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This page describes how to make FC and what influences the “quality”

Foam Concrete is also known as

Aircrete, Foamed concrete, Foamcrete, Foamed cement, Cellular lightweight concrete, Reduced density concrete, Lightweight concrete, Cellcrete, gasbeton, Foamkrete, air-cured lightweight concrete, gas concrete, insulated cellular lightweight concrete, insulating concrete, lightweight aggregate cellular concrete, low-density concrete, foamed mortar, mortar foam.

Foam Concrete (FC) is made by mixing foam into mortar. The mortar is a cement mix with sand and water. As a result you end up with a mix that is lighter than “normal” concrete. The mass, or density as we call it here, (weight per cubic meter) depends on how much foam is added to the mortar. The more foam we add the lighter it is, but also the weaker is becomes. The “Ideal” mix would have at least 20 MPa at a density of 1000 Kg/m3, however the best results I have found in studies was 18 MPa and a density of 1200Kg/ m3. A simple FC with no additives is usually around 5-8 MPa with a density of 1000 Kg /m3. There are FC “Brands” on the market that make the claim to be better than this.

The lighter the FC is, the better the thermal insulation becomes. The combination of the strength and the insulation value makes FC an ideal building material.  For the advantages and dis-advantages of FC go to:  Why Foam Concrete is an ideal building material

In this article we briefly describe:

Foaming agents

Foaming agents: properties and methods

Foam properties
Foaming agents and creating a foam is often overlooked in its importance to making FC It is however a very important aspect of the process, and if not done “correctly” it can go horribly wrong.

The characteristics of the foam that make a “good” foamed concrete are:

Stability, how long the foam keeps its bubbles.

This can be tested by making some foam and leaving it in a glass and see how long it is before you see shrinkage and liquid on the bottom. It should not collapse before the FC is set sufficiently to hold its form. However this test does not tell you how it behaves when mixed with the mortar and other reactions with additieves .

Bubble size:

Small bubbles are stronger than big ones, the optimum size is 0.5 mm. Good results can be achieves for bubble sizes between .05  and 1 mm and preferably the majority of a similar small size.

Uniformity and shape of the bubble:

The bigger bubbles tend to collapse first when mixing it in with the mortar. The more uniform the bubble size is the stronger the FC will be. The optimum shape of the bubble is a perfect round sphere. How much it can withstand deformation depends on the surface modules and the surface tension.

Connectivity of the bubbles:

Ideally all bubbles should be separated from each other when mixed in the mortar.

Bubble destruction

  • Bubbles can collapse because of reactions with other additives and the cement products we put in the mix.
  • Mixing the Foam Concrete longer than needed destroys the bubbles.
  • Pumping the FC over great lengths and heights can destroy the bubbles as well. Check the claims the Foam agent manufacturer makes.
  • The positive side of over-mixing and pumping is that the biggest bubbles tend to collapse first. Small (0.5 mm) homogenous bubble size makes stronger FC.

Below is a overview of the properties for a FC mix without any additives that can improve some of these aspects.

Types of foam agents

Foam agents can be divided into classes,

Synthetic surfactants (such as sodium dodecylsulfate) made from petroleum products

Protein based sodium and potassium salts of fatty acids (alkylcarboxylic acids), such as lauric and myristic acids. Usually made from animal offal.

Up till now the protein based agents are found to be better suited for making FC. Depending on how well they are refined, protein based can have a shorter shelf life and can produce a smell in the FC. Each manufacturer claims to have a superior product. Some synthetic foams claim to be more stable and pump-able than others. I found one study that compared 3 different foaming agents.

Plant origin

This type is an alternative if you do not want to use the other types.

FOAM AGENT ISOCEM S / BN 100% VEGETABLE, ISOCEM S / BN is the new product of the Isocem range, foaming agents for the production of Foam concrete. It is more concentrated and 100% plant origin.  https://www.isoltech.it This is the only brand I found so far of this type that is why I mention it by name here.

For suppliers of foaming agents go to Foaming agents

DIY Foam

Making foam from household products such as soft soap or shampoo is not recommended if you can get hold of special FC foam. The cost of even the most expensive foam agents is small compared to the cost of a failed foam concrete batch or whole house!  The biggest problem for the DIY is obtaining a small quantity of agent. I suggest to contact a local company that provides a foaming service or a FC product manufacturer. However their is a substance that is used to “strengthen” foam, Xanthan gum.  This is also used as a food additive.  I have no idea of the concentration to use, experiment with it , and let me know please.

