4

In making up cement-based mixtures, I always start with a water:cement ratio of 0.42:1, which is the optimal ratio. But whenever I make a mix that has a substantial quantity of sand in it (e.g. for mortar, or a self-consolidating pourable thing) the sand seems to gobble up all the water and turn the mixture into a gritty, unworkable paste. I always need to add a substantial amount more water to make it workable, which I know weakens the mix.

I've tried commercially-packaged masonry sand, sieved sand from my backyard, and the texturing sand I had in an old package of paint texture. Same diff, though it was worst with the paint texture stuff, which was very fine and dry.

I tried adding a commercial PCE superplasticizer (0.5% by weight) but it didn't seem to make any difference at all.

I don't have an electric mixer, so I have to do the mixing by hand. Could my technique be the problem?

Also I live in the desert so everything here is just naturally dry all the time; could it be that most people's sand has more moisture in it?

Improve this question
2
  • 2
    The last point seems most likely. But if we are to critique your hand-mix technique, you'd have to share what you are doing. Getting "no effect" from the superplasticizer does seem to point to a potential mixing issue. And mixing concrete well, dry enough, by hand is a bear - there's always a temptation to add water and make it easier (and weaker, as you know,)
    – Ecnerwal
    Feb 28, 2017 at 3:26
  • Ditto to what @Ecnerwal has said. Mixing optimal cement/sand/water mixtures in a wheelbarrow or mortar mixing trough is a strenuous task and takes more time than you would expect. There is a good solid reason that mixers were invented!! And every time I go to mix up several bags of Quikcrete in my wheelbarrow for some fence post holes I always wish at that time that I owned a mixer. :^)
    – Michael Karas ♦
    Feb 28, 2017 at 3:37

1 Answer 1

Reset to default
1

The cement/sand ratio for mixing mortar or mixed aggregate varies depending on what you are rendering or making. In the building industry different qualities of mortar/concrete are required. The usual standard is that water and cement are measured by volume and are the same amount. However in real life, proportions are not so exact. If for example you are laying aggregate on a dry base, for example earth, sand the water will be sucked out of the mix, and the mix will be weakened. Laying on dry earth or sand must be avoided, and the area must be soaked before laying, or the area should be covered in plastic to keep the water in the mix. As mentioned elsewhere, sand can be bone dry, or in any state up to totally water logged - you need to adjust the amount of water added to the mix to allow for this.

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.