Cement & its Constituents

1.Introduction

• Cement is the most vital material in building and other construction practices.
• Cement is manufactured from limestone and clay either by 'old wet process' or 'new dry process'
• Modern method of manufacturing is patented by Joshep Aspdin in 1824.
• Cement is available in grey powder form,which when mixed with water ,sets to hard durable mass

2. What actually does cement do?

Ans: Cement act as a binding material in mortar or concrete .

3. Why called Portland Cement?

Ans: As the resulting material is found to be similar in look to the stone found in Portland (United Kingdom) , Thats why it is called Portland Cement

4. What are Bogue's Compounds? 

Ans: Bogue's Compounds are constituents of cement that give cementing Properties 

a) Tri-calcium Silicate (C3S)  ⇒ (21- 45 %)

b) Di-calcium Silicate (C2S)   ⇒ (25- 31%)

c) Tri-calcium Aluminate (C3A) ⇒ (6 -12%)

d) Tetra-calcium Aluminoferrite (C4AF)⇒ (8 - 14 %)

Tricalcium silicate:
Hardens rapidly. Responsible for initial set and early strength.

Dicalcium silicate:
Hardens slowly and its effect on strength increases occurs at ages beyond one week.

Tricalcium aluminate:
Contributes to strength development in the first few days. It is the first compound to hydrate. The least desirable component because of its high heat generation and its reactiveness with soils and water containing moderate to high sulfate concentrations.

Tetracalcium aluminoferrite:
Lowers clinkering temperature. C4AF contributes very little to strength of concrete even though it hydrates very rapidly.

Note: If C3A & C4AF are kept low, then resistance to chemical increases.

•  C2S & C3S  are responsible for strength
•  Average content : C3S- 45% , C2S- 25%
• Total percentage of C2S & C3S is around 70%
• C3S- Responsible for "early strength"of concrete
• C2S- Responsible for "later strength" of concrete
• For producing low heat cement , C2S is increased and C3S & C3A decreased

5,  Heat of Hydration

• when water is added in cement, an exothermic reaction takes place between cement. and water and heat is liberated, The liberation of heat is called "Heat of Hydration".
• Heat is evolved due to the reaction of aluminates and sulphates.
• The early heat of hydration is mainly contributed from hydration of C3S.
• Hydrated aluminates doesn't contribute towards strength.

6. Chemical or OxideComposition  of cement

1. CaO-  (60-67%)
2. SiO2- (17-25%)
3. Al2O3- (3-8%)
4. Fe2O3 - (0.5- 6%)
5. MgO- (0.1- 4%)
6. Alkalies( K2o, Na2O) - (0.4 - 1%)
7. SO3- (1.3-3%)

7. Setting Of Cement

• The beginning of noticeable stiffening in the cement paste is known as the initial set. 
• The final hardening process which is responsible for its strength known as the final set. 
• The time from the addition of the water to the initial and final set are known as the setting times.
• Setting time is affected by: cement composition, cement fineness, rate of hydration, and the ambient temperature

a) Initial  Setting Time

• Initial Setting time - Generally 30 minutes
• It is the time interval throughout which the cement remains in plastic condition required for mixing,placing & compacting. 

b) Final Setting Time

• Final Setting Time- Generally 10 hours (600 Minutes)
•         It is the time at which the concrete looses its plasticity & start gaining strength.

Vicat Apparatus is Used to measure initial & final setting time