Concrete & Cement Flashcards
what is hydraulic cement?
hydraulic cement has the ability to set/harden when it reacts with water
some general info on concrete…
- 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
what are the 4 main components of concrete? (5 including air) - list in order of how much
aggregate sand water cement (air)
what are the 3 main stages of making concrete?
paste —> mortar —> concrete
paste = cement + water
mortar = paste + fine aggregate
concrete = mortar + coarse aggregate
what are the 3 main raw materials of Portland cement?
- limestone or chalk (lime CaO)
- clay or shale (SiO2, Al2O3, Fe2O3)
- gypsum (CaSO4)
what are the four steps of Portland cement production?
- grind raw materials
- burn in a large rotary kiln at 1450°C to
form balls known as clinker - cool clinker and grind to a fine powder
- add gypsum
what are the two main problems with Portland cement production?
- high carbon footprint (fuels for kiln operation and calcination of raw materials)
- consumption of natural resources
what is the issue with using fuels for kiln operation?
traditionally fossil fuels are used which leads to direction CO2 emissions
what is the issue with calcination of raw materials (burning CaCO3)?
CO2 is released due to decomposition of calcium carbonate into lime
what are the four phases in (Portland cement) PC?
- alite (C3S) - tricalcium silicate
- belite (C2S) - dicalcium silicate
- aluminate (C3A) - tricalcium aluminate
- alumina ferrite (C4AF) - calcium alumino ferrite
(final - gypsum)
what is the purpose of adding gypsum?
to slow down the hydration of C3A
what is the Portland cement notation?
C = CaO S = SiO2 A = Al2O3 F = Fe2O3 H = H2O š = SO3
what are the 2 hydration products from the first 2 silicate phases?
the C-S-H gel and Ca(OH)2
the calcium hydroxide is responsible for making the concrete alkaline
what is hydration and what does it do to the cement?
reaction with water results in setting and hardening (the two proposed mechanisms are in solution and solid state and both probably take place)
what percentage of the silicates (C3S and C2S) make up Portland cement?
approx. 70% therefore the contribution of these phases to concrete properties is large
what is the main hydration product of PC cement?
- C-S-H (gel) C3S2H3
- calcium silicate hydrate
- main source of concrete strength
what is the structure of the C-S-H gel?
very porous
what is the structure of the Ca(OH)2?
crystal structure
what is the main difference in structure between calcium hydroxide and ettringite?
calcium hydroxide = hexagonal plates
ettringite = hexagonal rods
what is ettringite?
rod-like crystals in the early stages of hydration (stage3)
what is monosulfate?
- occurs in the later stages of hydration, after a few days
- usually it replaces ettringite, either fully or partly
(stage 3)
what is the problem with adding gypsum at the C3A stage (stage 3)?
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)
what is the porosity of the cement paste usually?
approx. 28%
what does the cement gel do?
fills the voids and joins together the unhydrated cement particles
why is there an optimum water/cement ratio?
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)
what are the four types of water within the hardened cement paste? (ranging from progressively more difficult to remove from top to bottom)
- 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)
what factors affect rate of cement hydration?
- age
- cement composition
- cement fineness (finer = quicker)
- water/cement ratio
- temperature
- admixtures
why is it difficult to obtain PC?
because of the sustainability credentials
why are additions in PC important?
they are constituents (usually in powder form) that can be added at the concrete mixer to can improve the properties
what are the concrete properties admixtures effect?
- consistence (workability)
- set control
- strength
- durability
- special concretes
what do accelerators do?
they increase the rate of hydration, reduce setting time (useful in cold weather and road repairs)
what is the mechanism for accelerators?
- increase solubility of C3A
- over-rides retarding mechanism of gypsum
what do retarders do?
decrease the rate of hydration, delay the setting time
what is the mechanism for retarders?
- decrease solubility of C3A
- cement grains are coated in the admixture which acts as a protective layer
what do plasticizers (water reducing admixtures) do?
increases workability, becomes more fluid
what do super plasticizers do (or high range wanter reducing admixtures)?
creates a large spacing between cement particles, temporarily neutralizes the force of attraction, increasing workability and giving the concrete a more liquid consistency
what are aggregates?
- 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
is there any particular rock type which is preferred as aggregate?
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
are there any general requirements for an aggregate?
- hard
- strong
- free from impurities
are there other properties that influence
concrete performance?
- physical properties: specific gravity, hardness, strength, stability, shape, texture, size
- thermal properties
- chemical properties: chemical and mineralogical composition, petrographic description, colour
what is the purpose of the cement paste in the concrete?
- to fill the voids between the aggregate
- provide strength to hardened concrete
what is the purpose of the aggregate in the concrete?
- 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
what is Portland cement clinker?
made by reacting the raw materials at a high temp which is then grinded to make the portland cement
what are the hydration products of stages 1/2?
the alite and belite phases produce C-S-H gel and calcium hydroxide
what are the hydration products of stages 3?
in the early stages of hydration, ettringite is formed first and then monosulfate is produced usually after a few days
what are the hydration products of stage 4?
solid solution of sulfoaluminate and sulfoferrite
what is the general reaction for stage 4?
C4AF + calcium hydroxide + gypsum + water —> solid solution of sulfoaluminate and sulfoferrite (which gives rise to needle like crystals)
what is the order of the phases? (basic symbols only)
- C3S
- C2S
- C3A
- C4AF
what stages are most responsible for the compressive strength of the concrete?
C3S & C2S (stages 1/2)
is the reaction of portland cement with water exothermic or endothermic?
exothermic
order the stages from lowest to highest heat of hydration…
C3A (stage 3) = 837
C3S (stage 1) = 502
C4AF (stage 4) = 419
C2S (stage 2) = 251
what is the equation relating porosity to strength?
strength = k (1-P)^3
P = porosity (pore volume/total paste volume)
k = constant
[more pores = lower strength]
which reacts quicker, alite or belite? what do each effect?
- 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
what does the calcium hydroxide do to the paste?
makes the cement paste highly alkaline (pH 12.5)
what does an increase in fineness mean for the heat of hydration?
increased fineness = heat of hydration also increases
what are the 4 common variants of PC?
- RHPC (rapid hardening portland cement)
- white portland cement
- SRPC (sulfate resistant portland cement)
- LHPC (low heat portland cement)
what is RHPC?
finer ground than normal PC, with a slightly modified composition, leading to faster setting and earlier strength development
what is white portland cement?
portland cement in which the coloured ferrite phase has been eliminated
what is SRPC?
this is specially formulated for maximum resistance to attack by sulphate ions in ground water, otherwise similar to normal PC
what is LHPC?
this is specially formulated to have a low heat of hydration in order to avoid destructive thermal expansion effects in large structures
how is alternative cements created nowadays?
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
how can the porosity be divided (two categories)?
- 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
what does the volume of the capillary pores depend on?
it depends on the water/cement ratio and on the degree of hydration
what are the two types of additions (according to European standards)?
- 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.)
what are pozzolanic materials?
they are materials containing active silica (amorphous silica)
what does the term blended cement refer to?
blended cement is portland cement with pozzolanic additions
what are admixtures?
materials added during the mixing process of concrete (in small quantities) - usually in the form of a liquid