Quality of the foam

When making foam, the recommended “consistency” is 80 to 120 gram per liter, but I have seen publications where they used 45 g per liter.

The concentration depends on the brand. This is achieved by the correct dilution rate and the foam making process. This can be easily tested by filling an one liter container and weighing it. This need to be done before every batch! Once you decided what dilution you want to use, ensure that this is the same within 5% every time. The quality of your foam influences the quality of the FC!

The quality of the foam is also influenced by the type of foam generator. The desired bubble size is between 0.5 and 2 mm. The distribution of the bubble size seems also to have an influence on the MPa of the FC. Small (0.5 mm) homogenous bubble size makes stronger FC.

The type of foam generator has a large influence of the bubble size as well. So far from the literature I found that the “dry” foaming method produces a finer bubble size.

Adding super-plasticizers and accelerator to the mortar mix can also influence the bubble size and the distribution of the bubble size. Check if these products are compatible. Some additives contain an anti-foaming agent

The foam should be injected and mixed as soon as the mortar is ready, and preferably at the bottom of the barrel. An ingenues way is to use a paint mixing tool, put a pipe over the shaft with a T connection at the top and a plug at the top. The open end just above the stirrer blades. Pump the foam in at the T connection. For a picture go to  https://www.domegaia.com

Making the foam.

Ensure you have clean water, usually drinking water is suitable. The temperature of the water can have an influence on the result; keep it between 10 and 40 C.  If it is not ‘clean” than the organic matter can have a negative effect on the protein based foam agent quality, which would affect the FC mix formation.

There is a dry and a wet method to make foam, the dry method produces smaller bubbles.  Most foaming machines use the dry method, and it is possible to make a small one yourself. To produce consistent foam you need a reliable and controllable foaming method or machine. The dry method is also preferred as the water content is easier to control, and thus the effect it has on the mortar.

Having a foaming machine is probably the biggest hurdle in the process. They can vary from a home-made, see https: //www.etsy.com search for foam-generator etc. There are about 8 people selling various types.

Fully automated large volume commercial machine see Agents and Equipment. They all have one thing in common and is that they use compressed air to make the foam.

Each type of foaming agent has its own optimum foam density, for creating the desired FC density. The optimum water / cement ratio is also different for each type / brand of foaming agent. As the w/c ratio is extremely important to make a good FC is suggest you do some trials first.

Compressed Air

Not all compressors are the same! The most important issue for making the foam is the constant pressure that is delivered to the mixing chamber. If the pressure coming into the foaming machine varies, than the foam quality will be variable. I have not seen a study that has taken this aspect into account, but my experience tell me it does.

To avoid variable pressure and volume, the compressor capacity need to be big enough to keep up with the consumption when making foam! The pressure regulator need to deliver the same pressure all the time regardless if the compressor is running or has a fully pressurized tank.

The hose from the compressor need to be big enough in diameter, and not longer than needed.

The moisture and oil content of the compressed air could have an influence on the dilution rate, probably minimal, but keep it in mind. A good filter system helps. If you cool the compressed air coming out of the compressor before put the filter in the line, the filters do a better job. ( length (4M) of steel pipe is a simple way.

Calculating the quantities

Before you make the foam you need to do some calculations.

You need to decide how much volume of FC you want to make.

You need to decide on the density FC you want to make.

This gives you the volume of the diluted agent. The dilution depends on the type and brand of the foam. Most brands will give you a guideline on a dilution for a given density. This dilution need to be precise and consistent for every batch if you want the same result. Be on the safe side and have more solution than you need for the batch.

The density of the FC depends on how much foam you put in the mortar, there is a direct relationship. I have put a chart of this in my “E”-book. 

The capacity of the foaming machine has to be taken into account, and depends on the batch size or continues production. The production speed of the foam should be slightly faster than the mixing rate in batch production.

Other methods of making FC

High speed mixing

There is a method of making FC where they put the foaming agent in with the mortar mix when it is all in a special high speed mixer. Often used for FC with a density higher than 1800 Kg/m3. We leave this to the commercial specialists.

Just add water

There is a dry mix that only needs adding water and it starts to make a gas bubbles in the mortar. This is a chemical reaction between aluminium and an acid. The receipt of the mix is a trade secret! So far I found only one company, http://www.cellularfibroconcrete.com offering this product.

Mixing the foam into the mortar

This is the most exciting part of the process! An important part in the process and it need to be done correctly, use the same method every time you make a batch of FC.

Ensure your mixing vessel is big enough to hold the volume you want to make, plus a bit extra for correction and saving it from spilling over the rim.

