Portland cement is a tightly controlled chemical combination of calcium, silicon, aluminum, iron, and small amounts of other compounds, to which gypsum is added in the final grinding process to regulate the setting time of the concrete.
Some of the raw materials used to make portland cement are limestone, shells, and gypsum or marl, combined with shale, clay, slate or blast furnace slag, silica sand, and iron ores. Lime and silica make up approximately 85 percent of the mass that makes up this type of cement.
A little history of portland cement
The term “Portland” in Portland cement originated in 1824 when an English mason obtained a patent for his product, which he called Portland Cement. This was because his cement mix produced a concrete that resembled the color of natural limestone quarried on the Isle of Portland in the English Channel.
Composition and chemical properties
The three components of hydraulic cements are lime, silica, and alumina. In addition, most cements contain small amounts of iron oxide, magnesia, sulfur trioxide, and alkali.
There has been a change in the composition of Portland cement over the years, reflected primarily in increased lime content and a slight decrease in silica content.
An increase in lime content beyond a certain value makes it difficult to fully combine with other compounds.
Consequently, free lime will exist in the clinker and result in weak cement.
An increase in silica content at the expense of alumina and ferric oxide makes the cement difficult to fuse and form clinker.
Portland cement consists of the following chemical compounds:
| tricalcium silicate | 3 CaO.SiO2 (C3S) | 40% |
| calcium silicate | 2CaO.SiO2 (C2S) | 30% |
| tricalcium aluminate | 3CaO.Al2O3 (C3A) | eleven% |
| tetracalcium aluminate | 4CaO.Al2O3.Fe2O3 (C3AF) | eleven% |
There may be small amounts of impurities present, such as calcium oxide (CaO) and magnesium oxide (MgO).
When water is added to cement, C3A is the first to react and cause the initial set.
It generates a large amount of heat.
C3S hydrates early and builds strength in the first 28 days.
It also generates heat.
C2S is next to hydrate.
It hydrates slowly and is responsible for the increase in maximum strength.
C4AF is a relatively inactive compound.
Types of portland cement, characteristics and uses
Different types of portland cement are manufactured to meet different physical and chemical requirements for specific purposes.
The American Society for Testing and Materials (ASTM) designation C 150 establishes eight types of Portland cement:
Type I portland cement
Type I is a portland cement General purpose suitable for all uses where the special properties of other types are not required. It is used where the cement or concrete is not subject to specific exposures, such as sulphate attack from soil or water, or an objectionable temperature rise due to heat generated by hydration.
its uses they include sidewalks and sidewalks, reinforced concrete buildings, bridges, railway structures, tanks, reservoirs, sewers, sewers, water pipes, and masonry units.
Type II portland cement
Type II Portland cement is used where caution is important against moderate sulfate attack, such as in drainage structures where sulfate concentrations in groundwater are higher than normal but not unusually severe.
Type II portland cement will generally generate less heat at a slower rate than Type I.
With this moderate heat of hydration (an optional requirement), Type II cement can be used in structures of considerable mass, such as large piers, heavy abutments, and heavy retaining walls.
Their use will reduce temperature rise, which is especially important when concrete is placed in a hot climate.
Type III portland cement
Type III is a high early strength portland cement which provides high resistances in an initial period, usually a week or less.
It is used when the forms must be removed as soon as possible or when the structure must be put into service quickly.
In cold weather, its use allows a reduction of the controlled curing period. Although richer mixes of Type I cement can be used to obtain high early strength, the high early strength Portland cement, Type III, can provide it more satisfactorily and economically.
Portland cement type IA, IIA, IIIA
Specifications for three types of air-entrained portland cement (Types IA, IIA, and IIIA) are given in ASTM C 150.
They correspond in composition to ASTM Types I, II, and III, respectively, except that small amounts of the materials are crushed with the clinker during manufacture to produce minute, well-distributed, and completely separated air bubbles.
These cements produce concrete with improved resistance to freeze-thaw action.
Type IV portland cement
Type IV is a hydration cement Low heat for use where the rate and amount of heat generated must be minimized. It develops strength at a slower rate than Type I cement.
Type IV Portland Cement is designed for use in massive concrete structures, such as large gravity dams, where temperature rise resulting from heat generated during curing is a critical factor.
Type V portland cement
Type V is a sulfate resistant cement It is used only in concrete exposed to the severe action of sulfates, mainly where soils or groundwater have a high sulfate content.
A low tricalcium aluminate (C3A) content, generally 5% or less, is required when high sulfate resistance is required.
Physical properties of portland cement
The following physical properties must be verified before selecting a portland cement for civil engineering works. IS 269–1967 specifies the test method and prescribes the limits:
Fineness
It is measured in terms of percent weight retained after sieving the cement through a 90 micron sieve or per cement surface area in square centimeters per gram of cement. According to the IS code specification, the weight retained on the sieve must not be more than 10 percent. In terms of specific surface it should not be less than 2250 cm2 / gm.
Setting time
A period of 30 minutes as the minimum setup time for initial setup and a maximum period of 600 minutes as the maximum setup time is specified by the IS code, provided that the tests are performed according to the procedure prescribed by IS 269-1967.
Solidity or hardness
After the concrete has hardened, it is necessary to ensure that no volumetric changes occur. It is said that the cement is not solid, if it presents volumetric instability after hardening. The IS code recommends testing with the Le Chatelier mold to test this property. At the end of the test, the Le Chatelier mold indicator should not expand more than 10 mm.
Compressive strength
For this mortar, cubes are made with standard sand and tested in compression testing machines per IS code specification. The specified minimum strength is 16 N/mm2 after 3 days and 22 N/mm2 after 7 days of curing.
Manufacture of portland cement
Although there are several variations of commercially manufactured portland cement, each shares many of the same basic raw materials and chemical components.
The main chemical constituents of portland cement are calcium, silica, alumina, and iron.
Calcium is derived from limestone, marl, or chalk, while silica, alumina, and iron come from sands, clays, and iron ore sources.
Other raw materials may include shales and industrial by-products.
The basic manufacturing process heats these materials in a furnace to approximately 1,400 to 1,600 °C (2,600 – 3,000 °F), the temperature range in which the two materials chemically interact to form calcium silicates.
This heated substance, called »clinker«, is generally in the form of small gray-black balls approximately 12.5 mm (0.5 inches) in diameter.
The clinker is then cooled and pulverized into a fine powder that passes almost completely through a 0.075 mm (No. 200) sieve and is fortified with a small amount of gypsum.
The result is portland cement.
Portland Cement Uses
Portland cement is widely used for the construction of various structures. Some of them are listed below:
- Cement grout is used to fill cracks in concrete structures.
- Cement mortar is used for masonry, plastering and pointing work.
- Cement concrete is used for the construction of various structures like buildings, bridges. water tanks, tunnels, docks, etc.
- Cement is used to make lamp posts, telephone poles, railway sleepers, piles, etc.
- For the manufacture of cement pipes, garden seats, litter bins, flower pots, etc., cement is commonly used.
- It is useful for the construction of roads, trails, courts for various sports, etc.