Ordinary Portland Cement: A Comprehensive Guide

Ordinary Portland cement (OPC) is a highly popular and extensively utilized cement type. It possesses various types, properties, constituents, and manufacturing processes, making it versatile in its applications. OPC derives its name from Joseph Aspdin, who named it Portland cement due to its resemblance in color and quality to the white-grey limestone found in Portland stone, located on the island of Portland, Dorset. This cement offers numerous advantages and finds wide-ranging uses in various industries.

Constituents of Ordinary Portland Cement

Clays and shales, containing argillaceous or silicates of alumina, are combined with limestone, chalk, or marl (a mixture of clay and calcium carbonate) in a ratio of approximately 2:1. The mixture is crushed and ground either in dry ball mills or in a wet state. The resulting dry powder or wet slurry is then heated in a rotary kiln at temperatures ranging from 1400°C to 1500°C. After cooling, the clinker obtained from the kiln is further ground in ball mills, where gypsum is added. The grinding process achieves the desired fineness based on the product’s classification.

The chief chemical constituents of Portland cement are as follows:

Lime (CaO)	60 to 67%
Silica (SiO2)	17 to 25%
Alumina (Al2O3)	3 to 8%
Iron oxide (Fe2O3)	0.5 to 6%
Magnesia (MgO)	0.1 to 4%
Sulphur trioxide (SO3)	1 to 3%
Soda and/or Potash (Na2O+K2O)	0.5 to 1.3%

During burning and fusion, the constituents present in the raw materials undergo chemical reactions and combine to form specific compounds known as Bogue compounds.

Compound	Abbreviated designation
Tricalcium silicate (3CaO.SiO2)	C3S
Dicalcium silicate (2CaO.SiO2)	C2S
Tricalcium aluminate (3CaO.Al2O3)	C3A
Tetracalcium aluminoferrite (4CaO.Al2O3.Fe2O3)	C4AF

Portland cements have varying proportions of four compounds, with tricalcium silicate and dicalcium silicates being the main contributors to strength. Tricalcium aluminate initiates the initial setting of the cement. Tricalcium silicate rapidly hydrates, contributing more to early strength, while dicalcium silicate’s contribution occurs after 7 days and can continue for up to 1 year. Tricalcium aluminate hydrates quickly, generates significant heat, but only makes a small contribution to strength in the first 24 hours.

Tetracalcium alumino-ferrite is relatively inactive. All four compounds generate heat when mixed with water, with tricalcium aluminate generating the most heat and dicalcium silicate generating the least. As a result, tricalcium aluminate is responsible for most of the undesirable properties of concrete.

Cement with lower amounts of tricalcium aluminate will have higher ultimate strength, less heat generation, and fewer cracks. The table below presents the composition and percentage of these compounds in normal and rapid hardening, and low heat Portland cement.

Read Also: Measurement of Air Content in Concrete by Pressure Air Method

Portland Cement Composition and compound content

Portland Cement	Normal	Rapid hardening	Low heat
(a) Composition: Percent
Lime	63.1	64.5	60
Silica	20.6	20.7	22.5
Alumina	6.3	5.2	5.2
Iron Oxide	3.6	2.9	4.6
(b) Compound: Percent
C3S	40	50	25
C2S	30	21	35
C3A	11	9	6
C3A	12	9	14

Ordinary Portland Cement Properties

Table 2 : Properties of OPC cement

Properties	Values
Specific Gravity	3.12
Normal Consistency	29%
Initial Setting time	65min
Final Setting time	275 min
Fineness	330 kg/m2
Soundness	2.5mm
Bulk Density	830-1650 kg/m3

OPC cement Manufacturing

The manufacture of OPC cement involves five major steps.

1. Crushing and grinding of raw material

Cement manufacturing begins with crushing and grinding raw materials into small particles of a suitable size. The specific process used varies depending on the type of cement manufacturing: dry process, wet process, or semi-wet process. In the dry process, the raw materials are first dried before being crushed.

