Blog

Practical Technology of Concrete Admixtures

Concrete admixtures refer to chemical agents added to concrete before or during mixing, with a dosage generally not exceeding 5% of the cement mass (except for special admixtures). Their key feature is a small dosage and a large effect: they can improve concrete workability, adjust setting and hardening, and enhance durability without significantly changing the mix proportions. They are essential for pumping and high-strength concrete.

According to their functions, additives are mainly divided into five categories: improving workability (water-reducing agents), adjusting setting time (early strength agents, retarders), enhancing durability (air-entraining agents, rust inhibitors, waterproofing agents), improving volume stability (expansion agents, crack-resistance agents), and special functional categories (accelerators, coloring agents). Different categories correspond to different engineering scenarios.

How to select the right Concrete Admixtures?

Select water reducing agent according to strength grade

C15-C25 for low-strength concrete based on strength grade, while ordinary/sodium naphthalene sulfonate are chosen to balance fluidity and cost; C30-C60 medium-high-strength concrete, with priority given to polycarboxylate superplasticizer; C65 and above ultra-high-strength concrete, using high water-reducing polycarboxylate products, combined with mineral admixtures to reduce the water-cement ratio and ensure compactness.

Differentiated selection of coagulation additives for winter and summer construction

When winter temperatures are ≤ 5 ℃, select early-strength agents to accelerate early strength development and prevent freezing damage. Compound air-entraining agents to enhance frost resistance. When summer temperatures are≥ 30 ℃, use retarding components to suppress rapid slump loss; for pumping, prioritize retarding and slump-reducing water-reducing agents, balancing fluidity and setting time.

⚠️ Important reminder: The higher the dosage of additives, the better the effect. Excessive mixing can bring various quality hazards: excessive use of water-reducing agents can cause bleeding and segregation; excessive early-strength agent is prone to rapid solidification and shrinkage cracks; Excessive use of retarders can cause prolonged non-solidification and delay construction progress; Excessive use of air-entraining agents can directly cause a significant decrease in the compressive strength of concrete. All admixture dosages must strictly comply with the mix design.

On-site practice: The addition form and feeding sequence directly affect the final product effect

1.Workability for concrete

Many construction sites overlook material-feeding details, resulting in abnormal additive performance.

❌ It is not recommended to directly add dry powder additives: dry powder is prone to clumping, uneven dispersion, and local additive enrichment, causing local rapid solidification or long-term non-solidification.

✅ Correct method: Prepare into an aqueous solution, accurately feed through the mixing station metering system, and control the metering error within ± 2%.

The feeding sequence is equally crucial:

The recommended post-blending method for polycarboxylate superplasticizer can improve the water reduction effect and reduce slump loss;

Early strength and retarder are suitable for the co-blending method to ensure uniform dispersion;

Use the intermediate mixing method for the air-entraining agent to ensure fine, uniform bubbles.

When conditions are limited, dissolve the additive in water first, then add the aggregate and cement, while extending the overall mixing time.

Common on-site troubleshooting

Rapid slump loss (1-hour loss> 50 mm)

Causes include insufficient slump-retention performance of water-reducing agents, poor compatibility between cement and additives, high-temperature environments, and insufficient dosage. Solution: Replace the retarding and slump-retaining polycarboxylate; mix fly ash and mineral powder to improve compatibility; cool the aggregates and provide shading protection in high-temperature environments; fine-tune the additive dosage within the maximum allowable range.

After adding admixtures, the strength of concrete actually decreases

Four key points for investigation: excessive gas content in the air-entraining agent; Excessive bleeding and segregation of water-reducing agents; Poor compatibility between cement and additives; Chloride salt early strength agent causes steel corrosion. Adjust the dosage and replace the product with a compatible one based on the source of the problem, and strictly prohibit the use of chloride salt early strength agents.

Winter early strength agent triggers surface ‘white frost’

White frost is the crystallization of salts. Slight crystallization affects only appearance; severe sanding and peeling indicate excessive dosage or poor maintenance, and the damaged surface must be removed before repair. Control the dosage of early strength agent during construction, ensure proper moisture control and maintenance, and prioritize low-alkali, chlorine-free products for prevention.

Special requirements for external additives for C60 and above high-performance concrete

High-strength concrete has a higher threshold for comprehensive performance of external additives: it requires a high water reduction rate, excellent slump retention performance, and good compatibility with cement, silica fume, mineral powder, and other admixtures; Strictly control the chloride ion and alkali content to avoid the risk of steel corrosion and alkali-aggregate reaction. If conditions permit, use manufacturer-customized formulas.

Entry acceptance and storage management

How to determine the quality of additives upon entry?

Implement a four-step process of “data verification+visual inspection+test sampling+adaptation testing”. Verify the certificate of conformity and factory report, and compare the verification indicators with the GB8076 national standard; Observe the appearance of liquids and powders for any layering or clumping; sample and test key indicators by batch; It is necessary to use the actual raw materials of this project for concrete adaptation testing to avoid laboratory qualification and on-site use failure.

Improper storage can directly cause product failure

Store liquid additives in plastic and stainless steel tanks, avoiding iron containers; control storage temperature between 5-35 ℃ to prevent freezing at low temperatures; do not store for more than 6 months, and mix regularly to prevent delamination. Store powder additives in a dry, ventilated warehouse, elevated to prevent moisture, and use them as soon as possible after opening. Mixing different types of additives is strictly prohibited to avoid failure from chemical reactions.

Conclusion

Concrete admixtures are an indispensable component of modern concrete. Reasonable selection and correct use of admixtures directly affect concrete workability, strength, and durability. If you are looking for suitable polycarboxylate or naphthalene-based water reducers, please get in touch with us.