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EGB270 (completed)
Cement & Cementitious Materials
| Question | Answer |
|---|---|
| Hydration of Cement | Water causes the hardening of cement through a process called hydration |
| Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become... | hydrates or hydration products |
| Cement composition highly influences the hydration rate, the following properties are affected: | - time to stiffening; - setting time; and - hardening rate |
| Cement Types: | Type I Type II Type III Type IV Type V White |
| Type I | Classification: General Purpose Characteristics: Fairly high C_3_S content for good early strength development Applications: General construction (most buildings, bridges, pavements, precast units, etc) |
| Type II | Classification: Moderate sulfate resistance Characteristics: Low C_3_A content (<8%) Applications: Structures exposed to soil or water containing sulfate ions |
| Type III | Classification: High early strength Characteristics: Ground more finely, may have slightly more C_3_S Applications: Rapid construction, cold weather concreting |
| Type IV | Classification: Low heat of hydration (slow reacting) Characteristics: Low content of C_3_S (<50%) and C_3_A Applications: Massive structures such as dams, Now rare |
| Type V | Classification: High sulfate resistance Characteristics: Very Low C_3_A content (<5%) Applications: Structures exposed to high levels of sulfate ions |
| White | Classification: White colour Characteristics: No C_4_AF, low MgO Applications: Decorative (otherwise has properties similar to Type I) |
| Other types of cements include... | Portland Blast-furnace Slag Cement Portland Pozzolana Cement Ultra High Early Strength Cement |
| Portland Blast-furnace Slag Cement | Characteristics: 25% - 80% blast furnace slag is added in OPC, lower CO2 emissions Applications: Improved workability, higher long term strength, improved durability especially in aggressive environments |
| Portland Pozzolana Cement | Characteristics: 10% - 30% silaceous pozzolanic compounds such as volcanic ash, fly ash or silica fume are added in OPC, lower CO2 emissions Applications: Similar to OPC |
| Ultra High Early Strength Cement | Characteristics: HE Strength Cement is blended with high content of πΆπΆ3π΄π΄ and/or nano-sized particles of silaceous pozzolana such as silica fume Applications: Special high strength applications where early strength is also required |
| Supplementary Cementitious Materials (Natural) | β’ Volcanic ash β’ Burnt rice husk β’ Diatomaceous Earth (fossilised diatoms) |
| Supplementary Cementitious Materials (By-product pozzolanic materials) | β’ Blast furnace slag (by-product of steel production milled to a fine powder) β’ Calcined clay (Heating clay to > 1000β, πΆπΆππ2 drives off calcium carbonate leaving πΆπΆππππ) |
| Supplementary Cementitious Materials (By-product pozzolanic materials) (2) | β’ Fly ash (By-product of coal burning captured in flue gases β’ Silica fume (By-product of silicon production captured in flue gases) |
| Fly Ash (Positives) | β’ Typically increases water demand by 1%-10%; β’ Improves workability at a given slump; β’ Reduces bleeding; β’ Increases strength at later stage; and β’ Improves durability |
| Fly Ash (Negatives) | β’ Only low dosages (10%-15%) can be used; β’ Early strength may be affected; β’ Setting times may be affected; and β’ Curing time may need to be extended |
| Ground Granulated Blast Furnace Slag (Positives) | β’ High dosage up to 50% or more; β’ Typically increases water demand by 1%-10%; β’ Improves workability at a given slump; and β’ Increases strength at later stage |
| Ground Granulated Blast Furnace Slag (Negatives) | β’ Increased need for curing; β’ Increased bleeding; β’ Early strength may suffer; and β’ Setting times may be affected |
| Silica Fume (Positives) | β’ Improves workability at a given slump; β’ Reduces bleeding; β’ Increases strength; and β’ Improves durability |
| Silica Fume (Negatives) | β’ Typically increases water demand; β’ May increase plastic shrinkage; and β’ Cost |
