Cement

Question 1

What is Cement?

Answer 1

Cement is a binder made from burning limestone and other additives. Hydraulic, sets under water and is the result of an irreversible chemical reaction.

Question 2

How does cement compare to traditional binders?

Answer 2

The earliest lime binders where all non hydraulic, however romans discovered natural hydraulic limes.

Question 3

What was portland cement?

Answer 3

Although cement had been around for a while. Joseph Aspdin patented Portland cement in 1824. It began as a mixture of ground limestone and clay heated to 1400c.

Question 4

What is modent portland cement/ CEM1?

Answer 4

80%limestone or chalk (Calcium Carbonate)
20% clay or shale (Alumino Silicates, Shale is clay which has become rock)
Finely ground to 0.1mm (Raw meal)
Raw meal sent to pre heating chamber (up to 800c)(Calcination and CO2 release)
Feed into a sloping kiln at 1450c where they combine into Clinker
Clinker cooled quickly to 150c
Gypsum may be added to reduce set time
clinker is milled close to is sale

Question 5

Which cements set/ harden faster?

Answer 5

Rapid hardening SSA of 500-600m2/kg
rather that general purpose which is 350m2/kg

Question 6

What is the chemical structure of limestone?

Answer 6

CaCO3 / Calcium Carbonate

Question 7

What is the chemical structure of quicklime?

Answer 7

CaO = Calcium Oxide

Question 8

What is the chemical structure of slaked/ hydrated lime?

Answer 8

Ca(OH)2 = Calcium Hydroxide it then drys out to become Calcium carbonate again

Question 9

Types of cement rawmix?

Answer 9

Tricalcium Aluminate - VH
Tricalcium Silicate - H
Diacalcium Silicate - L
Tetracalcium Aluminoferrite - VL

Question 10

Whats a drawback of quicker set?

Answer 10

Bigger exothermic reaction causing more expansion and increased risk of cracking.

Question 11

Cem 1 or OPC can be made by?

Answer 11

3 to 1 sand to cement
and 1 to 2 water to cement

Question 12

What causes the chemical reaction in cement causing it to set/harden?

Answer 12

Water/ Hydration

Question 13

What happens when cement is hydrated?

Answer 13

irreversable exothermic reaction forming causing calcium silicate hydrate gel. Crystals grow and between 1 to 6 hours it becomes unworkable.

Question 14

What happens with cement hardening over time?

Answer 14

3 days 50% of 28 day strength
by the 28th day 90% concretes overall strength is usually attained

Question 15

Is concrete stronger with crushed aggregate?

Answer 15

Yes, its angular

Question 16

What detrimental effects can affect concretes hardening process?

Answer 16

Hot sun - evaporation reduces the amount of water required to hydrate the cement
Dry air - evaporation too
Frost - Cold surfaces may cause temperature gradiants leading to differential expansion and cracking.
Rapid hardening cements - more heat, more expansion, higher probability of early thermal cracking.

Question 17

What are the cement classifications for early strength?

Answer 17

L = Slow Set
N = Normal Set
R = Rapid Set

Question 18

How can we change the rate of setting?

Answer 18

Changing the raw mix
Adding gypsum
Grinding the cement finer
Chemical admixtures
Initial temperature of the ingredients

Question 19

What are CemII to Cem V?

Answer 19

Non Cem1 can contaion up to 50% supplementary cementitious Materials, Blastfurnace Slag (Iron byproducts), Pulverised Fuel Ash (Coal burning power), Ground Limestone

Question 20

What are Supplementary Cementitious Materials?

Answer 20

They are Pozolanic meaning they have no cementitious value. However, they react with alkali (given off by hydration of portland cement) in the presence of water to form compounds which are cementitious.

These reactions are slow and stregnth gain is slow. Known as Latent Hydraulic. Romans use volcanic ash.

Question 21

How much does cement account for in emmisions worldwide and in the UK?

Answer 21

About 7% worldwide
1.5% in the UK, however, the UK is a net importer of cement.

Question 22

How can we reduce the footprint of cement?

Answer 22

Changing the energy source for heat
Combining it with pozzolans or SCM’s
Reducing the need for it
CCS carbon capture and storage

Question 23

What are Chemical Admixtures? What are their uses? how much can we use?

Answer 23

They are chemical substances that are added when the concrete is being mixed.
They can be used as plasticisers, retarders, hardeners and air entrainers.
Plasticisers in paticular can reduce the amount of water required, increasing strengths whilst still being workable.