AAC blocks are precise: they come off the line within a millimetre or two of size. That precision is wasted if they are laid like bricks, with a thick bed of cement mortar. How you join the blocks decides how fast the wall goes up, whether it cracks, and how much of AAC's insulation you actually get.
What Is the Difference Between Adhesive and Mortar?
| Thin-bed AAC adhesive | Cement mortar (1:6) | |
|---|---|---|
| What it is | Factory-mixed polymer-modified cement; add water only | Cement and sand mixed on site |
| Joint thickness | 2 – 4 mm | 10 – 12 mm |
| Material per m³ of wall | Low | Much higher, plus sand |
| Bond to AAC | Strong, designed for it | Adequate if blocks are dampened |
| Cracking along joints | Less | More common |
| Thermal bridging | Minimal | Thick dense joints conduct heat |
| Speed of laying | Faster | Slower |
| Curing of joints | Not required | Required |
Why Does Thick Mortar Cause Cracks in AAC Walls?
AAC is light and slightly flexible; a 1:4 or 1:6 cement-sand mortar is dense and stiff. When the wall moves with temperature and drying shrinkage, the two materials move differently, and the weakest line — the joint — cracks. Thick joints also hold more water, and as that water dries the mortar shrinks more than the block around it.
Thick joints also undo some of AAC's insulation. AAC conducts heat slowly; a dense mortar joint conducts it much faster. With 10 to 12 mm joints around every block, a meaningful part of the wall is mortar, and the wall behaves less like an insulating AAC wall than its thickness suggests.
Is Adhesive More Expensive?
Per bag, yes: adhesive costs more than a bag of cement. But compare per cubic metre of wall:
- A thin bed uses a fraction of the material a thick mortar bed does.
- There is no sand to buy, carry or sieve.
- There is no mixing ratio to get wrong, so less material is wasted.
- Masons lay faster, so labour per sq ft falls.
- The joints do not need water curing.
On a whole-wall basis the two usually come out close, and the adhesive wall is built faster and cracks less. Manufacturers print the coverage per bag on the pack; use that figure, not a guess, when ordering.
How Do You Lay AAC Blocks With Adhesive?
- Lay the first course in mortar. The base course sits on a 10–20 mm bed of cement mortar to level out the floor. Get it perfectly level; every course above follows it.
- Mix adhesive as per the pack, with clean water, to a smooth paste.
- Apply with a notched trowel to the bed and vertical face, in a thin even layer.
- Place the block and tap it into position with a rubber mallet. Check line and level.
- Stagger vertical joints by at least a quarter of a block, ideally half.
- Cut blocks with a saw, not a chisel, for clean edges.
- Leave a 10–20 mm gap below the beam or slab soffit, filled later with a flexible filler or a sloped course, so the frame's deflection does not crush the top course.
- Fix galvanised mesh or ties at column junctions every few courses, so the wall stays tied to the frame.
If You Use Cement Mortar, How Do You Reduce Cracking?
Plenty of good AAC walls are laid in mortar. Do it carefully:
- Use a lean mix, about 1:6, not 1:4.
- Keep joints as thin as practical, no more than 10 mm.
- Dampen the block surface slightly before laying, but never soak blocks; a wet block shrinks as it dries.
- Cure the wall for several days.
- Wait before plastering — at least a week after the wall is complete, longer in a wet monsoon.
What About Plastering AAC Walls?
AAC has a fine, porous surface. For cement plaster, dampen the wall, apply a bonding coat or a spatter dash first, and keep plaster thin: 10–12 mm internally, 15–20 mm externally in two coats. Fix fibre mesh across the junction of the wall with columns and beams before plastering; that junction is where most visible plaster cracks start. Gypsum plaster is also common on internal AAC walls.
The Short Version
Lay AAC in thin-bed adhesive above a level mortar base course, stagger the joints, leave a gap at the beam soffit, tie the wall to the columns and mesh the junctions before plastering. If you must use cement mortar, keep it lean and the joints thin. Most AAC cracks come from the joint, not the block.
