AAC blocks. Brick is losing.

I was on a site in Jaipur three weeks ago. The walls were going up in autoclaved aerated concrete blocks — AAC — and the contractor was very pleased with himself. They were laying twice as fast as a brick crew. Lighter blocks, less mortar, fewer cuts.

I looked at the wall.

There were gaps between blocks the width of my thumb.

Not everywhere — but often enough that the next layer of plaster was going to have to fill them. Which it would, badly. Which is why the next layer of putty was going to crack along those joints. Which is why the paint job in two years would tell you exactly where every gap had been.

The block won. The installation lost.

This is the AAC blocks vs red bricks debate that nobody is having. Most articles compare the two on cost, weight, thermal performance — comparisons you can find on any manufacturer's website. What I'm going to talk about is the part that costs developers money and nobody writes about: the mortar.

Autoclaved aerated concrete. Cement, lime, sand or fly ash, and an aluminium powder that reacts during curing to create millions of tiny closed air pockets. The block is then steam-cured under pressure. The result is a block that is roughly one-third the density of fired brick — somewhere between 500 and 700 kg/m³ versus 1,800-2,000 kg/m³ for burnt clay.

That density drop changes everything. Lighter blocks, faster laying. Better thermal insulation — AAC reaches a thermal conductivity around 0.16 W/mK versus 0.81 W/mK for solid burnt brick. Lower load on the structure. Lower transport cost per cubic metre of wall.

Per IS 2185 (Part 3), AAC blocks are graded by density and compressive strength. Most residential AAC in India is grade 1 or grade 2 — compressive strength between 3 and 5 MPa. That sounds low compared to a fired brick at 7-10 MPa. But residential walls don't need brick-level compressive strength. They need to stand up and not crack.

Two things, both of which I've watched on site.

One. Efflorescence is dramatically lower. The white salt blooms that show up on Indian brick walls within months of construction — they barely appear on AAC. Burnt brick comes out of the kiln carrying soluble sulphates, and most contractors here pre-soak the bricks in water before laying. Whether the salts come from the brick, the water, the mortar, or all three, the result is the same: efflorescence. AAC blocks are factory-cured, dimensionally stable, low-suction, and they don't get pre-soaked the same way. The salt source isn't there. Walls stay cleaner for longer.

Two. The blocks are uniform. Burnt brick from rural kilns in Rajasthan varies in size, hardness and absorption from one batch to the next. AAC comes off a controlled production line at a consistent dimension. When the contractor lays AAC well, the wall is straight. When the contractor lays AAC badly — gaps and all — the wall is at least uniformly bad in a predictable way. That's not nothing.

The block is easy to place level. The masons here often don't.

I see three patterns.

Gaps between blocks because the mason hasn't troweled enough mortar across the bed joint. Vertical joints left half-empty because the block went in dry. And the worst one — using regular cement-sand mortar (1:4 or 1:6) on AAC at all. AAC has a suction profile that pulls water out of standard OPC mortar fast and unevenly. The mortar dries before it has time to hydrate properly. The bond is weak. The wall holds because gravity is on its side, not because the mortar did its job.

This is the AAC blocks vs bricks installation difference nobody mentions. With brick, the mason can mostly get away with sloppy mortar — the brick itself is forgiving. With AAC, sloppy mortar shows up months later as cracks, gaps in the plaster, and finishing failures.

Here's the part that doesn't get written about. So I will write about it.

The compressive strength of a bedding mortar should be reasonably matched to the substrate. For burnt brick at 7-10 MPa, a standard 1:6 cement-sand mortar at around 5 MPa works fine. For AAC at 3-5 MPa, that same mortar is too strong. Stress concentrates at the joints. The block cracks before the joint does, which is the wrong failure mode.

What we develop in the lab — and what some of my own work has been moving toward — is a thin-bed AAC mortar with a deliberately lower compressive strength, typically 2.5 to 4 MPa, and a higher polymer content. Redispersible polymer powder at 2-3 kg per tonne of dry mix. The polymer gives flexibility. The lower strength gives compatibility. The polymer-modified mix retains water against AAC's suction.

Joint thickness drops from the 12-15 mm of brickwork to 2-3 mm.

When that mortar is used correctly, the wall behaves. When it isn't — when the contractor uses leftover plaster mix because the AAC mortar bag costs more — you get the Jaipur wall I started with.

Brick is losing market share in residential India, and that is going to keep happening regardless of how well or badly the masons learn to lay AAC. The lighter block, the faster wall, the cleaner finish — those wins are visible to the developer at the gate of the site. The installation problems are visible months later when somebody has to repaint.

The mortar industry is adapting faster than the workforce. New formulations are coming out. Specifications are getting written. The bag is on the market.

What's missing is the same thing missing in every other story I've written here. Training. Specification enforcement. A masonry crew that knows the difference between cement-sand mortar for brick walls and thin-bed polymer-modified mortar for AAC blocks.

Until that catches up, the blocks will keep going up faster, and the next layer of finish will keep paying for it.

Next week — waterproofing. Why it fails, and why it's almost never the membrane's fault.

— Guillermo