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MAT1600 - Materials > Cement and Concrete > Flashcards

Cement and Concrete Flashcards

Lecture 14 and15

1
Q

What may cements and concretes be categorised as?

A

It may sometimes be categorised as ceramics BUT not produced by a firing of a green body.

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2
Q

Instead of raw materials being fired, what else do they use?

A

Instead, raw materials are fixed to produce powders that react with water:
- Setting reactions
- Strength develops with time

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3
Q

What kind of loads can cements and concretes withhold?

A

Can support compressive loads
- need reinforcing to support tensile loads

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4
Q

What are the different types of cement?

A

Portland cement, speciality cement and medical cement

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5
Q

What are the uses for portland cement?

A

Concrete and mortar
Buildings, infrastructure, roads, etc

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6
Q

What are the uses for medical cements?

A

Tooth and bone replacement (polymer or ceramic type)

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7
Q

What is Portland cement?

A

ASTM definition: “…consists essentially of hydraulic calcium silicates usually containing calcium sulfate”
- hydraulic: reacts with water and hardens under water
- i.e. cement sets, it doesn’t dry
The basis of most (>95%) concrete used worldwide

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8
Q

How is Portland cement made?

A

Raw materials:
- limestone: CaCO3
- clay/shale/sand / etc:
-> supply of AI2O3, SiO2 and Fe2O3
Heat to about 1500C in a rotary kiln
Limestone breaks down into CaO (lime) and CO2, CaO combines with SiO2, AL2O3 and Fe2o3 to form clinker phases

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9
Q

How much CO2 emissions are released per kg of Portland cement?

A

CO2 emissions of about 0.8kg per of cement:
- ~50% form fuel and ~50% from chemistry
- 5-8% of global human-derived CO2
-> Hence interest in blended cements

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10
Q

What is the cement chemistry notation for the oxide formula CaO?

A

C

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11
Q

What is the cement chemistry notation for the oxide formula SiO2?

A

S

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12
Q

What is the cement chemistry notation for the oxide formula Al2O3?

A

A

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13
Q

What is the cement chemistry notation for the oxide formula Fe2O3?

A

F

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14
Q

What is the cement chemistry notation for the oxide formula H2O?

A

H

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15
Q

What is the cement chemistry notation for the oxide formula SO3?

A

_
S

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16
Q

What is the cement chemistry notation for the oxide formula CO2?

A

_
C or c

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17
Q

What is the cement chemistry notation for the oxide formula Na2O?

A

N

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18
Q

What is the cement chemistry notation for the oxide formula K2O?

A

K

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19
Q

What happens when you mix cement with water?

A

Mixing cement with water makes it react
- particles dissolve
- ions rearrange in solution
- solids hydrate phases solidify and give strength

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20
Q

What happens during the hydration reaction process?

A

The reaction process is hydration, not drying!
- water forms an essential part of the solid phases in hardened cement
- drying is very bad for fresh cement
-> It slows or stops reactions
-> results in cracking and loss of strength
NB: heat is generated by the chemical reactions

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21
Q

What is paste?

A

PASTE: cement + water
- rarely used alone, usually combined with aggregate
-> similar materials: gypsum plaster/plaster of Paris, dental cement

22
Q

What is motar?

A

MOTAR: paste + sand (‘fine aggregate’, <5mm)
- used to join bricks together, or as a coating
-> similar materials: lime mortar, based on CaO

23
Q

What is concrete?

A

CONCRETE: paste + sand + coarse aggregate
- coarse aggregate usually gravel, crushed rock, up to a few cm in size

24
Q

What are aggregates?

A

Sand, gravel and rocks are much cheaper than cement
- 70 - 80 vol% of concrete
- use the cement as a ‘binder’ (like glue) to hold the aggregate together
- Ideally, choose aggregates that don’t react with the cement paste: quartz, basalt, granite
- Can also used slightly - reactive rocks (limestone)
- avoid glassy aggregates or sulphides
-> These react with cement, expand and cause concrete failure

25
Q

What is the approximate mix design values for cement?

A

Define mix constituents in kg/m^3 of concrete
- cementitious materials: 200-400 kg/m^3
- water: 120-240 kg/m^3
- aggregates: 1700-2000 kg/m^3
-> of which fine aggregate is ~40%, course aggregate ~60%
- admixtures and other additives: up to 5-10 kg/m^3

26
Q

What is the density of concrete?

