Cement: Constituents, Bogues Compound and Hydration of Cement

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Building MaterialsCement
cement

cement

Cement is an artificial building material that is used to impart binding property in construction that is being developed around (1824-25) by Joseph Aspedin.

Cement broadly consists of:

i)  Calcareous compounds: Compounds having Ca and Mg.

ii) Argillaceous compounds: Compounds having Si, Al, and oxides.

Calcareous compounds:

  • i)  Limestone
  • ii) Cement Rock
  • iii) Chalk
  • iv) Alkali waste
  • V) Marble shell

Argillaceous compounds:

  • i)   Clay
  • ii)  Shale
  • іїї) Slate
  • iv) Ash

Important Question

Q: What is the reason behind the name Ordinary Portland Cement?

Sol: The cement after setting resembles a stone that is being found in Portland. Thus, Joseph Aspedin named it Ordinary Portland Cement.

1.1 DIFFERENT CONSTITUENTS OF OPC

i) Lime (CaO) [62 - 67%]:

  • It imparts strength and soundness to the cements.
  • If it is in excess, it makes the cements unsound, causes it to expand, and finally disintegrates.
  • If it is in deficiency, it reduces the strength and causes the cements to set quickly.

Cement having an excess of lime causes unsoundness as follows

> Ca (OH)2 + H2O ---------> Ca(OH)2 + Heat

When quick lime is added to water, it leads to the formation of hydrated lime, and the reaction is known as slaking.

V' > V

The volume of hydrated lime is greater than the volume of quick lime

ii) Silica (Sio,) [17 - 25%]:

  • It also imparts strength to the cements.
  • If it is in excess, the strength of cement is increased but on the other hand, it also increases the setting time of cements (final setting).

Important Question

Q: In climatic conditions where the temperature is high and in underwater construction work, the amount of silica required will be increased or decreased?

Sol: If the temperature is higher, than the quick setting will take place, and to delay the setting time, the amount of silica required will be more. Whereas in underwater construction, we need quick setting thus the amount of silica required will be decreased.

iii) Alumina (Al2O3) [13 - 8%]:

  • It imparts quick setting property to the cements.
  • It acts as a flux and helps in reducing clinkering temperature.
  • If it is in excess, it weakens the cements.

iv) Calcium sulphate (CaSO), [3 - 4%]:

  • It is generally added in the form of gypsum [CaSO,2H,0].
  • It helps in increasing the initial setting time of cements.

v) Iron oxide (Fe, °) [3 - 4%]:

  • It imparts strength, hardness, and colour to the cement.

vi) Magnesia (MgO) [1 - 3%]:

  • It imparts strength, hardness, and colour to cements.
  • If it is in excess, it makes the cements unsound.

vii Sulphur trioxide (SO,) [1 - 3%]:

  • Sulphur in cement is also responsible for volume changes in it, and thereby it leads to its unsoundness.

vili) Alkali (Na, 0 and K,0) [0.2 - 1%]:

  • Alkalis in cements lead to efflorescence, thereby causing the development of strains over the surface of the structure in which it is used for construction.
  • Alkalis undergo expansive reactions with aggregates; thereby it leads to its disintegration.
  • Alkalis also accelerate the setting of cement paste.

Efflorescence: It is a whitish coloured powdered deposition of salts on the surface that is formed due to the evaporation of water from the cements/concrete.

Efflorescence


Fig. 1.1 Efflorescence


Bogues compound: When all the ingredients of cement are interground & burnt, they fuse & lead to the formation of complex chemical compounds termed as "Bogues compounds", which are responsible for the properties of cement.

Previous Years' Question

Question: Gypsum is typically added in cement to:
a) Prevent quick setting
b) Enhance hardening

c) Increase workability
d) Decrease the heat of the hardening
Sol: a

(GATE-2020, SET-I)

Example 1.1: Match List I with List II and select the correct answer.
List I List II
(Oxide of cement) (Function)
a) Si02 1) Deficiency of it causes the cement to set quickly
b) Cao 2) Excess of it weakens the cement
c) Al2 O3 3) Excess of it increases the setting time
4) Excess of it gives reddish-brown colour to cement
A B C
a) 3 1 2
b) 1 2 3
c) 1 4 2
d) 3 2 4

Sol: If SiO2, is in excess, the strength of cement increases & it also increases the setting time. CaO in deficiency causes the cement to set quickly. Al2 O3 in excess weakens the cement.
So, the correct answer is a).


1.2 BOGUES COMPOUNDS

i) Tricalcium aluminate [3CaO.Al2 03 [CA]:

  • It undergoes hydration within 24 hrs of the addition of water into the cements, hence is responsible for the flash setting of cement.
  • It produces maximum heat during its hydration process, thereby resulting in loss of water added in cement for hydration, hence leading to the development of cracks over the surface during the setting process moreover, it also reduces the strength by inhibiting complete hydration.
    It also reduces the resistance of cement against the attack of sulphur.
  • It is referred to as a harmful ingredient of cement.

