Formulas for Lightweight Concrete

Fernando Martinez Lewels has a M.S.C.E degree from the University of Texas at El Paso. He is now working with the Agartif company in Chihuahua, Mexico (about 170 miles from El Paso Texas). This Company has developed a type of lightweight aggregate that provides material for all types of construction needs, at reasonable cost and with good thermal insulation values. They manufacture the equipment required to do this according to the needs of their customers; the feed stock are common construction materials that should be available in most locations. Their philosophy in developing this type of aggregate is to be able to use this everywhere, without depending on a lightweight aggregate quarry, so you can have access to this material in any part of the world. In Mexico we have a saying that "we build our homes so we have to go outside in the summer to be fresh, and in the winter we go outside to catch the rays of sun to be warm". Lightweight concrete can help this situation by making available materials for more comfortable homes.

Questions and Answers

Q: Would you be kind enough to recommend a lightweight formula for our tabletops. Size: 48x48x1.5". These tops are tiled with marble mosaics. We can embed steel if necessary. Aesthetics are not important as they will be fully encased. However they must be strong enough to perform as tabletops without worry of breakage.

A: (Bruce Schundler) If it won't be exposed, we would recommend a one to four mix (one part Portland to four parts by volume of perlite or vermiculite) Add some fiber reinforcement (available at mason stores). The problem with the concrete if it is exposed is that it isn't very hard---and the surface would have to be hardened with some special coatings. Of course, I am presuming this concrete will have some kind of structural support under it. If not, lightweight concrete wouldn't work. All concretes are very bad in terms of tensile strengths--or their ability to support weight across spans. They are good for compressive strength, but not tensile strength. Often corrugated metal or metal forms have to be used if the concrete is spanning an area between two beams.

Q: You did not recommend the use of concrete on an exposed deck. We are planning to enclose our deck with a three seasons application. Our current deck boards are 5/4 treated lumber. We want to install radiant heat in the floor using pex tubing incorporated into a suspended slab of 1 1/2 " of concrete that we would lay tile on. Can you recommend a mix for this purpose?

A: (Bruce) I would recommend contacting a local redimix concrete supplier for his advice on what is available locally. Appropriate lightweight aggregates are different depending on what part of the country one is in.

Q: Can you tell me an easy way to aerate concrete with or without chemicals, as here in Indonesia chemicals & pumice etc. are hard to obtain!! I have made a large high speed mixer and am using an emulsifier (detergent) and have had mixed results. Problems in curing! Tried adding more lime etc. Can you advise?

A: (Bruce) Normally lightweight concretes need air entrainment--and normally the air entrainment can be purchased in the concrete by ordering Type 1A portland instead of regular Type 1. Or, in a pinch, people have used non-sudsing detergent (e.g. the type used in washing machines or dish washing machines--not the type used in kitchen sinks. But......different cements and different ingredients and different products can all affect what is used and how and why.

Q: I live in Eugene, OR, and every year this city has what can only be described as a "leaf problem". In other words leaves clog up the streets, drains, etc... Has anyone ever looked into turning leaves into paper products or building materials?

A: (Kelly)This website might interest you, since Phillip has been including various plant materials in his "hybrid adobe": hybridadobe.com

Q: Have you heard of this new building material yet? Its called hempcrete a mixture of concrete and hemp. The Oglala Sioux tribe who have been fighting for years to grow industrial hemp as a crop on their reservation want to also make hempcrete to help with the cost of housebuilding there. I was wondering what you think?

A: (Kelly) No, I have not heard of hempcrete, but I know that folks have been experimenting with adding straw to concrete to help with reinforcement, and hemp could be used for a similar purpose. The idea of making more insulating concrete with the addition of hemp could yield some improvement, but I would not expect much in this regard; any time you try to make a basically dense thermal mass material (such as concrete) serve as an insulation material, the compromise is rarely worth the trouble. If the hemp provided a significant portion of the volume, it might approach a good insulation material, similar to papercrete. I would suggest quite a bit of experimentation to ascertain how well it might insulate and what the properties of the material might be regarding durability, susceptibility to rot or deterioration from moisture, etc.

Q: I am having difficulty locating a source for a test quantity of cellular concrete foaming agent. Seems that that are not many suppliers in USA.....it has not 'caught-on' here yet. I have sent a few e-mail inquiries and seem to be to 'too little' to be of interest. I want to make homes that are non-traditional/contemporary......ie...no brick....no sheetrock....no asphalt shingles.....etc.

