Cements

Question 1

What is in portland cement?

Answer 1

Cement clinker
– Tricalcium silicate (“alite”), 3CaOꞏSiO2
– Dicalcium silicate (“belite”), 2CaOꞏSiO2
– Tricalcium aluminate (“aluminate”), 3CaOꞏAl2O3
– Tetracalcium aluminoferrite, 4CaOꞏAl2O3ꞏFe2O3
( “ferrite solid solution” – Al/Fe ratio can vary but usually ~1)
made from limestone and clays in a kiln, ground to powder

• Gypsum, CaSO4. 2H2O -> added during grinding
Ground mix of clinker + gypsum = cement

Question 2

Cement chemistry abbreviations

Answer 2

CaO --> C 
SiO2 --> S 
Al2O3 --> A
Fe2O3 --> F 
H2O --> H 
SO3 --> S

Question 3

So Tricalcium silicate (alite), 3CaO.SiO2 would become…

Answer 3

C3S

Question 4

How do you make cement?

Answer 4

• Limestone: CaCO3, supplies Ca
• Clay/shale/sand/etc: supply Al2O3, SiO2, Fe2O3
• Heat to ~1400°C in a rotary kiln
CaCO3  CaO + CO2
• CaO combines with SiO2, Al2O3, Fe2O3 to form clinker phases
• CO2 emissions ~0.8 kg per kg cement
~50% from fuel, ~50% from chemistry
> 4 billion tonnes of cement produced p.a.
–> ~8% of global human-derived CO2

Question 5

Normal clinker composition

Answer 5

• C3S – about 65% (45-75%) Alite
– Gives early (< 7 days) strength development
• C2S – about 20% (7-32%) Belite
– Gives longer-term growth in strength
• C3A – about 8% (0-13%) Aluminate
– Controls initial setting
• C4AF – about 8% (0-13%) Tetracalcium Aluminoferrite

Question 6

What is cement hydration

Answer 6

• Mixing cement with water makes it react
– Particles dissolve
– Ions rearrange in solution phase
– Solid hydrate phases solidify and give strength
• 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
–> slows or stops reactions, causes cracking and
loss of strength

Question 7

Calcium silicate hydrate (C-S-H)

Answer 7

– Formed by hydration of C3S and C2S
– Non-crystalline (amorphous/disordered), but resembles natural mineral tobermorite
– Porous on nanometre length scale
– No fixed chemical composition, but often written as approximately 1.6CaO. SiO2. 1.8H2O
(you may also see it given as 3CaO. 2SiO2 .3H2O, which is similar)
– Fills a lot of the space in hardened binders

Question 8

Calcium hydroxide (portlandite, Ca(OH)2)

Answer 8

– By-product of C3S and C2S hydration,
because these have more Ca than can be accommodated in C-S-H
– Makes large crystallites (tens of microns)
– Doesn’t contribute to strength
– Holds internal pH high to help durability
– Tends to grow around aggregate particles

Question 9

C3A reaction products

Answer 9

• Ettringite (C3A. 3C .S32H) (F can also replace some of the A)
– Also called “AFt“ (with “t” for tri-sulphate)
– Formed by fast reaction of C3A with water & gypsum
– Needles interlocking causes initial setting of cement
– Mostly converted to AFm at longer ages
• AFm phase family (approximately C3A. CS .nH)
– with n approx. 11-19, and some replacement of by
other negatively charged species
– Contributes a bit of strength

Question 10

What are Supplementary cementitious materials (SCMs)?

Answer 10

Any mineral component blended with cement that reacts to alter the properties of the binder
– Aggregates aren’t SCMs
– Organic admixtures aren’t SCMs
– Most SCMs are pozzolans
• Can be added to improve performance, improve sustainability, and/or reduce cos

Question 11

The pozzolanic reaction

Answer 11

Blend cement with other sources of
SiO2, with or without Al2O3
– Often waste materials – cheap and low-CO2
• These react with CH to form more C-S-H
– Fills pore spaces at longer age, refines pores
– Improves strength and durability
Approximately: CH + S + xA –> C-(A)-S-H

• Note that limestone (CaCO3) is an SCM but not a pozzolan – contains no Si or Al

Question 12

What are some pozzolanic additives?

Answer 12

• Fly ash (by-product from coal-fired power generation)
• Natural pozzolans (volcanic earth)
• Heat-treated (‘calcined’) clays
• Silica fume (by-product from semiconductor Si manufacture)
• Rice husk ash (by-product from burning agricultural waste)
• Other waste silicate glass materials

(Materials containing reactive Si, with or without Al)

Question 13

What are the influence of SCMs?

Answer 13

Pozzolanic reaction is generally slower than cement hydration
– Cement substitution with pozzolans usually reduces early strength (except with silica fume)
• Blended cements generally give better longterm properties
– Extra C-S-H fills in space, gives strength (remember CH doesn’t give much strength)
– Chemical durability – chloride/sulphate resistance
generally improved
– Almost all marine concreting is done with slag
blended Portland cements for this reason