Matrials- Cements and Concretes

Question 1

What are Portland, speciality and medical cements used for?

Answer 1

Portland: concrete, mortar, buildings, infrastructure
Speciality: immobilisation of toxic/radioactive waste, other specialised construction
Medical: tooth and bone replacements (polymer or ceramic type)

Question 2

What does Portland cement consist of?

Answer 2

Hydraulic calcium silicates, usually containing calcium sulfate. It is a very fine powder made of limestone and other minerals.

Question 3

What does hydraulic mean?

Answer 3

Reacts with water and hardens under water

Question 4

What is paste?

Answer 4

Cement and water. Rarely used alone.

Question 5

What is mortar?

Answer 5

Paste and sand or some other fine aggregate (<5mm). Used to join bricks together.

Question 6

What is concrete?

Answer 6

Paste, sand (or other fine aggregate) and coarse aggregate (usually gravel or crushed rock up to a few cm in size)

Question 7

Role of cement in concrete

Answer 7

Absorbs water and acts as a binder to hold the concrete together.

Question 8

Describe the mesostructure of concrete

Answer 8

Considered as a two-phase composite material with aggregate as inclusions and cement paste as a matrix.

Question 9

What are the crystal structures of aggregate and cement paste like?

Answer 9

Aggregates are more or less crystalline.

Hydrated cement paste is not crystalline (contains crystalline and amorphous phases).

Question 10

Which aggregates to avoid

Answer 10

Glassy aggregates or sulfites as these react with the cement, expand and cause concrete failure

Question 11

Examples of aggregates which can be used

Answer 11

Ones that won’t react with cement paste, e.g. quartz, basalt, granite.
Slightly reactive rocks, e.g. limestone.
Recycled aggregate, e.g. crushed old concrete.

Question 12

Roles of aggregates in concrete

Answer 12

Contribute to strength so need clean and strong aggregates.
Lightweight aggregates used for thermal insulation materials.
Heavy, dense aggregates used for radiation shielding.
Reduce thermal cracking (dilution of cement).
Increased cohesion (reduced segregation) in the fluid state.

Question 13

Why do mixer trucks keep rotating?

Answer 13

So the concrete can flow. Concrete is shear-thinning and so reduces in viscosity when a shear force is applied to it.

Question 14

What is sel-compacting concrete?

Answer 14

Concrete that flows like a liquid with no vibration because it uses chemical additives.

Question 15

What is the slump test?

Answer 15

Measure of flow of concrete. Full a cone with concrete. Put it on flat surface and lift cone off concrete. Measure change in height (slump) after it sinks slightly.

Question 16

What does increasing water content in concrete do?

Answer 16

Makes concrete flow more (increased slump). If there is extra water, it forms extra pores. This results in:
reduction in durability (more permeable so damaging agents from outside can get in)
reduction in strength (material with holes in is less strong)
delay in setting/hardening or cause segregation of particles from liquid.

Question 17

Compressive strength against water-cement ratio graph for different time periods

Answer 17

Decreases like decay curve with more water. Curve for lager time period is higher.

Question 18

Common water-cement ratios

Answer 18

0.5 +/- 0.2

Question 19

Why would more water be used in concretes for freezing conditions?

Answer 19

Extra pores in the concrete are produced. These act as spaces for water getting in to expand without causing the concrete to crack.

Question 20

What do superplasticisers do?

Answer 20

Cause high flow of concrete despite low water content. Has same workability as a higher water-cement ratio but higher strength.

Question 21

Cement chemistry notation for C, S, A, F, H, S bar, c or C bar, N, K

Answer 21

C is CaO. S is SiO2. A is Al2O3. F is Fe2O3. H is water. S bar is SO3. C bar or c is CO2. N is Na2O. K is K2O.

Question 22

What compounds are in Portland cement and how do they combine?

Answer 22

Limestone for CaCO3. Clay/shale/sand/fly ash for A, S and F. CaCO3 heated to 1400°C in rotary kiln to decompose to C and c. C combines with A, S and F to form clinker phases.

Question 23

How much CO2 is produced per kg of Portland cement?

