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The influence of expansion agent dosage on the performance of self compacting concrete
Blog The influence of exp
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One is the diversity and adjustability of molecular structures; the second is that it further concentrates and enhances the advantages of high-efficiency water-reducing agents, achieving green and pollution-free production processes. From the mechanism of action, the molecular structure of polycarboxylate superplasticizer is comb-shaped, with strong polar anionic “anchoring” groups in the main chain used to adsorb onto cement particles.
The outward-extending, comb-like structure supported by numerous branches provides sufficient spatial arrangement for further dispersion of cement particles. Compared to the double-layer electric repulsion effect of naphthalene-based water reducing agents, the steric hindrance effect results in a much longer dispersion and retention time.
By appropriately changing the comb structure of polycarboxylate superplasticizers, the density and length of side chains can be adjusted to obtain high water-reducing and high early-strength superplasticizers suitable for prefabricated components.
Polycarboxylate-based water reducers can be adjusted and changed in molecular structure according to requirements to achieve the goal of changing performance, rather than simply modifying by compounding. Based on this understanding, it may inspire us to improve our future application technology.
The saturation point of polycarboxylate superplasticizer varies greatly among different types of cement, so it is important to determine the saturation point for each type of cement. But if the user specifies that only 1.0% can be added, and the selected cement is not adaptable at this dosage, it will be difficult for the admixture provider to handle, and compounding methods often have little effect.
First-grade ash has good adaptability, while second- and third-grade ash are not suitable in many situations. At this time, even if the dosage of polycarboxylate is increased, the effect is not significant. Often, when a certain type of cement or fly ash has poor adaptability to external additives, and you are still not completely satisfied with switching to another additive, you may eventually have to replace the cementitious material.
When the mud content of sand is high, the water reduction rate of polycarboxylate superplasticizer will significantly decrease. The use of naphthalene-based water reducers often requires increasing the dosage to solve the problem. In contrast, polycarboxylate-based water reducers do not show significant changes when the dosage is increased. In many cases, the concrete begins to bleed before the flowability meets the requirements.
At this point, adjusting the sand content, increasing the gas content, or adding thickeners will not have a good effect. The best way is to reduce the mud content.
Polycarboxylate superplasticizers often retain some surface-active ingredients that reduce surface tension during the production process, so they have a certain degree of air-entraining properties. These active ingredients are different from traditional air entraining agents. In the production process of air entraining agents, some necessary conditions for generating stable, small, and closed bubbles are considered. These active ingredients are added to the air entraining agent so that the bubbles introduced into the concrete meet the requirements for air content without adversely affecting strength and other properties.
During the production process of polycarboxylate superplasticizers, the air content can sometimes reach up to about 8%. If used directly, it will negatively impact strength. Therefore, the current approach is first to defoam and then introduce air. Defoamer manufacturers often provide them, while air entraining agents sometimes need to be selected by the application unit themselves.
Polycarboxylate water reducer has low dosage, high water reduction rate, and good slump retention, but the following problems also occur in application:
① The dosage is very sensitive when the water-cement ratio is small, and shows a higher water reduction rate. However, when the water cement ratio is large (above 0.4), the water reduction rate and its changes are not so obvious, which may be related to the mechanism of action of polycarboxylate superplasticizers. Its dispersion and retention effects lie in the steric hindrance formed by the molecular structure. When the water-cement ratio is high, there is already sufficient spacing between water molecules in the cement dispersion system, so the steric hindrance effect of polycarboxylate molecules is naturally smaller.
② The effect of dosage is more significant when the amount of cementitious material used is large. Under the same conditions, the water-reducing effect when the total amount of cementitious material is less than 300 kg/m3 is lower than the water-reducing rate when it is greater than 400 kg/m3, and there is also a synergistic effect when the water-cement ratio is high. The amount of cementitious material is small.
Polycarboxylate water reducers are developed for high-performance concrete, so their performance and price are more suitable for application in high-performance concrete.
Polycarboxylate water reducers cannot be compounded with naphthalene-based water reducers. If two types of water reducers are used in the same equipment, they can also have an impact if not thoroughly cleaned. Therefore, nowadays it is often required that polycarboxylate superplasticizers should be used separately with a set of equipment.
In current use, the compatibility between air entraining agents and polycarboxylate salts is good, largely due to the low dosage of air entraining agents, which can be further compatible and complementary with polycarboxylate water reducers. The sodium gluconate in the retarder has good compatibility, but poor compatibility with other inorganic salt additives, making it difficult to compound.
The pH value of polycarboxylate superplasticizers is lower than that of other high-efficiency superplasticizers, some of which are only 6-7. Therefore, they must be stored in containers such as fiberglass and plastic and cannot be stored in metal containers for a long time. It can cause deterioration of polycarboxylate superplasticizers, and long-term acidic erosion can affect the lifespan of metal containers and the safety of storage and transportation systems.
The influence of expansion agent dosage on the performance of self compacting concrete
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