Cement

Cement is a binder, a substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Concrete is the most widely used material in existence and is behind only water as the planet's most-consumed resource

Use

1. It is used in mortar for plastering, masonry work, pointing, etc.
2. It is used for making joints for drains and pipes.
3. It is used for water tightness of structure.
4. It is used in concrete for laying floors, roofs and constructing lintels, beams, stairs, pillars etc.
5. It is used where a hard surface is required for the protection of exposed surfaces of structures against the destructive agents of the weather and certain organic or inorganic chemicals.
6. It is used for precast pipes manufacturing, piles, fencing posts etc.
7. It is used in the construction of important engineering structures such as bridges, culverts, dams, tunnels, lighthouses etc.
8. It is used in the preparation of foundations, watertight floors, footpaths etc.
9. It is employed for the construction of wells, water tanks, tennis courts, lamp posts, telephone etc.

Properties of Good Cement

It is always desirable to use the best cement in constructions. Therefore, the properties of a cement must be investigated. Although desirable cement properties may vary depending on the type of construction, generally a good cement possesses the following properties (which depend upon its composition, thoroughness of burning and fineness of grinding).

• Provides strength to masonry.
• Stiffens or hardens early.
• Possesses good plasticity.
• An excellent building material.
• Easily workable.
• Good moisture-resistant.

Composition of Cement

There are eight major ingredients of cement. The following image is showing the ingredients of cement: 

The general percentage of these ingredients in cement is given below:

Ingredient		Percentage in cement
Lime	CaO	60-65
Silica	SiO2	17-25
Alumina	Al2O3	3-8
Magnesia	MgO	1-3
Iron oxide	Fe2O3	0.5-6
Calcium sulphate	CaSO4	0.1-0.5
Sulphur Trioxide	So3	1-3
Alkaline	Alkalise ( K2O, Na2O)	0-1

 

Chemical Properties of Cement

The raw materials for cement production are limestone (calcium), sand or clay (silicon), bauxite (aluminium) and iron ore, and may include shells, chalk, marl, shale, clay, blast furnace slag, slate. Chemical analysis of cement raw materials provides insight into the chemical properties of cement.

1. Tricalcium aluminate (C3A)
   The low content of C3A makes the cement sulphate-resistant. Gypsum reduces the hydration of C₃A, which liberates a lot of heat in the early stages of hydration. C3A does not provide any more than a little amount of strength.
   Type I cement: contains up to 3.5% SO₃ (in cement having more than 8% C₃A)
   Type II cement: contains up to 3% SO₃ (in cement having less than 8% C₃A)
2. Tricalcium silicate (C₃S)
   C3S causes rapid hydration as well as hardening and is responsible for the cement’s early strength gain in an initial setting.
3. Dicalcium silicate (C₂S)
   As opposed to tricalcium silicate, which helps early strength gain, dicalcium silicate in cement helps the strength gain after one week.
4. Ferrite (C₄AF)
   Ferrite is a fluxing agent. It reduces the melting temperature of the raw materials in the kiln from 3,000°F to 2,600°F. Though it hydrates rapidly, it does not contribute much to the strength of the cement.
5. Magnesia (MgO)
   The manufacturing process of Portland cement uses magnesia as a raw material in dry process plants. An excess amount of magnesia may make the cement unsound and expansive, but a little amount of it can add strength to the cement. Production of MgO-based cement also causes less CO2 emission. All cement is limited to a content of 6% MgO.
6. Sulphur trioxide
   Sulphur trioxide in excess amount can make cement unsound.
7. Iron oxide/ Ferric oxide
   Aside from adding strength and hardness, iron oxide or ferric oxide is mainly responsible for the colour of the cement.
8. Alkalis
   The amounts of potassium oxide (K₂O) and sodium oxide (Na₂O) determine the alkali content of the cement. Cement containing large amounts of alkali can cause some difficulty in regulating the setting time of cement. Low alkali cement, when used with calcium chloride in concrete, can cause discolouration. In slag-lime cement, ground granulated blast furnace slag is not hydraulic on its own but is "activated" by the addition of alkalis. There is an optional limit in total alkali content of 0.60%, calculated by the equation Na₂O + 0.658 K₂O.
9. Free lime
   free lime, which is sometimes present in cement, may cause expansion.
10. Silica fumes
    Silica fume is added to cement concrete in order to improve a variety of properties, especially compressive strength, abrasion resistance and bond strength. Though setting time is prolonged by the addition of silica fume, it can grant exceptionally high strength. Hence, Portland cement containing 5-20% silica fume is usually produced for Portland cement projects that require high strength.
11. Alumina
    Cement containing high alumina has the ability to withstand frigid temperatures since alumina is chemical-resistant. It also quickens the setting but weakens the cement.

