Concrete and Cement

Question 1

Concrete composition?

Answer 1

Cement
Aggregate
Water
Admixtures
Additives

Question 2

What are the primary ingredients of cement? Where do they occur?

Answer 2

Lime, Calcium Oxide, CaO (in limestone, CaCO₃)
Silica, Silicon Dioxide, SiO₂ (Clay/Shale)

Question 3

What is the process of manufacturing cement?

Answer 3

1. Grading the raw materials with particles sizes of 75 mm or less
2. Mixing them in a certain proportion
3. Burning in large rotary kiln at temperature ~1400 °C

Question 4

What important compounds are formed in cement?

Answer 4

C₂S (Silicate)
C₃S (Silicate)
C₃A (Aluminate)
C₄AF (Aluminate)

Question 5

What is special about C₃S?

Answer 5

It occupies 30% ~ 60% of weight in cement.
When mixed with water, it hydrates rapidly, generating lots of heat, so it contributes to development of early strength (first 7 days).

Question 6

What is special about C₂S?

Answer 6

It occupies 15% ~ 40% of weight in cement.
It hydrates slowly and is mainly responsible for strength after 7 days. It may be active for a considerable amount of time.
Cement rich in C₂S results in greater resistance to chemical attack and has smaller drying shrinkage than other Portland cements.

Question 7

What is special about C₃A?

Answer 7

It occupies < 10% of weight in cement.
It combines rapidly with water so considerable heat is evolved.
It is the least stable, if cement contains > 10% of this, it becomes prone to sulphate attack.
It acts as a flux during manufacture, allowing C₂S and C₃S to be formed at low temperatures.

Question 8

What is special about C₄AF?

Answer 8

It occupies ~10% of weight in cement.
It is expensive to remove.
It acts as a flux during manufacture of cement.
It is responsible for grey colour of cement.

Question 9

Explain what calcium silicates do in the presence of water.

Answer 9

They form products of hydration, which in time produce a very hard and strong binding medium for the aggregate particles in concrete.

Question 10

Explain what calcium aluminates do in the presence of water.

Answer 10

Without gypsum, C₃A reacts very rapidly with water.
It results in a flash set, which immediately stiffens after mixing, making proper placing, compacting and finishing impossible.

Question 11

What is ettringite?

Answer 11

With the addition of gypsum, ettringite forms. The formation of ettringite occurs faster than the hydration of the calcium silicates, so it contributes to the initial stiffening, setting and early strength development (creates a diffusion barrier around C₃A)

Question 12

What is special about RHPC?

Answer 12

It has increased rate of strength development.
-because of high C₃S content and increased fineness.
–specific surface area of cement is increased -> rate of hydration is increased
Enables early removal of formwork
Used in very cold climate

Question 13

What is special about LHPC?

Answer 13

Its low rate of strength development is due to relatively high C₂S content and low C₃S and C₃A contents.
It reduces the heat concentration during hydration as excess heat will cause cracks in hardened concrete.

Question 14

What is special about SRC?

Answer 14

It has low C₃A content to increase resistance to sulphate attack.
But the reaction is expansive: may cause cracks in hardened concrete paste (HCP).

Question 15

What are the physical properties of Portland cement?

Answer 15

Fineness
Soundness
Setting time
Strength

Question 16

Explain fineness.

Answer 16

Affects hydration rate -> rate of strength gain
Smaller particle size, more surface area-to-volume ratio, more area available to water-cement interaction per unit volume.
These effects on strength seen during first 7 days.
Coase particles may not be completely hydrated. Causes low strength and durability.
Measured by the term “Specific Surface” (m²/kg).

Question 17

Advantages and disadvantages of fine cement?

Answer 17

Advantages:
Improves cohesiveness of fresh concrete
Reduce risk of bleeding effectively
Greater hydration
Develop strength and generate heat quicker than coarse cement

Disadvantages:
Cost more to manufacture as clinker must be more finely ground
On drying, cement gel shrinks, so finer cements have greater initial drying shrinkage at an early stage

Question 18

Explain soundness.

Answer 18

Refers to ability of HCP to retain its volume after setting
Expansion is caused by excessive amount of free lime (CaO) or magnesia (MgO).

Question 19

Explain setting time and its process.

Answer 19

Hydration results in formation of a gel around each cement particle. In time, they grow to the point that they touch each other. At this stage, cement paste begins to lose its fluidity.
Affected by:
Cement fineness
Water-cement ratio
Chemical content (especially gypsum)
Admixtures

Initial set: When the paste beings to stiffen considerably
Final set: When the cement hardened so that it can sustain some load

Setting mainly caused by C₃A and C₃S and occurs with temperature rising in the cement paste

Question 20

Explain strength.

Answer 20

Cement paste strength defined in 3 ways
Compressive
Tensile
Flexural

Affected by:
Water-cement ratio
Cement-fine aggregate ratio
Type and grading of fine aggregate
Curing conditions
Size and shape of specimen
Loading conditions and age