What is cement?

Cement is a fine mixture of limestone, chalk, or marl and clay which when mixed with water, becomes compacted due to the formation of strong molecular bonds. Due to hydration, cement turns into a hard mass when left for a sufficient amount of time upon mixing with water. The water turns the cement compounds into gel-like submicroscopic particles having higher surface areas. It is due to the hydration properties of cement, they are often known as hydraulic cement. For instance, Portland cement is a popular example in this category.

Cement is primarily not used alone but used as a constituent element in mortar and concrete. A mortar is a mixture of cement, sand, and crushed stones of approximately 5 mm in size, whereas concrete is an aggregate mixture of cement, sand, and other fine aggregates of stone, having size ranges of approximately 19 mm to 25 mm. Mortars are primarily used for providing bonds to bricks, blocks, and stones whereas concrete is used for different constructional purposes. Mixing soil with Portland cement is extensively used in road constructions.

Different types of cement

We get different types of cement by adding various ingredients and changing their proportions. These changes and additions make cement perfect for general construction and sulfate-resistant applications like sewage systems. One of five basic types of cement is Portland cement.

• Type 1 cement or Ordinary Portland cement (OPC) is a general-use material.
• Type 2 includes moderate sulfate resistance. There is also an MH variant for use in structures that comes in contact with sulfur in water or soil. This variant is moderately resistant to the heat of hydration. 
• An extra rapid hardening cement or Type 3 cement becomes harder more quickly.
• A low heat cement or Type 4 radiates less warmth as it sets and dries. 
• A highly sulfate resistant or Type 5 cement is used for contact with high alkaline soil and water.
• Types 1A, 2A, and 3A are other variants of type 1, 2, and 3 types of cement. Air-entraining materials are mixed in these to make them resistant to moisture damage.
• Some other types are IL (Portland-limestone), IS (Portland-slag cement), IT (ternary blended), and IP (Portland pozzolana) cement, with hydraulic and special properties. 

Portland cement

Portland cement is a type 1 cement that comes in the form of fine powder whose manufacturing process starts by heating limestone and clay materials in a kiln which forms a clinker. Clinker is the intermediate product that is formed in the generation of cement. Clinkers appear as lumps or nodes having few inches of diameter. The formation of clinker leads to the release of carbon dioxide gas from limestone.

Other materials are added to the clinker followed by the grinding process. This is the basic process of manufacturing Portland. The cement resembles the Portland stone found in the Isle of Portland.

Raw materials used in Portland cement

The basic raw materials which are used in the manufacturing of Portland cement are outlined below:

• Calcareous rocks, also known as Portland-limestone. These are majorly composed of limestone, chalk, and marl.
• Argillaceous rocks such as clay and shale.
• Argillocalcareous rocks such as clayey limestone and clayey marl.

Usually, during the production of Portland cement, any two of the raw materials are used based on suitability but should be mixed with proper proportions of lime, silica, and alumina.

For general construction purposes and most masonry work, we use ordinary Portland cement (OPC).

Apart from the common Portland cement, there are other different types of cement that are well used in building constructions and structural engineering applications, some of the common varieties of cement are discussed below.

Some other types of cement

White cement

White cement or white Portland is very similar to grey Portland cement but vary in degree of fineness and color. White is also commercially known as snowcrete. This cement is very important from the architectural point of view and the fineness and the color is uniform throughout the composition. Some of the key features of white cement are outlined below:

• The raw materials of this cement consist of very little iron oxide and manganese oxide which directly reflect the color of the cement.
• In the manufacturing of white cement, china clay is used along with chalk and limestone.
• Special types of oil from stored oil-well are used as a fuel source for the manufacturing of white cement to avoid contamination by coal ash.

It is used for architectural purposes as it is more expensive. Precast curtain walls, decorative interior and exterior work such as floorings, ornamental concrete, and garden paths are done by using white cement.

Portland-Pozzolana cement

This kind of cement falls under the category of blended cement. This type of cement is manufactured by either of the methods. In the first method, the ordinary Portland cement clinker along with gypsum and pozzolanic materials are thoroughly ground together to form a mixture. In the second method, all the individual components (the ordinary Portland cement, gypsum, and pozzolanic materials) are grounded separately and then certain proportions are mixed.

