Everything You Need to Know

As you may know, cement is the main component of mortar and provides strength and stability to materials in construction. If you’re interested in learning more about this building material, you’ve come to the right place. In this article, we will cover all the important aspects related to it.

What is Cement?

Cements are finely ground powders that harden into a solid mass when mixed with water. The setting and hardening occur due to hydration, which is a chemical combination of cement compounds with water, forming microscopic crystals or gel-like materials with a high surface area. Construction cements, which harden even underwater, are often called hydraulic cements due to their water-absorbing properties. The most important of these is Portland cement.

History of Cement

The origin of hydraulic cement dates back to ancient Greece and Rome. The materials used for producing cement included volcanic ash and lime, which would slowly react with water to form a hard mass. In fact, this cementitious material, Roman mortar and concrete, was used over 2,000 years ago in construction projects in Western Europe. The volcanic ash extracted near the present-day city of Pozzuoli in Italy, which was free from aluminosilicate minerals, led to the creation of the classic Roman pozzolan cement. To this day, the concept of pozzolana refers to the cement itself or any decomposed aluminosilicate that reacts with lime in water to form this construction material.

The term “cement” is also derived from the Latin word “cementum,” which refers to small stones used in Roman mortar, not the substance that causes adhesion itself.

Invention of Portland Cement

Portland cement is a replacement for a type of hydraulic lime. This lime was first developed in 1756 by John Smeaton and was advanced around 1800 in France and England. The invention of Portland cement is attributed to Joseph Aspdin from Leeds, Yorkshire, England. In 1824, he patented a material produced by a synthetic mixture of clay and limestone. He named this product “Portland cement” because, when applied, the material resembled Portland stone, the limestone used for construction in England. The same limestone that was used for construction in England. To obtain true Portland cement, Aspdin’s product had to be baked at a very low temperature. The initial version of it was likely the product that Isaac Charles Johnson produced in the southeastern part of England

around 1850.The production of this building mortar quickly spread across North America and European countries, and throughout the 20th century, it became widespread worldwide. Yes, by the end of 2019, China and India were recognized as the leaders in global cement production. Following them, countries like Vietnam, the United States, and Egypt also emerged as significant producers of cement. The cement industry has grown exponentially over the years, particularly in emerging economies, driven by infrastructure development and urbanization.

Applications of cement

In addition to being used on its own, cement is also used in mortar and concrete along with aggregates, which are inert materials. This building mortar is of great importance because it is the material used in the construction of the foundations of many roads. Cement-based mortar is used to bond building materials such as concrete blocks, tiles, stones, brick facades, pipes, railway tracks, and various extruded products. Additionally, some builders use its hardness to protect and cover certain structures, such as roofs and walls. Moreover, this construction material is popular for making concrete furniture like outdoor tables, chairs, and attractive tables used in interior home decoration.

The raw materials for cement production:

Portland cement is composed of calcium oxide (CaO), combined with silica (silicon dioxide, SiO2), and alumina (aluminum oxide, Al2O3). Lime is obtained from limestone raw materials, while the other oxides are derived from argillaceous (clay) materials. Additional raw materials such as silica sand, iron oxide (Fe2O3), and bauxite – which contains hydrated aluminum (Al(OH)3) – may be used in smaller quantities for processing the desired mix. The most common calcareous raw materials are limestone and gypsum, although other substances like shell or coral deposits can also be used.

Extraction and Processing

The raw materials used in cement production, such as hard rocks like slates, limestone, and some shales, are extracted either by blasting or from underground sources when necessary. Softer materials, such as gypsum and clay, can be directly extracted using mechanical excavators. Then, they are transported to the crusher plant by railway freight wagons, trucks, ropes, or conveyor belts. Alternatively, they may be transported as slurry through pipelines. In areas where limestone with sufficient lime content is not readily available, enrichment processes can be used. Froth flotation removes excess silica or alumina, thus separating the limestone, but this process is costly and is only used when extraction is unavoidable.

Production

Production and manufacturing of this building mortar are carried out in four stages:

1. The raw materials are crushed and ground.
2. The raw materials are properly mixed in the correct proportions.
3. The prepared mixture is fired and melted in a kiln.
4. In this stage, the baked product, called clinker, is ground with approximately 5% gypsum (to control the setting time of the cement).

• Crushing and Grinding

All materials except the soft ones are usually crushed in two stages before being ground. The grinding stage is often carried out in rotary mills or ball mills. Depending on the process used, this grinding can be done either wet or dry. For dry grinding, the raw materials may first need to be dried in rotary dryers. Soft materials are broken down by vigorous mixing with water in wash mills, and a fine slurry is produced, which passes through a sieve to remove larger particles.

• Mixing

Complete mixing of the dry materials in silos is ensured by intense stirring and circulation caused by compressed air. In the wet process, slurry tanks are stirred by mechanical means, compressed air, or both. The slurry, which contains 35

to 45 percent water, is sometimes filtered, reducing its water content to 20 to 30 percent, and then passed through a sieve before being fed into the kiln. This also helps reduce fuel consumption during the firing process.

• Firing

The first kilns where cement was fired in batches were bottle kilns, followed by chamber kilns and then continuous shaft kilns. Interestingly, the shaft kiln in its modern form is still used in some countries, but today, cement is mainly fired in rotary kilns.

These kilns can be up to 200 meters (660 feet) in length and six meters in diameter, and in wet-process plants, they are available in shorter heights. For the dry process, these kilns consist of a steel cylindrical shell lined with refractory materials. They rotate slowly on an axis that is slightly inclined toward the horizontal. The raw materials are introduced at the upper end of the kiln and gradually move downwards. The fuel for the kiln may be pulverized coal, oil, or natural gas, which is injected through pipes. The temperature at the firing end, depending on the raw materials being burned, typically ranges from 1350 to 1550 degrees Celsius (2460 to 2820 degrees Fahrenheit).

