What Is OPC Cement? Ordinary Portland Cement Explained Simply

Ordinary Portland Cement (OPC) is the most common ingredient used in the production of concrete, mortar, and other cement-based products. OPC is known for its versatility, strength, and durability, making it suitable for a wide range of construction applications. It uses a special production process that gives it significant properties that are distinct from other types of cement. Portland cement is a widely used cement in construction projects. Different construction projects may require different types of Portland cement. Portland cement is divided into three main categories: Ordinary Portland Cement (OPC), Portland Pozzolanic Cement (PPC), and Portland Slag Cement (PSC). OPC is a hydraulic cement made from Portland cement clinker and gypsum. OPC is used in almost all general construction projects. Ordinary Portland Cement (OPC Cement) is the most commonly used type of cement in the construction industry worldwide. It is a hydraulic cement, which means that it sets and hardens when mixed with water. It is a type of concrete widely used in the field of building renovation due to its adaptability and strength. It is prepared by heating a mixture of limestone, soil, and various materials to high temperatures and then beating the clinker into a fine powder. In the construction world, Ordinary Portland Cement (OPC) holds a prominent place as one of the most widely used building materials. OPC is a versatile binder that provides strength and durability to a variety of structures, making it a top choice for both residential and commercial projects. However, like any other material, OPC has its own pros and cons that builders and engineers must consider. In this article, we will take a deep dive into the meaning of OPC cement, its composition, types, applications, benefits, and industry trends.

Definition of OPC Cement

Ordinary Portland Cement (OPC) is a basic cementitious material in the construction industry. Its main components are clinker (limestone, clay, etc. produced by high-temperature calcination reaction) and gypsum. OPC has excellent bonding properties, high strength (especially compressive strength) and good durability. It is mixed with water and aggregate to produce concrete, mortar and plaster. It is widely used in various projects from residential to large infrastructure. OPC is also the basis for the production of mixed cements with added ingredients such as pozzolana or slag.

Raw materials used in OPC cement

The quality and performance of Ordinary Portland Cement (OPC) is closely tied to the selection and composition of its raw materials. Let’s take a closer look at the essential ingredients that form the basis of this indispensable building material.

• Limestone: Limestone is the main source of calcium in OPC cement. It reacts chemically during the manufacturing process to enhance the strength and durability of the cement. It is best to use high-purity limestone because impurities can affect the characteristics of the final product.
• Clay or shale: Clay or shale provides the necessary silica, alumina, and iron oxide. These elements combine to help improve the hydraulic properties of cement and enhance its performance. Different types of clay and shale are used depending on their availability and chemical composition.
• Iron ore: Iron oxide is added to iron ore, which can discolor the cement and affect its setting and hardening characteristics. The amount of iron ore must be controlled to prevent adverse effects on the final product.
• Gypsum: A small amount of gypsum is added. It can adjust the setting time of OPC cement and prevent flash setting, ensuring workability during construction. Accurate control of gypsum content is essential to achieve the desired setting characteristics.

The raw material selection process requires comprehensive testing and analysis to ensure that the raw materials meet strict standards. Variations in the composition of raw materials can affect the properties of cement, so strict quality control must be maintained throughout the production process.

Composition of Ordinary Portland Cement (OPC)

OPC consists of four main parts:

• Lime (CaO): Calcium oxide, which comes from limestone or chalk, is the main component of cement. Limestone and chalk are both calcium carbonates. It improves the strength and setting characteristics of cement.
• Silicon Dioxide (SiO₂): Silica, which comes from sand or clay, is the second most abundant chemical in cement. During cement production, silica reacts with calcium to form dicalcium silicate and tricalcium silicate. The silicate adds strength and durability to cement.
• Alumina (Al₂O₃): Alumina comes from clay and shale and helps speed up the hydration reaction. It increases its durability at high temperatures.
• Iron Oxide (Fe₂O₃): Iron oxide, which is extracted from iron ore, helps make cement with significant early strength. It improves the strength and color of cement.
• Gypsum (CaSO₄·2H₂O): Gypsum is added to cement to help regulate the setting time.

The main components of ordinary Portland cement are as follows:

Component	Content (%)	Description
Lime (CaO)	60 – 67	The primary component in Portland cement. Excessive lime can reduce the strength of the cement.
Silicon Dioxide (SiO₂)	17 – 25	Increases the strength of cement. However, too much silica can prolong the setting time of Portland cement.
Aluminum Oxide (Al₂O₃)	3 – 8	Helps in quick setting of cement. Excessive amounts may weaken the cement's strength.
Calcium Sulfate (CaSO₄)	3 – 4	Acts as a retarder, delaying the setting time.
Iron Oxide (Fe₂O₃)	3 – 4	Contributes to the color and hardness of the cement.
Magnesium Oxide (MgO)	0.1 – 3	Helps improve hardness, but excessive amounts can cause unsoundness.
Sulfur (S)	1 – 3	Excessive sulfur content may result in unsound cement.
Alkalis (Sodium and Potassium; Na₂O and K₂O)	0.5 – 1.3	Play a key role in alkali-aggregate reactions, but excessive amounts can lead to efflorescence.