Mixing the foam into the mortar is not easy as the mass of the foam and the mortar are very different. It also has to be done “gently” as not to collapse the foam. It is inevitable that some of the foam will collapse during the mix, thus influences the density.

Mixing the foam in the mortar is probably the last “additive” you want to put in the mix. All other ingredients should be mixed in already, otherwise more mixing is needed and more foam collapse will occur.

The best way is to inject the foam at the bottom of the vessel, close to the mixing implement if you make the mortar yourself. You can of course order a ready mix concrete truck and put the foam in the truck barrel. Now are verging on a professional job!

If you know the volume per minute of your foaming machine and how much foam you need you can time the process.

Knowing the total volume you need to achieve for the desired density is also a good measurement. This should give you the theoretical density, but you need to check this of course by weighting the FC before you pour!

Now you are ready to pour! But hang-on, this was the simple version! If you want to achieve a “stronger” FC there is a wide range of options. Varying from just adding another ingredient to the mortar during mixing and hoping for the best to really understanding what you are doing and getting it right.

Mixing the mortar

The basic ingredients for the mortar are Portland cement, sand and water. There are many different companies that make Portland cement up to standard to comply with Type I Portland Cement  specified in the British Standard (BS EN 197-1: 2000). We here make the assumption that it is up to standard.

Sand, the sand need to be clean river sand and preferably of equal size, it was found that increasing particle size of fine aggregate decreases its strength. Fine silica sand of different sizes 0.6, 1.18 and 2 mm is often used. Sand specified to be smaller than 2 mm may cost more.

The ratio of water and cement (w/c) is very important, this decides for a large part how “strong” your FC is going to be. It is now common practice to use a super-plasticiser to improve the mortar. The w/c depends on the plasticiser you are using. In one study they USED GLENIUM52, conforming to the ASTM standard specification (ASTM C494M–04). The superplasticizer is available in dark brown aqueous solution. The optimum mix proportion was designed based on target density, w/c and s/c (sand to cement ratio) of lightweight foamed concrete. The range of densities were 1500,1750 and 1800 kg/m3 . The range of w/c ratio used were 0.5, 0.45, 0.4, 0.35 and 0.3, while s/c was 1.0 for all mixes in this work.

The above example shows a very dense FC, for your purpose you may want to aim for 1000 Kg m3.

The W/C ratio to create the optimum strength FC with the used Foaming agent can vary. A study has shown that different agents require different W/C ratio’s for optimum strength. This maybe because the water could leach out of the foam, but this is just my speculation.

Measuring the quality of your mortar

Because you can have variations in the quality of sand and the moisture content, how much water you put in, and other minor variations in your ingredients, the outcome of the mix need to be consistent to make consistent good FC. You need to measure the consistency; one way of doing this is the slump test. The slump test is a measure of the consistency and work-ability of the concrete. The consistency therefore is a measure of the water content of the concrete. The water content controls and affects the cement content of the concrete. Since the slump test is important, do not substitute a guess for an actual test. The mortar should be flow able enough to be able to mix the foam into it. If it is too stiff, than the foam will collapse,

Equipment needed for the Slump Test: Slump test cone, non-porous base plate, measuring scale, temping rod.

The mould for the test is in the form of an open top and bottom cone 30 cm height, bottom diameter 20 cm and top diameter 10 cm.

The cone is placed on a hard non-absorbent horizontal surface. This cone is filled with fresh concrete in three stages. Each time, each layer is tamped 25 times with a 60 cm long bullet-nosed metal rod measuring 16 mm in diameter. At the end of the third stage, the concrete is wiped off flush with the top of the mould. The mould is lifted vertically upwards, so as not to disturb the concrete cone. The concrete then slumps. The slump of the concrete is measured by measuring the distance from the top of the slumped concrete to the level of the top of the slump cone.

The measurement is taken directly after lifting the cone. This should be the same within 5% of what you want to achieve.

If the slump test result is outside of the slump range, correct before placing the concrete in the work. Make corrections as follows: Slump too low: Add water in measured amounts to bring the slump within the specified range. Slump too high: Add additional cement to bring the slump within the specified range. Use same manufacturer as the batch. Record the added cement for future reference. After the addition of water, or cement, remix the batch for 50 revolutions at mixing speed to insure adequate dispersion of the materials throughout the batch. Retest to verify compliance with the range.

If your slump height is difficult to measure you can measure the diameter of the “slump”. To make this easier, mark concentric circles on the board and place the cone in the center. Ensure the board is horizontal and lift the pipe. Record the result for future reference.

