Concrete & Cement
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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