CEMENT

Portland cement is the basic ingredient of concrete.

Concrete is formed when portland cement creates a paste with water that binds with sand and rock to harden.

Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients.

Common materials used to manufacture cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore.

These ingredients, when heated at high temperatures form a rock-like substance that is ground into the fine powder that we commonly think of as cement.

Bricklayer Joseph Aspdin of Leeds, England first made portland cement early in the 19th century by burning powdered limestone and clay in his kitchen stove.

With this crude method, he laid the foundation for an industry that annually processes literally mountains of limestone, clay, cement rock, and other materials

into a powder so fine it will pass through a sieve capable of holding water.

Cement plant laboratories check each step in the manufacture of portland cement by frequent chemical and physical tests.

The labs also analyze and test the finished product to ensure that it complies with all industry specifications.

The most common way to manufacture portland cement is through a dry method.

The first step is to quarry the principal raw materials, mainly limestone, clay, and other materials.

After quarrying the rock is crushed.

This involves several stages. The first crushing reduces the rock to a maximum size of about 6 inches.

The rock then goes to secondary crushers or hammer mills for reduction to about 3 inches or smaller.

The crushed rock is combined with other ingredients such as iron ore or fly ash and ground, mixed, and fed to a cement kiln.

The cement kiln heats all the ingredients to about 2,700 degrees Fahrenheit in huge cylindrical steel rotary kilns lined with special firebrick.

Kilns are frequently as much as 12 feet in diameter—large enough to accommodate an automobile and longer in many instances than the height of a 40-story building.

The large kilns are mounted with the axis inclined slightly from the horizontal.

The finely ground raw material or the slurry is fed into the higher end.

At the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil, alternative fuels, or gas under forced draft.

As the material moves through the kiln, certain elements are driven off in the form of gases.

The remaining elements unite to form a new substance called clinker. Clinker comes out of the kiln as grey balls, about the size of marbles.

Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in various types of coolers.

The heated air from the coolers is returned to the kilns, a process that saves fuel and increases burning efficiency.

After the clinker is cooled, cement plants grind it and mix it with small amounts of gypsum and limestone.

Cement is so fine that 1 pound of cement contains 150 billion grains.

The cement is now ready for transport to ready-mix concrete companies to be used in a variety of construction projects.

Although the dry process is the most modern and popular way to manufacture cement, some kilns in the United States use a wet process.

The two processes are essentially alike except in the wet process, the raw materials are ground with water before being fed into the kiln.

WHAT ARE THE LABORATORY TESTS FOR CEMENT ?

1. Fineness test of cement
2. Standard Consistency test of cement
3. Initial Setting and Final Setting test of cement
4. Soundness test
5. Compressive Strength test of cement

1. FINENESS TEST OF CEMENT 

This test of cement is performed to check the fineness of cement according to standard specifications.

The fineness of cement can be measured either by the grain size of cement or by the surface area of cement.

The sieve Test (IS 4031- part-I) – 90 µ size sieve.

The fineness of cement has a significant effect on the hydration and in increasing the rate of gain strength.  The strength of cement is directly proportional to its fineness.

2. STANDARD CONSISTENCY TEST OF CEMENT 

It is used to find out the percentage of water required to produce cement paste of standard consistency.

It is also sometimes called as Normal Consistency (CPNC).

The Standard consistency of a cement paste is defined as  that consistency which will permit a Vicat’s apparatus plunger having10mm dia. and

50mm length to penetrate to a depth of 33-35 mm from the top of the mould.

3. INITIAL SETTING AND FINAL SETTING TESTS OF CEMENT 

Initial Setting time-

When the paste will start loosing its plasticity, the needle will penetrate only to some depth.

The period elapsing between the time when water is added to the cement and to the time at which the needle penetrates the test block to a depth

equal to 33-35mm from the top is taken as initial time.

For OPC it is generally taken as 30min.

Final Setting Time
The cement is considered fully set when the centre needle makes an impression while the annular attachment fails to do so.

The cement is said to be hard if it does not pierce more than 0.5mm.

It is generally taken as 10 hrs.

4. SOUNDNESS TEST 

The Cement is said to be unsound if it has excess of lime.

Due to high proportions of magnesium content or

Calcium sulphate content.

Because of inadequate burning of cement.

Due to insufficiency in fineness of grinding or thorough mixing of raw materials.

It cause appreciable change in the volume of cement after the cement has set causing disruption of the set and hardened mass.

Le-Chatlier’s Apparatus is used for the test.

In case the expansion is more than 10mm than the cement is said to be unsound.

5. COMPRESSIVE STRENGTH TEST OF CEMENT

It is Laboratory method to determine the strength of cement.

The size of cube mould is 70.6mm.

Cement and standard sand mortar is used to make the cube.

Three cubes are tested for the strength.

The average value is taken for the compressive strength of the three cubes for each period respectively.

