Early strength agents for concrete are a compound that may speed up the growth of strength in the beginning of concrete. Adding an appropriate amount of early strength agent to concrete can significantly shorten the curing cycle of concrete, improve construction efficiency, and enhance the frost resistance and durability of concrete.
Different early strength agents or the same early strength agent are added to different types of cement concrete, and their effects are not completely the same. Here, only a few early strength agents are taken as representatives to analyze their principles of action.
Chloride-based early strength agent: The main mechanism of action is the interaction between chloride and C3A in cement, generating insoluble hydrated chloroaluminates that can accelerate the hydration of C3A in cement. The calcium hydroxide that is formed through the hydration of chloride, as well as cement, creates calcium chloride that is insoluble, which reduces the level of calcium hydroxide present in liquid phases. It also speeds up the hydration rate of C3A, and the generated complex salt increases the volume of the solid phase in the cement slurry, forming an internal skeleton system, which is conducive to the formation of cement stone structure. At the same time, due to the fact that chlorides are mostly soluble salts, they have a salt effect. They can promote the dissolution rate of Portland cement clinker minerals and accelerate the hydration reaction process, thereby accelerating the hardening rate of concrete mixtures and improving the early strength of concrete.
Sulfate-based early strength agents, such as anhydrous sodium sulfate, dissolve in water and react with calcium hydroxide generated by cement hydration to produce calcium oxide and calcium sulfate. The newly generated calcium sulfate particles are extremely fine and have much higher activity than those doped with calcium sulfate, thus reacting with C3A to generate hydrated calcium sulfoaluminate at a much faster rate. Sodium hydroxide is an active agent that can improve the solubility of C3A and gypsum, accelerate the amount of calcium sulfoaluminate in cement, and lead to the hardening and early strength improvement of cement. However, sulfate early strength agents have a certain corrosive effect on the steel bars in concrete, including chloride early strength agents, and attenuate the later strength of cement mortar. Therefore, the dosage of chloride and sulfate early strength agents is gradually decreasing.
Impact on concrete mix
Early strength agents can generally shorten the setting time of concrete. Still, when the content of tricalcium aluminate in cement is low, or the content of tricalcium aluminate is lower than that of gypsum, sulfate will delay the setting time of cement. Early strength agents generally do not increase the air content in concrete. The air content of the water reducing agent determines the air content of early strength water reducing agents. If combined with sugar calcium water reducing agents, the air content does not increase, but when combined with wood calcium water reducing agents, the air content increases significantly.
Impact on concrete
Early strength agents can improve their early strength. The degree of improvement of the same type of early strength agent depends on its dosage, environmental temperature, curing conditions, W/C, and cement variety. The impact on the long-term strength of concrete is not consistent, ranging from high to low. Early strength agents have good effects within a reasonable dosage range, but when the dosage is large, it will have adverse effects on the later strength and durability of concrete. Early strength water reducing agents also have good early strength effects, and their performance is better than that of early strength agents, which can control the later strength changes. Triethanolamine can stimulate the early strength effect of cement, which can accelerate the hydration of tricalcium aluminate but delay the hydration of tricalcium silicate and dicalcium silicate. Excessive dosage can cause a decrease in concrete strength.
Durable sulfate based early strength agents have no corrosion effect on steel bars. Still, chloride early strength agents contain a large amount of chloride ions, which have a promoting effect on steel bar corrosion. When the dosage is large, it will also reduce the chemical corrosion resistance, wear resistance, freeze-thaw resistance, etc., of concrete, reduce the flexural strength of concrete, increase the early shrinkage of concrete, and have little impact on the later stage of concrete. At present, the use of admixtures containing chlorides is prohibited in the new national standard regulations. To prevent the impact of chloride salts on steel corrosion, rust inhibitors and chloride salts are often used in combination.
When using sulfate early strength agent, due to the increase in alkalinity in the concrete liquid phase, it should be noted that when the aggregate contains active silica, it will promote the reaction between alkali and aggregate, causing the problem of concrete damage due to alkaline expansion.
Triethanolamine will increase the shrinkage of concrete. When the dosage is greater than 0.05%, it will reduce the later strength, and the larger the dosage, the more it will decrease.
Early strength water reducing agent, due to its function as a water reducing agent, can compensate for the defects of early strength agent, change the microstructure of the inner pores of the concrete, improve its compactness, and thus improve the freeze-thaw resistance, admixture resistance, compression resistance, bending resistance, elastic modulus, and steel bar bonding of concrete. Within a reasonable dosage range, it has no adverse effect on steel bar corrosion, and early shrinkage slightly increases. It has been widely used in China.
A large number of tests have shown that sulfate series early strength agents can improve the impermeability of concrete.
The deformation: early strength agent generally has little effect on the deformation (dry shrinkage) of concrete.
Concrete early strength agents have been applied in many construction fields, and the following are several common application scenarios:
Low temperature environment construction: When conducting concrete construction in a low temperature environment with a daily minimum temperature of no less than minus 5 degrees Celsius, adding concrete early strength agent can accelerate the setting and solidification process of cement concrete, avoid freezing damage to cement concrete in colder environments, and accelerate the operation of formwork, improving work efficiency.
Rapid opening of road surface and bridge deck pavement layer: When constructing concrete for road surface or bridge deck pavement layer, especially on level crossings of Class 1, 2, and 3 highways, early strength agents can be added to accelerate the hardening process of concrete, enabling the road surface or bridge deck pavement layer to reach the opening conditions more quickly.
Rapid maintenance of concrete: For rapid maintenance of concrete pavement and bridges, the application of early strength agents can significantly shorten repair time and improve repair efficiency.
It is worth noting that for concrete with early strength requirements or steam curing, as well as concrete constructed with a daily minimum temperature below 5 ℃, it is generally not recommended to use early strength agents. Therefore, when using concrete early strength agents, it is necessary to select and add them based on specific construction conditions, requirements, and the properties of the concrete.
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