Note:
a) Flash setting means the immediate or instant setting of the cements which takes place due to the presence of alumina in cement.
b) To neutralise the instant setting of cement, gypsum is added, which forms a layer over C.A particles and avoids its interaction with water, but this layer is temporary and gets removed easily; thereby, it has no effect over the final setting time of cement.
c) The water of crystallisation of gypsum vaporises either completely or partially during the manufacturing of cement hence when water is added to cement, it first reacts with gypsum to fulfil its water deficiency due to which it hardens and gives the impression of a false setting of the cement, which can be identified by adding water furthermore into the cement.


ii) Tetracalcium alumino ferrate [4CaO.Al2 03.Fe2 03 [C4AF]:

  • It also undergoes hydration within 24 hrs of the addition of water into the cement hence is responsible for the flash setting of cement [Rate of hydration CAF > CA].
  • It is observed to have the worst cementing property amongst all the bogues compounds.
  • It also reduces the resistance of cement against the attack of sulplur.
  • It has no engineering use as it does not impart any property to the cement.
    Note: Attack of sulphur on C4AF is comparatively less than C3A due to the presence of Fe in it.

iii) Tricalcium silicate [CaO.SiO2] [C3S]:


It undergoes hydration within a week or two after the addition of water in cement; hence it is responsible for the development of early strength.


Note: If in any construction, early strength is required, the proportion of C,s is increased as in:
a) Pavement construction
b) Pre-fabricated structures
c) Cold weather concreting
d) Where formwork is to be reused for speedy construction

It is observed to have the best cementitious property amongst all the bogues compounds.

  • It also increases the resistance of cement against frost action [freezing and thawing (melting)].
  • In real terms, its effect on the heat of hydration is more than CA.
  • C,S + H2O → C-S-H + Ca(OH)2
  • C-S-H provides strength to the cements.


C-S-H Gel - Calcium silicate hydrate gel is also known as thombohyrite gel/tobermorite gel. It is a cementitious compound which possesses binding properties.

Q What is the positive effect of Ca(OH), released during the hydration reaction?

Sol: Ca(OH), released during the hydration makes the pH of concrete around 13 and reduces the tendency of corrosion in reinforcement.
C3S + H2O → C-S-H + Ca(OH)2


iv) Dicalcium silicate [2CaO.SiO,] [C,S]:

  • It undergoes hydration within a year or so after the addition of water into the cements hence is responsible for the development of ultimate or progressive strength in cement.
  • It also increases the resistance of cements against the attack of chemicals and acids.
  • If, in any construction, progressive strength is required, the proportion of C2S is increased.

1.2.1 Some important facts

i) Since (Ca(OH)2 is soluble in water & leaches out (drains out), making the concrete porous, particularly in hydraulic structures, thereby it reduces the durability of concrete.
ii) Ca(OH)2 also reacts with sulphate present in water or soil and leads to the formation of CaSO, which further reduces the durability of cements [by attacking C3A & C4AF]

ii) Leaching of Ca(OH)2 is about 20-30% in OPC.
iv) The percentage of C3S is reduced, and C2S is increased for cement to be used in hydraulic structures.
v) The rate of setting in cement is regulated by adjusting the proportions of Si02/Al2 03 + Fe203

vi) Binding property: C3S > C2S > C3A > C4AF

vii) Rate of hydration: C4AF >C3 A > C3S > C2S

1.3 hydration of cement

Hydration of cement is a chemical process that occurs when water is added to Portland cement, resulting in the formation of chemical compounds that provide strength and durability to concrete. This process is crucial in the construction industry because it is responsible for the setting and hardening of concrete. Here's an overview of the hydration of cements:

hydration of cement

Fig. 1.2 Hydration Of Cement

  1. Chemical Composition of Cement: Portland cement, the most common type of cement used in construction, is composed primarily of four major compounds:
    • Tricalcium silicate (C3S)
    • Dicalcium silicate (C2S)
    • Tricalcium aluminate (C3A)
    • Tetracalcium aluminoferrite (C4AF)
  2. Mixing with Water: When water is added to cement, it initiates a chemical reaction known as hydration. This process begins almost immediately upon contact with water and continues over a period of time.
  3. Formation of Hydration Products: During hydration, the four major compounds in cement react with water to form various calcium-silicate-hydrate (C-S-H) and calcium hydroxide (Ca(OH)2) compounds. The primary hydration products include:
    • Calcium silicate hydrate (C-S-H gel): Provides most of the strength and durability to concrete.
    • Calcium hydroxide (lime): Contributes to the alkalinity of concrete but does not provide significant strength.
  4. Setting and Hardening: As the hydration process progresses, the mixture gradually transforms from a liquid to a solid. This solidification is known as "setting," and it typically occurs within a few hours after mixing. The process of "hardening" continues for weeks or even months, during which the concrete gains strength.
  5. Heat Generation: The hydration reaction is exothermic, meaning it generates heat. This heat can be significant, especially during the first few days of curing. Proper curing is essential to control temperature rise and prevent cracking due to thermal stress.
  6. Curing: To ensure the best possible hydration and strength development, concrete should be adequately cured. Curing involves maintaining a moist and controlled environment, typically by covering the concrete with wet burlap, plastic sheeting, or applying curing compounds.
  7. Long-Term Strength Development: While concrete reaches its initial set within hours, it continues to gain strength over time. The long-term strength development of concrete can extend for months or even years, depending on the mix design and environmental conditions.

Proper hydration is essential for achieving the desired strength, durability, and other properties of concrete. The chemical reactions involved in the hydration process are complex, and they are influenced by factors such as temperature, water-cement ratio, curing conditions, and the specific composition of the cementitious materials used in the concrete mix.

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