Negative effects of admixtures on concrete performance

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Negative effects of admixtures on concrete performance

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Concrete admixtures have become an indispensable component of concrete and are widely used in concrete production. However, in practice, engineering quality accidents often occur due to improper selection and use of admixtures. The negative effects of commonly used admixtures on concrete performance mainly include the following aspects.

I. Negative Impacts of Admixtures type on concrete performance

1. Ordinary water-reducing agent

Lignin-based water-reducing agents are representative of ordinary water-reducing agents and represent the first generation. These agents have poor compatibility with cement. Especially when using anhydrite or industrial gypsum as a setting regulator in cement, these agents can affect the solubility of the gypsum, causing the cement to set too quickly. Furthermore, lignin has a certain air-entraining property; improper use or excessive dosage can lead to increased air content in concrete and decreased hardening strength, particularly noticeable in steam-cured concrete. Lignin-based water-reducing agents also have a certain retarding effect; therefore, dosage control is crucial to prevent retarding accidents.

2. High-efficiency water-reducing agent

High-efficiency water-reducing agents, represented by naphthalene-based and aliphatic water-reducing agents, are also second-generation water-reducing agents. Naphthalene-based and aliphatic water-reducing agents offer higher water reduction rates than lignin-based water-reducing agents, with saturated water reduction rates exceeding 20%. When the compatibility with cement and admixtures is good, the water reduction rate can even approach 30%. However, naphthalene-based and aliphatic high-efficiency water-reducing agents often experience significant slump loss in concrete over time during use. This phenomenon is often caused by poor compatibility between cement and admixtures. Therefore, attention should be paid to adjusting the compatibility between admixtures and cement during use. Generally speaking, high-concentration naphthalene-based high-efficiency water-reducing agents have better plasticizing and plastic-retaining effects than low-concentration products. However, when used in cement with high alkali content (or sulfur deficiency), although high-concentration naphthalene-based agents have a high effective component content, their plasticizing and plastic-retaining effects are not as good as low-concentration types. While aminosulfonate-based high-efficiency water-reducing agents have high water reduction rates, their use alone can increase concrete segregation and bleeding, requiring combination with thickening and water-retaining components. Aliphatic high-efficiency water-reducing agents can stain concrete and cause bleeding. They are not suitable for use on fair-faced concrete or concrete with surface finish requirements.

3. Early strength agent

Most concrete early strength agents are inorganic salts, and high dosages can negatively impact the later-stage strength of concrete. Chloride early strength agents can cause steel corrosion. Sulfate early strength agents may cause volume expansion, reducing the durability of concrete. Sodium early strength agents can increase the alkali content of concrete, increasing the risk of alkali-aggregate reaction.

4. Retarder

Sugar-based retarders (such as sucrose and molasses) can effectively inhibit the early hydration of C3A. Molasses has a good later-stage strengthening effect, but it has poor compatibility with cement. When used in cement with anhydrite or industrial gypsum as a setting regulator, it can cause false setting. Furthermore, sugar-based retarders are often temperature-sensitive; their retarding effect is more pronounced at lower temperatures, and the dosage should be adjusted promptly according to temperature changes. Citric acid, sodium tripolyphosphate, and zinc sulfate can increase the plasticizing effect of cement, but they also increase the bleeding and shrinkage of concrete. Sodium gluconate can effectively inhibit C3A hydration and has a high water-reducing effect, but its retarding effect is not as good as sugar-based retarders when used in cement with low C3A . In addition, acidic retarders (such as citric acid) are not suitable for high-alkali cement; alkaline retarders (such as sodium tripolyphosphate) are preferable.

5. Antifreeze

The early-strength and antifreeze components in antifreeze agents are mostly inorganic salts. Improper use can cause a decline in the later strength of concrete, steel corrosion, and alkali-aggregate reaction.

6. Expanding agent

High dosages and high alkali content of expansive agents often lead to significant slump loss in concrete, resulting in low later-stage strength and reduced durability. Temperature and humidity have a significant impact on the effectiveness of expansive agents; low-temperature, non-wet curing not only fails to compensate for shrinkage but may even increase the probability of cracking.

II. Negative Effects of Admixtures Dosage on Concrete Performance

1. High-efficiency water-reducing agent

When the dosage of high-efficiency water-reducing agents is normal, the slump of fresh concrete will increase with the increase of dosage. However, after exceeding the saturation point, further increases in dosage beyond a certain point will not only stop the water reduction rate from increasing, but will also increase the bleeding rate of the concrete and prolong the setting time. The dosage of polycarboxylate water-reducing agent is generally 0.15% to 0.25%; the dosage of aliphatic water-reducing agents and naphthalene-based water-reducing agents is generally 0.5% to 1% when using liquid with a solid content of about 40%, and the dosage is 1.5% to 2.5%.

2. Retarder

When the dosage of retarder is too low, the expected retarding effect cannot be achieved; excessive addition will cause the concrete to fail to set for a long time or increase the tendency for concrete to crack. The dosage of phosphate retarder is 0.06–0.1%. When using sodium tripolyphosphate, it should be noted that its initial solubility in water can reach 35g/110g, but after several days it decreases to 1/2–1/3 of the initial value, resulting in a white precipitate. The dosage of sodium gluconate is 0.06–0.1%.

3. Early strength agent

Excessive addition of accelerators, while providing good early-stage concrete performance, leads to significant later-stage strength loss, exacerbates salt precipitation affecting the concrete finish, increases electrical conductivity, and raises the risk of cracking. Salt-based accelerators are generally used in larger quantities, such as calcium sulfate at 0.5%–3.0%, while organic compound accelerators are used in smaller quantities, typically below 0.1%. In practice, a combination of both is often used.

4. Entraining agent

Excessive addition of air-entraining agents can actually reduce the workability of concrete and negatively impact its compressive strength, impermeability, and carbonation resistance. The typical dosage of fatty acid-based air-entraining agents is 0.005%–0.02%, with an air-entraining capacity of 2%–5%; the typical dosage of rosin resin acids is 0.003%–0.02%, with an air-entraining capacity of 3%–6%; and the typical dosage of saponins is 0.005%–0.05%, with an air-entraining capacity of 2%–4%.

In conclusion, correctly selecting the type and dosage of admixtures is crucial to ensuring concrete quality. When using admixtures, it is essential to consider the actual conditions (such as environmental conditions, construction conditions, material conditions, and structural design requirements for concrete performance) and conduct trial mixes and tests of concrete with admixtures to avoid adverse effects of admixtures on concrete performance.

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