cements and concretes

Question 1

Paste

Answer 1

cement + water
rarely used alone (expensive)
combined with aggregates

Question 2

cements

Answer 2

dry powder made by heating limestone, reactive

Question 3

mortar

Answer 3

paste+sand

used to join bricks together or as a coating

Question 4

concrete

Answer 4

paste+sand+coarse aggregate

coarse aggregate usually gravel, acts as bulk

Question 5

aggregates

Answer 5

sand, gravel, rock used as bulk/volume as cheaper than cement

• cement is the binder
• choose aggregates that dont react with cement paste eg quartz basalt granite
• slightly reactive rocks like limestone can be use to advantage
• avoid using glassy aggregates or sulfides - they react with cement and expand

Question 6

aggregates- role

Answer 6

strength , good quality clean strong aggregates need to be used, clay contamination will have a bad effect.
density, lightweight (foamed) aggregates for lightweight concretes, porous can store water for reactions
reduce thermal cracking - heat of hydration is reduced.
increased cohesion (less segregation in fluid state)
crushed up concrete can be used as an aggregate

Question 7

steel reinforcement

Answer 7

concrete strong in compression, but weak in tension
steel goo din tension used as reinforcement
cement stops steel from rusting, absorbs most of the water

Question 8

flow of concrete

Answer 8

needs to be able to flow, to be pumped
cant be like milk and wheatabix
concrete is shear thinning,flows more easily when a force is applied. must be kept in movement

Question 9

the slump test

Answer 9

1 fill a cone with concrete
2 lift off cone
3 measure change in height

high slump -flops down a lot
zero slump - no flopping
medium slump -intermediate flop

will indicate if something is v wrong

Question 10

water in concrete

Answer 10

needed for :-cement hydration
- makes concrete flow
too much water leads to :- creates more pores puddles in the concrete
concrete is more permeable
reduction in strength
can delay setting, separates liquid ans aggregates

Question 11

concrete durability

Answer 11

main cause is steel failure
when steel rusts it expands -causes cracks in the concrete
sea water is v bad

Question 12

making Portland cement PC

Answer 12

limestone CaCO3 
clay/shale/sand Al2O3 SiO2 Fe2O3 
heated in rotary kiln 
CaO produced reacts with clinkers 
CO2 released

Question 13

list cement clinker

Answer 13

• tricalcium silicate (3CaO.SiO2) C3S
• dricalcium silicate (2CaO.SiO2) C2S
• tricalcium aluminate (3CaO.Al2O3) C3A
• tetracalcium aluminoferite (4CaO.Al2O3.Fe2O3) C4AF

Question 14

gypsum

Answer 14

CaSO4.2H2O C$H2

Question 15

C3S

Answer 15

tricalcium silicate

• gives early strength <7 days
• can build things stronger quicker
• reacts quickly

Question 16

C2S

Answer 16

dricalcium silicate
-gives long term increase in strength
produces more C-S-H
-reacts slower for a longer time

Question 17

C3A

Answer 17

tricalcium aluminate
-fast reacting controls initial setting
-gypsu controls speed of this reaction
if reacted with water will flash set

Question 18

C4AF

Answer 18

tetracalcuim aluminoferite
similar reaction to C3A, but slower
-iron controls melting in the kiln helps transfer chemistry bits can dissolve into it
-gives it its grey colour

Question 19

list important hydrate phases

Answer 19

Calcium silicate hydrate (C-S-H)
Calcium hydroxide (portlandite)
Ettringite (AFt)
AFm family

Question 20

Calcium silica hydrate (C-S-H)

Answer 20

formed by hydration of C3S 
amorphous (non crystalline)
porous 
can take in some AL (C-A-S-H)
fills a lot of space 
MAIN strength giving phase

Question 21

Calcium hydroxide (portlandite)

Answer 21

by product of hydration
large crystalline
doesnt contribute to strength
holds internal ph high - protects steel reinforcement from corrosion

Question 22

Ettringite

Answer 22

AFt
fast reaction of C3A, water and gypsum, how water hardens
interlocking needles cause setting, if force is applied they move
convert to AFm after time

Question 23

AFm fam

Answer 23

contributes a bit to strength
fills space
provides somewhere for aluminium and sulphate to go

Question 24

Supplementary Cementitious Materials SCM

Answer 24

a powder which isnt cement - no useful strength
reacts and contributes to to material performance when blended with PC and water
Types:
-pozzolans
-ground granulated blast furnace slag
-limestone

Question 25

the pozzolanic reaction

Answer 25

• can blend PC with other sources of SiO2
• React with CH to form C-S-H
• over time fills pore space
• improves strength and durability
• slower than hydration
• can reduce early strength
• gives better long term properties
• reduces environ footprint
• reaction cant occur until other products are made

Question 26

pozzolanic additives

Answer 26

• fly ash (byproduct of coal fired powder generation
• volcanic earth
• calcined clays
• silica fume
• waste silica glass materials

Question 27

fly ash

Answer 27

medium sized spheres 
may be hollow 
reacts fairly slowly 
can be used up to ~55%
needs to have low content of unburnt carbon

Question 28

Calcined Clay

Answer 28

heat treatment at ~600-800C
not crystalline
highly reactive
creates pozzolanic and additional AFm phases
take up alot of water in concrete, ‘sticky’ concrete
~35%

Question 29

Silica fume

Answer 29

byproduct of semiconductor Si or ferrosilicon alloy manufacture 
creates C-S-H
v small particle 
-large sa so v reactive 
high water demand must be <10%
high early strength development 
densifies pores 
expensive - bc it makes concrete so much better

Question 30

potential pozzolanic wastes

Answer 30

-metallugial slags
-bottle glass
-window glass
-municipal solid waste incineration ash
-plasma virification residue
brick and ceramic waste

Question 31

Slag - special additive

Answer 31

ground granulated blast furnace slag
behaves like cement
reacts to form C-S-H
classified differently bc so reactive and so much better

calcium alumino-silicate glass
-by product of iron manufacture

can replace up to 95% of PC

Question 32

limestone

Answer 32

calcium carbonate CaCO3
can add unburnt finely ground limestone
cheapest add mixture
small amounts <5% can improve properties
with pozzolans can improve properties of PC