What does cement manufacturing involve?

Cement has been used for thousands of years, dating back to ancient Rome and Egypt. Today, it is a critical material in the construction industry, and the global demand for cement is high. The process of manufacturing of cement is a crucial one in the construction industry as it is a fundamental ingredient in the production of concrete, which is used in various structures ranging from homes and buildings to large infrastructure projects such as bridges and highways. The production of cement involves six critical manufacturing stages, and each step in this process contributes significantly to the quality and durability of the final product.

Source: Pinterest

See also: Testing Cement: Importance of testing and types of cement test

Stages of cement manufacturing

The production of cement involves six crucial stages in its manufacturing process that are:

1. Raw material extraction

To produce cement, various raw materials are required, including limestone (which is rich in calcium), sand, clay (which contains silicon, aluminium, and iron), fly ash, shale, bauxite, and mill scale. These materials are extracted by quarrying and then crushed into smaller pieces, typically around 6 inches in size. The smaller pieces are then reduced further by secondary crushers or hammer mills until they are about 3 inches in size. Once the raw materials are prepared, they are then ready for the crucial pyroprocessing stage, which plays a significant role in the chemical transformation of these materials into cement.

2. Grinding, proportioning, and blending

Before the raw materials can be transformed into cement, they need to be processed in the kiln. The first step in this process involves combining the crushed raw ingredients with additives and grinding them into a fine, homogenous mixture. The composition of the cement mixture is carefully proportioned at this stage to ensure that the final product has the desired properties. Typically, the mixture is made up of 80% limestone and 20% clay. Once the mixture is prepared, it is dried in the cement plant to reduce its moisture content to less than 1%. To achieve a consistent blend, heavy wheel-type rollers and rotating tables are used to blend the raw mix, which is then crushed into a fine powder by rollers before being stored in silos. This fine powder is then fed to the kiln, where the critical process of pyroprocessing takes place.

3. Pre-heating raw material

In order to reduce energy consumption and make the cement-making process more sustainable, a pre-heating chamber is used in the manufacturing process. The chamber comprises several cyclones that utilise the hot gases produced by the kiln to preheat the raw materials before they are introduced into the kiln. This process helps to reduce the energy requirements of the kiln, making the overall production process more efficient and environmentally friendly. During this preheating stage, the raw materials are transformed into oxides, which are then ready to be burned in the kiln.

4. Kiln phase

The kiln phase is perhaps the most important stage of the cement production process. It is during this phase that the raw mix is transformed into a clinker through a series of complex chemical reactions between calcium and silicon dioxide compounds. The events of the clinker production process can be broken down into the following sequence:

First, the free water in the mixture is evaporated. Next, the combined water in the argillaceous components is released through a process called evolution. Then, the calcium carbonate (CaCO3) in the mixture is calcined to calcium oxide (CaO). The reaction of CaO with silica results in the formation of dicalcium silicate. The reaction of CaO with the aluminium and iron-bearing constituents produces the liquid phase. The clinker nodules are formed as a result of this process. Volatile constituents, such as sodium, potassium, chlorides, and sulphates, are then evaporated. Finally, the excess CaO reacts with dicalcium silicate to form tricalcium silicate.

This series of reactions is critical to the formation of the clinker, which is an essential ingredient in the production of cement.

5. Cooling and final grinding

Once the clinker exits the kiln, it undergoes a rapid cooling process to reach temperatures between 100°C to 200°C by passing air over it. Following this, a variety of additives are combined with the clinker and ground to produce cement, the final product. Gypsum, which is added and ground with clinker, regulates the setting time of cement and is responsible for its most important property, compressive strength. Grinding aids, such as Triethanolamine, are also added to avoid powder agglomeration. In some cases, ethylene glycol, oleic acid, and dodecyl-benzene sulphonate may be used as additional additives. The heat produced during the clinker process is then recycled and utilised to save energy. Finally, in the last stage of cement production, the clinker is transferred to rotating drums with steel balls and ground into an incredibly fine powder, with each pound containing up to 150 billion grains, resulting in the final product, cement.

6. Packing and shipping

The final step in the cement production process is the transportation and packaging of the finished product. Once the grinding process is complete, the cement is conveyed to large storage tanks known as silos, where it awaits packaging and distribution. Typically, the product is transported in bulk by trucks, trains, or ships, with only a small percentage of the cement being packed into smaller quantities for customers who require it. This allows for the efficient delivery of large quantities of cement to construction sites and other destinations worldwide.

Cement testing

Cement testing is a crucial aspect of ensuring the quality and performance of construction materials. The evaluation process involves several key parameters laid down in national specifications. These specifications govern critical aspects like:

Fineness

While sieve tests were traditionally employed for assessing fineness, modern methods are more sophisticated. The prevalent method involves determining the surface area per unit weight of the cement by measuring the rate of air passage through a cement bed. Additional methods include measuring particle size distribution through sedimentation in kerosene or elutriation in an airstream.

Soundness

Soundness is a vital property ensuring that cement does not undergo significant expansion after setting, which could compromise mortar or concrete integrity. Testing involves subjecting set cement to boiling water or high-pressure steam to detect any unsoundness arising from excess free magnesia or hard-burned free lime.

Setting Time

Cement setting and hardening are continuous processes, but two distinct points are considered for testing. The initial setting time signifies when the mix loses plasticity, while the final setting time denotes when the set cement achieves sufficient firmness to resist defined pressure. These times are critical for construction timelines and are subject to specified limits in various standards.

Strength

Strength is a key indicator of cement performance. Testing involves assessing the rate at which cement develops strength, typically done on mortar or concrete specimens. Various methods, such as tensile, compressive, or transverse tests, are employed depending on international standards like ASTM, BSI, and ISO. These tests ensure the cement’s ability to withstand defined pressures and contribute to the structural integrity of the final construction.

FAQs