On Wed, 31 Aug 2005 22:55:58 +0100, Andrew Gabriel
wrote:

I called up Lafarge's helpline, and the chap there was very helpful.
He said to use 3:1, and sharp sand for best result. I asked about
PVA (which I do normally add when I want a strong mortar) and he
wasn't happy about that in mortar exposed to such moisture. He said
if I really wanted a bonding agent, use SBR(?). I commented that I
had waterproof PVA, and he was happy with that (he might have said
that is SBR -- didn't fully catch the comment). He also said to
coat the sand areas on the plastic gully with a slurry of
cement/waterproof-PVA/water in a 2:1:1 ratio to make the mortar
bond well, and apply the mortar whilst the slurry is still tacky.

PVA in a set cement is resistant to moisture, even submerged, as long as
it is used in the usual amounts or less. SBR is styrene-butadiene rubber,
a synthetic rubber, first developed in the 1930s. Due to the occupation of
Malaysia by the Japanese, and the consequent shortage of rubber (no tyres,
rubber dinghies ,Mae Wests etc.),this spurred the allies into producing it
for the war effort. As dried film, waterproof PVA (actually EVA - Ethylene
Vinyl Acetate) is little better than PVA. In set cement matrices,
performance is similar. Adhesives suitable for swimming pools contain
redispersible PVA. On that subject, I received an email asking why
redispersible PVA powder was being used on the creases of cricket pitches.
A bit of research revealed it was to stabilise the surface for more
consistent behaviour during a match. Yet another use for an amazing
material!

Anyway, I did one side of the gully. You know that nice feeling you
get when you're doing something and it's going perfectly? Well, I
just wasn't getting that. Mortaring in tiles further up was causing
the ones lower down to move and come unstuck. I had visions of this
last part of the job still leaving me with a leaking roof, screwing
up all the re-felting, replacement of battons and tiles I had done.

In hot weather it is a good idea to damp mortar and concrete down every so
often. If the mixture dries out, the hydraulic cure will not take place
properly.

John Schmitt



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