Physical properties of cement

• Soundness
• Strength
• Setting time
• Heat of hydration
• Bulk density
• Specific gravity (Relative density)

Soundness of Cement

Soundness refers to the ability of the cement to not shrink upon hardening. Good quality cement retains its volume after setting without delayed expansion, which is caused by excessive free lime and magnesia.

Tests:

Unsoundness of cement may appear after several years, so tests for ensuring soundness must be able to determine that potential.

• Le Chatelier Test
  This method, done by using Le Chatelier Apparatus, tests the expansion of cement due to lime. Cement paste (normal consistency) is taken between glass slides and submerged in water for 24 hours at 20+1°C. It is taken out to measure the distance between the indicators and then returned underwater, brought to boil in 25-30 mins and boiled for an hour. After cooling the device, the distance between indicator points is measured again. In a good quality cement, the distance should not exceed 10 mm.
• Autoclave Test
  Cement paste (of normal consistency) is placed in an autoclave (high-pressure steam vessel) and slowly brought to 2.03 MPa, and then kept there for 3 hours. The change in length of the specimen (after gradually bringing the autoclave to room temperature and pressure) is measured and expressed in percentage. The requirement for good quality cement is a maximum of 0.80% autoclave expansion.
  
  

Consistency of Cement

The ability of cement paste to flow is consistency.

It is measured by Vicat Test.

In Vicat Test Cement paste of normal consistency is taken in the Vicat Apparatus. The plunger of the apparatus is brought down to touch the top surface of the cement. The plunger will penetrate the cement up to a certain depth depending on the consistency. A cement is said to have a normal consistency when the plunger penetrates 10±1 mm.

Strength of Cement

Three types of strength of cement is measured – compressive, tensile and flexural. Various factors affect the strength, such as water-cement ratio, cement-fine aggregate ratio, curing conditions, size and shape of a specimen, the manner of moulding and mixing, loading conditions and age. While testing the strength, the following should be considered:

• Cement mortar strength and cement concrete strength are not directly related. Cement strength is merely a quality control measure.
• The tests of strength are performed on cement mortar mix, not on cement paste.
• Cement gains strength over time, so the specific time of performing the test should be mentioned.

Compressive Strength

It is the most common strength test. A test specimen (50mm) is taken and subjected to a compressive load until failure. The loading sequence must be within 20 seconds and 80 seconds.

Tensile strength

Though this test used to be common during the early years of cement production, now it does not offer any useful information about the properties of cement.

Flexural strength

This is actually a measure of tensile strength in bending. The test is performed in a 40 x40 x 160 mm cement mortar beam, which is loaded at its centre point until failure.

Setting Time of Cement

Cement sets and hardens when water is added. This setting time can vary depending on multiple factors, such as fineness of cement, cement-water ratio, chemical content, and admixtures. Cement used in construction should have an initial setting time that is not too low and a final setting time not too high. Hence, two setting times are measured:

• Initial set: When the paste begins to stiffen noticeably (typically occurs within 30-45 minutes)
• Final set: When the cement hardens, being able to sustain some load (occurs below 10 hours)

Again, setting time can also be an indicator of hydration rate.

Heat of Hydration

When water is added to cement, the reaction that takes place is called hydration. Hydration generates heat, which can affect the quality of the cement and also be beneficial in maintaining curing temperature during cold weather. On the other hand, when heat generation is high, especially in large structures, it may cause undesired stress. The heat of hydration is affected most by C₃S and C₃A present in cement, and also by water-cement ratio, fineness and curing temperature. The heat of hydration of Portland cement is calculated by determining the difference between the dry and the partially hydrated cement (obtained by comparing these at 7th and 28th days).

Bulk density

When cement is mixed with water, the water replaces areas where there would normally be air. Because of that, the bulk density of cement is not very important. Cement has a varying range of density depending on the cement composition percentage. The density of cement may be anywhere from 62 to 78 pounds per cubic foot.