What grades of OPC cement are there?

OPC cement is divided into three main grades based on its compressive strength after 28 days. The grade number represents the minimum strength (in MPa) achieved by the cement after 28 days of curing.

Grade	Compressive Strength (MPa)	Key Features	Common Applications
OPC 33	33 MPa	Lower strength, suitable for small-scale projects	Plastering, masonry, non-structural works
OPC 43	43 MPa	Medium strength with controlled setting time	General construction, residential and commercial buildings
OPC 53	53 MPa	High strength and excellent durability	Bridges, highways, high-rise buildings
OPC 53-S	53 MPa	Specifically designed for railway sleepers	Prestressed concrete railway sleepers

OPC 33 grade cement

1. Applications
   OPC 33 grade cement has traditionally been used for general building purposes under normal environmental conditions. It is suitable for plastering, flooring and masonry work where the structural loads are relatively low. However, due to its lower compressive strength than higher grade cements, its use has declined, and with the advent of stronger cements, OPC 33 grade cement has gradually fallen out of favor.
2. Compressive Strength
   The compressive strength of OPC 33 grade cement is determined by testing a mortar cube consisting of one part cement and three parts standard sand, with a surface area of ​​50 square centimeters. The strength is measured as follows:
   72±1 hours: not less than 16N/mm²
   168±2 hours: not less than 22N/mm²
   672±4 hours: not less than 33N/mm²
   These values ​​show that after 28 days, the cement reaches a minimum compressive strength of 33 N/mm².

OPC 43 Grade Cement

1. Applications
   OPC 43 grade cement is the most commonly used cement in India today. It is suitable for a wide range of construction activities including:
   Reinforced concrete (RCC) structures with concrete grades up to M30.
   Precast components such as blocks, tiles and asbestos products such as panels and pipes.
   Non-structural works such as plastering, flooring and decoration.
   Its balanced properties make it a versatile choice for both structural and non-structural applications.
2. Compressive Strength
   OPC 43 grade cement has a higher compressive strength than OPC 33 grade cement and is therefore suitable for more demanding construction projects:
   72±1 hours: not less than 23N/mm²
   168±2 hours: not less than 33N/mm²
   672±4 hours: not less than 43N/mm²
   This grade achieves a minimum compressive strength of 43 N/mm² after 28 days.

OPC 53 Grade Cement

1. Applications
   OPC 53 grade cement is suitable for projects that require high early strength. It is particularly suitable for:
   High-strength concrete, where cement needs to be saved without compromising strength.
   Concrete mixes designed to be grade M20 and above, which can save 8-10% of cement usage.
   Specialized construction projects such as prestressed concrete components, runways, bridges, reinforced concrete works with concrete grade M25 and above.
   Precast items that require high early strength, such as paving blocks and building blocks.
2. Compressive Strength
   OPC 53 grade cement has the highest compressive strength of the three grades:
   72±1 hours: not less than 27N/mm²
   168±2 hours: not less than 37N/mm²
   672±4 hours: not less than 53N/mm²
   The compressive strength reaches 53 N/mm² after 28 days, making it ideal for structures that require high durability and strength.

How is OPC cement made?

The production process of cement involves mixing lime and clay components in a ratio of 3:1. After mixing, two processes can be carried out to finally obtain ordinary Portland cement.
These two processes include：

• Wet process
• Dry process

Wet Process

• In this process, the crushed raw material is fed into a ball mill and a small amount of water is added.
• The steel balls inside the ball mill crush the raw material into a slurry.
• The slurry is fed into a silo where the ingredients are properly mixed. The slurry thus formed has a water content of about 40%.
• The slurry passes through a rotary kiln where it is formed into balls. The temperature inside the kiln is maintained at around 1600 degrees Celsius. At this temperature, the balls are converted into beaters.
• The clinker is then cooled and a small amount of gypsum (about 3%) is added. The addition of gypsum is very important as it avoids rapid setting of the cement.
• Finally, the cement is fed into silos and then supplied to the packaging plant.

Dry Process

This is a new process commonly used in modern times. This process is preferred due to the high energy and fuel consumption of the conventional wet process.

• In the dry process, the calcareous and clayey materials are reduced to 25 mm size with the help of mechanical crushers in ball mills.
• After proper grinding, both materials are mixed in proper proportions and then fed to the rotary kiln.
• Before feeding the mixture to the rotary kiln, it is preheated to 850°C for 2 hours.
• Subsequently, the mixture is fed to the rotary kiln.
• The product obtained from the rotary kiln is called clinker, which has a size between 3 mm and 20 mm.
• It is necessary to add 2 - 3% gypsum to the clinker as it reduces the flash properties and setting time of the clinker.
• The final product obtained is calcium silicate hydrate (CSH) gel, and calcium hydroxide is a by-product.

