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Is concrete sticking and bleeding really a problem with water reducing agents?
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A water reducing agent is an important additive that ensures concrete strength and pumping performance, playing a crucial role in improving the operability of concrete construction. This article accurately determines the optimal dosage of concrete water reducing agent based on the mechanism of action and influencing factors in concrete construction, finds the method of dosage of water reducing agent, verifies the compatibility and adaptability of water reducing agent with cement, ensures the quality of the project in a simple, accurate and reliable manner, reduces the cost of the project, and achieves the best results in the construction project.
After mixing cement with water, gravitational forces between cement particles form many flocculent structures that envelop 10% to 30% of the water and greatly reduce the fluidity of the concrete mixture. When an appropriate amount of water-reducing agent is added, the molecules of the water-reducing agent are directionally adsorbed on the surface of the cement particles, and the hydrophilic groups of the water-reducing agent are directed towards the aqueous solution. As the hydrophilic groups dissociate, the cement particle surface acquires the same electrical charge. As the concentration of the water-reducing agent increases, electrostatic repulsion increases, causing the cement particles to disperse, the coagulation structure to disintegrate, and the number of capillary pores to decrease significantly. The mixed water enclosed in it is released, which can effectively increase the fluidity and compactness of the concrete mixture.
After adding an appropriate amount of water-reducing agent to concrete, the water-cement ratio of concrete can be reduced, the amount of cement used can be reduced, the early strength of concrete can be improved, the demoulding time of concrete can be advanced, the turnover of formwork can be accelerated, the construction period can be shortened, and the project cost can be reduced.
High-efficiency water reducing agents have compatibility issues with various concrete components, with cement having the greatest impact. Compatibility varies significantly between the same cement type and different high-efficiency water-reducing agents, and between the same high-efficiency water-reducing agent and different cement types. These differences can affect the rheological properties of concrete, thereby affecting its workability, strength, and durability.
Therefore, a series of comparative tests on various types of concrete water reducers, including their performance and dosage, is necessary to determine the optimal dosage.
Different cement types have different clinker components, and higher MgO content in the clinker increases the impact of water reducing agents on concrete performance, making microcracks more likely.
Efficient water reducing agents selectively adsorb on mineral clinker. When C3A comes into contact with water, it immediately has a sufficient adsorption layer. The more C3A is present, the more efficiently the water-reducing agent can adsorb, while C3S, especially C2S, adsorbs less. This uneven adsorption results in varying dispersion effects of high-efficiency water-reducing agents. The higher the C3A content of cement, the worse the compatibility between cement and water-reducing agents, which can lead to insufficient or no water-reducing effect.
Experiments have shown that the setting time of concrete with added water-reducing agents is longer than that of concrete without water-reducing agents.
The setting and rheological properties of concrete systems with a low water-cement ratio and added high-efficiency water-reducing agents differ significantly from those of cement mortar without water-reducing agents.
Firstly, due to the decrease in water consumption, the amount of gypsum dissolved in the cement slurry is very small, and its concentration is not sufficient to control the hydration of C3A, resulting in rapid setting and rapid loss of slump; Secondly, the addition of high-efficiency water reducing agents interferes with the kinetics of cement hydration, reduces the solubility of gypsum, and lowers the concentration of gypsum in the liquid phase. At the same time, due to the lack of sulfate ions, the sulfonic acid groups on high-efficiency water-reducing agent molecules bind with C3A, reducing the dosage of high-efficiency water-reducing agents in the liquid phase, losing their dispersing effect on cement, and accelerating the loss of slump.
The specific surface area and particle-size distribution of cement affect the dispersion effect of high-efficiency water-reducing agents. Generally, for cement with a larger specific surface area, adding the same high-efficiency water-reducing agent can significantly improve its effect and increase flowability.
Many studies have shown that alkali content significantly affects the flowability of cement systems with water-reducing agents. Higher alkali content reduces the cement’s flowability. Experiments have shown that using high-efficiency water-reducing agents in high-alkali cement can reduce the water-reduction rate.
The adaptability of cement and water reducing agents is bidirectional. In this context, the type and quality of water-reducing agents also play a crucial role. Therefore, select water-reducing agents based on their compatibility with cement and construction requirements. When high-efficiency water-reducing agents are incompatible with a certain type of cement, the following situations may occur:
Avoid the above immiscible phenomena as much as possible when selecting water-reducing agents.
The dosage of concrete water-reducing agents depends not only on the amount of cement used, but also on the physical properties and chemical composition of the cement.
The traditional method for determining the dosage of concrete water reducers is mainly based on the water reducer product manual and the percentage of cement used. Because different varieties or batches of cement have different chemical compositions and contents, the compatibility, adaptability, and optimal dosage of water-reducing agents will also vary significantly. Determining the dosage of water-reducing agents solely based on the percentage of cement used makes it difficult to ensure the mixture’s optimal water-reducing effect. It can easily lead to wastage of water-reducing agents.
We conducted multiple experiments and used flowability measurements to determine the optimal dosage of concrete water reducer, achieving good results. The specific method is to accurately weigh the water reducer, water, and 300g of cement according to the recommended dosage in the water reducer manufacturer’s manual, and mix the water reducer, water, and cement slurry according to the standard method.
On clean glass, pour the mixed cement slurry into the test mold, lift the mold one by one, observe the slurry flow, and test whether the slurry’s fluidity meets the performance requirements for concrete pumping. Simultaneously record the water-reduction rate and dosage for different cement types and concrete water reducers to select the optimal combination.
Order of adding water reducing agents. Multiple comparative experiments on different water-reducing agents show that the order of addition affects their water-reducing effect.
After weighing a certain amount of cement, water-reducing agent, and water, pour them into the mixing pot in the order of cement → water-reducing agent → water, and the measured cement slurry expansion is 22 cm~23 cm. And in the order of adding cement first → water, after mixing for a certain period of time, the water-reducing agent is added, and the measured expansion degree of the cement slurry is 25 cm~27 cm.
From this, it can be seen that, with the same cement and water-reducing agent, the post-mixing method is superior to the pre-mixing method, as it improves compatibility between the water-reducing agent and cement and fully realizes the water-reducing agent’s effectiveness. Similarly, in actual concrete construction, water-reducing agents should be added after the concrete mixture has been mixed for a certain period to achieve the best water-reduction effect.
In summary, due to the differences in the characteristics and chemical composition of different varieties and batches of cement, the brand of water reducer and the optimal dosage of a water reducing agent should be determined through experiments based on different types of cement and water reducer, rather than solely relying on the manufacturer’s instructions and the percentage of cement dosage. Determining the dosage of concrete water reducer using cement slurry flowability is simple, accurate, and reliable, helping ensure project quality, reduce project costs, and achieve the best results in construction projects.
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