UKC

Any stonemasons or bricklayers in the house?

Paving in our garden, and a couple of flights of stone steps, is all made with rough offcuts. Might be something similar to Caithness flags, because it looks like sandstone and has a distinct horizontal grain. It'd likely be a local material to NE Scotland. Seems like pretty soft stuff.

As the occasional slab came loose last year I was bit by bit raking/chiselling out the original 20-year-old failed mortar and re-laying them. After winter we've again got a few random steps and slabs that need re-done. But now some of my last year's repairs have also begun to fall apart. Clearly this doesn't say much for my competence.

Done a few bits of garden stonework in previous houses and never had this issue.

The original mortar seemed to have simply given up adhering. In places it was very solid and took a lot of chiselling, while elsewhere it was basically damp sand. I deliberately went for a hard mix, hoping for a good long-lasting stick - 3:1 sharp sand to general purpose cement.

Pretty sure I was getting the consistency OK, and not doing any work in cold weather etc. But I definitely did something pretty fundamental wrong.

So I've got a few basic-level questions:

• My mortar from last year is very hard, but the bond between it and the stone seems like it was weak since at least some of the stones are coming loose again. I wonder if I should have gone softer. Is there an argument for a softer mortar mix with a softer stone?
• Could I consider using a plasticizer? Google tells me that's a thing.
• I was reluctant to muck about with fancy lime mortar for what seemed like a few basic repairs, but maybe that's where I went wrong? 
• And a penny pinching daft question: I've got an open half-bag of cement left from last year. I assume it would be a silly false economy to try using this?

Thanks

In reply to Dan Bailey - UKHillwalking.com:

Did you wet the slabs before contact with the mortar? Helps with adhesion. I use plasticiser but not entirely sure what is supposed to do😀

In reply to pasbury:

Plasticiser is so that you can get a workable mix with less water which affects concrete strength, how quick mortar can go off and all sorts of "exciting" things. For this I'd ignore plasticiser.

OP, follow pasbury advice about soaking slabs

In reply to Dan Bailey - UKHillwalking.com:

Ditto about wetting the slabs, that's one obvious thing I can think of that would weaken the bond. It's possible that the mortar only bonded to a thin layer of surfave dust, but again wash down and wet. You probably want to soak to a saturated but without surfave water. 

If it's flat paving, you really shouldn't need a rich, strong mortar, 8 - 10:1 is not unheard of.

Your old cem if fine if it's still loose. Plasticiser just makes it more "buttery" and easier to handle. For laying pavers it's not really worth it. Lime also works as a plasticiser, but if OPC cem doesn't bond, then lime isn't going to do any better.

Check out pavingexpert.com

In reply to pasbury:

Rookie error: no, I laid them dry. OK, so I'll know for this round of repairs. Thanks!

In reply to daWalt:

10:1 ... wow, that's not something I'd have remotely considered

Nice one, thanks I'll take a look at pavingexpert (coz I'm clearly not)

In reply to Dan Bailey - UKHillwalking.com: rather than wetting the slabs with water use an old  paint brush to work in a slurry made with cement , SBR  and wster . Mix to the consistency of thick paint . This has worked for me a few times for repairs of loose flags . Work it in really well . Tip from a landscaper I know . 

In reply to Dan Bailey - UKHillwalking.com:

It sounds like your mortar mix may have been a bit dry. When you lay a slab it should stick to the bed of mortar and if you try to lift it once in place, it should feel like it's magnertised to the ground. If it lifts straight up it'll never bond properly. 

Cement doesn't stick well to some stone. Porcelain, Slate and Sandstone need a bonding agent applied to the back of them to get a reliable bond. You mix the liquid bonding agent (£20 - £30 for 5 litres from any building suppliers) with cement power till you have a thick cream and apply it to the back of the slabs with a heavy paint roller or trowel. Then lay the slab in the normal way. 

Incidentally, when laying bricks, blocks or slabs the mortar should be weaker than the block or slab. That way any repairs can be made easily and the originals reused. 

In reply to Dan Bailey - UKHillwalking.com:

Hard mix is actually undesirable for this sort of thing as it has no flex. Soft sand and a weak mix is key.

Pavingexpert is the best of the internet. Check it out.

In reply to Dan Bailey - UKHillwalking.com:

You need to use Builders' Sand and not Sharp Sand.

In reply to mike123:

Another vote for SBR. Mix it with your mortar instead of plasticiser.