Troubles with a semi dry mix

[ohp]

I'm a bit of a cement novice, so I'm not really sure what to expect.

I'm trying to do a semi dry mix for the edge course. The sand has some moisture in it, it's not a crazy amount, but you could make a sand castle out of it.

I'm mixing it with cement in a 4 to one ratio. I'm then stomping down, topping up, and then tapping down the edge course pavers. It's up against a wall, so there's no haunching.

My concern is that after a couple of days it seems too crumbly. it has the feel of sugar that's been in a bag too long and has formed a crust: you can easily break it up with your fingers, and is very dry, and crumbles between your fingers.

It's in a shaded spot, but it is warm out there at the moment.

Should I just wait it out? will it harden over the next few days? can I help it along in any way? is it only ever going to be this crumbly? should I add a bit more water?

any comments gratefully recieved!

[ohp]

Well, I've decided to break it up and try again.

I'm guessing that it's probably just getting too dry, too quickly.. What little moisture there is is probably evaporating away, even though it's in a shaded area. I suspect these kinds of temperatures do this mix no favours.

Will try with added water next time next time, and wait until the weather is cooler, and also keep it moist with a tarp or something. Will let you know if it works!

[MikeG]

Too dry to begin with. Rule of thumb for initial moisture content is that it needs to hold itself together if compressed into a ball in your hand. I believe hydration needs to be available for at least 4 hours for the curing reaction to take place properly. I would definitely add more water to begin with, consistency enough to take the weight of the blocks, 4-1 strength is super strong for edging.

[Tony McC]

Apologies for being late to this - had all sorts of health issues with angina during the heatwave and now an arthritic knee that is driving me mad with pain.


I'd tend to agree with MikeG - it sounds as though there is insufficient moisture in the sand to start off with. The mixed mortar/concrete should clump into a firm ball when squeezed in the (gloved) hand, with no water dribbling out.