Concrete Production

Question 1

What is concrete made of?

Answer 1

Cement, admixtures, water(all make cement paste), fine aggregates(add to cement paste for mortar), and coarse aggregates

Question 2

How is lime made from calcium carbonate?

Answer 2

Lime produced by calcining (heating) natural calcium carbonate (limestone, chalk, marble, etc) at approx. 900 - 1200°C to decomposition
CaCO3 -> CaO + CO2

Question 3

What’s the reaction between quicklime and water?

Answer 3

Quicklime mixed with water

CaO + H2O -> Ca(OH)2

Question 4

How does slaked lime produce calcium carbonate again?

Answer 4

Slaked lime hardens slowly by reacting with absorbed CO2 from atmosphere
Ca(OH)2 + CO2  CaCO3 + H2O

Question 5

How was Hydraulic lime made?

Answer 5

1756 Eddystone Lighthouse, Plymouth, UK built by John Smeaton using hydraulic lime (burnt limestone + volcanic ash from Italy), superior cementing properties.

Question 6

What is Portland cement made of ?

Answer 6

1824 Joseph Aspdin patented ‘Portland cement’ made from limestone + fine clay heated to 1200°C (to produce mainly C2S and CO2).

Question 7

What sort of cement is Portland?

Answer 7

Inorganic hydraulic calcareous cement

Question 8

How is Portland cement manufactured?

Answer 8

~75% limestone + 25% clay or slate
or chalk from quarry

Heat to 1400-1500 degrees C in rotary kiln.

Form clinker from silicates and aluminates, put in grinder and add 4% gypsum

Question 9

What is cement clinker made from?

Answer 9

C3S, C2S, C3A, C4AF, and CŜH2

Question 10

What do C, S, A, F, H, and Ŝ consist of for cement clinker?

Answer 10

C=CaO S=SiO2 A=Al2O3 F=Fe2O3 H=H2O Ŝ=SO3

Question 11

What is a hydration product of C3S?

Answer 11

When C3S is hydrated it makes C-S-H gel

Question 12

What is a hydration product of C2S?

Answer 12

When C2S is hydrated it makes calcium hydroxide

Question 13

What is a hydration product of C3A?

Answer 13

When C3A hydrated, ettringite is made in middle (this can make concrete explode when concrete rehydrated)

Question 14

Which constituent of concrete hydrates fastest? what’s it followed by?

Answer 14

Almost all of C4AF hydrates, followed by c3A, then C3S then C2S

1 volume of cement  2.2 volumes of hydration products
Hydration continues if relative vapour pressure (relative humidity) in pores > 0.8 (80%)

Question 15

How does hydration and hardening work in concrete?

Answer 15

Setting : Portland cement and water, makes workable paste and has initial set. Becomes stiff unworkable paste and has final set. Hardening: strength rises over time.

Question 16

What happens when cement is hydrated?

Answer 16

Fluidity of cement falls while hydration and strength rise

Question 17

Which concrete constituents have greatest compressive strength?

Answer 17

C3S has greatest compressive strength as concrete ages, followed by C2S

Question 18

what can you see in a concrete microstructure?

Answer 18

In microstructure you can see CH which looks like plates, ettringite which looks like needles, capillary pores which look like coral reefs, sand, unhydrated cement, and C-S-H gel

Question 19

What are capillary pores?

Answer 19

Capillary pores:
Space not filled by hydration products
Typically 0.05 - 10μm in “diameter”
Influence most properties including strength and durability

Question 20

What is C-S-H known for?

Answer 20

C-S-H has sheets with interlayer water and gel pores contained. Has enormous specific index, porosity, and low permeability.

Question 21

What are the sizes of CSH gel pores, capillary pores, entrained air voids and entrapped air voids?

Answer 21

CSH gel pores have size of 1nm and capillary pores have 100nm to 10 micrometres. Entrained air voids 100 micrometres in size and entrapped airvoids 100 micrometres to 10mm in size

Question 22

What water cement ratio is used for structural forms?

Answer 22

w/c ratio of 0.45 used for structural forms. Lower w/c associated with greater performance

Question 23

Degree of hydration: how does a lower water cement ratio affect this?

Answer 23

For lower water cement ratios, water runs out faster meaning not all cement is used up and not all CH and CSH gel is made.