sodium hydroxide are generated when sodium sulfate interacts with free calcium hydroxide in the cement hydration process:
The formation of alkali increases the pH value of liquid phase, produces plasticizing effect, and increases the plastic strength of cement slurry.At the same time, the newly generated fine-grained dihydrate gypsum has a much faster hydration rate than the gypsum added during cement grinding, which promotes the formation of more hydrated calcium sulphoaluminate:
This further promotes the growth of early intensity.
Precautions for use of sulfate strength enhancers:
Because sodium sulfate strength enhancer and the new standard cement use strength enhancer effect is low, sodium sulfate strength enhancer, potassium ions in the defects of the concrete engineering community in recent years to talk about more.One of them is that K+ and Na+ do not combine with cement hydration products, and their salts are easily dissolved with water, so more remain in the liquid phase of concrete, which is the main cause of concrete surface whitening.Salt precipitation causes white pollution on concrete surface.This is extremely harmful to the surface of fair-faced concrete, and even more serious is the expansion of sodium salt in the surface of concrete crystals, which will cause cracking or falling off of the concrete surface, causing certain damage to the structure.Therefore, it is necessary to limit the application of sodium and potassium strength enhancer in concrete for the projects without decoration on the structural surface.
3. Organic series of strength enhancer
The organic series of strength enhancer are mainly triethanolamine, triisopropanolamine, methanol, ethanol, etc. The most commonly used triethanolamine.The early strength of triethanolamine is because it can promote the hydration of C3Al. In the system of C3Al -- CaSO4 -- H2O, it can accelerate the formation of ettringite, so it is beneficial to the early strength development of concrete.Because there is N atom in the molecule of triethanolamine, which has a pair of unshared electrons, it is easy to form covalent bonds with metal ions and form complex with metal ions.These complexes form many soluble regions in the solution, thus increasing the diffusion rate of hydration products.
Precautions when using organic series of strength enhancer:
Due to the formation of complex, C3Al hydrates formed on the surface of clinker particles and other products, such as calcium sulphate aluminate, will be destroyed in the initial hydration stage.As a result, the dissolution rate of C3Al and C4AlF is increased, and the reaction with gypsum is also accelerated to rapidly produce calcium sulphate aluminate, and the transformation between ettringite and calcium sulphate sulphoaluminate is accelerated.With the increase of the production of calcium sulfoaluminate, the concentration of Ca42+ and Al3+ in the liquid phase will be reduced, and the hydration rate of C3S will be further promoted.But in many engineering applications, people feel that the effect of strength enhancer of triethanolamine is not very ideal, because the mixing amount is very small, difficult to accurately grasp, such as bad control, mixing amount increase, but will make concrete retarding, strength is seriously affected.
4.Compound strength enhancer
Through the composition of various strength enhancer components, as well as the composition of strength enhancer and superplasticizer components, can be better than a single strength enhancer modification effect, such as: greatly improve the early strength of concrete development rate;It can not only improve the early strength of concrete, but also bring benefits to the later strength development of concrete.It can not only reduce water but also accelerate the early strength development of concrete.It can not only play a good effect of early strength, but also avoid some early strength components caused by concrete internal steel corrosion.
This shallow as you can see, salt and sodium chloride, potassium itself defects of concrete strength enhancer make the research and development of chloride and sodium, potassium strength enhancer is the urgent problem to be solved, although a lot of concrete science and technology workers at home and abroad have been concerned that chlorine salt strength enhancer about the quality of the concrete is more serious consequences, in recent years has been developing new products, strength enhancer without sodium chloride and good adaptability, K series of strength enhancer, the effect is obvious, and the cost is not high also, need this kind of product in the batch production and applied to the project as soon as possible.
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