A: (Bruce Schundler) Especially in Texas, there is a lot of cellular concrete. Many new flat commercial roof decks in the south are "poured" with cellular concrete. And it's been used for over ten years now. Probably redi-mix companies and commercial concrete/masonry supply stores would know more about it. The problem is that it's not usually the kind of concrete that is used or purchased by DIY or small contractors, and usually is manufactured on the job by the roof deck applicators. Another possibility would be the easiest---go to www.google.com and type in cellular concrete and you will find the manufacturers , information, etc, etc.

Q: Our Computer Aided Drafting class is looking for a lightweight concrete formula for making concrete canoes. We need a recipe that is less than 40lbs per cubic foot.

A: (Kelly) There are a number of mix designs on our web site at www.schundler.com/const.htm in the concrete section. Both perlite and vermiculite, mixed with other products, have been used in the concrete canoe "races".

Q: I am looking for a solution to a muddy fenced in dog yard (with lots of digging). Found cellular cement grassy pavers which are used in Europe for grassy parking areas. Thought that might prevent dogs from digging while still allowing for grass to grow. Have my own plastic forming machine (for signs) and thought I would create 2' x 2' x 2" molds to form cellular blocks for molding purpose. Wondering if lightweight cement might work (easier for me to handle). If so, what would be the suggested mix?

A: (Bruce Schundler) Lightweight perlite concrete could be used, but like cellular concrete, it does not have a lot of flexural or tensile strength. It probably should be mixed with fibers to give it more strength, and the mix could vary depending on a balancing of strength and weight. I wouldn't use a mix weaker than a four to one mix, however. (see our charts---especially those for special mix designs.)

Q: Would it work to use rice hulls as a concrete aggregate? I would think with their high insulation value, low moisture-rating capabilities, and economical value, they would be a good choice for green builders. Using fly ash as well as rice hulls would be only better, I would think. I would appreciate your thoughts on this.

A: (Kelly) I have not heard of anyone using rice hulls in cement. They might work, but I question the advantage of using them. Being encased in cement would severely limit their insulative qualities, and the product would not likely be as durable as mineral aggregate materials. I suggest that you run some experiments to test the results...and let me know how they turn out. I prefer to find ways to avoid using cement at all, since its manufacture is polluting. Rice hulls work well in earthbags.

Q: We are a company in the Dominican Republic who are looking for a lightweight concrete to build a number of affordable or low-cost housing. We have found a supplier in Romania of a product that allows us to build a house in 7 hours but it is very expensive. Do you have any suggestions for us to make a good project in affordable housing.

A: (Kelly) Lightweight concrete does not necessarily require the use of expensive or unusual additives. It is basically concrete that has replaced the standard heavy gravel aggregate with a lightweight substitute, such as crushed volcanic stone, perlite or vermiculite. There are special entrainment methods that introduce air pockets into the mix, and these do require additives.

Q: I have large supply of natural shell grit ranging in size from 3 to 15mm and a few 25 to 40mm shells. The type of shells closer to the 15mm end are generally broken bivalves while the smaller end are snail type shell and are generally complete. The grit weighs in at 700kg per cubic meter. Would this be suitable for lightweight concrete and what mixing ratio should I use. I have also much beach sand which is fairly fine also. Should I use this or not?

I have read that lime is used by Steve Kornher...for what reason? I would need to plaster the inside and outside on completion to seal it, and as the concrete would be porous, would this still have a good insulation value?

A: I would think that the smaller snail-type shells that contain a fair amount of trapped air would provide a reasonable aggregate for lightweight concrete; the larger broken bivalves are probably less useful in this regard, since they are more solid or hefty.

As for the beach sand, it may not be ideal for concrete, since beach sand is often rounded and not particularly sharp (which is the ideal)...but less than ideal sand does work for concrete in any case. I would not plan to use much of either type of sand with a lightweight concrete mix, as it will lessen the insulative value.

Lime is used for several reasons in concrete work:

1) It is cheaper than Portland cement.
2) It is lighter than Portland cement.
3) It provides a creamier, more adhesive mortar.
4) It combines well with Portland cement.

Interior and exterior plaster over lightweight concrete will not affect its insulative value. My suggestion is that you do some experiments with various mixes of these materials and see what works best.

Q: I am wondering if anyone has done anything with a cementitious mix with a mulch base. I am specifically interested in any experience with leftover corn silage.

A: (Kelly) I have never heard of anybody using mulch material this way...and probably for good reason: it would likely not stand up to the rigors of time. Other organic materials, such as straw and rice hulls have been used this way, but these are silica-rich and do not readily break down as would silage.

