Cement Manufacturing Fundamentals

Cement Definition and Production

• Cement is a crystalline compound of calcium silicates and other calcium compounds with hydraulic properties.
• Cement is produced by pulverizing clinker.
• Clinker is essentially a mixture of hydraulic calcium aluminates and silicates of varying compositions.
• When exposed to water or moist air, the clinker hydrates and hardens into a stone-like material.

History of Cement

• Lime and clay have been used as cementing materials for centuries.
• Romans are credited with developing hydraulic cement, notably by mixing volcanic ash from Mount Vesuvius with lime.
• The Pantheon in Rome (128 AD) is an example of a structure built using Roman cement techniques.
• Joseph Aspdin, of England, invented Portland cement in 1824.
• Portland cement has a similar color and strength to natural stone from the Portland area of England.

Raw Materials of Cement

• Calcareous Materials: Limestone, cement rock, and marine shells are examples.
• Calcareous materials should contain less than 3.3% magnesium oxide (MgO).
• Argillaceous Materials: Rich in silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3).
• Sources include clay, shells, blast furnace slag, ash, and cement rock.

Cement Manufacturing Process

• Calcium carbonate (CaCO3) is heated to produce calcium oxide (CaO) and carbon dioxide (CO2), typically between 700°C and 1000°C.
• Calcium oxide combines with silica (SiO2) above 1300°C to 1500°C, creating various compounds (e.g., C2S, C3S).

Components and Composition of Cement

• Tri Calcium Silicate (C3S): The main component responsible for early and later strength development.
• Di Calcium Silicate (C2S): Responsible for later strength.
• Tri Calcium Aluminate (C3A): Hydrates quickly, contributing to early strength. Gypsum is added to retard its hydration.
• Tetra Calcium Aluminoferrite (C4AF): Hydrates rapidly, but its contribution to strength is insignificant.
• Gypsum (CaSO4⋅2H2O): Added as a retarder to prevent immediate setting, control the hydration of C3A.
• Other minor constituents include free lime (CaO), chlorides, and alkalis.

Sulfate Attack

• High sulfate content in soil or water can lead to expansion and cracking in concrete.
• Sulfate attack is a major concern in certain environments.

Free Lime (Calcium Oxide)

• Free lime (CaO) is a component of cement when not combined in clinker.
• When present in large quantities, it can cause expansion and weaken the material.
• Free lime can cause the cement color to turn brown.

Chloride

• High chloride content can cause the corrosion of steel reinforcement within cement.
• The source of chloride in concrete is frequently from cement raw materials.

Alkalis

• Presence of alkalis can cause expansions and cracking within concrete due to their reaction with silica.
• Alkalis come from cement raw materials and fuel.

Effect of Alkalis

• Air content in concrete increases.
• Heat of hydration increases.
• Reactivity of special cement materials (e.g., SCM) also increases.
• Alkali-silica reaction (ASR) increases.
• Setting times may change.
• Shrinkage may decrease.
• Early and late strength may change.
• Water requirement increases.
• Workability decreases.

Insoluble Residue

• Insoluble residue is a non-cementing component.
• It negatively impacts the compressive strength of cement.
• Its level should not exceed 0.75%.

Physical Properties of Cement

• Fineness: affects hydration rate and strength gain within the material.
• Soundness: A measure of resistance to expansion or cracking, crucial for long-term durability.
• Setting Time: Both initial and final setting times are crucial properties indicating when the cement can be handled.
• affected by fineness, water-cement ratio, and admixtures like gypsum.
• Strength: Varies based on water-cement ratio, aggregate type & mixture, curing, and cement fineness.
• Specific Gravity: Used in mix design calculation.
• Heat of Hydration: Heat generated during cement hydration within concrete.
• Should be considered in concrete design.
• Loss of Ignition: The amount of volatile matter lost when cement is heated to a high temperature.
• Color: Can vary based on the raw material composition.

Type of Cement

• Ordinary Portland Cement (OPC): General-purpose cement.
• Portland Pozzolana Cement (PPC): Contains fly ash and results in lower heat of hydration and reduced leaching.
• Portland Blast Furnace Slag Cement (PBSF): Contains blast furnace slag, used for lower heat of hydration.
• Rapid Hardening Cement: This type of cement is used for emergency construction work.
• Low Heat Cement: Low heat liberated during setting.
• Sulfate Resisting Cement (SRC): Particularly suitable for environments with high sulfate content.
• White Cement: Raw materials are free from iron impurities.

Suitability of Different Cements

• Specific cement types are better suited for different applications depending on factors like environmental conditions, concrete strength requirements, and the intended structure.
• Different grades of OPC cement cater to differing strength and durability requirements.
• Cement type and quality directly impact the structural integrity and overall performance of concrete structures.

Cement Storage

• Portland cement is moisture sensitive.
• Proper storage conditions (dry, shaded areas) are necessary to maintain its quality.
• Bulk cement is typically stored in bins or silos.