Difference Between AAC and CLC Products
There is a great deal of misunderstanding in the marketplace about the differences between autoclaved aerated concrete (AAC) and cellular lightweight concrete (CLC) products. This article seeks to provide clarity.
What’s the difference in how they are manufactured?
AAC is typically made from sand, lime, cement, water and aluminum powder acting as an expansion agent. Sand, lime and cement is mixed followed by water. Aluminum powder is then added to cause a volume expansion. The aluminum reacts with the calcium hydroxide formed by the reaction of cement and water to produce tricalcium hydrate and hydrogen gas. The hydrogen bubbles out of the mixture and is replaced by air. Molds are filled with the mix, and, when expansion reaches the right point, each mold is opened and the material cut into blocks or panels. The material then enters an autoclave where it is exposed to heat and pressure. An autoclave is a strong, pressurized, steam heated vessel. The autoclave process takes about 12 hours resulting in fully cured product ready for packaging and shipping.
AAC manufacturing utilizes a lot of large scale expensive equipment including silos, conveyance systems and autoclaves. The initial capital investment is high. Fixed operating costs are also relatively high necessitating very high production volumes in order to make a profit. AAC is not suitable for small scale production. The production space needed for AAC is large but storage space is relatively small as the material is fully cured and ready to ship once removed from the autoclave.
In the manufacture of CLC a foam is produced by the action of compressed air on a liquid chemical foaming agent. This foam is mixed in a concrete mixer with regular sand, cement and water producing a porous concrete material. The entire process is under ambient conditions and curing takes about a day. Panels, blocks and a wide variety of product shapes and sizes can be produced.
A CLC plant is much cheaper to setup and operate than an AAC plant with concrete mixers and foam generators being relatively inexpensive. The production space for CLC is small compared to AAC, however, the space required for curing and storage is relatively large. For plants of similar capacity, CLC is cheaper per kilogram produced both in capital and operating costs.
CLC manufacturing is flexible enough to be done small scale, on site and even in remote locations. These options are not possible with AAC. With CLC manufacturing the foamed concrete can flow to fill just about any cavity making complex shapes and one-off moldings possible. A wide range of CLC densities can be produced by varying the amount of foam added. With AAC only a limited number of densities, shapes and sizes are economically feasible. Reinforcements such as polypropylene fibers can be added to CLC for added strength and crack resistance. Pigments can be added to CLC to mold colored product. With AAC such additives and reinforcements are not feasible. CLC output can be scaled and can be similar to AAC production output.
What’s the difference in their properties?
CLC properties for a given density can vary considerably depending on the mix design, in particular, the ratio of sand and cement used. For example, CLC having a density of 600 kg/cu m made only from cement has a compressive strength of 4 MPa while a one to one sand to cement ratio (of the same density) will have a 2 MPa compressive strength.
The strength of CLC is a bit lower than AAC, however, other physical properties are similar for the same density product. One exception is moisture absorption. As hydrogen escapes from AAC it can leave an interconnected pore structure mimicking the path taken by the hydrogen as it exits the mixture. This differs from CLC where the air bubbles are trapped resulting in a non-interconnected pore structure. The interconnected pores draw more moisture into the material resulting in a lower thermal insulation performance or lower R value for AAC. For a density of 1000 kg/cu m AAC picks up 30% moisture while CLC picks up only 12.5%. At 600 kg/cu m, AAC has a thermal conductivity of 0.16 W/mK while CLC has a value of 0.11 W/mK. CLC has a lower thermal conductivity or higher thermal insulating performance.
AAC has a much higher embodied energy than CLC. This is due primarily to the energy consumed in milling the sand (to make it finer) prior to mixing and the autoclave process which is very energy intensive. CLC is produced with unprocessed raw materials under ambient conditions. Process energy requirement (PER) is a measure of the energy directly related to the manufacture of a material. AAC has a PER value of 3.6 MJ/kg while CLC is about 2.9 MJ/kg.
Lime, a principal mix component in AAC, requires less energy to produce than cement but more carbon dioxide is produced per tonne (1,000 kg) versus cement (800-900 kg).
What’s the difference in the products that can be made from them?
With CLC a very wide range of densities, shapes and sizes are possible. Densities vary by the amount of foam introduced and, as long as the cellular concrete can flow into a mold cavity, virtually any shape and size can be cast. The same is not true of AAC. The high costs of creating each product line means that only a limited number of densities and shapes are economically feasible.
What’s the difference in how they are installed?
In theory, AAC and CLC of the same shape and size can be installed in essentially the same manner. However, regulations and restrictions on AAC in many countries means that the methods and materials used in installing AAC are often more limited than CLC.
AAC blocks require a special thin set mortar to attach the blocks during installation. With CLC, there are blocks such as SupaBlok™, that do not require any mortar to lay, they are dry stacked. AAC requires skilled and knowledgeable construction workers for proper installation. Mortarless CLC blocks such as SupaBlok™ can be laid with unskilled labor. In general, AAC block requires more skill and special tools and materials than a CLC block such as SupaBlok™. CLC and AAC can both be used for structural and non-structural applications.
Overall, CLC offers a lower manufacturing cost and more possibilities in terms of the many products that can be made and the many ways you can build with it.
SupaBlok™ CLC block