Q: I have heard of applications using Styrofoam as a structural form - but I have searched extensively and have not heard of Styrofoam used as an aggregate. I was under the impression that styrofoam was inherently " incompatible" property-wise.  Would like to know or read further as to how this could be accomplished?

A: (Kelly) I suggested that you investigate the Rastra icf system since they actually do use EPS as an aggregate in a cement slurry to make their forms. I really see no reason why you can't use EPS as an aggregate in concrete as long as there is sufficient slurry to surround the beads. It would certainly not be as strong as ordinary concrete made with gravel, but if there were sand in with it, I'm sure it would form a fairly solid block. Why not experiment, and report back?

Q: We are trying to develop a lightweight pumice concrete here in South Texas. We tried some mix designs, but the compressive strength yielded has been low (400-500 psi). I was wondering if someone has developed a good mix design to yield an 1200-1800 psi design. We are using pumice from New Mexico and cement. The ratio is 4:1 pumice to cement. Should we be using sand?

A: (Kelly) I have not personally mixed or used pumicecrete, so I can't speak from experience. The best information I have is: When mixing pumicecrete, the idea is to use just enough wet cement to coat the aggregate so it will adhere to the surrounding particles. Too much cement will defeat the purpose of maintaining all of that trapped air; about three bags of portland cement per cubic yard of aggregate is recommended. Once the material has set up a bit, the surface can be washed to expose the natural color of the stone. The rough texture of pumicecrete is ideal for adhering to further plasters that might be used.

I'm not sure what the compressive strength of the above mix would be expected to be, but it might be about the same as what you were able to obtain, since 3 bags of cement per cu. yard would yield about the same ratio as you were using. I expect that if you did add sand to the mix that it would increase the compressive strength, but would also diminish the insulation value. Perhaps your design needs to be altered so that bond beams or columns can help distribute the weight.

Q: We are an organization embarking on a project for the homeless through low cost housing. I would like a specific lightweight concrete formula for house walls that has some structural qualities reasonable insulation qualities and can acheive a smooth finish. A mix that does not reqire sophistcated machinery to prepare is low cost and durable; above all easy to prepare. Can you help us? We are in Australia.

A: (Robert Alexander) Below is the recipe for the mix we used to build the pumice-crete house

5 - 5 gal buckets (25 gal.) of crushed, graded (3/8" ) and washed (no fines) pumice (dripping wet)

1/3 of a 5 gal bucket Portland cement

Approx 2 Gal. clean water

Mix just long enough ( ~2 min. after everything is in) to coat pumice.

Mix should look "slick" but, not "wet". If more than about a cup of water runs out when you pour mix into form, it is too wet.

Scoria or cinders may be used in place of pumice if available. Beware that there is a possibility for Radon problems using cinders.

Over-mixing lowers R value which is somewhere between 0.8 - 1.2 R/inch depending on mix. We used a gasoline powered paddle batcher which worked well.

Our slip forms were made from 16" wide strips of 3/4" ply with U-shaped brackets which slipped over the top to keep forms from spreading. Small L brackets were installed at the bottom which rest on wall for next tier.

Mix has the consistency of tooth paste and does not "flow" readily. It will hold about a 45 degree "angle of repose".

Q: I had some questions about whether straw or rice hulls could be mixed with lime and some cement. I have seen this done in hempcrete and the builders claim a consistent 65-70 degrees F 24/7. If a straw/lime mix could be incorporated I'd assume the cost would be far less than hempcrete because of the abundant amount of straw and hulls available in this country and even overseas. I am going to begin my own experimenting on formulas and making bricks out of these mixtures. How do you think this will work?

A: (Kelly) I suspect that you could make a lightweight concrete using rice hulls with cement, lime, or some combination, as has been done with straw to some extent. The result would likely not be as insulating as the rice hulls alone placed in earthbags. Any claims about temperature ranges is highly dependent on both climate and the density of the material. Doing some experimentation is the way to go...and let us know your results.

Q: How do they make the small, lightweight porous bricks in Mexico that are used to build vaulted roofs? I can get low-fired clay bricks, but they are not porous like the ones in Mexico. What can we do to make the bricks here more porous?

A: What I have done is to prepare the clay with a chemical agent to generate a porous wet brick, then I have fired it up, resulting in a hard, strong lightweight brick.

Q: Can you tell me about aggregate from Styrofoam for lightweight concrete? And how about the cost?

