Cement and Concrete Flashcards
Lecture 14 and15
What may cements and concretes be categorised as?
It may sometimes be categorised as ceramics BUT not produced by a firing of a green body.
Instead of raw materials being fired, what else do they use?
Instead, raw materials are fixed to produce powders that react with water:
- Setting reactions
- Strength develops with time
What kind of loads can cements and concretes withhold?
Can support compressive loads
- need reinforcing to support tensile loads
What are the different types of cement?
Portland cement, speciality cement and medical cement
What are the uses for portland cement?
Concrete and mortar
Buildings, infrastructure, roads, etc
What are the uses for medical cements?
Tooth and bone replacement (polymer or ceramic type)
What is Portland cement?
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
How is Portland cement made?
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
How much CO2 emissions are released per kg of Portland cement?
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
What is the cement chemistry notation for the oxide formula CaO?
C
What is the cement chemistry notation for the oxide formula SiO2?
S
What is the cement chemistry notation for the oxide formula Al2O3?
A
What is the cement chemistry notation for the oxide formula Fe2O3?
F
What is the cement chemistry notation for the oxide formula H2O?
H
What is the cement chemistry notation for the oxide formula SO3?
_
S
What is the cement chemistry notation for the oxide formula CO2?
_
C or c
What is the cement chemistry notation for the oxide formula Na2O?
N
What is the cement chemistry notation for the oxide formula K2O?
K
What happens when you mix cement with water?
Mixing cement with water makes it react
- particles dissolve
- ions rearrange in solution
- solids hydrate phases solidify and give strength
What happens during the hydration reaction process?
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
What is paste?
PASTE: cement + water
- rarely used alone, usually combined with aggregate
-> similar materials: gypsum plaster/plaster of Paris, dental cement
What is motar?
MOTAR: paste + sand (‘fine aggregate’, <5mm)
- used to join bricks together, or as a coating
-> similar materials: lime mortar, based on CaO
What is concrete?
CONCRETE: paste + sand + coarse aggregate
- coarse aggregate usually gravel, crushed rock, up to a few cm in size
What are aggregates?
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
What is the approximate mix design values for cement?
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
What is the density of concrete?
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
What is the role of aggregates?
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’
What is a mesoscale of concrete?
Composite of aggregate (coarse and fine) in a matrix of cement paste
What is Rheology?
Study of flow characteristics
What does it mean when concrete is “shear-thinning”?
If it flows more readily when a force is applied to it hence, concrete mixer trucks keep rotating
What helps the placement of concrete?
Vibrations help the placement of concrete. Makes it flow nicely into place
What does “self-compacting” concrete mean?
“self-compacting” concrete flows like a liquid with no vibration -> uses chemical additives
What is water needed in concrete?
Water is needed for 2 main reasons:
- essential for cement hydration reactions
- makes concrete flow (increased slump)
Why is too much water in concrete bad?
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
How does water affect the strength of cement?
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
What are some chemical admixtures used to control cements and concretes?
“plasticisers”, superplastics, air content, rate of setting
How do plasticisers affect cements and concretes?
adding sugar to reduce the water needed for the desired flow
How do superplastics affect cements and concretes?
superplastics have very dramatic effects -> high flow at low water content
How does air content affect cement and concrete?
Gives freezing resistance
How does the rate of setting affect cements and concretes?
accelerators = faster set
retarders = slower set
What is cement?
Widely used in mortars and concretes
- mortars include sand
- concrete includes sand and coarse aggregate
most cement is Portland cement
What are the major phases in Portland cement?
Alite (C3S), belite (C2S), aluminate (C3A), ferrite (C4AF), gypsum (CSH2)
How do cements hydrate?
drying at early stages is detrimental
water is important for flow and setting
-> but too much water will reduce the strength
What do most blended cements have?
Most cement sold in the UK and EU are blended with “supplementary cementitious materials” (SCMs)
What do SCMs include?
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
How do blended cements contribute to the reaction process?
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
What is a major benefit of SCMs?
Importantly SCMs reduce total CO2 emissions from cement
What are the three types of SCMs?
Pozzolans (various different types)
Ground granulated blast furnace slag
-> low iron calcium aluminosilicate
-> hydraulic but requires an activator (CH)
-> slow reaction
limestone
What are the hydration products used?
C-S-H gel
calcium hydroxide (CH)
Ettringite (AFt) which converts to mono sulfate (AFm)
Facts about supplementary cementitious materials?
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
What is the hydration phase for calcium silicate hydrate (C-S-H)?
What is the hydration phase for calcium hydroxide (CH - portlandite, Ca(OH)2)?
What is the hydration phase for Ettringite (C3A.3CS.32H)?
What is the hydration phase for AFm (C3A.CS.nH)?
What is the heat evolution for the cement graph?
What is cement microstructure?
What is the summary hydration of C3S?
What is the summary hydration of C2S?
What is the summary hydration of C3A?
What is Supplementary Cementitious Materials (SCMs)?
What is the pozzolanic reactions?
What are the pozzlanic additives?
What is a slag?
What is a limestone?
What are the different types of cement?
What is reinforced concrete?
What are some issues with reinforced concrete durability?
How do we prevent the rusting of steel in reinforced concrete?
What are the forms of degradation in reinforced concrete?
What is pre-stressed concrete?
What is pre-tensioning?
What is post-tensioning?
