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

NY · 2021-05-30T16:48:10+00:00

Our house has a patio of stone slabs (slightly irregular size, shape and thickness - not rectangular concrete slabs). They seem to have been laid on bare earth, as far as I can see from looking into the gaps, without levering up a slab to look.Some of the mortar "grouting" between the slabs has cracked and disintegrated, so I'm trying to re-mortar the broken bits. I've removed the broken bits and brush away as much of the powder residue that remains. I'm using ready-mixed mortar from B&Q, with water added to form a stiff paste which is thick enough to adhere to a palette knife so I can direct it into the gap (typically 5-20 mm) and then tamp it down with my fingers (wearing rubber gloves in case the cement causes skin irritation).I then use the hosepipe to spray a fine mist over the sections every few hours (given that it's hot and sunny here) so it has enough water to cause the cement to set.But some sections never set: even after a couple of days the mortar can be rubbed away in a powder of sand and cement. Other sections have set rock solid. The colour of the dry mortar varies from slate grey where it's set to almost white (maybe with golden sand granules on top) where it remains powdery after several days.I've tried various alternative techniques to the stiff-paste consistency:- adding a bit more water to the mix so it's a bit more runny, and letting it fall off the palette knife into the gap, building up the level gradually and then tamping it down to make it roughly level with the slabs- filling the section with dry ready-mix and then spraying a mist of water over to start the setting process: repeating the misting every few hoursI've checked that all the dry powder is thoroughly wetted, to avoid dry sections.No technique seems to give better or worse results. Sometimes sections set, sometimes they dry to a firm dust that can be rubbed or washed away.I don't know what the proportions of sand and cement are - just the standard mortar mix.I'm making up small...

May 30, 2021 Last reply: 5 years ago 25 Replies

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Andrew 5 years ago

It is possible that the bag had been in store for awhile and had a small hole or other damage which has allowed the cement fines to start to set but without turning the whole lot hard. It's possible instore staff had noticed some harness and bashed it to break up any fine lumps. This is never going to be any use for a decent mortar.

Also, when mixed and a proper admix used it should give a nice 'fatty' mortar. If it doesn't, don't use it. If the soil is liable to 'heave' in cold weather, a lime-based mortar might be better but this needs weather protection while it achieves full strngth.

Also possible that the whole batch was poorly mixed by the store supplier and has less cement than it should.

For grouting slabs, it must be protected from frost for at least

7 days.

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Tricky Dicky 5 years ago

I have used this stuff just to seal up a mowing strip, it is easy peasie to use and gives a good hard wearing finish and importantly no staining.

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Richard

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charles 5 years ago

Could it be that the product is not properly mixed in the bag?

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The Natural Philosopher 5 years ago

Of course that's the problem. But it takes 30 armchair experts to completely confuse the OP

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RobH 5 years ago

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.

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