Cement goes off too quickly

[leen...]

Hi all,

I am laying some blocks and the cement is drying out /going off after about 25 mins or so. Not completely dry but dry enough to make it almost impossible to lay blocks. If I add a little water it is ok again - I don't think I am supposed to do this though. I am very slow at laying these so that covers about a block!

I am using plasticiser ( a good splash in half a bucket), wetting the ground before mixing and wetting the table I use to use the cement from. I have also soaked the blocks for a few minutes before use and pouring some water on the blocks below and that seems to help.

Any idea what I am doing wrong?

Thanks

Lee.

[RJH]

In my limited experience, the cement is old - is there a date on the packet?

[Tricky Dicky]

That sounds like out of date cement, as Rob has said check the date on the bag and get back to your supplier. Adding water is useless as once it starts to go off you have lost the bonding element of the cement and you are in effect laying with an almost pure sand mortar.

Take apart any blocks already laid even if they feel bonded, over time the mortar will simply crumble away.

Richard

[leen...]

Thanks both. The cement has a use by date of end 2021. When I checked with the manufacturer, they said that the date relates to some chemical in it that helps prevent skin irritation etc. So as long as I wear gloves, the cement will be fine.

[The Natural Philosopher]

It's simply warm weather. You can add water later on if you don't mind a slightly weaker bond

worthwhile to keep the blockwork wet after laying - cement that dries out before setting takes time to reabsorb moisture and set properly

[Fredxx]

When I read up about old cement I came to the same conclusion. My experience is that if the cement is still powdery it should be ok. It really ought to be kept in a plastic bag, to stop moisture and CO2 getting to the stuff.

[leen...]

Thanks NP. When you say keep the block work wet do you mean like spray it with a hose or something periodically?

[Chris Hogg]

Retarders are available that slow the setting time. Gypsum, calcium sulphate dihydrate, is one such, added at less than 4%. Whether plaster of Paris in its unhydrated state also works (i.e. builders' 'plaster'), I don't know.

[Andrew]

You don't want the make the blocks actually wet because that will cause mortar to run down the face and stain it.

Just use a plant sprayer to dampen the mating surfaces

Could it be because you are mixing your mortar with insufficient water to start with ?.

[Mike Rogers]

Sounds to me like its drying out due to sun/heat rather than going off. It sets by a chemical reaction that requires it to email moist for some time. Try shading your mortar from direct sunlight and cover laid blockwork with sacking or similar.

Mike

[The Natural Philosopher]

yeah. or piss on it. or throw a bucket over it. block work is not an exact science.

[The Natural Philosopher]

I thought gypsum and plaster of Paris speeded it up...seem to recall hydrated line was useful in this context