"Grouting" between existing patio slabs: how to avoid dry powdery mortar Featured

I forgot to say: the mortar is brand new and the bag was only opened a few days ago and has been kept in a dry garage.

It's made with grey Portland cement, not white lime.

Another thing I forgot to say. Where I've dug up the loose mortar from when the patio was first laid (about 10 years, according to neighbours), it's been a thin crust, in some places only about 5 mm thick, with a large void below it down to the level of the base on which the slabs have been laid. Is this normal? I'd have thought the whole depth of the crack between slabs would have been filled with mortar.

When I laid my patio , about 20 years ago, I put all the pavers / slabs on a bed of sharp sand / cement mixed at about 7 or 8:1. I have had to re point the gaps about 3/4 times since and I use a 3:1 mix of builders sand and cement with just enough water to make it damp and no more. .

That works of for me.

Laying the slabs on bare soil is not a good idea for a start.

"Just enough water to make it damp and no more" sounds similar to the consistency I've used. I found that any more runny was very difficult to direct into the gap without it dribbling over the surface of the slabs and having to be wiped off afterwards; it also led to patches of golden sand in the wet mixture, rather than the grains of sand remaining coated in grey cement.

There seems to be no rhyme or reason to which parts set like stone and which remain the consistency of damp sand. The very first batch I made was partly used to fill a couple of holes in the rendering on the brick wall of the house (and that has set solid), but the rest of the same batch has set hard in places but there are patches which haven't set -- and that's for the same batch, all used within about 30 minutes.

I'm getting a success rate of about 1/3 ie 2/3 of what I've done will have to be gouged out with a screwdriver and washed out so I can redo it .

How long does mortar remain workable after it has been made? I presume if it was starting to set before I'd finished using it, it would get noticeably more stiff and lumpy, rather than remaining a smooth paste. I'm sure I'm getting each batch used within about 30 minutes of adding water to the dry mix.

Is the porosity of the base that the slabs are laid on relevant? Am I right in wondering if the "just enough water to make it damp and no more" mortar is drying out by water leaching into the ground (or evaporating from the surface on a hot day) before the water has had chance to react with the cement to make it set around the sand?

The ready-mix is made by Tarmac (ie it's not B&Q's own brand of dubious quality) and the label says "cement 15% aggregates 85%". That's a ratio of about 5.5:1 - quite a bit less cement than the figure of between 3:1 and 4:1 that I've seen mentioned everywhere. Surprisingly the mixture ratio is not advertised in large letters - I had to search the various paragraphs of small print to find the 15%/85% figure.

A lot of what I see when I google "grouting/filling gaps in patio slabs" makes it sound very easy, so there's obviously something I'm not doing right!

It's many years since I've pointed slabs but the advice used to be: to brush-in the dry mix, brush any cement powder off the surface of the slab and then to sprinkle a *small* amount of water on the joints and let nature (humidity and/or light rain) take its course. By doing that you fill the gaps, get a strong mortar and don't smudge the slabs.

Wait until it's been dry weather for a few days, then tip a good mortar mix, dry, onto the slabs, and brush it around until it goes into the cracks. Then go round bashing the slabs with a big lump of wood to vibrate them, then brush some more in. Then leave it alone for a couple of weeks, unless you're in a very dry area, in which case sprinkle a bit of water gently all over and cover it with polythene.

Bill

if you are getting non setting crumbly s*it there is not enough cement

the problem with a pre iced bag is that it may after a while not have the mixture uniform and if you use it in small quantities you may get all cement or no cement.

It better to buy separately for small jobs and it doesn't cost a lot more

I've been doing some of this recently, I have separate cement and sand and I've been using a 'strong' mix of maybe 2 or 3 sand to 1 cement. I've also been using a pretty wet sort of mix which makes it easy to get it into the gaps, they are cement slabs so a bit of splash round the edges doesn't matter at all. The results seem pretty solid so far (but it has only been done for a month or so).

That was over a 20 year period tho'. I'm not a builder so I have been learning as I go along so to speak. Also I put it down to frost / extremely cold weather that has caused the mortar to break up or go crumbly on the surface

When we moved into this house 44 years ago, the previous owner had left a pile to irregularly shaped paving for a path at the side of the house. He's already laid a terrace at the back of the house. I laid the path and

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.

No I know that you don't get that with brickwork, but as I'm not a builder what should I have done, ie method, mix or anything else, so it doesn't crack, crumble or break up with cold weather.

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.

No he needs to bite the bullet and look under one and see what was done properly. If its on soil, depending on the composition, round here its got a lot of clay, the soil swells and contracts, as I found at a feeble attempt at crazy paving some years ago! That is enough to crack any cement between stones. As for drying but not setting, is it the narrowest slits that dry out? I would imagine that would be the case. Cement is not very strong unless under compression and the less there is the less the compression and the more it turns to just powder again. Brian

Have you ruled out joint compound?

I've just laid a new patio and i'm deciding what to do as the jointing compound is expensive, but if it outlives mortar it might be worth it.

Ok, so what is the correct way of pointing the slabs, if not with a trowel What is the correct mix if not 3:1 What is the correct water level if not just enough to dampen the mix And finally what is the correct time of year to do the job.

Thanks

AIUI, don't work with cement if the ambient temperature is 5C or less - it won't set properly.

Thanks for not answering the questions about mix and water level.

Bricklaying is a different matter as it is vertical, slab laying is horizontal and therefore more chance of frost and ice adhering to the mortar.

The cement is not washed away, as I did not say it was. Never mind, if you don't know, some one else will tell me.