Physical properties of OPC cement

In addition to compressive strength, OPC must also meet certain physical property requirements to ensure that it performs adequately in construction applications. These properties include fineness, strength and setting time.

Fineness

• Definition: Fineness refers to the particle size of the cement. The finer the cement particles, the greater the surface area available for hydration and the faster the strength growth.
• Requirement: All grades of OPC should have a surface area of ​​not less than 225m²/kg when tested by the Blaine permeability method.

Soundness

• Definition: Soundness ensures that the cement does not undergo a significant volume change after setting, which would cause cracking or disintegration.
• Le-Chatelier method: For unaerated cement, the expansion should not exceed 10 mm.
• Autoclave test: The expansion rate of unaerated cement should not exceed 0.8%.
• These tests ensure the dimensional stability of the cement after setting.

Setting time

Initial setting time:

• Requirement: Not less than 30 minutes.
• Meaning: Allow enough time for mixing, transportation and placement before the cement begins to harden.

Final setting time:

• Requirement: Not more than 600 minutes (10 hours).
• Significance: To ensure that cement sets within a reasonable time so that construction activities can continue without unnecessary delay.

OPC Chemical Requirements

The chemical composition of OPC significantly affects its properties and suitability for various applications. The main chemical requirements for OPC grades 33, 43 and 53 are outlined below:

Property	OPC 33 Grade	OPC 43 Grade	OPC 53 Grade
Lime Saturation Factor (LSF):
Ratio of lime content to the combined content of silica, alumina, and iron oxide	0.66 – 1.02	0.66 – 1.02	0.80 – 1.02
Silica Modulus:
Ratio of alumina content to iron oxide content	≥ 0.66	≥ 0.66	≥ 0.66
Insoluble Residue (% by mass)	≤ 4	≤ 2	≤ 2
Magnesium Oxide (MgO) (% by mass)	≤ 6	≤ 6	≤ 6
Sulphur Trioxide (SO₃) Content (% by mass):
– When Tricalcium Aluminate (C₃A) ≤ 5%	≤ 2.5	≤ 2.5	≤ 2.5
– When Tricalcium Aluminate (C₃A) > 5%	≤ 3	≤ 3	≤ 3
Loss on Ignition (% by mass)	≤ 5	≤ 5	≤ 5

Key points

• Lime saturation factor (LSF): controls the ratio of lime to other oxides and affects the strength and durability of cement.
• Silica modulus: affects the formation of silicate phases that contribute to strength development.
• Insoluble residue: represents impurities that do not contribute to strength; the lower the value, the better.
• Magnesium oxide content: excess magnesium oxide causes expansion retardation; therefore, the limit is 6%.
• Sulfuric anhydride (SO₃): affects setting time and strength; the limit is set based on the C₃A content.
• Loss on ignition: indicates the presence of volatile matter; high values ​​may affect the properties of cement.

Advantages of OPC

• Versatility: OPC is known for its versatility and can be used in a variety of construction applications. It can be used to build buildings, bridges, dams, roads, and other infrastructure projects. Its adaptability makes it an ideal choice for different construction needs.
• Strength and durability: OPC has excellent compressive strength, which enables the structure to withstand heavy loads. In addition, it has good weather resistance, making it suitable for building long-lasting and durable buildings. This strength and durability ensure the durability and stability of structures built using OPC.
• Adequate supply and affordable price: OPC is widely available in the market and can be easily applied to construction projects. Its large-scale production and wide supply make it affordable, making it an affordable choice for builders.
• Compatibility with other materials: OPC can be easily mixed with other materials such as aggregates, water, and additives, allowing builders to customize its properties. This compatibility makes it possible to produce various types of concrete and mortar mixtures according to specific project needs.
• Reliable performance: OPC has a long history of successful application in construction projects around the world. Its performance has been extensively studied and documented, providing builders with a reliable and predictable building material.

FAQ

Conclusion

Understanding the grade of OPC cement is essential to ensure the strength and durability of a building project. OPC 33 is suitable for small projects, OPC 43 is suitable for general construction, while OPC 53 is best suited for high-performance structures. Selecting the right grade ensures that the building is safe, cost-effective, and long-lasting. However, it is important to weigh its advantages and disadvantages. While OPC has many advantages in terms of structural stability and performance, it also brings challenges such as environmental impact, slow setting time, and susceptibility to cracking and shrinkage. With the increasing importance of sustainability and environmental protection, it is crucial to explore alternative cement options or take measures to mitigate the disadvantages of OPC to achieve more sustainable construction practices.