The most important thing is to be consistent in your method.

Testing your FC mix

You tested your foam and your mortar, now you need to verify that you have the correct density.

You can use the same cone, but fill it in one go and do not tamper. Your slump height will be too difficult to measure, measure the diameter of the “slump” instead. To make this easier, mark concentric circles on the board and place the cone in the center.

If it is too “thin”, change your mind of what you are going to make as adding mortar in is not a good practice. Not “thin” enough put more foam in the mix.

It is also a good idea to make a test sample(s) of each batch. Ensure you identify each sample. Even if you make brick, as the test samples should be of even size.

Formwork

The simplest way is to make bricks. The size depends on your building method and all the other factors that impact in the thickness of the wall. In my opinion, the less bricks you need to use to build a wall the better it is. The deciding factor may be the weight you can lift and place them, and make a straight wall. The less bricks, the less mortar you need, less finishing work and chance of water ingress through a joint.

The simplest way to make a form for bricks is using plywood and screws. These can last a long time, one hundred times is possible, done this myself.

The first rule is that the form needs to be strong enough to hold the weight on the FC. I never used anything less thick than 16 mm, also because the screws need a bit of thickness to hold and it needs to stay straight.

You need to be able to remove the form sideways from the FC. You cannot lift it straight up without damaging the FC if using a plywood mould. So making a long form with plywood dividers does not work for plywood form-work!

It is best to paint the plywood to stop it absorbing water. Every unevenness in the wood will show up in your brick !

I always use a mould release agent for “normal” concrete on the form as the concrete can stick to the form and pull out. The cheapest release agent is sugar water, but I am not sure what it does to FC. Try it and let me know. Look on the foaming agent label for compatibility!

If you want to use a metal mould, check the FC equipment suppliers, go to Foam agent and equipment suppliers

There are some interesting interlocking systems available.

Pouring your FC

Pouring your FC

Even more exciting than making the FC and can be just as tricky!

The critical points in this process are:

  • The mould is clean and prepped with releasing agent.
  • Sits perfectly horizontal, and stays that way under the weight.
  • You have enough moulds for your batch, plus some spare!
  • You can reach all the moulds comfortably when pouring.
  • Place the mould in such a position that you can easily take it apart.
  • We use to have to moulds on a table, but we had to transfer the concrete from the wheel barrow onto the table. With FC you may be able to mix the FC in a barrel that is placed above the moulds and have a hose attached to the bottom.
  • Control to pouring as not to spill.
  • Fill the mould to the correct level every time!

Clean your equipment every time! Did I mention to clean your foaming machine with (preferably) warm water!

Last but not least keep it tidy, this avoids accidents. Did I mention to clean up after pouring a batch?

Curing the FC

This is the process of the FC hardening. As you now have discovered, making FC is like baking a pastry cake, not just an ordinary cake. Now comes the best part, because you do not have to do too much. As for baking a cake you need a good reliable oven. It is the same for FC. Curing is a chemical process. The water reacts with the ingredients in the mix! All your efforts can be ruined if this does not happen as it should be.

Keep the poured form wet, or do not let it go dry, cover what you poured. Even if this is a whole house! Do not let go dry! You can also keep it wet after it has set with sprinkling water onto it. If you make blocks cover them until you take them out of the mould, then wrap the blocks in cling wrap. Leave them to cure for at least a week, four weeks is better. This curing process will go on for years.

The rules for curing FC are the same as for “normal” concrete, go to https://www.wikihow.com/Cure-Concrete

Some more on this, 8 pages and some interesting points.

http://www.ce.memphis.edu/1101/notes/concrete/PCA_manual/Chap12.pdf

Removing the FC from the mould.

This is best done when it is set enough to holds its own shape and strong enough to withstand the force you may put on it when removing the form.

This can vary from a couple of hours to more than a day. This depends on the ambient temperature.

Internal curing

Curing FC is a chemical process! It needs water. When all the available water is used for the curing, the process stops. Some of the ingredients may not have fully reacted with its neighboring component due to the lack of water. As a result, the FC is weaker at that point. The advantage of FC is that the bubble “skin” contains water and becomes available for internal curing. Some foaming agents may do this job better than others, but this is a subject for further research.

Internal curing can be assisted by the use of materials that absorb water quickly when whetted, but releases it slowly, or needs forces to suck the water out of the material. Super Absorbent Polymer (SAP) is such a material and can be added to the FC mix. Some lightweight aggregates absorb water and release it easily too, this makes it more difficult to get the correct water cement ratio, and this is utmost important.

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