A

The density of concrete (i.e. total mass of ingredients) is usually around 2300 - 2500 kg/m^3
-> This changes if lightweight or heavyweight aggregates are used, and don’t include the mass of steel reinforcement

27
Q

What is the role of aggregates?

A

Strength
-> Concrete cannot be stronger than aggregate, because it fails at its weakest point
-> need good quality, clean, strong aggregate, clay contamination tends to have a bad effect
Weight
-> lightweight (foamed) aggregates for lightweight concrete -> expanded pearlite, pumice, vermiculite
Reduce thermal cracking
-> Dilution of cement
can also use crushed old concrete as ‘recycled aggregate’

28
Q

What is a mesoscale of concrete?

A

Composite of aggregate (coarse and fine) in a matrix of cement paste

29
Q

What is Rheology?

A

Study of flow characteristics

30
Q

What does it mean when concrete is “shear-thinning”?

A

If it flows more readily when a force is applied to it hence, concrete mixer trucks keep rotating

31
Q

What helps the placement of concrete?

A

Vibrations help the placement of concrete. Makes it flow nicely into place

32
Q

What does “self-compacting” concrete mean?

A

“self-compacting” concrete flows like a liquid with no vibration -> uses chemical additives

33
Q

What is water needed in concrete?

A

Water is needed for 2 main reasons:
- essential for cement hydration reactions
- makes concrete flow (increased slump)

34
Q

Why is too much water in concrete bad?

A

If there is extra water, it forms extra pores, resulting in:
- reduction in durability -> more permeable, so damaging agents from outside can get in
- reduction in strength (materials with holes in it is less strong)
- delayed setting/hardening
- can cause segregation of particles from liquid

35
Q

How does water affect the strength of cement?

A

more water = more pores
this reduces strength
water content is measured as water/cement ratio (w/c)
for blended cement use water/binder ratio (w/b)
typically w/c ~ 0.45-0.6
-> In theory only w/c ~0.35 is required for hydration but this leads to problems with mixing and flow

36
Q

What are some chemical admixtures used to control cements and concretes?

A

“plasticisers”, superplastics, air content, rate of setting

37
Q

How do plasticisers affect cements and concretes?

A

adding sugar to reduce the water needed for the desired flow

38
Q

How do superplastics affect cements and concretes?

A

superplastics have very dramatic effects -> high flow at low water content

39
Q

How does air content affect cement and concrete?

A

Gives freezing resistance

40
Q

How does the rate of setting affect cements and concretes?

A

accelerators = faster set
retarders = slower set

41
Q

What is cement?

A

Widely used in mortars and concretes
- mortars include sand
- concrete includes sand and coarse aggregate
most cement is Portland cement

42
Q

What are the major phases in Portland cement?

A

Alite (C3S), belite (C2S), aluminate (C3A), ferrite (C4AF), gypsum (CSH2)

43
Q

How do cements hydrate?

A

drying at early stages is detrimental
water is important for flow and setting
-> but too much water will reduce the strength

44
Q

What do most blended cements have?

A

Most cement sold in the UK and EU are blended with “supplementary cementitious materials” (SCMs)

45
Q

What do SCMs include?

A

Industrial byproducts
-> e.g. ground granulated blast furnace slag from iron production, fly ash from coal combustion
Minerals
-> e.g. limestone or calcine (burnt) clays

46
Q

How do blended cements contribute to the reaction process?

A

These contribute to the cement reaction process but mostly react more slowly than Portland cement. This can reduce early strength but can give improvements in final properties - strength and durability

47
Q

What is a major benefit of SCMs?

A

Importantly SCMs reduce total CO2 emissions from cement

48
Q

What are the three types of SCMs?

A

Pozzolans (various different types)
Ground granulated blast furnace slag
-> low iron calcium aluminosilicate
-> hydraulic but requires an activator (CH)
-> slow reaction
limestone

49
Q

What are the hydration products used?

A

C-S-H gel
calcium hydroxide (CH)
Ettringite (AFt) which converts to mono sulfate (AFm)

50
Q

Facts about supplementary cementitious materials?

A

reduced CO2 emissions
Reduced heat evolution
Pozzolans (various different types) - sources of silica (and/or alumina) which react with CH to form additional C-S-H
Ground granulated blast furnace slag
limestone

MAT1600 - Materials (19 decks)

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