On 31 May 2021 at 14:40:04 BST, ""NY"" wrote:

"Andrew" wrote in message
...
If the bag says 85% aggregates then you have bought the wrong stuff.
That sounds more like postcrete or concrete mix.

You need a bag of sand and a smaller bag of portland cement and
make up a 5:1 mix of sand:cement.

85% sand : 15% cement is 5.7:1 (ie 85/15 : 1)

That's not *too* far from the 5:1 that you suggest. If I need to buy any
more (depends how long the 25 kg bag of pre-mixed lasts) then I might buy
separate sand and cement and make up a slightly stronger (more cement) mix.
Is 5:1 enough? I've seen some web sites which recommend 4:1 or even 3:1.

The bag is definitely labelled "mortar". B&Q did also sell other bags
labelled "post mix".

I've done some more mortaring today, so I'll see how it sets. A patch of the
mortar that I did three days ago had dried to an almost white solid which
was actually cement/sand dust (almost like the un-wetted stuff I bought!)
even though I'd mixed it to a stiff paste just like all the other batches. I
prodded it with a screwdriver and I was left with dust - I was *almost*
tempted to mix it back to a paste with water and reuse it ;-)

I presume I want builder's sand, if I buy it separately.

Is there any significance in the colour of the "set" mortar? Dark grey
(after three days) is rock solid. Much lighter grey seems to be powder. And
that's light grey all the way through, so it's not just the surface that has
more cement to sand that what's underneath.

I'll see how it goes. It's a long slow job because the more I look the more
I find grouting that has cracked and can be pushed down into the void
between the slabs, or has turned to dust. Not sure how long ago it was
done - maybe 10 years. The cracking all occurred this more recent winter
(frost?), after looking fine before that.




When I've finished that, I need to make up wooden ramps so we can wheel a
trolley (*) up and down between three different levels of patio - and the
step height is different in each case.


(*) For bringing up loads of wood/peat/sawdust-blocks for the wood-burning
stove, and for moving loads of plant pots of bulbs onto the patio in time
for daffodils etc to flower, and then off the patio to make way for other
things in the summer.

Two practical points. Firstly, the 5;1 mix is traditionally in volumes, and
bears no simple (AFAIK) relation to proportions by weight. And secondly, a
bag of mortar mix separates in store (if indeed the contents were ever mixed)
and the whole bagful needs thorough mixing in a tub or barrow before taking
some out to mix with water.

--
Roger Hayter