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Materials Science 1 > Concrete & Cement > Flashcards

Concrete & Cement Flashcards

1
Q

what is hydraulic cement?

A

hydraulic cement has the ability to set/harden when it reacts with water

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2
Q

some general info on concrete…

A
  • most widely used construction material worldwide
  • 2 tonnes of concrete per head of population made globally each year
  • vital to society
  • bridges, roads, railways, homes, schools and hospitals
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3
Q

what are the 4 main components of concrete? (5 including air) - list in order of how much

A 
aggregate
sand
water
cement
(air)
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4
Q

what are the 3 main stages of making concrete?

A

paste —> mortar —> concrete
paste = cement + water
mortar = paste + fine aggregate
concrete = mortar + coarse aggregate

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5
Q

what are the 3 main raw materials of Portland cement?

A
  • limestone or chalk (lime CaO)
  • clay or shale (SiO2, Al2O3, Fe2O3)
  • gypsum (CaSO4)
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6
Q

what are the four steps of Portland cement production?

A
  1. grind raw materials
  2. burn in a large rotary kiln at 1450°C to
    form balls known as clinker
  3. cool clinker and grind to a fine powder
  4. add gypsum
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7
Q

what are the two main problems with Portland cement production?

A
  • high carbon footprint (fuels for kiln operation and calcination of raw materials)
  • consumption of natural resources
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8
Q

what is the issue with using fuels for kiln operation?

A

traditionally fossil fuels are used which leads to direction CO2 emissions

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9
Q

what is the issue with calcination of raw materials (burning CaCO3)?

A

CO2 is released due to decomposition of calcium carbonate into lime

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10
Q

what are the four phases in (Portland cement) PC?

A
  1. alite (C3S) - tricalcium silicate
  2. belite (C2S) - dicalcium silicate
  3. aluminate (C3A) - tricalcium aluminate
  4. alumina ferrite (C4AF) - calcium alumino ferrite
    (final - gypsum)
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11
Q

what is the purpose of adding gypsum?

A

to slow down the hydration of C3A

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12
Q

what is the Portland cement notation?

A 
C = CaO
S = SiO2
A = Al2O3
F = Fe2O3
H = H2O
š = SO3
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13
Q

what are the 2 hydration products from the first 2 silicate phases?

A

the C-S-H gel and Ca(OH)2

the calcium hydroxide is responsible for making the concrete alkaline

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14
Q

what is hydration and what does it do to the cement?

A

reaction with water results in setting and hardening (the two proposed mechanisms are in solution and solid state and both probably take place)

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15
Q

what percentage of the silicates (C3S and C2S) make up Portland cement?

A

approx. 70% therefore the contribution of these phases to concrete properties is large

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16
Q

what is the main hydration product of PC cement?

A
  • C-S-H (gel) C3S2H3
  • calcium silicate hydrate
  • main source of concrete strength
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17
Q

what is the structure of the C-S-H gel?

A

very porous

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18
Q

what is the structure of the Ca(OH)2?

A

crystal structure

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19
Q

what is the main difference in structure between calcium hydroxide and ettringite?

A

calcium hydroxide = hexagonal plates

ettringite = hexagonal rods

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20
Q

what is ettringite?

A

rod-like crystals in the early stages of hydration (stage3)

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21
Q

what is monosulfate?

A
  • occurs in the later stages of hydration, after a few days
  • usually it replaces ettringite, either fully or partly
    (stage 3)
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22
Q

what is the problem with adding gypsum at the C3A stage (stage 3)?

A

the gypsum although delays hydration, it adds possible long term reactions with external sulphates (a PC containing less C3A is more desirable for a sulphate resistant PC)

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23
Q

what is the porosity of the cement paste usually?

A

approx. 28%

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24
Q

what does the cement gel do?

A

fills the voids and joins together the unhydrated cement particles

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25
Q

why is there an optimum water/cement ratio?

A

you need a high amount of water to produce more C-S-H gel but an excess of water will then evaporate and leave pores which reduces the compressive strength of the concrete (porosity has a big effect on the compressive strength)

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26
Q

what are the four types of water within the hardened cement paste? (ranging from progressively more difficult to remove from top to bottom)

A
  • chemically combined in the hydrates (non-evaporable)
  • interlayer water (present in narrow gel pores and under the influence of attractive surface forces)
  • adsorbed water (adsorbed on to the solid surfaces of the gel)
  • capillary water (present in the pores beyond the surface forces of the solids)
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27
Q

what factors affect rate of cement hydration?

A
  • age
  • cement composition
  • cement fineness (finer = quicker)
  • water/cement ratio
  • temperature
  • admixtures
28
Q

why is it difficult to obtain PC?

A

because of the sustainability credentials

29
Q

why are additions in PC important?

A

they are constituents (usually in powder form) that can be added at the concrete mixer to can improve the properties

30
Q

what are the concrete properties admixtures effect?

A
  • consistence (workability)
  • set control
  • strength
  • durability
  • special concretes
31
Q

what do accelerators do?

A

they increase the rate of hydration, reduce setting time (useful in cold weather and road repairs)

32
Q

what is the mechanism for accelerators?

A
  • increase solubility of C3A

- over-rides retarding mechanism of gypsum

33
Q

what do retarders do?

A

decrease the rate of hydration, delay the setting time

34
Q

what is the mechanism for retarders?

