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I'm currently mixing render to cover ugly brick walls in my front garden.

Let's say I want to use this recipe for the top coat, just for an example:

  • Cement 1 parts
  • Lime 1 parts
  • Sharp Sand 2 parts
  • Soft Sand 2 parts

The exact recipe isn't critical but my question is ... well a 25 kg bag of lime is much bigger in volume than a 25 kg bag of cement or sand, about twice or close enough, so are the recipes I'm finding online by weight or by volume? None of them say.

Now when I'm mixing mortar I just use shovels (Volume), and that seems to be the advice I'm finding online and cement and sand are pretty close in weight/volume so it's fine, but now I'm seeing this huge difference in the weight/volume of lime I am concerned that the recipes will be way off if I don't break this question once and for all.

Also, I've read all the articles that say it doesn't matter because sand and cement have similar density, eg: Understanding render mix ratios but please remember that this won't cut it when lime is involved because the densities are definitely different.

spl
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  • Usually they are ratios, it could be weight, volume ect, still ratio – Ruskes Jul 06 '22 at 00:40
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    Think the word "parts" means more volume than weight. If weight most people would tend to use weight measures(lbs, kgs), especially if the densities were much different. Good question, hope an expert chimes in. – crip659 Jul 06 '22 at 00:52
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    @knowitall which weighs more: a pound of sand or a pound of feathers? Now how about a cubic foot of sand vs a cubic foot of feathers? So can you really say that the ratio is the same when comparing volume to weight? – brhans Jul 06 '22 at 01:31
  • @brhans If it says 1 pound of concrete and 5 pounds of lime, how to you express that in ratio. ??? yes, you are right 1:5 – Ruskes Jul 06 '22 at 01:48
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    My *hunch* is weight, because when it comes to these kinds of quantities, you typically don't *measure* it so much as "add 'x' bags of each item". And the bags (at least what I have seen) of these types of materials are measured by weight, not volume. But that's just a hunch, because until I saw this post, I thought render had to do with chicken fat. – manassehkatz-Moving 2 Codidact Jul 06 '22 at 01:51
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    @knowitall the problem you're skipping over is that the common "recipes" do not give measurements in pounds or shovels or buckets. They use "parts". So if you're measuring these "parts" in volume and you mix 2 gallons of sand with 1 gallon of lime you get a different ratio of sand to lime compared to measuring "parts" by weight and using 2 pounds of sand with 1 pound of lime - because sand and lime have different densities. – brhans Jul 06 '22 at 01:54
  • @manassehkatz-Moving2Codidact nobody uses weight for mixing concrete. If at all it will be volume. One bucket of concrete with 5 buckets of Lime – Ruskes Jul 06 '22 at 02:50
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    The density of cement is 1.44 g/cm³, while the density of hydrated lime is 2.21 g/cm³. If I mix 2 parts cement with 1 part lime by weight - lets say 20g cement + 10g lime and then look at their respective volume I have 13.9cm³ of cement and only 4.5cm³ of lime. So even though I mixed 2:1 by weight I now have roughly 3:1 by volume. – brhans Jul 06 '22 at 02:52
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    The comparison I think of is food recipes. Typical (US) consumer recipes are by volume for many ingredients - 1 cup sugar, 2 cups flour, 1/2 cup oil, etc. But as I understand it - and as I have seen myself in *commercial* bakeries (well, bakery - I had one as a customer once) measuring by *weight* is far more consistent, particularly for things like flour. But I don't know what is "normal" for mixing concrete, etc. A quick Google search found "parts" again :-( – manassehkatz-Moving 2 Codidact Jul 06 '22 at 02:54
  • @brhans newer seen a scale to measure concrete mixing. A "part" has no measurable definition, what is a Part ? You do not use scale in concrete mixing, you do that for making cookies. In construction they use volume, buckets – Ruskes Jul 06 '22 at 02:55
  • The Ratio is either in Volume or Weight (or something else), but for all ingredients. You can not mix (use) the volume for one and weigh for other. In construction typical a volume is used (shovel, buckets), newer seen a scale used when mixing cement. A "part" could be anything you are using, a cup a shovel a bucket (a tee spoon). – Ruskes Jul 06 '22 at 03:21
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    Nobody is suggesting to use volume for one 'ingredient' and weight for another 'ingredient'. Neither is anyone suggesting to use a scale to measure them by weight. Have you never seen a 50lb bag of cement or building sand? That's measured by weight not volume. – brhans Jul 06 '22 at 03:24
  • All the brickies I know and worked with mean shovel full aka "parts" and then from the basic mix 1:1:2:2 they would then say use 5 so that becomes 5:5:10:10 and each would be counting shovel fulls. everything from cement mixes to concrete to limecrete. – Solar Mike Jul 06 '22 at 07:35
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    When they make the recipes in a lab, it is probably done by weight, more exact. They then convert those recipes to volumes to be used in the field/home/construction site, to make life easy. – crip659 Jul 06 '22 at 09:52

2 Answers2

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Disclaimer: I am NOT an expert on cement, mortar, render or other brick laying techniques/technology.

