concrete admixture

1. High Alkali Cement

The soluble alkali in cement is usually expressed as Na2O equivalent, mainly from the clay and admixtures used to produce cement. The right amount of soluble alkali is beneficial to promote cement hydration and the early strength development of concrete. Experiments have shown that the fluidity of cement concrete increases with the increase of alkali content. However, when it reaches a certain amount, cement will hydrate rapidly, and the fluidity of cement slurry will drop significantly. The plasticizing effect is also reduced considerably after adding a water reducer. Over time, the slump loss rate of water reducers used in commercial and pumped concrete construction increases.

The reason for the above phenomenon is generally believed that the alkali in cement promotes the dissolution of tricalcium aluminate (C3A). At this time, cement quickly forms a certain amount of AFt crystals with the participation of setting regulator CaSO4 and wraps on the surface of C3A, inhibiting the direct hydration of C3A to form calcium aluminate and improving the fluidity of cement slurry. However, if the alkali content in cement is too high due to the initial formation of a large number of AFt crystals, the fluidity will decrease instead, and the adaptability of the water reducer for the above cement will inevitably decrease. The main manifestations are insufficient water reduction rate, poor plasticization effect, and high slump loss rate over time.

When using high-alkali cement, the use of a water reducer with a low sulfate content yields poor results. However, employing a water reducer with a high sulfate content (sodium sulfate content of more than 20%) significantly enhances the effect. This is primarily due to the fact that the CaSO4 in the low-concentration water reducer, produced during synthesis and neutralization, has excellent water solubility. It dissolves in water in large quantities before gypsum is dissolved in cement. When higher alkali accelerates the dissolution of C3A, a large amount of SO3 already exists in the water, which reacts with C3A to form AFt, thereby preventing the decrease in fluidity caused by the formation of calcium aluminate and reducing the slump loss. It is evident that water reducers with high sodium sulfate content are more suitable for high-alkali cement.

Many polycarboxylic acid water reducers have a low pH value, making it challenging to adapt to high-alkali cement when used in combination with acidic retarders such as citric acid. The main issue is that when acidic admixtures are added to high-alkali cement, an acid-base neutralization exothermic reaction occurs rapidly, causing the temperature to rise sharply. This not only leads to quick cement hydration but also triggers a vicious cycle of large amounts of hydration heat release. The prepared concrete not only has poor fluidity but also risks losing its slump in a very short time. However, this issue can be circumvented by using other alkaline retarders.

2. Low-alkali and sulfur-deficient cement

The optimal content of soluble alkali in cement is 0.4% -0.6 %. Cement with less than 0.4% alkali content is usually called low-alkali cement. Water-soluble alkali mainly exists in alkali sulfate, so low-alkali cement is also called sulfur-deficient or under-sulfur cement.

Sulfur-deficient cement usually has poor fluidity when added to a water reducer. Although increasing the amount of water reducer has a specific effect, it will increase the bleeding of concrete, and the prepared concrete has poor homogeneity and fast slump loss. Therefore, it is difficult for commonly used water reducers to adapt. Even if the retarder is doubled, it will have no effect.

3. Cement with high C3A content

The main components of cement are C3S, C2S, C3A and C4AF. The order of adsorption activity of these mineralized components is generally considered C3A>C4AF>C3S>C2S. Among them, C3A has the most considerable adsorption amount on water reducer. Therefore, when the amount of water reducer is constant, the fluidity of concrete decreases with the increase of C3A content. The slump loss rate over time also increases. This is mainly because most of the water reducers added will be adsorbed by C3A, while the main mineralized component, C3S, does not have enough water reducers to adsorb and disperse, reducing the cement paste’s fluidity. Many tests have shown that when the C3A content in cement exceeds 8%, it will hurt the fluidity of concrete.

4. Cement with high admixture content

High-quality fly ash should be highly active (i.e., have high active SiO2 and AL2O3 content), have low ignition loss, low fineness, and low water demand. The ignition loss has the greatest impact on the compatibility of admixtures.