| Impacts of supplementary cementitious materials: Durability is improved due to: | β’ Reduction in CH, calcium hydroxide; β’ Decrease in pore structure; β’ Reduction in w/c ratio; β’ Increased sulfate resistance; and β’ Reduced chloride diffusion |
| Chemical Admixtures (How?) | β’ Added to cement by cement manufacturer; β’ Added to concrete at batch plant; and / or β’ Added to concrete on site |
| Chemical Admixtures (Why?) | β’ To reduce water requirements or improve workability: plasticising admixtures. β’ To improve freeze / thaw performance: air entraining admixtures. |
| Chemical Admixtures (Why?) (2) | β’ To control the set time of concrete: set-controlling admixtures (Retarders/Accelerators) β’ Other reasons: Shrinkage reduction; Viscosity modification; Corrosion inhibitors |
| Plasticisers allows for... | reduction of w/c ratio while maintaining workability: Increased strength, impermeability and durability |
| Plasticisers increase slump without... | adding water: to facilitate difficult placements |
| Plasticisers achieve desired slump with lower cement content without... | changing w/c (reduce water requirement at least 5-10%): cheaper |
| Superplasticisers reduce water... | requirement by 15% -40% |
| Superplasticisers makes mix highly fluid that can be placed with... | little or no vibration or compaction |
| Superplasticisers effects lasts only 30-60 mins and is followed by rapid... | loss of workability |
| Superplasticisers are... | usually added at jobsite |
| 3 Ways to use Plasticisers | 1) improve workability using same w/c ratio 2) increase strength using lower w/c ratio 3) reduce cost at same w/c ratio by reducing both water & cement |
| Air-entraining Admixtures are recommended for all concrete exposed to... | freeze/thaw cycles |
| Air-entraining Admixtures produce tiny, dispersed air bubbles into... | the concrete: β water expands as it freezes causing internal stress that cracks the hardened cement paste and greatly reduces durability; and β air entrainer provides space for the water to expand |
| Air-entraining Admixtures improve | workability |
| Air-entraining Admixtures improved resistance to... | de-icing chemicals, sulfates and alkalis |
| Air-entraining Admixtures typical air values are... | 5% - 8% |
| Air-entraining Admixtures decreases strength but can be compensated with lower... | w/c ratio |
| Air-entrained Concrete should not be confused with... | entrapped air in concrete, which causes lower strength |
| Air-entrained Concrete's Frost resistance improves with decreasing... | bubble size |
| For Air-entrained Concrete, small bubbles have less effect on... | strength than larger bubbles |
| Set-controlling Admixtures (Retarders) | Delay or retard initial set β’ During hot weather to reduce heat of hydration; β’ Long haul time; β’ When extra time is needed for placement/finishes; β’ May reduce early strength; and β’ Usually doesnβt reduce final set time by much |
| Set-controlling Admixtures (Accelerators) | Speed up the time for initial and final set β’ Reduce the time for finishing operations to begin; β’ Reduce curing time; β’ Increase rate of strength gain; β’ Used in shotcrete applications; and β’ Plugging leaks under hydrostatic pressure |
| Cement Summary - Manufacture: | β Take Limestone + Clay + Iron Oxide + Silica; and β Heat to ~1450β to get clinker |
| Cement Summary - Hydration: | β Ettringite forms rapidly; β Calcium Hydroxide crystals form; and β Calcium Silicate Hydrate gel binds |
| Cement Summary - Composition: | β Clinker: β’ Alite πΆ_3_π; β’ Belite πΆ_2_π; β’ Aluminate πΆ_3_π΄; and β’ Ferrite πΆ_4_π΄F β The above^^^ compounds are added with Gypsum (3%) πΆπΜ H and grounded to make cement powder |
| Cement Summary - Types of Cement: | β I β V, Blended cements |
| Cement Summary - Supp Cementitious Materials: | β Volcanic Ash, Rice Husk Ash, Diatomaceous Earth, Fly Ash, GGBFS, Silica Fume and Calcined Clay |
| Cement Summary - Chemical Admixtures | β Plasticisers; β Air-entraining; and β Set-controlling |
| Clinker + Gypsum = | Cement |
| Clinker has the following compounds: | Alite πΆ_3_π; Belite πΆ_2_π; Aluminate πΆ_3_π΄; and Ferrite πΆ_4_π΄πΉ |
Created by:
Asher - S
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