Cement | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Cement:

Cement is a binding material used in construction that sets, hardens and adheres to other materials, binding them together. Cement is seldom used solely, but is used to bind sand and gravel (aggregate) together. Cement is ordinarily combined with fine and coarse aggregates to produce mortar (with sand) or concrete (with sand and gravel).

Storage of cement

Cement is highly hygroscopic and readily absorbs moisture from the atmosphere. It must therefore be protected from dampness during packing, transportation and storage. Improper storage reduces the strength of concrete made from that cement. For example, cement stored for about three months may cause roughly a 20% reduction in 28-day concrete strength; cement stored for two years can lead to a reduction in 28-day strength of the order of 50%. Cement that has been stored for long periods should be tested in a laboratory before use.

Composition of cement

• Lime (CaO) : 60% - 65%
• Silica (SiO2) : 20% - 25%
• Alumina (Al2O3) : 4% - 8%
• Iron oxide (Fe2O3) : 2% - 4%
• Magnesium oxide (MgO) : 1% - 3%
• Sulphur trioxide (SO3) : about 1% (varies, gypsum addition)
• Alkalis : 0.3% - 1%

Functions of ingredients

• Lime (CaO) - Main constituent; provides cementing property. Its proportion is to be maintained carefully, excess lime makes cement unsound (expansion, disintegration). Deficiency of lime reduces strength and accelerates setting.
• Silica (SiO2) - Contributes to strength and imparts hardness; controls setting rate in combination with lime.
• Alumina (Al2O3) - Increases the rate of reaction (quick setting). Excess alumina weakens the cement.
• Calcium sulphate (gypsum) - Controls the initial setting time by regulating the activity of tricalcium aluminate.
• Magnesia (MgO) - Small amounts add hardness and affect colour; excess MgO causes unsoundness.
• Sulphur (as SO3) - Small amounts are useful; excessive sulphur causes unsoundness.
• Alkalis - Most alkalis volatilise during burning; excessive alkalis in cement can cause efflorescence and affect durability.

Bogue's compounds (principal clinker phases)

• Lime, silica and iron oxide mainly contribute to strength.
• Alumina accelerates setting; excessive alumina reduces ultimate strength
•  Magnesium oxide, when present in small amounts, adds hardness and colour to cement.
•  A very small amount of sulphur is useful in producing sound cement.
• Small amounts of magnesia and sulphur may be beneficial; in excess they are harmful.
• Excess alkalis may produce efflorescence.

Formation and role of Bogue compounds during hydration

• Tricalcium aluminate  [3CaO. Al₂O₃] [4-14%] C₃A - Forms rapidly on contact with water (within 24 hours). Responsible for a rapid early heat evolution. It controls early set and early heat generation.
• Tetracalcium aluminoferrite ( [4CaO, Al₂O₃. Fe₂O₃] [C₄AF] [10-18%]) - Also forms early (within the first day) and contributes to early heat of hydration; its contribution reduces with time.
• Tricalcium silicate ([3CaO. SiO₂] [C₃S] [45- 65%]) - Reacts more rapidly in the first few days (formed/acts within a week of hydration). Responsible for early strength development.
• Dicalcium silicate  [C₂S] [15-35%] - Reacts slowly (may continue for months to a year) and is chiefly responsible for long-term (later age) strength gain.

C3S and C2S

• C₃S (Tricalcium silicate) - Gives initial/early strength and higher early heat of hydration; used where early strength is required: cold weather concreting, precast and emergency repair.
• C₂S ( Dicalcium silicate)- Gives later/ultimate strength and produces lower heat; used in massive structures such as dams and bridges where low heat generation is desirable.

  3 Days        90 Days

  C3A               212          310         I              24 hrs C4AF            69            98           II             24 hrs C3 S              58            105         III            1 weak C2 S              12            42           IV             1 year

Heat of hydration

The heat of hydration is the heat released when Portland cement reacts chemically with water. The heat evolved is influenced primarily by the proportions of C3S and C3A in the cement, and is also affected by water-cement ratio, cement fineness and curing temperature. Increasing any of these increases the heat of hydration.

