1. Adaptability issues of polycarboxylate superplasticizers to cementitious materials
There is a significant difference in the saturation point of polycarboxylate superplasticizers for different types of cement, so it is very important to find the saturation point for different cements. However, if the user stipulates that only 1.0% can be added, it is very difficult for the supplier of the admixture if the cement selected at this dosage has poor adaptability. The method of compounding often has little effect.
The adaptability of first-grade fly ash is good, but there are many cases of inadaptability for second and third-grade fly ash. At this time, even if the dosage of polycarboxylate superplasticizer is increased, the effect is not significant. It often happens that when a certain type of cement or fly ash has poor adaptability to the admixture, even if you switch to another admixture, you still cannot be completely satisfied, and ultimately you may have to change the cementitious material.
2. Problem of silt content in sand
When the silt content of the sand is high, the water-reducing rate of polycarboxylate superplasticizers will decrease significantly. Increasing the dosage of naphthalene-based superplasticizers is often used to address this, while the polycarboxylate superplasticizers show little change when the dosage is increased. In many cases, before the fluidity reaches the required level, the concrete has already started to bleed. Adjusting the sand ratio, increasing the air content, or adding thickening agents will not be very effective. The best solution is still to reduce the silt content.
3. Problem of air entrainment
Polycarboxylate superplasticizers often retain some surface active components that reduce surface tension during the production process, thus they have a certain degree of air-entraining properties. These active components are different from traditional air-entraining agents, as the production process of air-entraining agents takes into account the necessary conditions for producing stable, fine, and closed bubbles. These effective components are added to the air-entraining agents to ensure that the bubbles introduced into the concrete meet the requirements for air content without adversely affecting strength and other properties.
In the production process of polycarboxylate superplasticizer, the gas content can sometimes reach about 8%, which is detrimental to strength if used directly. Therefore, the current practice is to defoam first and then introduce air. Defoamers are often provided by defoamer manufacturers, while air-entraining agents sometimes need to be selected by the application unit.
4. Issue of dosage of polycarboxylate superplasticizer
Polycarboxylate superplasticizer has low dosage, high water-reducing rate, and good slump retention, but the following issues have also emerged in its application:
① The dosage is very sensitive to water-cement ratio when it is low, and it shows a higher water-reducing rate. However, when the water-cement ratio is high (above 0.4), the water-reducing rate and its change are not as obvious, which may be related to the mechanism of action of polycarboxylate superplasticizer. Its dispersion and retention effects are due to the spatial steric hindrance effect 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 spatial steric hindrance effect of polycarboxylate molecules naturally becomes smaller.
When the dosage of the cementitious material is large, the influence of the dosage becomes more obvious. Under the same conditions, the water-reducing effect of the cementitious material total amount less than 300kg/m3 is less than that of more than 400kg/m3, and there will be a synergistic effect when the water-cement ratio is high and the dosage of the cementitious material is small.
The polycarboxylate superplasticizer is developed for high-performance concrete, so its performance and price are more suitable for application in high-performance concrete.
5. Concerning the compound issue of polycarboxylate superplasticizers
Polycarboxylate superplasticizers cannot be compounded with naphthalene-based superplasticizers. If the two superplasticizers are used in the same equipment without being thoroughly cleaned, they will also have an impact. Therefore, it is often required now that polycarboxylate superplasticizers be used with a separate set of equipment.
According to the current usage, the compatibility between air-entraining agents and polycarboxylate blends is relatively good, a major reason being the low dosage of air-entraining agents, which can only be “soluble” with polycarboxylate-based water-reducing agents to further achieve compatibility and complementarity. The compatibility of sodium gluconate in retardants is also good, while the compatibility with other inorganic salt-type admixtures is poor, making it difficult to blend.
6. Regarding the pH value of polycarboxylate-based water-reducing agents
The pH value of polycarboxylate-based water-reducing agents is approximately 5-7, therefore, they are required to be stored in glass fiber, plastic, and other containers, and should not be stored long-term in metal containers. This can cause the polycarboxylate-based water-reducing agents to deteriorate, and after long-term acidic erosion, it can affect the lifespan of metal containers and the safety of the storage and transportation system.
Post time: Apr-01-2025