Understanding how to evaluate the quality of cement is crucial before incorporating it into your construction.

Cement is a fundamental construction material globally, serving as a binding agent for concrete and mortar.

As we unravel the intricacies of cement quality assessment, consider registering to receive regular construction insights.

Now, let’s delve into the process of checking cement quality and its importance.

Understanding Cement Composition

Cement, commonly found in grey powdery form within sturdy paper bags, is a composition of various materials.

Primary components include limestone, sand, iron ore, and bauxite. Additional elements like chalk, clay, and slate can also be part of its composition.

These raw materials undergo processing and are subjected to high temperatures around 1500 degrees celsius.

The resulting fine grey powder is then packaged and sold in the market.

The significance of cement quality check

Checking cement quality is a pivotal step for builders and masons to ensure the structural integrity of constructions. While these checks might not encompass all cement properties, they offer insights into its strength and overall quality. Fortunately, you don’t need to be an expert to conduct these tests. Here are some straightforward field tests that anyone can perform to ascertain cement quality:

1. Manufacturing date check:
Inspect the manufacturing date printed on the cement bag, typically located on the back or side.

It’s crucial to use cement within 90 days from its manufacturing date. Cement’s strength can diminish over time due to moisture absorption.

Freshly produced cement has a high affinity for moisture in the air, which accelerates its chemical reaction called “Hydration.”

using older cement can result in reduced binding effectiveness and overall structural strength.

3 months: The strength of cement can typically decrease by around 20% to 30% from its initial strength during the first three months of its manufacturing date.

6 months: By the six-month mark, the strength reduction becomes more pronounced, ranging from 30% to 40% of its original strength.

12 months: After a year, the ageing process further affects the cement’s strength, leading to a reduction of approximately 40% to 50% from its initial strength.

2. Colour examination
Quality cement possesses a uniform grey colour with a faint greenish tint. This colour is an indicator of the burning process and composition.

Dark greenish shades could suggest an excessive amount of lime or clay, affecting the cement’s performance and setting characteristics.

A consistent, light grey hue is generally an indication of well-processed and high-quality cement.

3. Lumps inspection
Upon opening the cement bag, carefully examine its contents for any lumps. The presence of lumps suggests that the cement has come into contact with moisture.

A bag with lumps should be rejected, as it might compromise the construction’s integrity.

4. Cement float test
Take a handful of cement and gently release it into a bucket of water. Good-quality cement will initially float on the water’s surface before eventually sinking.

Impure or low-quality cement, on the other hand, will rapidly sink due to the presence of heavy impurities.

This test assesses the density and impurity level of the cement.

5. Adulteration test
Rub a small amount of cement between your fingers. Quality cement should feel smooth and fine to the touch.

If you feel grittiness or roughness, it could indicate adulteration with excessive amounts of sand or other impurities.

A smooth texture suggests that the cement is well-processed and has a consistent composition.

6. Cement temperature test
Insert your clean and dry hand into the cement bag, and touch the cement powder.

If it feels cool to the touch, it suggests that the cement hasn’t undergone hydration due to moisture exposure.

However, if the cement feels warm, it indicates that hydration has started, which may lead to reduced strength and performance.

Opt for cement that feels cool, indicating its freshness and quality.

7. Cement strength test
For this test, create a cement block measuring 25mm x 25mm x 200mm and immerse it in water for seven days.

After the immersion period, place the block on supports about 15 cm apart and apply a weight of around 34 kg on top of the block.

A good-quality cement block should remain intact and show no signs of cracks or failure under this load, indicating its strength and durability.

8. Cement odour test
While being cautious, smell the cement to detect any unusual odours.

Quality cement should be odourless. If you detect an earthy smell, it could be an indication of excess clay and sand in the cement composition.

An abnormal odour might imply a compromised mixture, affecting the final construction quality.

9. Cement setting test
Prepare a thick paste of cement and water, resembling a cake-like structure, and place it on a glass plate. Submerge the glass plate in water for 24 hours.

After this period, examine the cement cake for cracks or deformities

. A well-structured, crack-free cake signifies that the cement has a consistent setting quality and will perform well in construction.

10. Consistency test
In this test, mix cement with water to form a thick paste. Spread the paste onto a glass plate to create a uniform, flat shape.

After 24 hours, observe the consistency and shape of the cement.

A uniform and consistent shape indicates that the cement maintains its desired properties during the setting process, ensuring a high-quality construction outcome.

Final word

Checking cement quality on-site during preliminary inspection is essential.

While these tests provide a rough idea of cement quality, not all engineering properties can be assessed through them.

Remember, the proper quality of cement contributes to the durability and strength of your construction project.

Whether you’re building your dream home or embarking on a renovation journey, these simple field tests can empower you to make informed decisions about

the cement you use, ensuring your project stands the test of time.