A FILOFACTS ON CEMENT MIXES
The Filofacts is a work in progress, or at least due to be in progress again soon, at http://tinyurl.com/oscthjf/
The original idea was to produce lists ready-made to fit your famous-brand personal organiser, but I haven’t quite got there yet, so meanwhile it’s over to you to cut and paste as required.
Latest on this is good old Jeff Howell, Sun. Tel 16.6.13, on patching render …
“ The correct mix depends on a number of factors, including the substrate (i.e. the wall itself), the surrounding finish that you want to marry up to and even the weather conditions at the time of application.
“When repairing an older building, I would always try to replicate the existing remaining render, first in terms of sand grading (i.e. coarseness) and second, in terms of the binder that holds it together. If the existing render is lime and sand, then there’s no point using a strong cement-and-sand mix, as it will behave differently, and cracks will open up at the junction of the two materials.
“A general principle is that the render should be softer than the substrate, so you could try a 6:1:1 mix of washed sharp sand, lime and cement, but if the underlying brickwork is very soft – say, soft red bricks in lime mortar – then less cement and more lime might work better. Brick joints should also be raked out first to provide a “key”.
“In dry weather the substrate needs to be damped down and the render kept damp with wet hessian sheeting until cured. In wet conditions a dryer mix should be used, and the finished render should be protected from rain and frost. “
The Filofacts archive offers:
Most CEMENT today is Portland cement – soft stone ground up and dried out. It’s mainly grey but you can buy white. Mix it with SAND and other ingredients for most purposes
SOFT SAND best for mortars, for sticking things together, and renders, for covering things over, and
SHARP SAND for concrete, for building things with, or for bedding mortar for paving. But if pointing (between bricks and blocks) is to be very wide, some sharp sand will give it a bit of body. Watch the colour of the sand if colour of finish matters.
Mix in gravel or stones, too, to make CONCRETE – choose stone size to suit finish required.
Remember – wetter it starts, weaker it dries.
In the old days, LIME was used instead of Portland cement and some builders still like a shovel of lime in a mortar mix to give it more flexibility – but nowadays a commercial plasticiser or a squirt of Fairy liquid is more common.
RECIPES (C = cement; S = sand; A = aggregate; L = optional lime.)
STANDARD MORTAR – 1C, 1/4L, 4S
REPOINT MORTAR (weaker but more flexible) – 1C, 1/2L, 5S
STRONG MORTAR – 1C, 1/2L, 3S
GRANOLITHIC mortar (for strong patching,building steps, etc) – 6 fine A, 2C, 1
BEDDING mortar for paving – 10 gritty S, 1C
CHEAP CONCRETE – 6A, 3S, 1C
STANDARD CONCRETE (3, 2, 1) – 1C, 2S, 3A
HYPERTUFA (moss-friendly mix for plant containers) – equal parts sharp sand, peat, cement
MORE ON MORTAR
Further to previous Filofacts on Cement Etc. …
I am adding in a link to a good guide to mortar and renders which answers the questions …
* What mortar above damp-proof course?
* Will Fairy liquid do instead of custom-made plasticiser?
* How much mortar do I need for 60 bricks or 10 blocks?