cements and concretes Flashcards
Paste
cement + water
rarely used alone (expensive)
combined with aggregates
cements
dry powder made by heating limestone, reactive
mortar
paste+sand
used to join bricks together or as a coating
concrete
paste+sand+coarse aggregate
coarse aggregate usually gravel, acts as bulk
aggregates
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
aggregates- role
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
steel reinforcement
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
flow of concrete
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
the slump test
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
water in concrete
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
concrete durability
main cause is steel failure
when steel rusts it expands -causes cracks in the concrete
sea water is v bad
making Portland cement PC
limestone CaCO3 clay/shale/sand Al2O3 SiO2 Fe2O3 heated in rotary kiln CaO produced reacts with clinkers CO2 released
list cement clinker
- tricalcium silicate (3CaO.SiO2) C3S
- dricalcium silicate (2CaO.SiO2) C2S
- tricalcium aluminate (3CaO.Al2O3) C3A
- tetracalcium aluminoferite (4CaO.Al2O3.Fe2O3) C4AF
gypsum
CaSO4.2H2O C$H2
C3S
tricalcium silicate
- gives early strength <7 days
- can build things stronger quicker
- reacts quickly
C2S
dricalcium silicate
-gives long term increase in strength
produces more C-S-H
-reacts slower for a longer time
C3A
tricalcium aluminate
-fast reacting controls initial setting
-gypsu controls speed of this reaction
if reacted with water will flash set
C4AF
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
list important hydrate phases
Calcium silicate hydrate (C-S-H)
Calcium hydroxide (portlandite)
Ettringite (AFt)
AFm family
Calcium silica hydrate (C-S-H)
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
Calcium hydroxide (portlandite)
by product of hydration
large crystalline
doesnt contribute to strength
holds internal ph high - protects steel reinforcement from corrosion
Ettringite
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
AFm fam
contributes a bit to strength
fills space
provides somewhere for aluminium and sulphate to go
Supplementary Cementitious Materials SCM
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
the pozzolanic reaction
- 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
pozzolanic additives
- fly ash (byproduct of coal fired powder generation
- volcanic earth
- calcined clays
- silica fume
- waste silica glass materials
fly ash
medium sized spheres may be hollow reacts fairly slowly can be used up to ~55% needs to have low content of unburnt carbon
Calcined Clay
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%
Silica fume
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
potential pozzolanic wastes
-metallugial slags
-bottle glass
-window glass
-municipal solid waste incineration ash
-plasma virification residue
brick and ceramic waste
Slag - special additive
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
limestone
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
