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The main reasons and countermeasures for the unsuitability of concrete admixtures

Concrete performance depends not only on the properties of the constituent materials, but also on factors such as material compatibility and the concrete mix ratio. As the fifth component of concrete, admixtures are used in small proportions but significantly affect performance. They can improve slump, adjust setting time, enhance construction performance, or reduce costs.

During cement hydration, a coagulation structure forms to encapsulate water. To achieve more complete hydration and improve concrete performance, more water needs to be added. Additives can adsorb directionally on the surface of cement particles, causing them to carry the same charge and repel each other, releasing water encapsulated by the cement coagulation structure, allowing more water to participate in the hydration reaction, and improving fluidity.

However, for various reasons, additives and cement can also be incompatible. Mainly manifested in:

(1) Admixtures do not significantly improve the workability of cement;
(2) Excessive loss of concrete slump or rapid setting of concrete;
(3) Causing cracks that are more likely to occur in concrete structural components.

These problems seriously affect the quality of cement concrete, pose hidden dangers to engineering quality, and can even lead to engineering accidents, causing significant economic losses. This article analyses the main factors and related countermeasures for incompatibility between concrete admixtures and cement, providing a reference for engineering construction quality and engineering safety management.

The main factors causing unsuitability of concrete admixtures

The main factors contributing to the incompatibility between concrete admixtures and cement are:

Factors of additives themselves

The main reasons for the concrete admixtures themselves are as follows:

(1) different varieties;

(2) Differences in structural functional groups;

(3) Different degrees of polymerisation;

(4) the components of the compound are different.

These factors affect cement adaptability in different ways.

There are also many differences in the additives produced by different manufacturers, mainly due to:

(1) production and manufacturing processes;

(2) the technical level of the manufacturer’s manufacturing process;

(3) Quality management level.

Therefore, products from different manufacturers will inevitably differ.

The influence of mineral composition of cement on concrete admixtures

The mineral composition of cement significantly affects the use of external additives. The mineral composition of cement mainly includes tricalcium aluminate (C3A), tetracalcium aluminate (C4AF), tricalcium silicate (C3S), dicalcium silicate (C2S), etc. Different mineral compositions are mainly determined by the raw materials and production processes used in cement production. The factors affecting cement mineral composition, in order, are C3A > C4AF > C3S > C2S. C3A hydrates quickly, early strength increases rapidly, water demand is high, C3A content is too high (mass fraction greater than 8%), C3A adsorbs a large amount of additives, and the concrete admixtures effect is largely lost.

The influence of adding adjusting gypsum varieties to cement clinker

The final step in cement production requires adding gypsum to adjust the setting time. The factors affecting the external additives used by cement manufacturers, in order, are hard gypsum (industrial anhydrous gypsum) > hemihydrate gypsum > dihydrate gypsum. Cement manufacturers often use industrial anhydrous gypsum to save costs; it does not affect cement quality requirements and does not react adversely with ordinary concrete without additives. However, for modern concrete with additives, cement with hard gypsum requires more water, adsorbs more concrete admixtures, and loses more concrete admixtures. Hard gypsum has a greater impact on wood calcium and may even cause rapid setting (false setting).

The influence of cement fineness and particle size distribution

Cement manufacturers often intensify grinding and increase cement fineness to meet new cement standards, improve market competitiveness, and enhance strength. If cement is too fine and requires more water, it will also adsorb more additives, resulting in greater additive loss. At the same time, finer cement reaches a higher temperature during grinding, which can cause more hydrated gypsum to decompose into anhydrous gypsum, increasing anhydrous gypsum content and reducing its adaptability to additives. Cement with a poor particle-size distribution and a high water-leakage rate has poor adaptability to additives.

Alkali content of cement

Cement with high alkali content (alkali content > 0.8%) or low alkali content (alkali content < 0.5%) also performs poorly with additives. The alkali in cement mainly comes from the raw materials used, especially lime and clay. Cement with excessively high or low alkali content, when combined with certain additives, can change gypsum solubility, accelerate the hydration rate of the cement mineral component C3A, increase water demand, and cause faster loss of workability. Adding soluble Na2SO4 at this time can improve its compatibility with additives. Fly ash and mineral powder can undergo a secondary reaction with the hydration product Ca(OH)2 of cement, reducing concrete alkalinity and improving additive compatibility with cement.

Adaptability of Fly Ash and concrete admixtures

If fly ash is too fine, it requires more concrete admixtures to disperse the particles. The greater the loss on ignition of fly ash (i.e., the higher the carbon content), the greater the water demand and the greater the impact on external additives. The carbon particles are coarse and porous, so they absorb water easily and strongly adsorb additives, increasing additive dosage, especially for air-entraining agents.

Effects of Aggregates

High clay content and clay block content in aggregates absorb more moisture and consume more additives, resulting in poor fresh concrete workability, easy segregation, significant slump loss, and reduced concrete strength; improper mix proportions and unreasonable sand ratios can also increase slump loss. The sand ratio is relatively small, and the concrete is also prone to segregation and bottom climbing, resulting in significant loss of concrete slump; The sand ratio is too high, and excessive sand requires more moisture to wet, which reduces the slump of concrete and also affects its strength; Poor grading of aggregates, especially the lack of intermediate particle size aggregates, can easily cause concrete segregation, bottom climbing, and significant loss of concrete slump, affecting the quality of concrete.

Related Countermeasures

To address incompatibility between concrete admixtures and cement, the focus is on prevention, material selection, and on-site material testing. Admixture compatibility with cement is complex, and if admixtures and cement are mismatched, concrete manufacturers should take timely measures. Based on the situation and experiments, analyse and identify the reasons, adjust the concrete mix proportion, improve the factory slump, and reduce slump loss.

Usually, the fly ash dosage can be adjusted, the admixture dosage can be increased, the liquid-phase residue of admixtures in concrete can be increased, the water-cement ratio can be kept unchanged, and the cement dosage can be increased. However, this undoubtedly increases the cost per cubic meter. During production, the secondary addition method can also be used, which controls the slump at the factory at 80-100, and adjusts it to 140 by vigorously stirring the admixture solution for about 2 minutes before use on the construction site. This treatment is more economical and effective.

Concrete manufacturers often require additives to adapt to the large inventory of cement. Therefore, additive manufacturers must adjust the formula, including the types and dosages of water-reducing agents and retarders, to match the cement used by the concrete manufacturer, and may add plasticisers, air-entraining agents for bubble stabilisation, etc. When determining the concrete mix ratio during construction, consider the concrete’s setting time, and ensure the admixture includes a retarding component.

Sudden temperature drops, excessive additive use, and failure to adjust the mix promptly can delay setting and seriously reduce strength. Summer construction should avoid high temperatures and strong winds at noon and cool the raw materials. Adjust the sand ratio based on sand fineness and coarse aggregate porosity.

Conclusion

This article elaborates on the role of concrete admixtures in cement concrete and the main manifestations and hazards of the mismatch between concrete admixtures and cement. It analyses the main factors that cause the mismatch between additives and cement in concrete, summarises and proposes corresponding countermeasures for production and construction processes, and offers some reference values for engineering construction quality and safety management.