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I've started making some small decorative concrete objects. Like trays, potters, table legs, etc. But recently my pours all go straight to the garbage bin.

The problem (or the symptom) is that the poured concrete breaks by looking at it, it breaks like a cookie, that soft.

The concrete that I used is bought, about, 8 months ago. It's translated called extra strong concrete mix. It has a water/mix ratio of 1,8/2 Liters per 20 Kg Mix. But since I use small amounts I've scaled it down to 0,09/0,1L per 1Kg.

Here are some of my thoughts and more detail:

  • In most molds I use black pigments. Could I over do it with the ratio and make it brittle?
  • I've got 60Kg of ~8 months concrete mix. Could it be old? I do have to crush big chunks from the bag.
  • Since I don't use the entire bag, and I work my way down the bag, could it be the cement is in top and sand in the bottom? Resulting in not getting a good mix scooping it out of the bag?
  • I've tried bigger and smaller water ratios. (the ration mentioned on the bag was way too little. It made it a bit wetter than damp sand.

Any tips are more than welcome :)

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2 Answers 2

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It’s too old.

An obvious clue is you stating you have to break up large chunks.

You may find it better to buy the separate components and mix your own mix.

This would mean you can get the cement fresh per batch. The other components won’t « age » so much or at all - thinking of gravel sand etc but any moisture content needs accounting for.

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    Do the ingredients age when stored on their own, or only when combined (prior to adding water)? I'm certainly no chemist, but I don't see how Portland cement stored on its own will age any less than it would when stored in a mixture with sand & gravel. Honest question, not questioning your assertion.
    – FreeMan
    Dec 11, 2020 at 14:43
  • Yeah, I'm not sure I follow this reasoning either. I'd think Portland cement would age (cure due to water in the air) whether it's stored with aggregate or without.
    – isherwood
    Dec 11, 2020 at 15:49
  • Due to poor planning , I have had bags resealed with tape, turn solid in a couple years from humidity in the garage.
    – blacksmith37
    Dec 11, 2020 at 16:22
  • @isherwood what’s difficult to have sand & gravel etc « in stock » and buy a small bag of portland to do a little amount as needed and then 2 weeks or months later buy another small bag of portland.
    – Solar Mike
    Dec 11, 2020 at 16:35
  • No one said that was difficult. I'm asking about the chemistry of what you said. I think FreeMan has the same question.
    – isherwood
    Dec 11, 2020 at 16:47
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Any pigment used can weaken the mix, there is a maximum amount that should be used. Check the suppliers recommendation for that.

Your concrete may me drying out too rapidly which will cause symptoms that you have. Once the concrete is set, keep it covered with wet rags to keep it moist at all times for the first 2 days. This should increase its strength.

You may want to consider the "dry pack" method. It is used with cast cement products for building products and shower bases built on site.

No, the age of the mix in the bag will not matter. I would sieve the dry mix to get the rubble out and keep it in a large durable plastic bin with a snap on top.

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  • That’s funny ´cos I have bags of concrete mix that are now useless as they have self hardened over time - useful if you want a concrete pillow though.
    – Solar Mike
    Dec 11, 2020 at 17:47
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    @SolarMike, Yes it will harden in the bags on its own if left to take in the humidity. If left outside in an out building it will harden in a couple of months. I have had it happen to concrete and thinset. If it is never exposed to any moisture at all, it will last indefinitely. Keeping it inside where the humidity is better controlled, which may be the case for the OP since it is being used in form for casting that it may be stored indoors. Even if the OP is not working or storing the materials indoors. Keeping the moisture content to 0% will keep the concrete from hardening.
    – Jack
    Dec 12, 2020 at 2:42
  • Ah yes, I was thinking of buying a sealable containter to store it. This way I can also make sure it it mixed well before using a small portion of one bag. I've also recently had issues with concrete without the pigment. And on the other hand also had way too much in one pour because I wanted REALLY black concrete. That went bad :) And yes, I have all my casts covered with plastic. Had my share of cracked casts in the beginning :)
    – Tim
    Dec 13, 2020 at 11:51
  • As an example, I was clearing out some stuff in my unheated workshop and came across a bag of fast setting cement in a sealed bag. Still as fresh as can be with no clumps
    – Jack
    Dec 13, 2020 at 19:11

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