The word CEMENT can simply mean something that joins things, living or inanimate, together in a strong bond which is almost unbreakable.

So much so that we use the word to describe anything that is firm and solid. We use the word in

DENTAL CEMENT

 BONE CEMENT

To a Dentist, cement is a mixture of bonding substances that are placed in the mouth as a viscous liquid and set to a hard mass, perfect for filling cavities.

To an Orthopaedic Surgeon, cement refers to Bone Cement (Polymethyl methacrylate or PMMA) which they use for fixing implants. And to an Engineer and to ordinary folk, CEMENT is simply a construction material.

Read on, to cement your knowledge about this material, which is ubiquitous to all types of construction.

THE HISTORY of CEMENT

Cross-section of a typical Roman cement casting

Hydraulic cement made with lime and volcanic ash which set into a hard mass when mixed with water was the cementing material of the Roman mortars and concretes of 2,000 years ago. The term CEMENT meanwhile, derives from the Latin word CAEMENTUM which meant stone chippings such as were used in Roman mortar—not the binding material itself.

THE CHEMISTRY of CEMENT

Water and cement initially form a cement paste that begins to react and harden (set). This paste binds the aggregate particles through the chemical process of hydration.

Cement does not set by ‘drying out’. On the contrary, appropriate moisture content has to be maintained during the ‘curing phase’ of the setting process. Keeping the surface wet for at least seven days is the common practice.

Improperly cured cement mortar or cement concrete, turns out significantly weaker and is not desirable.

MODERN CEMENT

There are several types of cement (some for special uses), but we will only explore the two types which are used in general construction - Ordinary Portland Cement (OPC) and Portland Pozzolona Cement (PPC)

OPC

OPC was manufactured by burning and grinding a mixture of limestone and clay or limestone and shale. The inventor Joseph Aspdin, of England, patented the basic process in 1824, naming it for the resemblance of the cement when set, to portland stone, a limestone from the Isle of Portland in the English Channel.

OPC is now manufactured by crushing and grinding argillaceous (containing clay) and calcareous (containing Calcium Carbonate) materials which are burnt in a rotary kiln. The ‘clinker’ obtained is cooled and ground with gypsum to the required fineness (see the process chart).

PPC

Volcanic ash mined near what is now the city of Pozzuoli, Italy, was particularly rich in essential aluminosilicate minerals, giving rise to the classic pozzolana cement of the Roman era. To this day the term pozzolana, or pozzolan, refers either to the cement itself or to any finely divided aluminosilicate that reacts with lime in water to form cement.

Modern PPC is a type of Portland cement characterized by the presence of pozzolana particles like fly ash which are added to OPC in the ratio of 15% to 35%. Some of these above-mentioned materials are industrial by-products from other processes but they and are ideal in producing PPC Cement.

Cement Manufacturing Process Chart

WHAT CEMENT? WHAT GRADE?

Being bombarded with advertisements of cement which is as strong as an elephant, which have ‘bonding particles’ or with water repellent properties and so on, we are bound to be confused or even misled.

I advise you to take all the ‘hard’ selling (pun intended) with a pinch of salt or better still with some good old Rajnikanth humour.

Once upon a time Rajnikant used tooth powder to get strong teeth …

Today that powder is known as XXX CEMENT!!!

Cement Grades, denoted 33, 43, 53 etc. is a reference to the compressive strength (in Newtons per Sq mm) attained by the cement on the 28th day of casting. Grade is stated only for OPC.

PPC being a blended cement does not have a stated grade. However, the type of pozzolanic material used (say fly ash) is printed on the bag. It is best to use the cement within three months of its manufacture

Choosing a well-known brand assures you of consistent quality, but watch out for spurious cement as well. Check if the manufacturer’s name and registered trademark are properly printed.

Reference IS code number should be printed on the cement bag as well. For example, if you are buying OPC 53 grade cement, then ‘IS-12269-1987’ should be printed and for PPC, ‘IS 1489-1991’ should be printed on the bags.

            Good Cement is smooth and fine to the touch and free of lumps

PPC

Use : Best choice for mass concreting, plastering and masonry because of low heat of hydration

Setting Time: Setting time   of PPC is higher than OPC. Its slower setting time gives better workability

Cost: Less expensive

Strength: It has a   higher strength than OPC in the long term, better resistance to chemicals

OPC

Use : OPC is better for reinforced concrete columns but not ideal for mass concreting due to its high heat of hydration.  

Setting Time: Lower Setting time of OPC allows faster removal of formwork

Cost: More Expensive

Strength: Initial strength is higher than PPC, less resistant to chemicals

The thumb rule for building construction is, use OPC for elements like RCC columns and beams or other applications where there is a need to develop strength quickly.

Use PPC for all other applications.

Cement is just one (vital nevertheless) component of concrete or of mortar, both widely used in construction.

We will explore all that and more in the upcoming articles.

As the American inventor R. G. Le TOURNEAU said, The pebbles of knowledge must be bonded together by the CEMENT of experience.

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