Construction Cost Estimating


What is Portland Cement and How is it Used in Construction

What is Portland Cement

The Portland Cement is a particular type of cementing material used in building construction. It is essentially an amalgamation of clay and chalk. This blend, when subjected to water, hardens up and when it is hard, mimics the portland stone in color. The Portland Stone is found in quarries in Portland, Dorset in England initially. This type of hydraulic cement was patented in 1824.

The portland cement is exceptional in giving strength to structural properties. Most commonly, it is used in making concrete. However, the portland cement can also be directly used in creating stucco or be used as a mortar. Some non-specialty grout also uses this type of cement as one of the main ingredients.

ASTM 150 defines the Portland Cement as ?hydraulic cement (cement that not only hardens by reacting with water but also forms a water-resistant product) produced by pulverizing clinkers which consist essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an inter ground addition.?

How is Portland Cement Made

Usually created from heating limestone and clay minerals in a kiln, portland cement needs a little bit of gypsum in it to give it the desired setting qualities and to prevent flash setting. The chemical constituents of portland cement and their ratio are as follows:

1. Calcium oxide (61-67%)
2. Silicon dioxide (19-23%)
3. Aluminum oxide (2.5?6%)
4. Ferric oxide (0?6%)
5. Sulfur oxide (1.5?4.5%)
6. Gypsum (2-10%)

The Mix is generally a thin, light powder. Depending upon the ratio of materials in the clinker and the cement, this powder can be gray, white or something in-between. The most common form, called the OPC, displays a soft gray color. After hardening, it resembles the portland stones found in Dorset.

How does the Portland Cement Set

When the portland cement is used in construction, it needs to be mixed with an appropriate amount of water set it. The hardening process is basically a set of complicated chain reactions, resulting in forming crystallization of the constituents. The interlocking of these crystals is what gives the concrete its strength. After the initial setting, warm water can be used to speed up the hardening. However, it needs to be seen that the paste does not flash-set or quick-set in some areas. That will cause maldistribution of tensile strength and the structure may crack under load. This is why the gypsum is added, generally only 5%, so that the cement hardens uniformly.

Uses of Portland Cement

Production of concrete is the most common use of portland cement. A composite of gravel, sand, cement, and water, concrete is the prime building material these days. Since it is a paste, to begin with, concrete can be cast in any shape and form and once it hardens it can bear immense loads. This gives concrete the ability to form structural support elements like columns, piers, piles, beams, sections, etc. It takes only a few hours (six hours on average) to set the water/cement paste, and then it hardens over weeks as long as the set structure is kept hydrated. Depending upon the mix of the product and for how long it was watered, it can develop massive compressive strength over time.

Portland cement can also be used to form mortar, which is comprised of sand, cement, and water only. It can also be mixed with water making grouts that can be injected into gaps or crevices of a structure to increase its consolidation. The portland cement is also applied in plastering and screeds.

Types of Portland Cement

According to ASTM C150 standards, there are five types of Portland Cements, as described below.

Type I: This is the most commonly used portland cement when nothing else is specifically mentioned. It is best to be used in building precast or prestressed sections that may not come into contact with water or soil.

Type II: This type of portland cement is most favored in North America because of its resistance to ground or water sulfates. The cement mix has a low amount of aluminates which strengthens it against sulfur-related corrosive effects. This makes the products of this cement very good to be used in contact with soil or water.

Type III: Almost similar in composition to Type I, this type of portland cement is ground much thinner and that gives it a much faster strength aggregation. The amount of compressive strength gathered by the types I & II in four weeks can be achieved by Type III in one week only. However, that makes the concrete produced by this cement slightly more brittle, sacrificing longe-term strength. This makes the type 3 portland cement best usable in construction that needs to be done fast, like in emergencies or in producing precast concrete.

Type IV: This is the complete opposite of type III. With a higher amount of silicates and ferrites and a lower ratio of aluminates, this type of portland cement sets and strengthens much slowly but in the end gives vastly better load-bearing strength. The type 4 portland cement is not commonly manufactured or stocked, so this needs to be specially ordered. The application of the concrete made from this sort of cement is in building very large constructions with low surface-to-volume ratios. However, with the advancement in construction chemistry, type IV is becoming obsolete.

Type V: A much better sulfur-resistant cement than the type 2, the Type 5 portland cement sees building uses in rather badly alkaline soil and sulfurated waters. This type is mostly used in Western USA and Canada. Again, this type is being gradually replaced with other advanced tertiary blends containing fly ash and slag.

White Portland Cement: The white cement is in properties very close to the common grey portland cement, but with much less ferric contents which gives it the fair color. Other types of chemicals may also be used to increase the whiteness of the cement or to change its color altogether. Since this requires mining much purer raw materials and different methods in production, the white cement is much costlier and is mostly used for its aesthetic value only.

Disadvantages of Portland Cement

Because of the nature of the chemicals that are used to make portland cement, the resultant composition is quite caustic. This portland cement mix can cause chemical burns or skin irritation. If inhaled, the fine light powder may cause lung cancer over prolonged exposure to it. This is why proper precautions and protection must be taken while producing the portland cement in factories or when using it in concrete or in construction.

To be kept in mind that the production process of portland cement is highly polluting and it causes a lot of greenhouse gas emission. Even 10% of the total world CO2 emission is contributed by portland cement manufacturing.

However, Portland Cement has always been favored in choosing construction materials because of its low cost, versatility and widespread availability of base materials such as limestone, shales and other natural minerals. The world?s most amount of concrete is produced from Portland Cement.

What is Portland Cement and How is it Used in Construction