AAC (Autoclaved Aerated Concrete) blocks have become the dominant wall construction material in South India over the last decade. Lightweight, thermally efficient, and easy to cut, they have largely replaced traditional burnt clay bricks in new residential and commercial construction.
One of the most common questions we get: "Can gypsum plaster be applied on AAC blocks?" The answer is yes — in fact, gypsum plaster is one of the best-performing finishes for AAC block walls. Here is what you need to know.
Why AAC Blocks and Gypsum Plaster Are a Good Match
AAC blocks have a highly porous, low-density surface. This creates challenges for cement sand plaster — the high suction of AAC can cause cement plaster to lose water too quickly, resulting in poor bonding, cracking, and delamination.
Gypsum plaster, by contrast, actually benefits from this characteristic. The porous AAC surface provides a mechanical key for the gypsum, and gypsum's rapid set time means it is less dependent on controlled moisture curing. The result is excellent adhesion and a crack-free finish.
AAC blocks + gypsum plaster = one of the most durable, low-maintenance interior wall systems available in modern construction.
The Correct Application Process for AAC Blocks
While gypsum plaster works well on AAC, the application process requires specific steps that differ from brick or RCC surfaces:
Step 1: Surface Preparation
AAC block joints and any surface irregularities should be filled and levelled before plastering. The block surface should be clean and dust-free. Do not wet the surface excessively — AAC's high suction will draw moisture away from the plaster, but pre-soaking causes different problems.
Step 2: Bonding Agent Application
This is the critical step. Apply a gypsum-compatible bonding agent (such as Gyproc GypBase or equivalent) to the AAC block surface before plastering. This creates a consistent suction surface and improves adhesion. Skipping this step is the most common cause of gypsum plaster failures on AAC blocks.
Step 3: First Coat (Undercoat)
Apply the undercoat plaster at 8–10mm thickness. Set screeds for level control. Allow to set fully (45–90 minutes) before applying the finish coat.
Step 4: Finish Coat
Apply the finish coat at 2–3mm and trowel smooth. This produces the final paint-ready surface.
Common Issues and How to Avoid Them
| Problem | Cause | Solution |
|---|---|---|
| Plaster delaminating from AAC | No bonding agent applied | Always use bonding agent on AAC surfaces |
| Cracks at block joints | Joint not filled before plastering | Fill and level all joints before applying plaster |
| Bubbling or blistering | AAC was wet when plaster was applied | Allow AAC to dry after wetting — check moisture before plastering |
| Uneven surface | AAC surface not levelled before plastering | Fill large voids with patching compound first |
| Cracking at RCC-AAC junction | Different thermal expansion rates | Apply alkali-resistant mesh at all RCC-AAC junctions before plastering |
AAC Block Junctions: The Most Important Detail
Where AAC blocks meet RCC columns or beams, there is a significant difference in thermal expansion and structural behaviour. This junction is the most common point of cracking in plastered walls.
The correct solution is to embed an alkali-resistant fibreglass mesh (100mm wide, centred on the junction) into the first coat of plaster at these points. This distributes stress across a wider area and prevents the crack from propagating through the surface plaster.
Does Gypsum Plaster Add to AAC's Thermal Performance?
Yes. AAC blocks are renowned for thermal insulation, and gypsum plaster complements this. Gypsum has a thermal conductivity of approximately 0.25 W/mK — lower than cement mortar plaster (0.7–1.0 W/mK). Replacing cement plaster with gypsum on AAC block walls contributes to better overall wall thermal performance, contributing to cooler interiors in South India's climate.
Why Kanish Plasters Recommends Gypsum for AAC
We have applied gypsum plaster over AAC blocks on hundreds of projects across South India — homes, apartments, hospitals, and schools. When applied correctly with bonding agent, mesh at junctions, and appropriate thickness, gypsum on AAC produces walls that are crack-free, smooth, and durable for the life of the building.
We use only Gyproc-certified materials and trained applicators who follow a documented process checklist on every project.