Cement / mortar keeping and setting time

[Harry Bloomfield]

Around 6 years ago I bought a couple of bags of ready mixed mortar and at the same time I bought a bag of cement. Both have been kept in a hut and also in double plastic bags, both open as they have been used - sometimes even left open to the air.

Even when new, the ready mixed took a week to set up, where as when I mixed sand and cement to make mortar, it would be set up in 24 hours.

Fully expecting it to be beyond use, I used up the remnants of both last week. Using the bag of cement I mixed up a small batch of concrete I needed and it worked fine setting solid in 24 hours, driveable over in 48 hours.

The ready mixed mortar however, took the best part of ten days before it was set enough to be firm to the touch.

I'm curious how it remained usable for so long and the big variation in setting time.

[Andrew Gabriel]

I find a big variation in how long it keeps.

At the beginning of this storm/widy period, the lead flashing blew off the apron roof where the garage projects a little in front of the house. I bought some lead, made new lead wedges, and hammered it back in. There was about a 6 hour break in the rain, and I used some readymix I already had. It didn't set. The reason is the cement in it has already set in place as a powder, and it won't set again if you wet it.

Next day, I jetted it all out with the hose, and rushed around to Wickes for another bag. Had to go through several completely solid ones in the pile (which I can only imagine is the result of dunking in water) before I found one which was still powder. That's still no guarantee the cement hasn't set like my old one had, but that's all they had and I was in a hurry before it rained again. Fortunately, that set hard enough in 4 hours so the rain couldn't wash the cement out of the pointing before it set completely.

The readymix is a very strong mix - I would guess 3:1. Even if half the cement has gone off, providing you mix it so it's evenly distributed, the remaining effective 6:1 mix will be more than good enough for most things. It's only when the last bit has gone off before you use it, that it becomes completely useless.

I've kept the old bag in case we need any sand on the hill, but I don't see any sign of snow here anytime soon.

[Harry Bloomfield]

Andrew Gabriel explained :

We have met office warnings of snow from lunchtime tomorrow.

Thanks - I've never really had the time to stand over it and watch it set before, but now I'm retired, I took more interest in this time, which was why it aroused my curiosity. The ready mix, as said, took a very long time to set - so long I was on the point of cleaning it out and starting again. Then it suddenly set very rapidly over the final 12 hours. I was reluctant to rip it out, because it had taken hours to form it, a drainage channel.

[Tim Lamb]

In message , Harry Bloomfield writes

Ready mix mortar might include lime. If the cement component had degraded, the remaining lime would be very slow to harden.

>

[Lobster]

On 10 Feb 2014, Harry Bloomfield grunted: The ready mix, as said, took a

But why would you even think of taking the risk of using an ancient bag of cement in the first place, especially on a labour-intensive job like that?

[PeterC]

Might be expensive - and no good for mortar unless sieved )and some sand added?) - but one of the 'post-mix' products could be of use for some jobs. Seems to set rapidly (too?) and strongly. Could be OK for a channel, if worked into shape in about 30 seconds!

[Andrew Gabriel]

What you decribe is for plaster. Mortar is somewhat the opposite, except it won't ever reach the intended strength either, besides taking ages to noticably start setting at all.