A: (Kelly) It is possible to make lightweight concrete using Styrofoam as aggregate, and this has many possible uses, especially in situations where there is a need for lighter concrete that is better insulating, but you cannot expect this to be as durable or hold up very much weight, so you need to be careful how you use it. I doubt that there is much difference in the cost.

Q: I am sending you an update on some experiments I have been doing at NJIT with the concrete engineering students. We have done about 6 different sample mixes that were compacted in cylinder molds and will be crushed in 28 days to test compressive strength. After that, if the results are satisfactory some cylinders will be sent to test insulation value.
Our formula used rice hulls in place of hemp hurds but I'm sure either can be used.

We worked out formulas by volume and no more than 50 percent rice hulls were used in any formula. We used a magnesium /phosphate cement. I am not sure what the proportions were so we mixed it 50/50 magnesium oxide with soluble rock phosphate. We had about 30 percent sand in the mix and I think maybe this could be lowered and the cellulose raised. I realize if the formula is a little porous because of less sand than the plaster and mineral paint should make up for any porosity on the exterior. We also did some formulas using portland cement and lime, and one using lime with magnesium phosphate. I'm hoping this will give us a good idea of where we can stand with strength/insulation relationship and tweak the formulas accordingly. We tried to keep the cement levels to 20 to 25 percent to maintain high strength. If the strength factor is there after testing this first batch I may opt to raise the cellulose level, cut the sand to 15 percent and raise the cement level.

I am wondering if pea gravel can be used as aggregate to raise strength and cut the sand out all together or drop it to 10 percent. Since most concrete formulas are done by weight we had to do ours by volume because of the light weight of the rice hulls. I'm not sure if the pea gravel would stop it being breathable but I don't think it should be that much of a factor if the rice hulls are used as fine aggregate instead of sand. If you can offer any advice I'd appreciate it.

The formulas are intended for mortarless concrete blocks. They will be reinforced with galvanized rebar but I would not like to include the strength of steel into our block. I know the steel will give it more than enough strength for code purposes but it is important for me to develop a block formula that can be stacked without reinforcement for single story homes.

A: (Kelly) This is fascinating and should yield some really interesting and useful results. I am very pleased that you are proceeding with these tests. I don't really have any specific advise, although I do expect that the pea gravel experiment will work well enough, as you suggest.

A: (Mike Collins) Try pumice; it is way better at bonding and has the pozzolanic effect, and what's more, you can go quite light on materials...start as high as 1-10 and work down. Also note that it pays to use all grain sizes of pumice. Compressive strength is excellent as we have a bench in my house in Mexico and it feels like wood in its thermal and strength characteristics. I also make doors and sculptures and roofs of this material. I recommend aggregates with sharper and more bonding surfaces. You can add a little borax; that helps the reaction proceed.

Q: My question is regarding perlite versus vermiculite for lightweight concrete: I know that perlite will wick water and vermiculite will absorb water, so how will this affect the concrete? Is it better to use one or the other or a combination of the two?

A: (Kelly) Typically only one of those lightweight aggregates is used in any given application. In all cases, it is best to damp-proof the concrete so that the embedded aggregate does not get wet and retain moisture, which would diminish the insulative value of the concrete.

Q: I am building a family home for 6 with little cash (pay check to paycheck method) and I want to use a pumicecrete. Its important that I "pour" the house in 1 day, wait 3 days, remove the forms, and put on the roof. I understand the basic premise of using pumicecrete but how do you get the pumice into the forms? ( I know it is not poured.)

A: (Kelly) Basically it is poured. When mixing pumicecrete, the idea is to use just enough wet cement to coat the aggregate so it will adhere to the surrounding particles. Too much cement will defeat the purpose of maintaining all of that trapped air; about three bags of Portland cement per cubic yard of aggregate is recommended. This is poured into forms. If you do this all in one day, you might want to use some sort of a conveyor or pump arrangement.

Comment: The Russians and Polocks call it EPS (extruded polystyrene) Concrete. The ferro boys call it epscrete. The unclassified random weirdos call it styrocrete/styracrete. The NM Papercrete website guy calls it paracrete. A few Siberians mix 20 parts EPS to 1 Portland, and say it gets surprisingly hard and can be trowled and polished nicely or stuccoed. A quick mad scientist diy EPS grinder blowing the ground pearls into 55 gallon trash bags is in order for testing. There are only a couple masons really using this technique in our fair land and they are only doing 5:1 or so mixes for pours. I think 20:1 plasters would be awesome for sculptural roofs, but only have been done in wall pours...