• Relative heat generation (approximate order): C₃A > C₃S > C₄AF > C₂S
• Typical heat of hydration: about 89-90 calories/g for 7 days and about 90-100 calories/g for 28 days (values may vary with cement composition and test conditions).
• Approximately 23% (by weight of cement) of water is chemically required for complete hydration.
• About 15% of the added water may be retained as pore/entrapped water and does not participate chemically; hence total water required for full hydration is often taken as around 38% by weight of cement (practical values vary).

Manufacturing of cement

Cement manufacturing is carried out by two principal routes:

• Dry process (modern, energy-efficient)
• Wet process (older; requires more fuel because of water evaporation)

Dry process (New Method):

Calcareous and argillaceous raw materials are crushed separately in the gyratory crushers to small pieces (2-5 cm) and then ground in ball or tube mills to obtain a fine powder. The finely ground materials are stored in hoppers after screening and mixed in the required proportions to produce a dry raw mix. The dry raw mix is stored in silos and fed to the rotary kiln for clinkering. Careful control of proportions and grinding produces the required clinker chemistry.

Wet process (Old Method):

The raw materials are firstly crushed and made into powdered form and stored in silos. The clay is then washed in washing mills to remove adhering organic matters found in clay.
The powdered limestone and water washed clay are sent to flow in the channels and transfer to grinding mills where they are completely mixed and the paste is formed, i.e., known as slurry. The grinding process can be done in ball or tube mill or even both. Then the slurry is led into collecting basin where composition can be adjusted. The slurry contains around 38-40% water that is stored in storage tanks and kept ready for the rotary kiln.

Comparison of dry process and wet process of Cement Manufacture:

Testing of cement

Field tests

• Colour - Cement should have a uniform grey colour.
• Physical feel - Cement should feel smooth when rubbed between fingers.
  (i) the cement should feel smooth when rubbed between finger.
  (ii) if a small quantity of cement is thrown in bucket of water, it should sink and does not float over the surface.
  (iii) cement should be free from presence of any lumps which are formed due to absorption of moisture by the cement from the atmosphere leading to its hydra
• Free-flowing - A small quantity thrown into a bucket of water should sink; it should not float on the surface.
• No lumps - Presence of lumps indicates absorption of moisture and partial hydration; such cement is suspect.
• Simple strength check - A standard 25 × 25 × 200 mm mortar bar cured for 7 days, when supported 150 mm apart and subjected to a central load of 340 N, should not show failure (field check approximate).

Laboratory tests

Fineness test

Fineness of the cement is tested to check the grinding of the cement. Fineness affects rate of hydration, early strength development and heat evolution. Two common methods are the sieve test and the air-permeability test.

• Sieve test
  • Take 100 g cement and place on IS sieve No. 9 (90 μm).
  • Break any air-set lumps by hand and sieve for at least 15 minutes.
  • Residue on the sieve for Ordinary Portland Cement (OPC) should not exceed 10% by mass.
  • Note: sieve test is seldom used in modern practice.
• Air-permeability test

  Also called the Blaine specific surface test. It measures specific surface area (total surface area per unit mass) in cm2/g by relating the flow of air through a bed of cement particles to particle surface area. Typical specific surface for OPC is not less than 2250 cm2/g.

Strength tests

Compressive strength of cement is determined by making mortar of cement and standard sand (commonly in the proportion 1 : 3 by weight). For preparation of mortar the standard sand weight is normally 550 g for a cement sample; water-cement ratio commonly used for standard tests is 0.40. Mortar is compacted into moulds (75 mm cubes or other standard sizes) and cured; compressive strength is determined on a compression testing machine.

Tensile strength - Direct tensile testing of cement or mortar is difficult because of brittleness. Tensile strength of cement mortar may be determined using briquette specimens; tensile strength (in MPa) is calculated as failing load divided by cross-sectional area. A typical expression used in some test arrangements is:

Tensile strength = failing load / 6.45 (this formula reference depends on unit and specimen geometry; follow the specified standard test procedure).

Standard consistency test

The standard consistency is the water content at which a cement paste allows the Vicat plunger (10 mm diameter, 50 mm height) to penetrate to a depth of 33-35 mm from the top of a standard mould (and 5-7 mm from the bottom). The consistency is expressed as percentage of water by weight of cement (denoted p%).

Procedure summary: take 500 g cement and add 24% of water by weight, mix into a paste, fill the mould, remove entrapped air, place the Vicat plunger and note penetration. Repeat trials until penetration is 33-35 mm. The percentage of water giving this penetration is the standard consistency (p). The test is normally performed at 24 ± 2 °C and ~90% relative humidity.

