Cement: how to use it, and how much you need

Cement is the binder in plaster, brickwork and blockwork mortar, RCC slabs, beams, columns and footings, and the base course under flooring. The same 50 kg bag does all of it in a Kanpur house — only the mix ratio and the grade change. It is not a finish and not a waterproofer: cement plaster still needs putty or paint over it, and a leaking roof or damp wall needs a proper waterproofing system, not a richer cement mix.

What you can do with it, and how much it takes

Plastering a wall

150 sq ft per 50 kg bag, 12 mm plaster in 1:6

Source: PWD Analysis of Rates 2018, Government of Arunachal Pradesh — Ch. 13 Finishing item 4 (E0904) read with Ch. 3 Mortar item 6 (E0007)

Brickwork

1.1 bags of cement per 35 cft of brickwork in 1:6 mortar

Source: PWD Analysis of Rates 2018, Government of Arunachal Pradesh — Ch. 6 Brick Work item 4 (E0289: 0.22 cu.m mortar per cu.m) with Ch. 3 Mortar item 6 (E0007: 0.25 t cement per cu.m mortar)

Concrete slab (M20)

8 bags of cement per 35 cft of M20 concrete (1:1.5:3)

Source: PWD Analysis of Rates 2018, Government of Arunachal Pradesh — Ch. 4 Concrete Work items 8-9 (E0136-E0138: cement concrete 1:1.5:3, 0.40 tonne cement per cu.m)

Floor bed / PCC (1:2:4)

6.4 bags of cement per 35 cft of 1:2:4 concrete

Source: PWD Analysis of Rates 2018, Government of Arunachal Pradesh — Ch. 4 Concrete Work item 7 (E0134-E0135: cement concrete 1:2:4, 0.32 tonne cement per cu.m)

How to use it

Wet the brickwork the evening before and again in the morning — dry brick pulls water out of the mortar and the plaster cracks in a week. Fix level dots, then batch with a farma (gauge box), not by spade count: one 50 kg bag fills roughly 1.25 cft, so one box of cement to six boxes of sand gives a true 1:6. Mix only what you will use in 30 minutes, lay 12 mm in one pass on internal walls, level with the aluminium patti between the dots, and finish with the thapi. Never add water to mortar that has started to stiffen — re-tempered mortar loses strength permanently. Start wet curing the next morning and keep the surface damp for seven days; IS 456 requires not less than 10 days in dry hot weather, which is most of April to June in Kanpur.

Tools you'll need

Farma (gauge box) for batchingask us to source it
Karni (mason's trowel)ask us to source it
Thapi (wooden float)ask us to source it
Aluminium patti (6 ft straight edge)ask us to source it
Ghamela / tasla (steel pan)ask us to source it
Phawda (mixing spade)ask us to source it
Sand sieve (chalni)ask us to source it
Spirit level, plumb bob and dori setask us to source it
Curing hose or pressure sprayerask us to source it
Plumb bob, spirit level and dori setask us to source it
Needle vibratorask us to source it
Tamping rod / plate compactorask us to source it

Related material

Fine sand (plaster grade)ask us to source it
Integral waterproofing compound (Dr Fixit Pidiproof LW+ or equivalent)ask us to source it
Gunny bags / hessian cloth for curingask us to source it
Chicken mesh (GI jaali) for junction stripsask us to source it
Red clay bricksask us to source it
Fine sand for the mortarask us to source it
20mm gitti (coarse aggregate)ask us to source it
10mm gitti (coarse aggregate)ask us to source it
Coarse sandask us to source it
TMT steel — quantity from your bar scheduleask us to source it
Shuttering ply and propsask us to source it
Binding wireask us to source it
Cover blocksask us to source it

Safety

Rubber glovesask us to source it
Dust maskask us to source it
Safety gogglesask us to source it
Gum bootsask us to source it
Safety helmetask us to source it

Common mistakes

Ordering external plaster on the internal figure. Internal is 12 mm, external is 15-20 mm, so a bag drops from 150 sq ft to 125 or 95. Buy the whole house on the 150 figure and you will be 30-55 percent short on the outside face.
Buying 53 grade for plaster because it sounds stronger. UltraTech points builders at 43 grade or PPC for plaster and masonry and keeps 53 for RCC. 53 sets fast and runs hotter, so in a thin plaster coat it gives you more shrinkage cracks, not a better wall.
Volume-batching above M20. IS 456 clause 9.3 permits nominal mixes only for M20 and lower; M25 and above must be a design mix. A site mix called 1:1:2 is not M25 and will not test as M25.
Cutting curing short. IS 456 clause 13.5.1 asks for at least 7 days wet curing and not less than 10 days in dry hot weather. A slab that was not cured never reaches its grade, and adding extra cement does not buy it back.
Buying two months of cement in one lot and stacking it on the floor against a wall. Cement takes up damp and loses strength; IS 1489 clause 8 requires weather-proof storage. Stack on a wooden platform clear of walls and use bags in the order they arrived.

Buying tips

Read the bag face before you accept the load. It must carry the ISI mark with the BIS licence number (CM/L-...), the IS number for the type you asked for (IS 269 for OPC, IS 1489 Part 1 for PPC, IS 455 for PSC), and the week and year of manufacture. Cement is under mandatory BIS certification, so no licence number, or a smudged or reprinted date, is a refuse.
Weigh two or three bags at random off the truck. IS 1489 Annex B sets the average net mass at 50 kg, allows no more than 5 percent of bags to be over 2 percent light, and no single bag more than 4 percent light. A bag under 48 kg is out of spec, and across 200 bags that shortfall is a real bill.
Buy the grade the job needs, not the grade the shop is pushing. PPC or OPC 43 for plaster, brickwork and general work; OPC 53 where you want early strength in slabs, beams and columns. PPC gains strength slower but keeps gaining, which suits masonry and plaster in a hot climate.
Squeeze a handful from an opened bag. Fresh cement is cool, free-flowing and crushes straight to powder. Hard lumps that will not break between your fingers mean the bag has taken damp in the godown and the strength is already gone — that stock is not worth a discount.