A
  • decrease solubility of C3A

- cement grains are coated in the admixture which acts as a protective layer

35
Q

what do plasticizers (water reducing admixtures) do?

A

increases workability, becomes more fluid

36
Q

what do super plasticizers do (or high range wanter reducing admixtures)?

A

creates a large spacing between cement particles, temporarily neutralizes the force of attraction, increasing workability and giving the concrete a more liquid consistency

37
Q

what are aggregates?

A
  • generally they are granular materials
    derived from natural rock or natural gravels
  • aggregates comprise 70-80% of the volume
    of concrete and therefore exert a significant
    influence on concrete properties
  • synthetic aggregates include expanded
    clays and shales
  • increasingly recycled materials, including
    demolition waste are being considered as
    aggregates
38
Q

is there any particular rock type which is preferred as aggregate?

A

no - but certain constituents should be avoided e.g…
reactive silica – leads to alkali-silica reaction
weak, friable aggregate – low hardness
porous aggregates – poor freeze/thaw resistance

39
Q

are there any general requirements for an aggregate?

A
  • hard
  • strong
  • free from impurities
40
Q

are there other properties that influence

concrete performance?

A
  • physical properties: specific gravity, hardness, strength, stability, shape, texture, size
  • thermal properties
  • chemical properties: chemical and mineralogical composition, petrographic description, colour
41
Q

what is the purpose of the cement paste in the concrete?

A
  • to fill the voids between the aggregate

- provide strength to hardened concrete

42
Q

what is the purpose of the aggregate in the concrete?

A
  • provides ‘cheap’ fill
  • provides a mass of particles which are suitable for resisting the action of applied loads, abrasion, percolation of moisture and action of weather
  • reduces volume changes resulting from the setting and hardening process and moisture changes in the cement paste
43
Q

what is Portland cement clinker?

A

made by reacting the raw materials at a high temp which is then grinded to make the portland cement

44
Q

what are the hydration products of stages 1/2?

A

the alite and belite phases produce C-S-H gel and calcium hydroxide

45
Q

what are the hydration products of stages 3?

A

in the early stages of hydration, ettringite is formed first and then monosulfate is produced usually after a few days

46
Q

what are the hydration products of stage 4?

A

solid solution of sulfoaluminate and sulfoferrite

47
Q

what is the general reaction for stage 4?

A

C4AF + calcium hydroxide + gypsum + water —> solid solution of sulfoaluminate and sulfoferrite (which gives rise to needle like crystals)

48
Q

what is the order of the phases? (basic symbols only)

A
  1. C3S
  2. C2S
  3. C3A
  4. C4AF
49
Q

what stages are most responsible for the compressive strength of the concrete?

A

C3S & C2S (stages 1/2)

50
Q

is the reaction of portland cement with water exothermic or endothermic?

A

exothermic

51
Q

order the stages from lowest to highest heat of hydration…

A

C3A (stage 3) = 837
C3S (stage 1) = 502
C4AF (stage 4) = 419
C2S (stage 2) = 251

52
Q

what is the equation relating porosity to strength?

A

strength = k (1-P)^3
P = porosity (pore volume/total paste volume)
k = constant
[more pores = lower strength]

53
Q

which reacts quicker, alite or belite? what do each effect?

A
  • alite reacts fast and therefore is responsible for early strength of the hardened cement
  • belite reacts more slowly, mainly contributing to the long-term strength of the hardened cement paste
54
Q

what does the calcium hydroxide do to the paste?

A

makes the cement paste highly alkaline (pH 12.5)

55
Q

what does an increase in fineness mean for the heat of hydration?

A

increased fineness = heat of hydration also increases

56
Q

what are the 4 common variants of PC?

A
  • RHPC (rapid hardening portland cement)
  • white portland cement
  • SRPC (sulfate resistant portland cement)
  • LHPC (low heat portland cement)
57
Q

what is RHPC?

A

finer ground than normal PC, with a slightly modified composition, leading to faster setting and earlier strength development

58
Q

what is white portland cement?

A

portland cement in which the coloured ferrite phase has been eliminated

59
Q

what is SRPC?

A

this is specially formulated for maximum resistance to attack by sulphate ions in ground water, otherwise similar to normal PC

60
Q

what is LHPC?

A

this is specially formulated to have a low heat of hydration in order to avoid destructive thermal expansion effects in large structures

61
Q

how is alternative cements created nowadays?

A

by combining PC with alternative ‘cementitious’ or ‘pozzolanic’ materials - because these materials are industrial by-products this is a step towards mote sustainable use of concrete materials

62
Q

how can the porosity be divided (two categories)?

A
  1. gel pores with a diameter between 0.5 - 5nm

2. capillary pores which are the remains of water filled cavities which have not filled with gel, range between 5 - 10μm

63
Q

what does the volume of the capillary pores depend on?

A

it depends on the water/cement ratio and on the degree of hydration

64
Q

what are the two types of additions (according to European standards)?

A
  • type I (inert additions that do not react with water or cement)
  • type II (pozzolanic or hydraulic additions which include silica fume, fly ash, ggbs etc.)
65
Q

what are pozzolanic materials?

A

they are materials containing active silica (amorphous silica)

66
Q

what does the term blended cement refer to?

A

blended cement is portland cement with pozzolanic additions

67
Q

what are admixtures?

A

materials added during the mixing process of concrete (in small quantities) - usually in the form of a liquid

Materials Science 1 (10 decks)

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