Setting time tests

Setting time indicates the loss of plasticity of cement paste and is reported as initial and final setting times.

• Initial setting time - Time from adding water until the paste starts losing plasticity. Take 500 g cement sample.
  Add 0.85p% water (where p = water required for standard consistency) and prepare cement paste.
  Fill the paste in the Vicat mould properly.
  Bring the square needle gently to touch the top surface and release it quickly.
  Note the time when the needle penetrates only up to 33–35 mm from the top (or remains 5–7 mm above the bottom).
  This time is taken as the initial setting time.
  For OPC, initial setting time is about 30 minutes.
• Final setting time - Time from adding water until the paste attains sufficient firmness to resist a specified needle.
  To determine it, the cement paste is prepared and filled in the mould (Vicat test).
  The needle with annular collar is lowered gently on the paste surface.
  Final setting time is the time when the needle makes an impression, but the annular collar does not leave any impression on the paste.
  For OPC, final setting time is about 10 hours.

Soundness tests

Soundness of cement is the property that ensures no appreciable expansion occurs after setting; unsound cement causes cracking and poor durability. Main causes of unsoundness are free lime (CaO) and free magnesia (MgO).

• Le Chatelier test (for free lime) -
  The apparatus consists of a small split cylinder made of spring brass, about 30 mm diameter and 30 mm height.
  It has two indicator arms of length 165 mm fixed on either side of the split.
  For the test, cement is gauged with 0.78p water (p = standard consistency) and filled into the mould.
  The mould is covered with glass plates on top and bottom.
  The whole assembly is immersed in water at 27–32°C for 24 hours.
  After 24 hours, the mould is removed and the displacement of the split is measured using the indicator arms.
  The assembly is again immersed in water and boiled at 100°C for 3 hours.
  It is removed again and the final displacement is measured.
  The difference between the two readings should not exceed 10 mm for OPC.
• Autoclave test (sensitive to magnesia and lime) - Cement paste specimens (e.g., 25 mm cubes) are autoclaved at elevated pressure (e.g., steam pressure ~21 kg/cm2) for 3 hours. The change in size should not exceed 0.8% (or about 2 mm on the original dimension) for sound cement.

Chemical composition checks

• Typical control limits and requirements used in practice include:
• The ratio of Al2O3 to Fe2O3 should not be less than about 0.66 (this ensures proper balance of alumina and iron oxide for clinker chemistry).
• Lime saturation factor (LSF) - A key parameter in clinker chemistry defined as

LSF = CaO / (2.8 × SiO2 + 1.2 × Al2O3 + 0.65 × Fe2O3)

• Manufacturers control LSF to a specified range to obtain the desired clinker composition (typical design targets are selected to produce desired fractions of C3S, C2S, C3A and C4AF).
• Other typical specification limits (examples; apply the relevant standard specification used locally): total sulphur content, insoluble residue, loss on ignition, MgO content, etc. (Input values often used in practice: total sulphur around 2.75% as an upper reference in some tests, insoluble residue ~1.5%, loss on ignition ~4%, MgO allowed up to about 5% - check the applicable standard before final acceptance).

Types of cement

1. Rapid hardening cement

This cement develops strength at a faster rate than ordinary Portland cement (OPC); it should not be confused with quick-setting cement which only sets rapidly.
Rapid hardening cement attains in 3 days approximately the strength OPC attains in 7 days.
 Applications: precast concrete, cold weather concreting, emergency repairs and where early removal of formwork is required.

2. Extra rapid hardening cement

Manufactured by intergrinding rapid hardening clinker with a small percentage of calcium chloride (normally ≤ 2% by weight). It must be used immediately (within about 20 minutes of mixing). It attains much higher early strength (e.g., 25% higher than rapid hardening cement at 1-2 days). Strength difference reduces with age; by 90 days its strength approaches that of OPC.

3. Sulfate resisting cement

Designed to resist sulfate attack (e.g., from MgSO4 or Na2SO4). Sulfate attack can form expansive products (e.g., ettringite) and cause cracking. Sulfate resisting cements are produced by limiting C3A and C4AF contents. OPC is highly susceptible to the attack of sulphate especially to that of MgSO₄ which reacts with both Ca(OH)2 to form CaSO₄ and calcium aluminate to form calcium - sulphur aluminate volume of which is approximately 227% than the original volume of the constituent.
The cement is manufactured by reducing the:
% of C₃A and C₄AF. [C₂A 5% and C₄AF + 2C₃A 25%]
Typical applications: marine structures, sewage works, foundations in sulfate soils, pipes laid in marshy areas.

