Construction Materials – Cement Questionnaire

When you are about to start the construction work, you may have so many doubts which are not clear to you. This site attempts to address some common questions. If you have any other questions, you are welcome to write to us. Our panel of experts will be happy to provide you with the answers.
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What’s the role of cement in construction?
You use Sand, Jelly, Water, Bricks, Steel and such other material in your construction. Cement binds these elements together to provide strength and durability to your constructions.
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What are the different types of cement?
Cement is generally of two types. Ordinary Portland cement which is available in 33, 43 & 53 grades. The other variety is known as Blended cement which is available as PPC or slag cement.
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What is grade?
Grade is a number that indicates the minimum compressive strength gained by the cement-sand mortar mix in 28 days time.
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What is PPC?
Portland Pozzolana cement is obtained by intergrinding pozzolanic material or slag with clinker and gypsum.
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What are the advantages of using Blended cement?
When water is added, pozzolana material combines with lime and alkalies that are present in cement and forms a compound which contributes to strength. Additionally, it makes the concrete less impermeable and sulphate resistant. It also contributes to the workability, reduces bleeding and controls destructive expansion from alkali aggregate reaction. Leaching of free lime is also reduced.It also reduces heat of hydration, thereby controlling temperature differentials and minimizes the incidence of cracks.
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What is concrete?
Concrete is nothing but a mixture of cement, sand, stone aggregates (jelly) and water.
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What are the different types of mixes used?
Reinforced cement concrete is a mix that is placed around a structure of steel rods, like you see on the roof.  For binding the bricks as well as plastering mortar mix is prepared using cement, sand and water in recommended proportions.
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What are the recommended proportions?
The ratio of cement / sand / aggregates for various types of mixes are as follows:As in Leaflet
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What care should be taken during preparing concrete mixes?
Use only prescribed ratios of various ingredients. Changing the ratios will not yield the desired strength.
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How soon should the prepared concrete or mortar be used?
Once water is added to the mix, the hydration process begins. Normally the mix once prepared, should be used within 45 to 50 minutes.
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What is compacting?
When you mix various material together, there will be air which is entrapped between them. This air has to be removed to make concrete impervious and attain maximum strength. Otherwise there will be voids in the concrete which reduce the strength substantially.
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What is the best way of compacting?
One can use steel rods or paddling sticks, but mechanical vibrators are ideal. Any compacting device has to reach the bottom of the form and should be small enough to pass between reinforcement rods.
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Ideally when should the concrete work be done?
Concreting work should be done preferably during morning or evening time. If concreting work is done during high temperatures, the water that is present in the mix will evaporate affecting the material ratios. Since water is essential for hydration process, avoid concreting in the afternoon time. If unavoidable, cover the concrete with a plastic sheet to reduce evaporation losses.
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What is curing? How is it done?  Normally for how many days should the curing be done?
Curing is nothing but maintaining a favourable environment for moisture retention in concrete or supplying of moisture to the concrete from an external source.For walls, water should be sprinkled from the top and kept wet.For slab or roof, make ponds of 1m x 1m and gently pour water.For Beams and Columns should be kept wet by covering a wet sack over them.Curing should begin just after the surface becomes dry and ideally curing should be done for 21 days.
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What is formwork?
During concreting work you need metal shuttering frame work to support the construction. Shutterings are made either with wood or steel. Shuttering should be placed in such a way that there are no gaps in between. The timeframe for de-shuttering differs depending on the type of work.
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Why cracks appear and what precautions can be taken?
There are many reasons for cracks.

    • Sand contains clay. If clay is not washed and removed, it absorbs water from the concrete mix, leaving lesser water for cement hydration. Therefore wash sand thoroughly and dry it before use. Don’t forget to sieve the sand.
    • When you use bricks which are not properly burnt or are chipped at the edges, they absorb water from the mix. Therefore use good quality bricks which are properly burnt. Soak them in water for 24 hours and use them in wet condition.
    • If the curing process is not started on time or if the curing is not done for a specified period, the mixture faces water stress and causes cracks. Therefore start curing as soon as concrete dries. And continue curing for the prescribed time. Avoid concreting during hot times and cover concrete with plastic to avoid water evaporation.
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What is cement and how is cement made in India?
Cement is a powdery-type substance. It is manufactured using mixture of elements that are found in natural materials such as limestone, clay, sand and/or shale. Materials such as sand, jelly, water, bricks, and steel are used in construction; cement binds these elements together to provide strength and durability to these constructions.