Answer 23

0.8kg

Question 24

What is cement clinker made of?

Answer 24

Tricalcium silicate or alite (C3S). Dicalcium silicate or belite (C2S). Tricalcium aluminate or aluminate (C3A). Tetracalcium aluminoferrite or ferrite solid solution (C4AF).
Clinker made from limestone and clays in a kiln then ground into a fine powder. Gypsum (CaSO4•2H20) added during grinding.

Question 25

Typical PC clinker composition

Answer 25

45-75% C3S
7-32% C2S
0-13% C3A
0-13% C4AF

Question 26

Roles of components of clinker

Answer 26

C3S gives early (<7 days) strength development
C2S gives long term strength
C3A gives initial strength
C4AF is similar to C3A but reacts slower and doesn’t contribute much strength. Controls melting. Gives grey colour.

Question 27

How does C4AF control melting in the kiln?

Answer 27

Means can make clinker at 1400°C because only Fe melts at this temperature and helps to distribute heat around the clinker composition to hell reaction of minerals.

Question 28

What happens when cement is mixed with water?

Answer 28

Particles dissolve. Ions rearrange in solution phase. Solid hydrate phases solidify and give strength. Reaction process is hydration not drying.

Question 29

Graph shape for weight % of phases in clinker against hydration time (log scale x-axis)

Answer 29

Starts top left. Decreases slightly, then steeply, then less steeply approaching x-axis.

Question 30

Describe the calcium silicate hydrate phase and it’s role

Answer 30

Known as C-S-H. Formed by hydration of C2S and C3S. It is amorphous. It is porous on nanometer length scale. Can take in some Al as well to form C-A-S-H. No fixed chemical composition. Fills in a lot of the space in hardened binders. It is the main strength giving phase. The reaction with C3S mostly occurs early to give substantial strength increase and reduction in porosity. C2S less soluble so hydration rate slower so makes substantial contributions to strength of mature cement paste and concrete.

Question 31

Describe the calcium hydroxide phase and its role

Answer 31

Symbol CH or Ca(OH)2 aka portlandite. By-product of C3S and C2S because these have more than enough Ca than can be accommodated in C-S-H. Makes large crystallite (tens of microns). Doesn’t contribute to strength. Holds the internal pH high to help durability (protects reinforcing steel from corrosion).

Question 32

Describe the ettringite phase (hydrate) and its role

Answer 32

Formed by reaction of C3A with water and gypsum. Symbol is
C3A•3CSbar•32H (or F can replace some of A) or aka AFt where t means tri-sulfate
Forms very early in reaction process. Forms needle-shaped crystals which interlock as they grow to cause initial setting of concrete (applying force breaks them apart to make concrete more liquid). Mostly converted to AFm at longer ages. If not enough gypsum added, doesn’t form so AFm forms instead (flash set).

Question 33

Describe the AFm phase (hydrate) and its role

Answer 33

Approximately C3A•CS•nH (some of all of sulfate can be replaced by carbonate or hydroxide) and n is usually 11-14. m means monosulfate. Forms gradually during hydration. Ettringite can convert to AFm at later ages. It is important as a sink for Al but can cause problems (sulfate attack). Contributes a bit of strength. Fills in some space.

Question 34

Describe heat evolution rate against time graph

Answer 34

Initially very high (initial wetting) but comes almost vertically down to a minimum point (dormant period). Curves back up to a peak during acceleration period (when setting happens). Curves down a bit then new peak (sulfate depletion peak). Curves down more then becomes more level with a gentle bump on the way down (AFm formation).

Question 35

What is in Portland cement paste of moderate maturity (days/weeks after mixing)?

Answer 35

Large chunks of unreacted cement. Smaller chunks of portlandite. Lots of C-S-H. Some ettringite (AFt). Pore space throughout. Some AFm.

Question 36

What happens if gypsum in cement reacts completely before the C3A?

Answer 36

Concentration of sulfate ions in pore solution decreases drastically and ettringite becomes unstable and converts to different solid phase with less sulfate.
3C4ASbarH12.