4. Supersulphated cement

Made by intergrinding large proportions (e.g., ~80%) of granulated blast furnace slag with 15% hard burned gypsum and 5% cement clinkers. It gives low heat, good sulfate resistance and is used where these properties are required.

5. Portland slag cement

Made by intergrinding Portland clinker, gypsum and granulated blast furnace slag in definite proportions. It is a low-heat cement with improved sulphate resistance and reduced permeability; suitable for marine and hydraulic structures.

6. Quick setting cement

Produced by increasing cement fineness and adjusting gypsum and certain additives (e.g., small aluminium salts). It sets rapidly and is used for grouting and concreting under running water.

7. Low heat cement

This cement is produced by reducing the proportion of C3A and C3S and increasing proportion of C₂S [C₃A – 5%, C₃S – 46%, C₂S – 56%]. It develops strength slowly and is used in mass concrete to limit temperature rise. Specification limits on heat evolution are used (e.g., 7-day and 28-day heat values, is not greater than 75 calories/gm).

8. Portland pozzolana cement (PPC)

Produced by intimate grinding or blending of pozzolanic material (natural pozzolana or fly ash) with Portland cement clinker and gypsum. PPC offers improved chemical resistance (sulfates and chlorides), better long-term strength development, lower heat evolution and higher workability. Applications: marine works, sewage works, underwater concreting, bridges, piers and mass concrete.
• This cement possess:
(a) higher tensile strength.
(b) attains compressive strength with age.
(c) is more plastic and workable.
(d) offer higher resistance to attack of sulfur and chlorine.
(e) after higher resistance against explosion.
(f) low heat cement.
(g) after higher resistance to penetration of water.
(h) is more economical.

9. High alumina cement

 This cement is manufactured by intergrinding the clinker obtained from calcination of bauxite and lime. Contains high alumina (Al2O3 ≥ ~32%) and an Al2O3/CaO ratio typically in the range ~0.85-1.3. It sets rapidly and attains high early strength, can resist high temperatures and chemical attack by acids. Initial setting time may be > 3.5 hours and final setting about 5 hours depending on grade and temperature.This cements attains higher earlier strength such that 20% of ultimate strength is attained in one day and substantial strength is attained within 6-8 hours.

Other special cements and brief notes

• Portland pozzolana cement (continued) - PPC is generally more economical, more workable, tends to gain strength with age and has lower permeability than OPC.
• Super-sulphated and slag blended cements - Useful where low heat and high sulfate resistance are required; used in marine and hydraulic works.

Important points and typical conversions

• Floor area occupied by one bag of cement (approx.) = 0.3 m2 up to a height of 0.18 m (practical approximate figure used for on-site estimations).
• One bag of cement (typical Indian 50 kg bag) ≈ 0.0345 m3 (34.5 litres). (Note: bag weight and volume may vary by region and bag size; verify local standard bag weight.)
• Voids in bulk cement ≈ 40% (approximate for storage calculations).
• During hand mixing an additional cement allowance of about 10% is sometimes used to account for wastage and inconsistency; machine mixing requires less allowance.
• Water required for theoretical complete hydration ≈ 20 - 30 litres per 50 kg bag (practical water for mixing is governed by workability and mix design rather than only hydration requirement).
• Small amounts of certain admixtures (for example, sugar in trace amounts) have been reported to influence strength development in some contexts; exercise caution and follow standards when using chemical admixtures.
• Compressive strength test: typical mortar proportion for bench tests is 1 : 3 (cement : standard sand). For the 1 : 3 mix, standard sand mass ≈ 555 g for a 185 g cement sample (proportion examples follow the applicable standard procedure). 
  Water required =%,where - P is consistency.
• Tensile strength tests use briquette specimens; water quantity similarly related to standard consistency. Water required =  %.
• Testing room conditions for standard curing and testing: about 27 ± 2 °C and 90% relative humidity are commonly used reference conditions for particular standard tests (follow the relevant test standard used locally).