The ignition loss is the content of unburned carbon in fly ash. The greater the ignition loss, the higher the unburned carbon content and the worse the compatibility with admixtures. A higher carbon content will further deteriorate the performance of concrete. Unburned carbon is mainly made up of porous particles that readily absorb water and have a high water demand in concrete. Overflowing will increase the water seepage of concrete, increase the shrinkage and deformation of concrete, and affect the bonding performance of the cement paste and aggregate interface. When carbon meets water, it may also form a hydrophobic film on the surface of the particles, hindering further water penetration and affecting fly ash’s activity. Studies have also found that the carbon in fly ash has a strong adsorption capacity. After the water reducer is added, it will compete with cement for adsorption, affecting the fluidity of the cement slurry.

The current standard method for solving the compatibility of fly ash with high ignition loss, pozzolanic cement, and admixtures mainly involves increasing the number of admixtures and adding a certain amount of high-quality air-entraining agent.

Since slag contains more aluminates, it requires more gypsum setting agents, and slag cement produced by ordinary Portland cement is more prone to sulfur deficiency. Therefore, using a water reducer with a high sulfate content is more suitable. When mixed with high-quality air-entraining agents, tiny and dense bubbles can also reduce the adsorption of aluminates on water reducers to a certain extent, but the amount of addition needs to be increased.

5. Adding gypsum cement with poor water solubility

Gypsum is used as a cement setting agent, and its addition amount matches the C3A content in cement. After adding water, gypsum forms a certain amount of calcium sulfonate in cement, which is adsorbed in C3A to control the hydration of C3A and regulate the cement’s setting time. Among the commonly used gypsums, dihydrate gypsum (CaSO4.H2O) has the best water solubility, so dihydrate gypsum is mainly used in cement production. However, gypsum is often ground with cement clinker in cement production. When the temperature is too high during grinding, a large amount of dihydrate gypsum will be converted into hemihydrate gypsum (CaSO4.1/2H2O) or anhydrous gypsum (CaSO4), i.e., hard gypsum. Some cement plants also directly use anhydrous or industrial waste gypsum, such as fluorite, desulfurized gypsum, phosphogypsum, etc. Hard and waste gypsum have poor water solubility and dissolve slowly in water. In the admixture, a cost-effective slow-setting water-reducing agent, such as wood calcium or sugar calcium, is usually added, and the addition of these water-reducing agents will further affect the solubility of gypsum. Since gypsum cannot be dissolved quickly, C3A in cement will quickly hydrate, producing a large amount of calcium aluminate crystals, causing the false setting of concrete (i.e., a small amount of cement has solidified while a large amount of cement particles have not yet hydrated and solidified, and the cement slurry loses fluidity).

To prevent the false setting of cement mixed with anhydrite or another gypsum with poor water solubility, it is best not to use water reducers such as wood calcium, wood sodium, sugar calcium, etc., that affect gypsum dissolution. Experiments have shown that controlling the amount of the above water reducers has a specific effect. It is also possible to prevent false settings by adding many admixtures that can supplement SO3 in cement.

6. Fresh cement and cement with a large specific surface area

The storage time and specific surface area of ​​cement out of the kiln will also affect the adaptability of the admixture. Usually, we call cement with a short storage time after production “fresh cement.” Because the above cement has a short storage time, the cement temperature is high, the cement hydration speed is breakneck, and because the cement generates charges during the grinding process, the particles adsorb each other, which affects the dispersion of the water reducer and increases the slump loss rate of the concrete.

Prolonging the storage time of cement until the temperature drops below 50°C is conducive to improving the compatibility with admixtures. If the storage time of cement cannot be extended, increasing the amount of retarder can also have a specific effect.

The specific surface area of ​​cement influences the adaptability of admixtures. Cement with a larger specific surface area requires more water and admixtures to achieve a certain fluidity. It is generally believed that the most suitable specific surface area of ​​cement is about 5000CM2/g. Cement with a larger specific surface area develops faster in the early stage, but it will have an adverse effect on the later strength and collapse retention performance of concrete.