Which of the following constituents is present in the blast furnace slag?
🔬 Understanding Blast Furnace Slag
Blast furnace slag is a non-metallic byproduct obtained during the manufacturing of pig iron in a blast furnace. It consists primarily of silicates and aluminosilicates of calcium and other bases. When this molten slag is rapidly cooled or "granulated," it forms glassy, granular particles that have cementitious properties, making it a valuable raw material for producing slag cement.
📝 Detailed Analysis of Composition
(b) 45% calcium oxide and 35% silica
This is the correct answer. The chemical composition of blast furnace slag is similar to that of Portland cement but with different proportions. A typical composition suitable for cement production includes approximately 45% Calcium Oxide (CaO) and 35% Silica (SiO₂). This specific balance of constituents gives slag cement its unique properties.
(a), (c), and (d) Incorrect Compositions
These options present incorrect proportions of the key chemical compounds. While alumina (Al₂O₃) and magnesia (MgO) are present in slag, their percentages are typically lower (around 10-15% for alumina and <10% for magnesia). The dominant components that define its cementitious value are calcium oxide and silica in the ratio specified by option (b).
🔧 Properties and Use of Slag Cement
Slag cement is manufactured by grinding granulated blast furnace slag with Portland cement clinker. The proportion of slag typically ranges from 35% to 65%.
- Strength Development: Slag cement has a slower rate of strength gain in the initial 28 days compared to ordinary Portland cement. However, its long-term strength (after 28 days) continues to increase and can surpass that of ordinary cement.
- Improved Durability: The refined pore structure of slag cement concrete provides excellent resistance to chemical attacks, particularly from chlorides and sulfates.
- Primary Application: Due to its high resistance to saline water, slag cement is extensively used for marine construction, such as ports, harbors, and structures in coastal areas. It's also used in mass concrete works due to its lower heat of hydration.
💡 Study Tips
- Key Components: Remember that the two main components of blast furnace slag are Calcium Oxide (CaO) and Silica (SiO₂).
- Memorize the Ratio: The approximate ratio of 45% CaO to 35% SiO₂ is a common point of examination.
- Link Property to Use: The slow initial strength gain and high ultimate strength, combined with excellent chemical resistance, are its defining features. This makes it perfect for marine works.