I think the confusion comes from the fact that you're looking to mix in small quantities.

If you were mixing a large quantity, you'd mix by the bag:

  • 1 bag of cement
  • 1 bag of lime
  • 2 bags of sharp sand
  • 2 bags of soft sand

and you wouldn't think twice about, nor would you care, that the bag of lime was much bigger (other than grumbling as you tried to heft it into the mixer).

If you need to make a "half recipe" because you don't need that much, you'd use 1/2 bag of cement & lime and 1 bag of each of the sands.

Since you seem to be interested in making even smaller quantities, you now need to measure, somewhat accurately, your materials. Since you're simply working in "parts", and you now want to measure small quantities, you need to know how to measure the parts.

I think it stands to reason that you'd continue to measure in fractional bags. Since the bags are labeled by the manufacturer by weight, not volume, it would stand to reason that you want to continue to measure your material by weight and not make the switch to volume.

So, a small recipe would now be:

  • 1/25 bag (or 1 Kg) of cement
  • 1/25 bag (or 1 Kg) of lime
  • 2/25 bag (or 2 Kg) of sharp sand
  • 2/25 bag (or 2 Kg) of soft sand

There's no switching of units of measure because your original "bag" measures were all 25Kg units.

NOTE: If the bags are labeled by volume not by weight, then the exact opposite is true: You'd need to make your partial recipe by volume, but it would still be a "fractional bag".

Disclaimer: Again, I am NOT an expert on this matter, I've just worked through it in a way that seems logical to me.

FreeMan
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  • ...but if mixing large quantities, you don't have **bags** of sand, normally. You have a pile deposited by a dump truck. And a mixer. And a shovel... – Ecnerwal Jul 06 '22 at 13:11
  • @Ecnerwal and as a result you'd probably mix it wrong at least '(technically' anyway). But since the relative densities of the common materials aren't that much different, and it's probably rare to use materials with a very different density, the resulting mix when done by volume is usually close enough. If you're making a mortar mix with something like lime, then you almost certainly don't have a pile of lime lying on the ground, but rather have it in bags portioned by weight. – brhans Jul 06 '22 at 13:15
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    valid point, @Ecnerwal, but I've also seen plenty of videos of guys pulling bags of material from large stacks and dropping them into the mixers, too. And, if you've got dump truck loads of material, you may also have a bucket loader instead of a shovel. (At least, I'd hope so!) – FreeMan Jul 06 '22 at 13:20
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    Well done. It's interesting to note that ready-mix concrete is _sold_ by volume (cubic yards in USA) but is _batched_ by weight at the plant (have a look at a delivery ticket!). One could (and I have) done the same with a bathroom scale and bucket. That said, for small jobs it's common practice to use an informal "lots of this, some of that, mix with water until it looks right" recipe. The reality is that concrete works with a wide range of ratios. Often it's not necessary to optimize for cost, nor to guarantee the correct 28-day strength, and so "good enough" is ... good enough. – Greg Hill Jul 06 '22 at 16:03
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    Good points, @GregHill. If you're pouring the foundation for a 52 story sky scraper then you'd better get the mix right. If you're making a skim coat for an existing brick wall as a decorative overlay, it's probably not quite as critical. – FreeMan Jul 06 '22 at 16:08
  • I'd almost added in that the landscape material I've purchased in the last few years is labeled in both cu/ft or cu/yd _and_ weight, but the volume is the prominent measure and the weight is more of a "oh, BTW" note. I've always seen that concrete premix bags are labeled by weight, but the individual ingredients could be bagged by volume. – FreeMan Jul 06 '22 at 16:12
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It's nearly always volume with building materials. The reason this is true is mostly one of practicality. It's much more of a hassle to weigh raw materials on a jobsite than to measure or estimate volume. While recipes depend on density to some extent, they're set up to be applied by volume.

Some examples...

  • Masonry is mixed by shovelful, bag, or cubic yard (sand, gravel, cement). While bags are filled by weight, the quantity of bags in a recipe is more about volume.
  • Tile mortar is mixed by volume parts (dry powder and water), though some products imply both (quarts of water to a bag of powder weighing N lbs.). Really you're dealing with volume in either case.
  • Paint is colored and thinned by liquid volume, irrespective of density (base, colorants, thinners).
isherwood
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