Air-entraining cement

The cement industry adds special air-entrainment agents to this variety of cement to enhance certain properties. During the manufacture of this category of cement, the cement industry injects air bubbles into the composition which allows space for minute drops of water to be expanded during the freezing and thawing process. This protects the concrete from cracks and damages.

Non-hydraulic cement

Unlike ordinary cement, which hardens due to hydration, a non-hydrating cement hardens when kept dry. Non-hydrating cement does not need to pass through the process of hydration to attain strength. Lime, gypsum, plaster, and oxychloride are the main components of a non-hydrating cement.

Sulfate-resistant cement

In this type of cement, the percentage of tri-calcium aluminate is lower than 5%, which restricts the formation of Sulphate salts which might lead to a Sulphate attack on the concrete. This gives a Sulphate-resisting property to the cement which strongly minimizes the formation of calcium sulpho-aluminate. Calcium sulpho-aluminates are known as the primary damaging compounds of concrete.

Rapid hardening cement

This cement possesses more tri-calcium silicate, special formulations that give the cement a quick compressive strength than other ordinary cement which requires waiting on cement (WOC) time. This category is specially used for repairing concrete cracks that require quick drying and strength.

Water-repelling cement

This kind of cement falls under the category of type 1 cement which contains different additives that induce hydrophobic properties to the cement. This cement is the best for waterproofing repairs but follows a laborious procedure during its manufacturing, hence they are expensive too.

Low heat cement

Low heat cement needs less water to mix and set. There are lower amounts of tricalcium illuminate (around 5%), and higher amounts of dicalcium silicate (around 46%) added in OPC to obtain this variant. It is used for floors and surfaces in chemical plants, dams, wind turbine feet, and large footings and raft slabs of concrete.

Expansive Cement

Expansive cement expands slightly with time rather it does not shrink during and after the time of hardening. It has calcium sulfate, and sometimes tricalcium aluminate added to Portland cement clinker. It is used overcome shrinkage loss, sealing joints grouting anchor bolts, and prestressed concrete ducts.

High-alumina cement

High alumina is prepared by baking ground up bauxite and lime in a kiln, then grinding the clinker again. It is used in marine, sewer construction, and refractory applications. Due to low pH due it is chemical resistant, including sulfuric acid.

The water-cement ratio

The water-cement ratio signifies the weight of water to the weight of cement for the preparation of concrete. A lower ratio leads to low strength, reduces the viscosity of the mix, while a higher ratio induces more strength to the concrete but becomes difficult to work with. Ratios in the range of 0.40 to 0.60 are generally used for most purposes.

Context and Applications

This topic is extensively taught in various undergraduate and postgraduate degree courses of:

• Bachelors of Science in Civil engineering
• Masters of Science in Civil engineering
• Masters of Science in Building construction and materials

Practice Problems

Q1) Which of the following is the primary constituent of Portland cement?

1. Sulfate
2. Types of Cement
3. Limestone
4. All of these

Answer: Option c

Explanation: Limestone forms the primary constituent of Portland cement.

Q2) Which of the following is the damaging chemical for concrete?

1. Sulpho-aluminate
2. Sulfate
3. Blast furnace slag
4. Calcium

Answer: Option a

Explanation: Sulpho-aluminates are the most damaging chemicals for concrete.

Q3) Which of the cement provides a rapid setting and quick compressive strength?

1. Hardening cement
2. Quick-setting cement
3. White cement
4. None of these

Answer: Option b

Explanation: Quick setting cement provides a rapid setting and quick compressive strength.

Q4) Which of the following cement is used for waterproofing?

1. Water-repelling cement
2. Quick-setting cement
3. Air-entraining cement
4. Portland cement

Answer: Option a

Explanation: Water-repelling cement is used for waterproofing purposes.

Q5) Which of the following gas is produced during the formation of clinker?

1. Carbon dioxide
2. Hydrogen
3. Oxygen
4. None of these

Answer: Option a

Explanation: During the preparation of clinker, carbon dioxide gas is produced.