Types of gray cement

According to chemical properties gray cement can be classified as follows.

• Ordinary Portland cement (for general use)
• Rapid-hardening Portland cement (where high early strength is required)
• Low heat Portland cement (in mass concrete for dam etc.)

Manufacturing of gray cement

Specifications of gray cement

Fineness

• Method 1: After sieving, the cement on IS test Sieve No.9 (B.S. Sieve No 170) should not      exceed 10% in case of Ordinary Portland Cement.
• Method 2: Specific surface by Air Permeability Method should not be less than 2150 sq. cm/  kg in case of Ordinary Portland Cement

Soundness

Setting time

• Initial setting time Not less than 30 minutes
• Final setting time: Not more than 600 minutes i.e. 10 hours

Compressive Strength

Compressive strength of cement concrete mix

S. NO.	Type of Mix	Ratio of Mix	Compressive strength
1.	M 10	1:3:6	100 kg. Per cm square
2.	M 15	1:2:4	150 kg. Per cm square
3.	M 20	1:1.5:3	200 kg. Per cm square

General guidelines for gray cement

Cement is the most important raw material used in construction of a building. You can ensure the quality of ‘Cement’ with following guidelines.

• Cement bag bears date of manufacturing
• One cement bag has 50 Kg. weights
• Cement bags are not hand stitched
• Consignment must have identification mark on the package
• Cement is packed in bags of synthetic jute or polypropylene bags
• Factory manufactured instead of plant manufactured
• Bears manufacturer's name or his trade mark , grade and type of cement
• Not older than six weeks from the date of manufacturing
• Not partially set due to moisture or have small lumps in bags
• Not pressed or have lumps due to high pressure of stack, having more than 10 to 12 bags
• The bags are not torn by side and stitched in later

Precautions while transporting cement

• Carry bags in clean vehicle which is not dustier or on earth etc. as it reduces the strength of cement.
• Take cement bags under covered Polythene or ‘Tarpaulin’ during monsoon season.
• Labor should not tear bags while loading and unloading cement bags.

Precautions while stacking/storing gray cement

• Store cement in moist proof area otherwise quality of cement will be affected
• Stack cement bags in dry, leak proof and moisture proof shed/fumes
• Place cement bags on dry brick floors, wooden crates or planks
• Don’t place cement bags on the earth in any case
• Don’t make stack of bags higher than 10 bags to avoid lumps under pressure
• Don’t stack up cement bags with outer walls to protect it from dampness
• Pile up cement bags from different manufacturer separately
• Make use of cement on first come first serve basis
• Cover cement bags with polythene during monsoon season
• Keep cement bags close to each other to reduce air circulation

Caution for storage/stacking

• When construction work is assigned to a contractor, make sure he takes proper care of cement. Many times contractors do not take care for proper storage of cement at site as it costs them labor and money.  This results in reduction in strength of cement as shown in the table below.
• In some cases, it is observed that after laying RCC slab on some parts of building, the contractor dismantles his temporary store at site and places cement bags on ground floor of building which is under construction. It is not good as cement bags may draw moisture from floor and walls of new building and results in reduction of strength in cement because new building is wet due to construction work in progress.

Reduction in cement strength due to storage

S. No	Storage Period	Reduction in Strength
1.	Fresh cement	Nil
2.	Three months old	20%
3.	Six months old	30%
4.	12 Months old	40%
5.	24 Months old	50%

Caution for cement strength

• Cement retards in hardening and reduces strength due to absorption of moisture during storage.
• It is okay for cement to absorb up to 1.2% moisture as this level of moisture is acceptable but if absorption exceeds 5%, the cement is destroyed for all practical purposes.
• Different types of cements should not be mixed while using them for constructional purposes.