Kumar IronUpdated
In this guide
Thin bed mortar or cement mortar
Block jointing mortar is a factory mixed powder of cement, fine sand and polymer additives. You add water, mix, and spread a thin layer. Ordinary cement sand mortar is much thicker, takes more material, and the wide joint conducts more heat than the block itself.
Thin bed joints work only because AAC blocks are cut to tight size tolerance. If the blocks are chipped, out of shape or uneven, thin joints will not hide it. Reject damaged blocks at delivery.
| Point | Block jointing mortar | Cement sand mortar |
|---|---|---|
| Typical joint thickness | About 2 to 3 mm (some brands say up to 3 to 5 mm, check the sheet) | 10 mm or more |
| Where it is used | Second course upward | First course on the slab or beam, levelling |
| Mixing | Powder plus clean water, small batches | Site mix of cement and sand |
| Speed | Fast, one trowel pass per course | Slower, more mortar handling |
| Extra benefit | Less thermal bridging, less shrinkage crack risk | Can absorb level errors |
Start with the first course
The first course decides how straight the whole wall will be. Clean the slab or plinth beam of dust and loose concrete. Mark the wall line and check levels with a water level or laser.
Lay the first course on a cement mortar bed. Standards such as IS 6441 Part 2 allow ordinary cement mortar for AAC masonry, and most site practice uses a leveller bed of cement sand mortar, commonly 1:4 to 1:6, of roughly 10 to 30 mm. This is the only course where the floor tolerance can be corrected. Place the corner blocks first, run a string line, then fill in between and tap each block level with a rubber mallet.
In ground floor walls, keep AAC above a damp proof course. Do not start AAC directly on soil or below plinth level.
Laying the second course onward
- Mix only as much mortar as you can use in the pot life printed on the bag. Do not add more water to revive a stiff mix.
- Brush off AAC dust from the top of the course. Most manufacturer sheets ask for a clean surface that is dust free, and some ask for a light mist of water in hot dry weather. AAC sucks water fast, but do not soak blocks. Follow the bag.
- Spread the mortar with a notched trowel (or a sliding mortar box) sized for the block width. The ridges should stay visible. If the ridges merge, the mix is too wet.
- Press the block down and slide it against the previous one. Tap level. Wipe out squeezed mortar with the trowel edge.
- Stagger the vertical joints. A common rule is an overlap of at least one third of the block length, and not less than about 100 mm.
- Vertical joints are usually filled as well, either by buttering the block end or by tongue and groove blocks if you buy them.
- Check plumb and level every two or three courses with a spirit level.
Tools that make the job easy
- Notched trowel and mortar scoop or mortar box
- Rubber mallet
- Hand saw for AAC (the toothed type), or a band saw for big jobs
- Scraper or sanding board to smooth uneven faces
- Water level, spirit level, string line and square
- Groove cutter for electrical conduit and small pipes
Cut blocks with a saw, not a chisel and hammer, so the edges stay square. Plan cut pieces so you always have a full block at corners. Do not chase deep grooves for pipes. Shallow grooves for conduit are fine, deeper ones should be avoided or boxed in.
Bond beams, lintels and openings
Over doors and windows, AAC blocks cannot span on their own. Use RCC or precast lintels with bearing on both sides as your engineer specifies, typically 200 mm or more. A band at sill level and at lintel level (a concrete or reinforced band running through the wall) helps control cracks, especially in tall walls.
The top course below a slab or beam should leave a gap of about 10 to 20 mm, filled later with a compressible or non-shrink material, so that slab deflection does not crush the wall. Ask the structural engineer for the detail. Fix heavy items such as geysers or cabinets with AAC anchors, not plain plugs.
Plastering on AAC
Wait until the wall has settled. The jointing mortar should reach its strength, usually a few days per the data sheet. Dust the wall and apply the primer or bonding coat if the plaster maker asks for it. Normal rough plaster does not grip smooth AAC well.
Use a plaster designed for AAC, or a lean sand cement mix such as 1:5 or 1:6 as discussed in our plaster ratio guide. Apply in one thin coat or two thin coats, with chicken or fibre mesh across the joint between block and RCC column or beam. Cure for a few days as for any plaster.
Frequently asked questions
- Do I need to soak AAC blocks before laying?
- No. Soaking makes them heavy and can cause shrinkage cracks later. Brush off dust, and in hot dry weather a light mist is fine if the mortar data sheet allows it.
- Why is the first course in cement mortar and not jointing mortar?
- Slabs are never perfectly level, and thin bed mortar cannot correct an error beyond a few millimetres. A cement mortar bed levels the base so the rest of the wall can use thin joints.
- Can I use normal cement mortar for all courses?
- You can, as IS 6441 allows it, but joints become thick, use more mortar and give weaker thermal performance. Many masons then see more cracks. Jointing mortar is the better match for AAC.
- How thick should the joint be?
- About 2 to 3 mm is the usual range in manufacturer sheets. A few brands allow slightly more. Follow the bag you buy.