Six Main Raw Materials of Concrete and Their Functions
1. Cement: The cementitious material of concrete, accounting for 10-15% (according to the "General Portland Cement Standard" GB 175-2020). A common type is Portland cement, which hardens and bonds other materials through hydration.
2. Aggregate: Accounting for 60-75% of the total volume, divided into coarse aggregate (such as crushed stone, particle size > 4.75mm) and fine aggregate (such as river sand, particle size < 4.75mm), providing skeletal support and reducing shrinkage cracks.
3. Water: Accounting for 15-20%, it must comply with the "Standard for Water Used in Concrete" JGJ 63-2006. Water quality affects the hydration reaction; impurities may reduce strength.
4. Admixtures: Such as water-reducing agents and retarders, accounting for 0.2-5% (data from the China Building Materials Association), which can improve fluidity or shorten setting time.
5. Admixtures: Industrial byproducts such as fly ash and slag, replacing 10-30% of cement to improve durability and reduce costs.
6. Fiber Materials: Steel fibers or polypropylene fibers, added at 0.1-2% (ACI 544 report), enhancing crack resistance and toughness.
Composition and Proportioning Logic of Concrete Concrete is a composite material, and its composition design must meet three major objectives: strength, durability, and workability. For example, a typical mix proportion for C30 standard concrete is: water 175kg, cement 461kg, sand 512kg, and aggregate 1252kg (refer to the "Specification for Mix Proportioning Design of Ordinary Concrete" JGJ 55-2011).
- Cement paste: Coats the aggregate to form a bonding layer. The water-cement ratio (water/cement mass ratio) is a key parameter, typically 0.4-0.6.
- Aggregate gradation: Continuous particle size distribution reduces porosity and increases density. Excessive coarse aggregate content can lead to segregation, while excessive fine aggregate increases water consumption.
Raw Material Adjustments in Engineering Applications
1. High-Temperature Environments: Retarders need to be added to delay initial setting time, or low-heat cement (such as slag silicate cement) should be used.
2. Freeze-Thaw Resistance Requirements: Adding air-entraining agents (4-6% air content) can improve freeze-thaw cycle resistance (GB/T 50082-2009).
3. High-Strength Concrete: Reduce the water-cement ratio to below 0.3 and add silica fume (5-10% of cementitious materials) to fill micropores.
By optimizing the raw material combination, concrete can adapt to diverse scenarios such as bridges and high-rise buildings. Understanding the characteristics and interactions of core materials is fundamental to ensuring project quality.
