Understanding Cement Properties and Manufacturing

Posted on Nov 26, 2024 in Geology

ITEM 6: CEMENT

1. Content of a Standard

Purpose: This section outlines the purpose of the standard and describes specimen testing procedures, including sample characteristics (quantity, volume, weight), equipment, and tools used in the process. It also explains how results are obtained.

2. Definitions

  • Pozzolanic Activity: Finely ground pozzolan reacts with Ca(OH)2, creating stable compounds with binding properties.

  • Hydraulic Activity: Hydraulic binders, when mixed with water, set and harden upon contact with the liquid.

  • Stability: Limits CaO (2%) and MgO (5%) in cement and concrete to prevent expansion.

  • Strength Class: Defines the minimum compressive strength of concrete at 28 days, expressed in N/mm2. Classes include 32.5, 42.5, and 52.5.

  • False Setting: Premature stiffening of cement paste due to partial dehydration of gypsum during clinker grinding or alkali carbonation during storage. Plasticity can be restored by remixing.

  • Flash Setting: Rapid hardening of concrete within 24-48 hours due to C3A hydration and heat release. Controlled by gypsum addition.

  • Strength Regression in CAC-R: Under high temperature and humidity, CA recrystallizes into C3A, causing long-term strength loss. Requires caution in use, especially in prestressed concrete.

Constraints Specified in RC-03 for Sulfate-Resistant and Seawater Cement

Influence of Clinker Components on Heat of Hydration and Mechanical Resistance

  1. C3S (Tricalcium Silicate/Alite): Contributes to high initial strength (60% of clinker). High heat of hydration (~120 cal/g).

  2. C2S (Dicalcium Silicate/Belite): Contributes to medium and long-term strength. Low heat of hydration (~62 cal/g).

  3. C3A (Tricalcium Aluminate): Contributes to initial strength when combined with silicate. High heat of hydration (~207 cal/g). Gypsum controls rapid setting.

  4. C4AF (Tetracalcium Aluminoferrite): Minimal contribution to strength. Acts as a flux, lowering clinker formation temperature. Heat of hydration ~100 cal/g.

Additional Features of Common Cements

  1. SR: Sulfate-resistant

  2. MR: Seawater-resistant

  3. BC: Low heat of hydration

Significance of “X” in Masonry Cement

Indicates an air-entraining additive that creates air bubbles, reducing capillary action and preventing frost damage. Improves workability but may reduce strength due to increased porosity.

3. Major Oxides in Raw Materials and Clinker Formation

  • Raw materials (limestone, marl, clay) provide SiO2, Al2O3, CaO, and Fe2O3.

  • Reactions:
    900°C: Calcination (clay decomposition)
    1200°C: Clinkering (oxide reactions form C2S, C3S, C3A, and C4AF)

4. CEM II AP 42.5 / UNE 80,306 BC

CEM II: Portland cement with additions
P: Natural pozzolan addition
A: High clinker content
42.5: Minimum compressive strength at 28 days
BC: Low heat of hydration
Conforms to UNE 80,306

5. Constituents Influencing Cement Shrinkage

Shrinkage, a contraction during setting and early hardening, is influenced by grain size (finer grains lead to greater shrinkage), not constituents.

6. Constituents Influencing Cement Hardening

Hardening, an expansion due to water absorption, occurs due to CaO and MgO hydration or reactions between C3A and gypsum, forming expansive ettringite.

7. Dry Process Cement Manufacturing

  1. Limestone crushing and drying, clay drying

  2. Material dispensing

  3. Pulverization

  4. Homogenization in silos

  5. Clinkering and grinding with gypsum

  6. Cement storage and packaging

8. Wet Process Cement Manufacturing

  1. Grinding, decanting, and slurry preparation

  2. Material dispensing

  3. Refining

  4. Homogenization in silos

  5. Wet paste enters kiln

9. CEM II Cement Additions and Their Actions

  • Slag (S): Hydrates to form resistant constituents. Higher silica/lime ratio than Portland cement, improving acid water resistance and reducing heat of hydration.

  • Silica Fume (D): Fine amorphous silica dust with similar action to slag.

  • Pozzolans (P/Q): Natural (P) or calcined (Q) siliceous compounds that react with Ca(OH)2, improving chemical resistance.

  • Fly Ash (V/W): Fine powders with pozzolanic activity.

  • Limestone (L): Finely ground inert rock that improves strength, reduces shrinkage, and increases workability.

10. Clinker Component Effects on Cement Properties

  1. Heat of Hydration: C3A > C3S > C4AF > C2S. Higher C2S and C4AF content (from iron and silica) reduces heat but also initial strength.

  2. Mechanical Resistance: C3S provides initial strength, C2S provides long-term strength, C3A contributes to initial strength with silicate, C4AF has minimal effect.

  3. Chemical Resistance: C3S and C3A are more vulnerable. Higher C2S and C4AF content improves chemical resistance.

11. Effects of High C3A Content

  • Increased workability

  • Rapid setting due to high heat of hydration

  • Susceptibility to chemical attack (especially sulfates)

12. Clinker Components and Resistance

  • High C3S and C3A: High early strength, lower chemical resistance, high heat of hydration.

  • High C2S and C4AF: Low initial strength, higher long-term strength, higher chemical resistance, lower heat of hydration.

13. Factors Affecting Hardening

  • Excess mixing water: slows hardening

  • Cold weather: slows hardening

  • Hot weather: accelerates hardening

  • Finer grinding: accelerates hardening

14. Influence of Grinding on Portland Cement

  • Finer grinding accelerates reactions, increases heat of hydration, increases shrinkage, and increases vulnerability to alkali-silica reaction.

15. Advantages and Disadvantages of Additions

  • Advantages: Improved durability

  • Disadvantages: Lower density, reduced reinforcement protection, delayed setting, lower initial strength, requires intense curing

16. Types of Additions

  • Active Additions: React with Ca(OH)2, improving chemical resistance. Includes pozzolans, slag, fly ash, and silica fume.

  • Inactive Additions: Inert materials that do not react. Includes limestone fillers.

17. Pozzolanic Cement

Portland cement with pozzolans (P or Q). Requires more water and intense curing. Pozzolans slow setting, reducing initial strength but improving long-term strength and chemical resistance. Lower heat of hydration improves volume stability.

18. Calcium Aluminate Cement

  • Made from limestone and bauxite, melted at 1500°C.

  • Used for high early strength or chemical resistance in acidic, sulfate, or saltwater environments.

  • Main component: Monocalcium aluminate (AC)

  • Hydration releases Al(OH)3, improving acid resistance but increasing vulnerability to alkaline waters.

  • Slow setting, high heat of hydration, suitable for cold weather concreting.

  • Prone to strength regression under high humidity and temperature.

19. “Cements with Additional Features” (RC-03)

Cements meeting specific requirements beyond their type and class, such as sulfate resistance (SR), seawater resistance (MR), or low heat of hydration (BC).

20. Hydration of Clinker Components

  • C3S: Hydrates to form silicates and Ca(OH)2.

  • C2S: Hydrates slowly, releasing less Ca(OH)2.

  • C3A: Hydrates rapidly to form aluminates.

  • C4AF: Hydrates rapidly to form aluminates and ferrites.

  • Free CaO and MgO: Hydrate to form hydroxides.

21. Setting and Hardening

Setting: Increased viscosity and reduced workability due to hydration and precipitation.

Hardening: Progressive strength gain due to crystal interposition and colloidal adhesion.