Mortar mix ratios for masonry and plaster work

At a glance

The mortar choice for most residential brick masonry is a one-to-six cement-sand mortar by volume, which aligns with a Type N mortar under ASTM C270 and delivers a compressive strength of approximately 5 N per square millimetre at twenty-eight days. For load-bearing work in seismic zones III and above, including coastal Karnataka and Kerala, the mortar steps up to one-to-four, which corresponds to an ASTM Type S at roughly 12.5 N per square millimetre. Internal plaster typically uses one-to-four or one-to-five for the first coat and one-to-six for the finishing coat, with the leaner mix on the outside so that cracking propagates away from the substrate rather than into it.

The Indian reference for mortar proportioning is IS 2250 for code of practice on preparation and use of masonry mortar, read with IS 1542 on sand for plaster and IS 2116 on sand for masonry. The European equivalent is EN 998-2 for masonry mortar and EN 998-1 for rendering and plastering mortar. The performance classification under EN 998-2 uses the letter M followed by the compressive strength in N per square millimetre, so an M5 mortar and a Type N mortar and a one-to-six nominal mix all describe approximately the same material.

Mix table

Sand grading and bulking

Sand that is used as received from a river bed or a manufactured-sand plant is rarely dry. A site-stored heap of fine sand at fifteen percent moisture bulks by roughly twenty-five to thirty percent over its dry volume. A one-to-six mortar measured by box on bulked sand therefore delivers a true proportion closer to one-to-seven-point-eight, which falls outside the strength envelope of a Type N mortar. The correction is either to reduce the sand box by the measured bulking percentage or to saturate the sand the previous evening so that bulking is eliminated by the time of use.

Grading requirements under IS 2116 for masonry and IS 1542 for plaster specify a zone II to zone III grading with a fineness modulus between 2.2 and 2.8 for masonry and between 1.5 and 2.2 for plaster. A sand that is too coarse produces a harsh mortar that is difficult to trowel, a sand that is too fine needs a much higher water content and shrinks excessively on drying. Manufactured sand from crushed granite performs well in both applications provided the fines content below 150 microns is controlled below fifteen percent by mass.

Worked example: hot-weather internal plaster in Tumakuru

Consider a 12 mm thick internal plaster on brick masonry over a wall area of 100 square metres, applied in May when the ambient air temperature at site is 38 degrees Celsius and the relative humidity is 35 percent. The specified mix is one cement to five sand by volume, with a target twenty-eight-day compressive strength of 7.5 N per square millimetre.

Volume of plaster equals 100 multiplied by 0.012 equals 1.2 cubic metres. Dry volume allowance of 1.30 gives 1.56 cubic metres of dry mortar. At a one-to-five ratio, cement is 1.56 divided by 6 which equals 0.26 cubic metres, and sand is 1.56 multiplied by 5 divided by 6 which equals 1.30 cubic metres. Converting cement volume to weight at a bulk density of 1440 kg per cubic metre gives 374 kilogrammes, which rounds to eight bags. Sand at 1.30 cubic metres with a bulking of twenty-five percent needs 1.62 cubic metres of bulked sand at the site.

Hot-weather practice under ACI 305 requires that fresh mortar temperature be kept below 32 degrees Celsius. At 38 degrees ambient this is achieved by shading the sand stockpile, using water that has been stored overnight in an insulated tank, and adding ice to the mix water where required. The plaster must be cured by fine water spray at least twice daily for seven days. A plaster applied on a wall whose substrate temperature exceeds 40 degrees Celsius will suffer plastic shrinkage cracks regardless of the mix design and should be scheduled for early morning or late afternoon.

Common failures and their remedies

Map cracking on the surface of a freshly completed plaster almost always traces back to over-rich mortar combined with inadequate curing. The cure is a leaner mix and a disciplined curing regime, not a thicker application. Efflorescence on brickwork a few weeks after completion typically reflects the use of salt-contaminated sand or unwashed bricks; the preventive measure is a sand wash and a twenty-four-hour brick soak before laying. Hollow sounding plaster indicates poor bond at the substrate interface, usually from a dry substrate, a lean first coat or a delay in applying the second coat; the remedy is to dampen the substrate, apply a spatterdash of neat cement slurry and observe a maximum gap of twenty-four hours between coats.

Reference standards

The following standards are cited in this article: IS 2250 code of practice for preparation and use of masonry mortars, IS 2116 specification for sand for masonry mortars, IS 1542 specification for sand for plaster, IS 2386 methods of test for aggregates, ASTM C270 specification for mortar for unit masonry, ASTM C780 preconstruction and construction evaluation of mortars, EN 998-1 and EN 998-2 for rendering, plastering and masonry mortars, ACI 305 guide to hot weather concreting and mortar practice, and ACI 530 / TMS 402 building code for masonry structures.

Related pages

See the companion technical articles on concrete curing, rebar development length and MCB and RCCB sizing. The cement and binders category page carries the grade selection table that feeds the mix designs above.

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