Cement Industry

Cement Industry

INTRODUCTION

Cement is broadly described as material having adhesive and cohesive property with capacity to bond the material like stone, bricks, building blocks etc. Cement is a binder, a substance that sets and hardens independently, and can bind other materials together. Cements are inorganic material that shows the cementing properties of setting and hardening when mixed with water. Cement is prepared from calcareous (Ca) material and argillaceous (Al + Si) material.

Cement has property of setting and hardening under water by virtue of chemical reaction of hydrolysis and hydration. Therefore, cements are generally divided into two types hydraulic and non-hydraulic that is on the basis of their setting and hardening pattern. Hydraulic cements harden because of hydration, chemical reactions that occur independently of the mixture’s water content; they can harden even underwater or when constantly exposed to wet weather. The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water-soluble. Non-hydraulic cements must be kept dry in order to retain their strength. Portland cement is example of hydraulic cement material while ordinary lime and gypsum plaster are consider as example of nonhydraulic cement.

Cement is used for structural construction like buildings, roads, bridges, dam etc. The most important use is the production of mortar and concrete the bonding of natural or artificial aggregates to form a strong building material that is durable in the face of normal environmental effects.

Both cement and concrete are different, because the term cement refers to the material used to bind the aggregate materials of concrete. Concrete is a combination of a cement and aggregate.

In the last couple of decades of eighteenth century, modern hydraulic cements began to be developed due to fulfill following requirement

• * For finishing brick buildings in wet climates
• * Development of strong concretes
• * Hydraulic mortars for masonry construction of harbor works, etc., in contact with sea water

As the good quality building stone became expensive and construction of prestige buildings from the new industrial bricks, and to finish them with a stucco to imitate stone became the common practice. Hydraulic lime was favored for this, but the need for a fast set time encouraged the development of new cements.

James Parker developed cement from clay minerals and calcium carbonate and patented as Roman cement in 1796. It was made into a mortar with sand, set in 5–15 minutes. The success of “Roman Cement” led other manufacturers to develop competing products by burning artificial mixtures of clay and chalk.

In the first decade of nineteenth century, it was proved that the “hydraulicity” of the lime was directly related to the clay content of the limestone from which it was made first by John Smeaton and then by Louis Vicat. Vicat produce artificial cement by burning of chalk and clay into an intimate mixture in 1817. Also, James Frost produced “British cement” in a similar manner around the same time, and patented in 1822. At the same time Portland cement, was patented by Joseph Aspdin in 1824.

“Setting time” and “early strength” are important characteristics of cements. Hydraulic lime, “natural” cements, and “artificial” cements all rely upon their belite content for strength development. Belite develops strength slowly. Because they were burned at temperatures below 1250°C, they contained no alite, which is responsible for early strength in modern cements. In early 1840s the first cement to consistently contain alite was made by William, who is son of Joseph Aspdin. This was what we call today “modern” Portland cement. Vicat is responsible for establishing the chemical basis of these cements, and Johnson established the importance of sintering the mix in the kiln.

William Aspdin’s innovation has high manufacturing costs but the product set reasonably slowly and developed strength quickly, thus opening up a market for use in concrete. The use of concrete in construction grew rapidly from 1850 onward, and was soon the dominant use for cements. Thus Portland cement began its predominant role.

But in the early 1930s it was discovered that, Portland cement had a faster setting time it was not durable especially for highways. These leads to development of some specialty cement based on the application and requirement of strength and setting time.

CLASSIFICATION

Based on source of cement

Natural cement

Artificial cement

1) Natural cement

Natural cement is obtained by burning and crushing of 20-40{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} clay, carbonate of lime and small amount of magnesium carbonate. It is brown in colour and best variety known as Roman cement. The natural cement resembles very costly element hydraulic lime and sets very quickly and strongly as compare to artificial cement. It finds very limited application

2) Artificial cement

Artificial cement is obtained by burning of calcareous mixture at very high temperature. Mixture of ingredients should be intimate and they should be in correct proportion. Calcined product is known as Clinker. A small quantity of gypsum added to clinker and pulverized to fine powder is known as cement or ordinary cement or normal setting cement. After setting, this cement closely a variety of sandstone which is found in abundance in Portland in UK. Therefore, it is also known as Portland cement.