Why does lime fall of sandstone blocks?

[chuckey]

I am sorting out some dodgy lime plastering in a rock and rubble built house. The lime comes of in great big lumps, 1 1/2" thick, really hard, BUT where it should have been adhering to the stone work there is just a fine layer of dust. Often this dust is very dark in colour showing that the bond failed 100 years ago and the blackness is just house dust that has got blown into the crack. There are spots of lime that have adhered to the blocks in isolated very small patches ( < 3/4" diameter) over about 5 % of the total block surfaces.
Is it that the acid contents of the rock has chemically changed the lime back to chalk? The actual stone is mainly limestone but some sandstone and millstone grit. Where I am working there is no great amount of dampness, other then atmospheric (first floor level).
Frank

 

[middi]

It is quite possible that the lime plaster was applied during or just after the industrial revolution where the original walls may have been subjected to pollutants absorbed into the sandstone therefore speeding the "bonding" decay ( ??).

 

[MdB]

When I removed the multi layers of c*m**t, plaster and other rubbish on our sandstone walls, I found a few odd bits of lime plaster showing that it was used originally. However, it looked like it hadn't stuck which was the same as the c*m**t that replaced it. All came off very easily. The same was true with the outside render. I guess sandstone is not the best for sticking stuff to.

 

[Gareth Hughes]

There is a theoretical chemical issue about the compatibility of lime and sandstone, which was discussed here many moons ago, but just at the moment I can't find the thread or remember much detail :?

 

[Lime]

Adhesion to sandstone depends on several things.
How clean or damp the stone, how wet the mixture (i.e. how fast the glue-area dries) and how hard the sandstone (as some is quite soft and comes apart relatively easily so the adhered layer separates).

Then there is the question of chemical reaction of the mortar to the integrity of the stone.
I suspect the sandstone also has a catalytic reaction on the adhesion layer mortar.

 

[andrew gayton]

There is a theoretical chemical issue about the compatibility of lime and sandstone, which was discussed here many moons ago, but just at the moment I can't find the thread or remember much detail :?

lime will eventually corrode sandstone, hence water run-off from lime/stone/mortar will corrode sandstone - sandstone therefore should be laid in block above limestone if both used in the same section of walling. So, applying that rationale, wet lime plaster onto a sandstone substrate could well lead to a debonding due to the decay and subsequent receeding of the sandstone substrate?

and/or

a dry/dusty substrate leads to the shrinkage/poor bonding of the plaster scratch coat?

Answers on a postcard.

 

[biffvernon]

lime will eventually corrode sandstone,

This could get complicated. There's sandstone and then there's sandstone. The common feature is that much of it is made of grains of quartz, SiO2. But what sticks the quartz grains together, the stuff in the pore spaces between the sand grains, varies greatly, from pure quartz (The rock is called quartzite and is rarely used for building as it is very difficult to cut), to calcite (lime) to clay minerals (mud). Usually its a mixture of a host of minerals. The weathering characteristics depend on this mixture. No two stones from different quarries (and often from within the same quarry) will be quite the same.

 

[Gareth Hughes]

There is a theoretical chemical issue about the compatibility of lime and sandstone, which was discussed here many moons ago, but just at the moment I can't find the thread or remember much detail :?

lime will eventually corrode sandstone, hence water run-off from lime/stone/mortar will corrode sandstone - sandstone therefore should be laid in block above limestone if both used in the same section of walling. So, applying that rationale, wet lime plaster onto a sandstone substrate could well lead to a debonding due to the decay and subsequent receeding of the sandstone substrate?

and/or

a dry/dusty substrate leads to the shrinkage/poor bonding of the plaster scratch coat?

Answers on a postcard.

The first of the two was the one I recalled seeing debated here before, but although I grew up in a sandstone area (where the problem is avoided by only using cement, especially in the form of beautiful snail-track ribbon pointing :roll: ), I've done very little work on sandstone buildings (about three), so it's not a problem I've any worthwhile experience of to know how true the "chemical incompatibility with lime mortar / plaster" story is.

 

[ian]

There's sandstone and then there's sandstone. The common feature is that much of it is made of grains of quartz, SiO2. But what sticks the quartz grains together, the stuff in the pore spaces between the sand grains, varies greatly, from pure quartz (The rock is called quartzite and is rarely used for building as it is very difficult to cut), to calcite (lime) to clay minerals (mud).

Biff has highlighted something here as to why lime may take on differing characteristics depending on the substrate. Pure lime plus some clays can be a pozzilanic lime.