2. Mixing or Blending

The raw material, limestone, is ground and then combined with clay in the desired proportion of 75% limestone and 25% clay. Compressed air is used to thoroughly mix the two ingredients, creating a homogeneous blend. In the dry process, the mixture is stored in silos, while the wet process utilizes slurry tanks. The resulting mixture, called slurry, contains approximately 35-40% water.

3. Heating

In the manufacture of OPC cement, one crucial step involves conveying the mixed materials into the Kiln via conveyor belts. Initially, the mixture undergoes preheating at 550°C, which eliminates moisture and transforms clay into silica, aluminium oxide, and iron oxide.

Subsequently, the temperature is raised to 1500°C, allowing the oxides to form silicate, aluminates, and ferrite compounds. Finally, the product is cooled to 200°C, resulting in the formation of cement clinkers, which appear as greenish black or grey colored balls within the kiln.

4. Grinding

The cement clinkers and necessary gypsum are combined and ground into fine particles in this stage. These particles are then stored in silos and subsequently packed into cement bags for distribution. OPC cement typically has a shelf life of three months.

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Ordinary Portland Cement Types

OPC cement differentiation relies on country-specific codes. Each country has its own set of codes that define and distinguish various types of OPC cement. These codes serve as guidelines for manufacturers and users to ensure consistency and quality in cement production and usage. By adhering to these specific codes, countries can maintain standardized practices within their respective construction industries.

1. As per ASTM 150 (American Standards)

Portland cement is categorized into different types based on its properties and intended use. Type I is commonly referred to as general-purpose cement and is widely used unless another type is specified. Type II cement offers moderate sulfate resistance and generates less heat during the hydration process.

Type III cement possesses high early strength and is similar to Type I, but it is ground to a finer consistency. Type IV cement is primarily recognized for its low heat of hydration. Type V cement is specifically used in situations where sulfate resistance is crucial, as it contains a significantly low amount of C3A, which contributes to its high sulfate resistance.

2. As per EN 197 Norm ( European norm)

CEM I is a type of cement that contains Portland cement and a small amount, up to 5%, of additional minor constituents.

CEM II is another type of cement that includes Portland cement and can have up to 35% of a single constituent other than Portland cement.

CEM III is a type of cement that contains Portland cement and a higher percentage of blast furnace slag.

CEM IV is a type of cement that consists of Portland cement and can have up to 55% of pozzolanic constituents.

CEM V is a type of cement that combines Portland cement with blast furnace slag or fly ash and pozzolana.

3. As per  CSA A3000-08 ( Canadian standards)

There are several types of cement available for various applications. General use cement (GU, GUL) is commonly used for general construction purposes. Moderate sulphate resistant cement (MS) is suitable for environments with moderate levels of sulfate exposure. Moderate heat cement (MH, MHL) is designed to generate less heat during the hydration process.

High early strength cement (HE, HEL) is known for its rapid development of strength in the early stages. Low heat cement (LH, LHL) minimizes heat generation during hydration. High sulphate resistant cement (HS) is specifically formulated to resist high levels of sulfate exposure, although it generally takes longer to develop strength compared to other types.

Uses of Ordinary Portland Cement

It is commonly utilized in general construction applications that do not demand specific properties, such as reinforced concrete structures, bridges, and pavements. This type of concrete is suitable for use in normal soil conditions and finds extensive use in various concrete masonry units.

Advantages:

OPC (Ordinary Portland Cement) has excellent resistance to cracking and shrinkage, but it is less resistant to chemical attacks. It also has a faster initial setting time compared to PPC (Portland Pozzolana Cement), making it suitable for projects where formwork needs to be removed early. Additionally, OPC has a shorter curing period, resulting in reduced curing costs. However, one disadvantage of OPC is its lower resistance to chemical attacks.

Disadvantages:

OPC, or Ordinary Portland Cement, has limitations for mass concreting due to its high heat of hydration in comparison to PPC (Portland Pozzolana Cement). Concrete made with OPC is less durable than PPC concrete. Additionally, OPC produces less cohesive concrete, making concrete pumping more challenging. The lower fineness of OPC leads to higher permeability, reducing its durability. Finally, OPC is more expensive than PPC.

FAQs about Ordinary Portland Cement