Specific Gravity (Relative Density)

Specific gravity is generally used in mixture proportioning calculations. Portland cement has a specific gravity of 3.15, but other types of cement (for example, Portland-blast-furnace-slag and portland-pozzolan cement) may have specific gravities of about 2.90.

 

Depending on the composition and characteristics there are many types of cement.  Followings are the other cement types:

´  Ordinary Portland Cement (OPC)

´  Portland Pozzolana cement (PPC)

´  Rapid hardening cement

´  Quick setting cement

´  Low heat cement

´  Sulphate resisting cement

´  White cement

Ordinary Portland Cement (OPC)

Portland cement clinker is a hydraulic material that shall consist of at least two-thirds by mass of calcium silicates, (3 CaO·SiO2, and 2 CaO·SiO2), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The ratio of CaO to SiO2 shall not be less than 2.0. The magnesium oxide content (MgO) shall not exceed 5.0% by mass.

The composition of Ordinary Portland Cement:

• Argillaceous or silicates of alumina (clay and shale)
• Calcareous or calcium carbonate (limestone, chalk, and marl)

Uses of Ordinary Portland Cement

• It is used for general construction purposes.
• It is also used in most masonry works.

Rapid Hardening Cement

·         When finely grounded Tri-calcium silicate (C3S) is present in OPC with higher content, it gains strength more quickly than OPC. This type of OPC is called Rapid Hardening Cement. It’s initial Setting Time 30 minutes and Final Setting Time 600 minutes.

·         Uses

o   Rapid hardening cement is mostly used where rapid construction is needed like the construction of pavement.

o   It also gives high strength

 

Quick setting cement

Quick setting cement is the cement that sets in a very short time. The initial setting time is 5 minutes and the final setting time is 30 minutes. The composition of Quick Setting Cement:

´  Clinker

´  Aluminum sulfate (1% to 3% by weight of clinker)

´  The aluminium sulfate increases the hydration rate of silicate.

Uses

It is used in underwater construction. It is also used in rainy & cold weather conditions.It is used a higher temperature where water evaporates easily.Used for anchoring or rock bolt mining and tunnelling

Portland Pozzolana Cement (PPC)

Pozzolans are natural or synthetic materials that contain silica in reactive forms. It reacts with calcium hydroxide generated by hydrating cement to form additional cementations materials when it is finely divided. The composition of Portland Pozzolana Cement:

´  OPC clinker

´  Gypsum

´  Pozzolanic Materials (Fly ash, volcanic ash, and Calcined clay or silica fumes.)

Uses

´  PPC is usually used in hydraulic structures, marine structures, construction near the seashore, dam construction, etc.

´  It is also used in pre-stressed and post-tensioned concrete members.

´  As it gives a better surface finish, it is used in decorative and art structures.

´  It is also used in the manufacture of precast sewage pipes.

 

Low heat cement

´  It is a spatial type of cement which produces low heat of hydration during the setting. Some chemical composition of Ordinary Portland Cement is modified to reduce the heat of hydration. The chemical composition of low heat cement:

´  A low percentage (5%) of tricalcium aluminate (C3A)

´  A higher percentage (46%) of declaiming silicate (C2S).

´  Uses

´  It is used for the construction of dam’s large footing, large raft slabs, and wind turbine plinths.

´  It is also used for the construction of chemical plants.

 

Sulphate resisting cement

Sulphate resisting cement is used to resist sulfate attacks in concrete. Due to the lower percentage of Tricalcium aluminate, the production of calcium sulpho-aluminates gets reduced.

Uses

Construction in contact with soils or groundwater having more than 0.2% or 0.3 % g/l sulfate salts respectively. Concrete surfaces subjected to alternate wetting and drying such as bridge piers, concrete surface in the tidal zone, apron, Building near the seacoast.Effluent treatment plans, Chimney, Chemical industries, water storage, sumps, drainage works, Cooling towers, Coastal protective works such as sea walls, breakwaters, tetrapods, etc.

 

White cement

White cement is quite similar to Ordinary Portland Cement except for colour. Amounts of iron oxide and manganese oxide are low in White Cement. It is expensive then OPC so not economical for ordinary work.

´  Uses

It is usually used in decorative work. It can also use for traffic barriers, tile grouts, swimming pools, roof tiles patching materials, and terrazzo surfaces.