Differences Between AAC Block Jointing Mortar and Traditional Cement Mortar

March 25, 2025

Deep in a construction project, the kind of mortar you use may not feel like much of an issue. But ask any seasoned contractor and they'll advise you—this is one choice that can affect your building's performance for years to come. If you're considering AAC block jointing mortar versus conventional cement mortar, knowing their essential differences will guide you to the best choice for your particular requirements.

The Makeup Matters

Conventional cement mortar is incredibly simple—cement, sand, and water with proportions such as 1:3 or 1:4. It has been used on countless construction projects across the globe for centuries, and its simplicity is one of its attractions.

AAC jointing mortar goes a more specialized route. When you break open a bag, you can see it is finer and finer to a more adhesive-like consistency. That's because producers have designed it specifically for Autoclaved Aerated Concrete blocks, with polymers and chemical additives that improve bonding qualities.

The Thickness Test

You'll quickly see the contrast in application thickness between these choices. With standard mortar, you're generally dealing with joints that are 10-15mm thick. These fat joints take more material and produce large thermal bridges across your wall system.

Switch to AAC jointing mortar, and you'll use layers a mere 2-3mm thick. That drastic decrease isn't just conservative on materials—though it saves materials—it creates more thermally stable walls with fewer points of weakness. You'll also find it simpler to maintain your blocks perfectly aligned using these thinner joints, creating straighter, professional-looking walls.

Flexibility When You Need It

Structures shift more than you realize. They settle into their foundations, grow in summer heat, and shrink in winter cold. Old-fashioned mortar can't handle these shifts very well—it's stiff and cracks when made to bend.

AAC jointing mortar provides you with that essential measure of flexibility. Those polymer additives permit slight movement without cracking, keeping your structures intact through changing seasons and settling. When you're working to prevent callback repairs for hairline cracks, that flexibility is invaluable.

Maintaining Warmth Where It Belongs

The modern building codes and client demands of today put greater focus on energy efficiency. Mortar is now a liability in this regard, with thermal conductivity ratings of about 0.6-1.0 W/mK. Those wide joints can decrease your wall's thermal efficiency by as much as 20%.

Select AAC jointing mortar, and you get about half the thermal conductivity. In combination with the inherently insulating AAC blocks, you have a more uniform thermal envelope that traps heating and cooling in their proper place—inside the building. Your customers will feel it on their utility bills every month.

When Rain Comes Calling

Water resistance may not be foremost on sunny construction days, but it becomes absolutely essential over the life of a building. Old mortar soaks up water easily, which can cause dampness problems in the future.

AAC jointing mortar also commonly contains water-resistant properties that will help safeguard your entire wall system. This increased resistance to water intrusion lowers the maintenance headaches and lengthens the functional lifespan of your structures. In more humid environments, this aspect alone might be the justification to make the transition.

On high-traffic construction sites, workflow speed is important. Old mortar leaves you with a fairly limited 30-60 minute working time before setting takes place, typically leading to a hasty application process and waste of materials.

AAC jointing mortar gives you a 2-3 hour working time, which means more measured application and much less waste. You'll also discover that applying thinner layers accelerates the overall building process, potentially trimming days off big projects.

The Environmental Angle

As customers more and more request sustainable building practices, the green implications of your material selection become more important. Conventional cement manufacturing is famously carbon-heavy, playing a major role in the environmental impact of construction. Several AAC jointing mortars contain supplementary cementitious materials that minimize their carbon footprint. Coupled with the fact that you're using less material in general because the joints are thinner, you're facing a quantifiably more environmentally friendly choice that satisfies sustainability requirements without sacrificing performance.

Making Your Decision

For regular masonry applications, old mortar is still an effective, economical choice. But when dealing specifically with AAC blocks, there's specialized jointing mortar that isn't merely a substitute—it's an engineered product that maximizes the overall wall system.

The initial cost and learning curve associated with AAC jointing mortar generally reap rewards in the form of quicker construction, improved thermal efficiency, and less maintenance. As building standards increasingly move toward higher efficiency and sustainability, these niche products increasingly reflect not only best practice, but adaptation that is necessary.

The next time you're designing an AAC block project, think through these differences seriously. The proper mortar selection will affect your building's performance well after the construction dust settles.

AAC jointing mortar goes a more specialized route. When you break open a bag, you can see it is finer and finer to a more adhesive-like consistency. That's because producers have designed it specifically for Autoclaved Aerated Concrete blocks, with polymers and chemical additives that improve bonding qualities.