A type of heat exchanger is typically installed at the back of the kiln to increase heat transfer to the incoming raw materials and thus reduce the heat loss in the exhaust gases. The burned product exits the kiln as small clinker lumps. The clinkers are then transferred to coolers, where heat is transferred to incoming air, and the product is cooled. The clinker may be immediately ground into cement or stored in warehouses for later use.

In the semi-dry process, the raw materials, in the form of lumps containing 10-15% water, are fed into a shorter rotary kiln designed for the wet process via a moving chain grate. Hot gases coming from the kiln are drawn through the lumps on the grate, pre-heating them.

Modern cement plants are equipped with instrumentation to control the firing process. In some plants, the raw materials are automatically sampled, and a computer calculates and controls the raw mix composition. The largest rotary kilns have outputs of over 5000 tons per day.

• Grinding

The clinker and the required amount of gypsum are ground in horizontal mills, similar to those used for grinding raw materials into fine powder. These materials may pass directly through the mill (open circuit grinding), or coarser materials may be separated from the ground product and returned to the mill for further grinding (closed circuit grinding). The finished cement is pneumatically pumped into storage silos, from where it is either packaged in paper bags or sent in bulk containers for shipment.

Types of Portland Cement

Cement is a versatile material with interchangeable compositions and properties. There are six different types of cement, each with specific behaviors and unique chemical compositions. Below, we take a look at each type and their distinctive features.

• Type I

Ordinary Portland Cement (OPC) is an all-purpose cement without any specific properties. It is not resistant to sulfates and generates a moderate amount of heat during the hydration process.

• Type II

This cement is suitable when moderate sulfate resistance or medium heat of hydration is required. It is particularly useful for foundation works in areas with moderate sulfate levels in groundwater. It typically sets at a slightly slower rate than Type I cement and produces less heat during the hydration process. Let me know if you’d like further refinements!

• Type III

This cement, classified as high-early-strength cement, is finer than Type 1 and has a higher C3S content and sulfate level. It also achieves strength within 7 days. It is useful in situations where forms need to be removed quickly or where early access is required for road surfaces.

• Type IV (SR Cement, Sulfate-Resistant Cement)

This type of cement has very low heat of hydration and gains strength more slowly than Type 2 and Type 1. It is used in soils containing high levels of sulfate/alkali compounds, as well as in groundwater, sewage systems, piers, and coastal platforms.

• Class ‘G’ Oil Well Cement

This is a specialized cement designed for the oil and gas industry, with specific behavioral properties for high-temperature and high-pressure applications. It is highly resistant to sulfates and can be combined with various additives to modify behavior under unique well conditions.

• Blended Cement (Pozzolan)

This is also a general-purpose cement, mainly used for concrete pouring and plastering. Its advantages include: increased long-term strength, improved workability and slump retention, reduced impact of reactive aggregates, decreased risk of cracking due to lower heat of hydration, more durable concrete, and reduced greenhouse gas emissions.

Important Points Before Purchase

Cement is the most important construction material that has made the construction of modern buildings possible. This construction material is widely used in the world of construction. The strength and durability of a structure mainly depend on the strength of its mortar. Naturally, the cost of cement is also higher than other construction materials (coarse aggregates, fine aggregates, etc.) and it affects the overall cost of your project. Nowadays, many cement manufacturers operate in our country. Before purchasing this construction mortar, you should check some basic standard specifications. The information that follows outlines the important points you must pay attention to when buying cement.

• Check the Manufacturing Date

Always check the manufacturing date before purchasing a bag of cement because its strength decreases over time. Ideally, this construction mortar should be used within 90 days from the date of manufacture.

• Check for Trademark, ISI Mark, and Brand Name

To ensure quality, remember to check the bag of cement for a registered trademark and ISI mark. Also, be sure to buy the highest quality cement from a reputable brand. While there are many good cement sellers in Iran, purchase your cement from a trusted seller to ensure a healthy structure.

• Check the Net Weight and Raw Materials

Examine the mortar bag for the correct quantity as printed. To avoid losing cement, do not purchase torn bags. Additionally, check the raw materials before buying.

• Check the Grade of Cement Different

types of structures require different grades of cement. Verify its grade—whether it is OPC 43 grade cement, OPC 53 grade, or the best PPC cement—use the cement that suits your construction needs.

• Check the Grain of Cement

Fresh cement should always be free of moisture and lumps. When rubbed between fingers, it should feel smooth. Thoroughly check the cement for any lumps and do not purchase it if it is clumped, as it may affect the strength of the building.

• Check If Cement Floats

High-quality cement will float in water for some time before sinking. To ensure strong and durable construction, check the quality of cement by pouring a small amount into a bucket of water before mixing for concrete. If you plan to use

OPC 43 grade, OPC 53 grade, or PPC cement, make sure they float in water for a while before sinking.

• Check the Quality

Take a small amount of quality cement, mix it with water to create a thick paste, and spread it on a glass plate. Keep this plate under water for 24 hours. When you take the cement out, if it is of good quality, the concrete will be completely free of any cracks.

• Check the Temperature

When you put your hand into the cement bag, you should feel a specific coolness. If the construction material is cool, its quality is considered satisfactory. However, if your hand feels warm, it indicates poor quality.

Sources:

https://www.britannica.com/technology/cement-building-material/The-major-cements-composition-and-properties