Response (Kelly): This is confusing me a bit, because EPS usually means Expanded Polystyrene; extruded polystyrene (XPS) is the kind of blue or green board that can be buried and has a closed cell structure. I think that XPS has better insulation values than EPS. EPS will absorb moisture over time, not that that is necessarily bad for the application intended (although it might be, depending on the use). I've often thought that one could quickly grind up large chunks of styrofoam in a small home shredder of the sort that is used to shred small branches to make mulch. Let me know how your experiments turn out.

Q: I would like to make some planters that are long lasting and that will be exposed to both rain and sun. I would like to use a 3-2-1ratio of perlite-cement-sand respectively. Can you tell me if this mix will be long lasting?

A: (Kelly) Common cement to aggregate ratios for lightweight perlite concrete run from 1:4 to 1:8. The ratio that you propose would be 1:2, so you would likely be using at least twice as much cement as necessary.

Q: I am hoping it can help me with a plan I have. I am wanting to level a wood floor in an old cabin and want to simply pour a mix of portland and perlite or vermiculite into the low spot and have it self-level itself. I will then cover the entire floor with 3/4" plywood. What would be a good ratio of portland, water and perlite to ensure it self-levels?

A: What you suggest does seem possible to me, but I would have a few concerns. The original wood floor needs to be solid enough to support the weight of the new materials and you would need to be convinced that it would remain intact over time. The fact that the floor is so uneven would suggest to me that there may be serious problems with the foundation or floor joists under the floor. Also, you cannot expect the lightweight concrete to puddle into a completely level  surface with some troweling and checking with a level. A 1:6 (one part Portland cement by volume to 6 parts perlite by volume) mix would be appropriate and add enough water to make it easily spreadable, but not so wet that it is dripping.

I am curious, based on your experience what a 1:6 ratio would weigh to level that section?

Perlite concrete can weigh between 20 and 09 lbs. per cubic foot, depending on how much sand or other aggregate is added to the mix. You wouldn't need a whole lot of compressive strength for your application so it could be on the lighter end of that scale.

Q: I want to make lightweight concrete railroad ties. 4" thick x8" wide x4' the only weight on these would be me walking on them. I want to try and line my garden with them. What would my light weight mix be?

A: (Kelly) Typical mixes of cement:lightweight aggregate range from 1:4 to 1:8, with the 1:4 ratio being more dense, which might be better for your application. Some of the aggregate can be sand, which also helps make it more dense. Then you would add enough water to make a reasonable wet mix.

Q: I need to know if it's a good idea to add fibers in aircrete?

A: (Chris Steen) Not if you plan on utilizing a lightweight design, as it has little structural value. For denser mixes, or put more practically, structural cretes/cements, I always favor a steel matrix where the reinforcement is aligned in the direction needed. Fibers are so randomly oriented that in my judgement, they only matter in exterior dome plasters subject to freeze thaw, or even surface bonded drystack block such as Qbond. Slabs, walls and masonry roofs have little benefit from fiber when a properly oriented reinforcing matrix could be utilized.

I strongly recommend an engineered quality commercial aircrete surfactant, not some dish soap. Dawn soap is for cleaning ducks after crude oil spills. It has no place in a masonry mix.

Q: I would like to enquire about aircrete using soap foam. I have tried a few mixes and have failed using regular portland cement. Is there a way you can help me out with some advice as to soap mix ratio to water etc? Id really appreciate your assistance since here in Belize, roofs are made of tin which extremely heats houses making them unbearable. I have made a soap machine that operates at 20 psi.

A: (Chris Steen) Quality surfactant (soap) is a game changer. See http://alliedfoamtech.com/Appconc.htm You could also try bumping up your PSI and maybe fiddle with how air is introduced and mixed. I would also think that traditional / locally available insulations under the roof would be easier to install and maintain.

I have some more samples curing to see what the results will be. The reason I'd like to do aircrete is to be able to master the construction of domes. This would eliminate the use of tin roofing material. Hopefully its more affordable than cinder blocks.  But yes, will increase psi next and keep mixing my samples in a small container with a spoon or fork.

Q: We are wondering about using aircrete on the upper deck of our house covered with tile, and also plastering the exterior of the earthbags with aircrete. What do you think about this?

A: Aircrete is not suited as plaster. You could pour, membrane seal and thinset roof tile. But the stakes are high considering tooling up, getting a good mix, labor, impact. Especially considering the abundance of used commercial poliso on the market.

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