CEE 311L Portland Cement
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What is Portland cement?
A hydraulic binder that bonds aggregates together to form concrete.
Why is Portland cement called Portland cement?
It was named after the limestone and chalk cliffs on the Isle of Portland.
What is the primary function of cement in concrete?
To act as the glue or binder that holds aggregates together.
What are the three major steps in cement production?
Crushing/grinding raw materials, kiln heating to form clinker, and grinding clinker with gypsum.
What raw material supplies calcium oxide for cement production?
Limestone, chalk, or oyster shells.
What raw materials supply silica and alumina?
Clay, shale, and blast furnace slag.
At what temperature is clinker formed?
1400-1650°C (2500-3000°F).
Why is gypsum added during cement production?
To delay setting time.
Why are cement particles ground very fine?
To increase surface area and improve hydration.
What are the four main compounds in Portland cement?
C3S, C2S, C3A, and C4AF.
What does C3S stand for?
Tricalcium silicate.
What does C2S stand for?
Dicalcium silicate.
What does C3A stand for?
Tricalcium aluminate.
What does C4AF stand for?
Tetracalcium aluminoferrite.
Which two compounds contribute most to concrete strength?
C3S and C2S.
What is calcination?
The kiln process that changes the molecular structure of cement ingredients.
What alkalis in cement can cause expansion and disintegration?
Na2O and K2O.
What is alkali-silica reactivity (ASR)?
A reaction between cement alkalis and reactive silica that causes expansion and cracking.
What are the approximate percentages of lime, silica, and alumina/iron oxide in Portland cement?
60-65% lime, 20-25% silica, and 7-12% alumina/iron oxide.
What is fineness?
The particle size and surface area of cement.
How does fineness affect hydration?
Finer particles hydrate faster.
Why is excessive fineness undesirable?
It increases cost and heat of hydration.
What test measures cement fineness through air permeability?
Blaine Test.
What test measures fineness through sedimentation?
Wagner Turbidimeter.
What is hydration?
The chemical reaction between cement and water.
Is hardening the same as drying?
No, hardening results from hydration while drying is water evaporation.
What happens when concrete dries before hydration is complete?
Hydration and strength gain stop.
What hydration mechanism dominates early hydration?
Through-solution hydration.
What is topochemical hydration?
A solid-state reaction occurring at the cement particle surface.
Which hydrates faster, aluminates or silicates?
Aluminates hydrate faster than silicates.
What does hydration of C3S and C2S produce?
C-S-H and calcium hydroxide.
What does C-S-H stand for?
Calcium-silicate-hydrate.
What is the primary source of strength in cement paste?
C-S-H.
What is the main product responsible for concrete strength?
C-S-H gel.
Is hydration a short-term or long-term process?
Both; rapid initially but can continue for decades.
What major byproduct of hydration can reduce durability?
Calcium hydroxide.
What percentage of hydrated C3S becomes C-S-H?
About 61%.
What percentage of hydrated C3S becomes calcium hydroxide?
About 39%.
What percentage of hydrated C2S becomes C-S-H?
About 82%.
What percentage of hydrated C2S becomes calcium hydroxide?
About 18%.
Which compound contributes more to long-term strength and durability?
C2S.
How does increasing C2S relative to C3S affect concrete?
Increases ultimate strength and durability.
What occurs during Stage I of cement paste development?
Cement particles begin hydrating in water.
What occurs during Stage II of cement paste development?
Hydration products begin growing outward and initial set occurs.
What occurs during Stage III of cement paste development?
Cement particles become locked in place.
What occurs during Stage IV of cement paste development?
Spaces fill with hydration products, increasing strength and durability.
What are interlayer hydration spaces?
Spaces between atomic layers in hydration products.
What can occur if humidity drops below 11% in interlayer spaces?
Shrinkage.
What causes capillary voids?
Excessively high water-cement ratio.
How do capillary voids affect concrete?
Reduce strength and increase permeability.
What are trapped voids?
Large air pockets caused by handling or poor consolidation.
How do trapped voids affect concrete?
Reduce strength and increase permeability.
What is entrained air?
Microscopic air bubbles intentionally introduced by admixtures.
How does entrained air affect concrete?
Increases durability and freeze-thaw resistance.
What is setting?
The stiffening or change from plastic to solid state.
What is hardening?
The strength gain of cement paste after setting.
What test is commonly used to measure setting time?
Vicat test.
What other test measures setting?
Gillmore test.
How is initial set determined using the Vicat test?
Needle penetration of 25 mm or less.
How is final set determined using the Vicat test?
Needle penetration of zero mm.
What is soundness?
The ability of cement to retain volume after setting.
How is cement soundness measured?
Using the autoclave expansion test.
What conditions are used in the autoclave soundness test?
420°F and 295 psi.
What ASTM standard measures mortar compressive strength?
ASTM C109.
What specimens are used in ASTM C109?
Three 2-inch mortar cubes.
Can concrete strength be accurately predicted from cement strength alone?
No.
What is the most important property of hydrating cement paste?
Water-cement ratio.
How much water is required for hydration alone?
w/c = 0.22-0.25.
Why is additional water often added beyond hydration requirements?
To improve workability.
What effect does excess water have on concrete strength?
It decreases strength.
What effect does excess water have on durability?
It decreases durability.
What effect does excess water have on permeability?
It increases permeability.
What effect does excess water have on bond with reinforcement?
It decreases bond strength.
What is the typical water-cement ratio range for PCC?
0.4 to 0.7.
What is Type I cement?
Normal Portland cement.
What is Type II cement?
Moderate sulfate-resistant cement.
What is Type I/II cement?
Cement that meets both Type I and Type II requirements.
What is Type III cement?
High-early-strength cement.
What advantage does Type III cement provide?
Earlier strength gain and earlier form removal.
What is Type IV cement?
Low heat of hydration cement.
When is Type IV cement used?
For massive concrete placements.
What is Type V cement?
High sulfate-resistant cement.
What type of water is generally acceptable for concrete?
Potable drinking water.
Can seawater be used for concrete?
Yes for plain concrete, but not reinforced concrete.
What ASTM standard provides criteria for mixing water quality?
ASTM C94.
What strength requirement must non-potable water meet under ASTM C94?
Mortar cube strength must be at least 90% of that made with fresh water.
What environmental concern exists with concrete wash water?
It is considered hazardous wastewater.
How can wash water be reused?
As mixing water with stabilizing admixtures.
What is an admixture?
A material added to concrete to improve fresh or hardened properties.
What are the six major admixture categories?
Air entrainers, water reducers, retarders, hydration controllers, accelerators, and specialty admixtures.
What is the purpose of an air-entraining admixture?
To create tiny air bubbles for freeze-thaw protection.
When are air entrainers recommended?
For all concrete exposed to freezing conditions.
How do air entrainers improve durability?
They provide space for freezing water to expand.
How do air entrainers affect workability?
They improve workability.
How much strength reduction can air entrainment cause?
About 20%.
How can strength loss from air entrainment be compensated?
By lowering the water-cement ratio.
What is the difference between entrained and entrapped air?
Entrained air is beneficial and microscopic; entrapped air is larger and harmful.
What is a water reducer?
An admixture that increases cement particle mobility.
What property is primarily improved by water reducers?
Workability.
What test typically measures the workability improvement from water reducers?
Slump test.