Why can't I make cement?

MidlandsGlory

When I try to make small amounts of cement, 1 measure standard cement, 3 measures building sand, well mixed, add a splash of plasticiser and enough water to make a smooth but firm fudgy mix. Yet when its set is like gritty shortbread, so weak you can crumble it to dust between your fingers. I don't get it.
We had a builder do a wall a few years ago and he just shoved some stuff in a small mixer, the cement on that wall is so smooth and as hard as steel!
It seems such a simple thing yet I don't understand where I am going wrong!

FreeBear

Try a 5:1 or even a 6:1 mix of sharp sand (not soft/plastering/builders sand) and fresh cement. If you are only mixing a small bucket at a time, leave out the plasticiser - It is very difficult to measure out the precise quantity, and you could well be overdoing it.

Mix the sand & cement dry until you have an even colour, the start adding small quantities of water. Leave to stand for five minutes, then give it another mix.

If you are laying bricks or repointing, give everything a good soak before starting - The brick needs to be damp, but not dripping wet. Once the job is complete, cover with damp hessian and leave it well alone for two days at least. Cement will harden within a few hours, but takes weeks to reach full strength. But after two days, it should be hard enough to do another section.

MidlandsGlory

I want to make a strong mix for pointing quite big gaps between cotswold slabs that will survive a bit of jet-washing in future..

grumbler

Is the cement fresh enough?

Also, if the slabs are porous, the weather is dry and the mortar in the gap dries fast, it can't set properly as it has to be wet to set. Wet the slabs well first and then try covering them with something to prevent drying (or spay with water periodically).

FreeBear

MidlandsGlory said: I want to make a strong mix for pointing quite big gaps between cotswold slabs that will survive a bit of jet-washing in future..

Mix it too strong, and the mortar will just crack. Water will penetrate the cracks, freeze over the winter months, and cause further cracking. Once fully cured, a 5:1 mix will resist a jet wash just fine.

Mistral001

grumbler said:

Is the cement fresh enough?

Also, if the slabs are porous, the weather is dry and the mortar in the gap dries fast, it can't set properly as it has to be wet to set. Wet the slabs well first and then try covering them with something to prevent drying (or spay with water periodically).

This is most likely your problem.  Either old cement or putting it on dry porous bricks, or both.  Dampen bricks before applying mortar to prevent the bricks sucking out moisture which is needed in the chemical reaction to harden the mortar.

stuart45

What type of tool are you using to finish the joints with? 

clive0510

A  good 5 to 1 mix should be just right for what you are doing.if it hasnt rained for a few days, spray water in the gap and also over the slabs before you lay the  muck in there, its stops it going off too quick,

MidlandsGlory

Thanks everyone, I will give it another go next weekend,  this is the sand I am using, says its for cement but what do I know!
https://www.buildland.co.uk/product/building-sand-poly-bag-each/
the cement I compacted into a couple of test joints isn't too bad, its the left over clumps that worry me as seem so crumbly, does cement need quite a bit of compacting to set strong?

stuart45

It's better when compacted. Finishing with a metal jointer or trowel makes a more watertight finish.

[Deleted User]

always add a good squirt of fairy liquid, my builder FIL taught me that one.