Flashcards in SCMs Deck (23):
What are SCMs?
A material that, when used with portland cement contributes to the properties of the hardened concrete through: Hydraulic Activity, Pozzolanic Activity or both.
Describe what pozzolans are?
A siliceous or alumino-silliceous material that, in a finely divided form and in the presence of moisture, chemically reacts with a hydration product of portland cement to form compounds possessing cementing properties.
Describe what natural pozzolans are?
A natural material which may also be calcined and/or processed.
What is CSH?
Calcium Silicate Hydrate: is the binder (glue) responsible for strength.
What is CH?
Calcium Hydroxide: does not have an important contribution to strength
What are the hydration products?
What does SCMs do?
- Enhancement to mechanical properties and durability
- Refinement of porosity
- Densification of aggregate-cement paste interface
What is Transition Zone?
Is a thin layer between the bulk hydrated cement paste and the aggregate particles in concrete. This zone is the weakest component in concrete, and its also the most permeable area.
What are the different SCMs?
- Silica Fumes
- Fly Ash
- Natural Pozzolans
What are silica fumes?
- Smoke by-product from furnaces used in the production of ferrosilicon and silicon metals
- Very Fine non crystalline silica: high silica content, extreme small particle size, large surface area and highly reactive pozzolan
Forms of silica fumes?
- As-produced powder
- Silica fume slurry (50-52% solids)
- Densified Silica Fume
- Increases Strength (S+CH = CSH)
- Reduces Permeability thus enhances durability
- Increases concrete cohesiveness
- Good Curing
What are Fly Ashes?
By-product of coal burned power plants.
What are the two types of fly ash?
- Class F Fly Ash
- Class C Fly Ash
Describe Class F Fly Ash
- low calcium oxide fly ash
- produced by calcination of bituminous coal
- has pozzolanic properties
- Moderate Heat
- High concrete
- Sulfate resistance
Describe Class C Fly Ash
- High calcium oxide fly ash
- Produced by calcination of subbituminous coal or lignite.
- Has pozzolanic + cementitious properties
- Higher early strength
- Soil Stabilization
What is the effect of fly ash on setting time?
- Fly ash usually hydrates slowly
- Setting time is moderately or severely extended depending upon the properties of fly ash, temperature and mixture proportions
- Finer fly ash results in shorter setting time (C)
- Higher CaO content results in shorter setting time (C)
- Increases later strength
- Reduces permeability thus enhances durability
- Increase workability
- Reduce heat of hydration
- Increase concrete ASR resistance
- Increase concrete sulfates resistance
- Good curing
What is slag?
By-product of the metallurgical industry. The most used in cement is iron blast furnace slag.
What are the uses of slag?
- Used as aggregate or disposed as waste
- Hydraulic Binder
- Hydraulic slags give similar hydration products to OPC but react slowly with water.
What are the effect of slag on fresh properties?
- At low temperatures, setting time is increased (slower hydration of GBFS)
- Strength development of slag cement is slower at early stage than that of OPC but faster at later ages.
- At one year slag cement achieves significantly higher strength
- comparable strength to OPC at 28 days
- Improve the concrete resistance to fire damage
- penetration resistance
- Reduces AAR