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Materials > Cements > Flashcards

Cements Flashcards

1
Q

What is Portland cement?

A

Hydraulic cement capable of setting, hardening and remaining stable under water. It consists essentially of hydraulic calcium silicates.

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2
Q

What are the 4 cement clinkers?

A
  • Alite (Tricalcium silicate), 3CaO.SiO2
  • Belite (Dicalcium silicate), 2CaO.SiO2
  • Aluminate ( Tricalcium aluminate), 3Cao.Al2O3
  • Teracalcium aluminoferritre, 4CaO.Al2O3.Fe2O3
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3
Q

How is portland cement made?

A
  • Made from limestone and clays in a kiln, ground to a powder + gypsm (CaSO4.2H2O).
    -limestone provides Ca
    -heat to 1400C in rotary kiln
    CaCo3 = CaO + CO2
    -CaO combines with SiO2, Al2O3, Fe2O3 to form clinkers
    -Co2 emission 0.8Kg per Kg of cement
    -8% of global CO2 emissons
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4
Q

What are the cement chemistry notations ?

A 
CaO = C , SiO2 = S
Fe2O3 = F,  Al2O3 = A
SO3 = $ or S_
Alite = C3S
Belite = C2S
Aluminate= C3A
Teracalcium aluminoferritre = C4AF
Gypsum = C$H2
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5
Q

What are the clinker composition?

What does each clinker provide?

A

C3S - about 65%
- Gives early strength development

C2S- about 20%
-Gives long term gain of strength

C3A- about 8%
-Controls initial setting

C4AF
-Similar hydration products to C3A, but reacts slower
and doesn’t contribute much strength
– Important in controlling melting in the kiln
– Gives grey colour

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6
Q

What is cement hydration?

A

Mixing cement with water makes it react:

  • Particles dissolve
  • ions rearrange
  • solid hydrate phases solidify and give strength
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7
Q

What are the important hydrate phases?

A

Calcium Silicate Hydrate (C-S-H)

Calcium hrdroxide (Portlandite, Ca(OH)2)

Ettringite (C3A.3C$.32H)

AFm phases family (C3A.C$.nH)

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8
Q

What is Calcium Silicate Hydrate ?

A

C-S-H:

  • formed by hyrdatiuon of C3S and C2S
  • Non-crystalline
  • porous on nanomtre length scale
  • No fixed chemical composition.
  • Fills a lot of the spoace in hardened binders
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9
Q

What is Calcium hydroxide?

A

Portlandite, Calcium hrdroxide:

  • By product of C2S and C3S
  • Makes large crystallites
  • doesn’t contribute to strength
  • Hold PH high
  • tends to grow around aggregate particles
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10
Q

What is Ettringite?

A

Ettringite (C3A.3C$.32H):

  • also called AFt
  • formed by fast reaction of C3A with water and gypsum
  • Needles interlocking causing initial setting
  • Mostly converted to AFm at longer ages.
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11
Q

AFm phases family

A

AFm phases family (C3A.C$.nH):

  • with n approx. 11-19 and some replacement of $ by the other negatively charged species
  • contributes a bit to strength
  • Fills in some space.
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12
Q

What are Supplementary cementitious materials (SCMs)?

A
  • Any mineral component blended with cement thats react to alter the properties of the binder.
  • Most SCMs are Pozzolans.
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13
Q

What is the Pozzolanic reaction?

A

-Blend cement with other Sources of SiO2, with or without Al2O3.
-these react with CH to form more C-S-H (improves strength and durability)
- Approx:
CH + S + xA = C-(A)-S-H
-Limstone is a SCM but not a Pozzolan

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14
Q

Name some Pozzolanic Additives:

A
  • Fly Ash
  • Natural Pozzolans (volcanic earth)
  • Heat-treated clays
  • Silica fume
  • Rice husk Ash
  • Other waste silicate glass materials.
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15
Q

what are the Influences of SCMs?

A
  • Poxxolanic reaction is slower than hydration so usually reduces earth strength.
  • Blended Cements general;ly give better long term properties.
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16
Q

What is paste , Mortar, Concrete?

A

Paste = Cement + water

Mortar = Paste + sand

Concrete= Paste + Sand + coarse aggregate.

17
Q

Why is water important in concrete and why should you be careful how much you put in?

A
  • Required in cement hydration reactions
  • Makes concrete flow

BUT, Too much water forms extra pores:

  • reduces durability ( increases permeability)
  • Reduced strength
  • delay setting
  • cause bleeding
  • increase drying shrinkage
18
Q

how would you reduce water content?

A

add (super)plasticisers to improve flow properties.

19
Q

What is segregation?

A

Concrete is too wet and the coarse aggregates are on show.

-to avoid add less water and/or use fine aggregates.

20
Q

Whats is Plastic shrinkage?

A

rapid water evaporation from the surface makes the paste shrink - and water bleeds to the surface. Aggregate particles stay in place and restrain the shrinkage causing cracking.
-to avoid stop the concrete drying and control the bleed

21
Q

What is plastic settlement?

A

Solid aggregate paricles can sink through the paste, leaving water pockets under agreegates and R bars and cracks on surface. Fatal for durability.

22
Q

Name some binder other than portland cement:

A
  1. Alkali-activated cement (Aluminosilicate materials + alkaline solution) - less CO2 emissions
  2. Calcium aluminate cement (CAC), sometimes catastrophic strength loss if used in wrong conditions
  3. Magnesium oxychloride cement. (high early strength but sensitive to water).
  4. Bitumen concrete (a mixture of organic molecules) used to bind stones on solid hardened materials (for roads). Not technically and cement.
23
Q

What is the easiest way to test concrete? and what should the results be?

A

You place and a square or cylindrical sample of concrete into ta compression test and see what stress the sample breaks at.

should ‘apple core’ and not touch the top or bottom.
Should not shear or chip this means it was applied unevenly.
Discard bad tests

24
Q

How are cements standardised?

A

They are standardised according to strength grade. e.g. CEM II/A-V 42.5R is common.
CEM II/A-V - is a type of fly ash blended cement.
42.5 is the Grade (28-day compressive strength.)
R means rapid strength development and N would mean normal strength development.

25
Q

How to test splitting tensile and bending strengths?

A

Splitting is a sphere sample getting compressed by two plates.

Bending is a 3 point test with two supports and a force being applied in the middle of the rectangular sample.

26
Q

Name 4 Non-destructive tests for concrete:

A
  1. Electrochemical testing
  2. Radiographic or radar-based methods
  3. Air or water permeability
  4. testing surface hardness - hammer bound / pullout a screw test
27
Q

describe creep testing

A

Longterm test
load samples at different ages (2- 90 days)
Need to be careful of differential creep

Materials (3 decks)

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