I want to make a few special coving bricks to replace broken ones on a
chimney. I shall make a wooden mould first. Is it practical to speed
curing by gentle baking in an electric oven or even in a microwave?

peejos wrote:
I want to make a few special coving bricks to replace broken ones on a
chimney. I shall make a wooden mould first. Is it practical to speed
curing by gentle baking in an electric oven or even in a microwave?

Definately not.
Concrete cures better when wet, lots of people have the mistaken notion that
'dry' means hard, but it's the opposite that is true, the longer it stays
wet, the harder it becomes, drying out too quickly causes cracks and stops
the cement from hardening properly, resulting in a crumbly texture

--
Phil L
RSRL Tipster Of The Year 2008

Phil L wrote:
peejos wrote:
I want to make a few special coving bricks to replace broken ones on
a chimney. I shall make a wooden mould first. Is it practical to
speed curing by gentle baking in an electric oven or even in a
microwave?

Definately not.
Concrete cures better when wet, lots of people have the mistaken
notion that 'dry' means hard, but it's the opposite that is true, the
longer it stays wet, the harder it becomes, drying out too quickly
causes cracks and stops the cement from hardening properly, resulting
in a crumbly texture

Agreed!

Tanner-'op

On Sep 14, 8:57*pm, peejos wrote:
I want to make a few special coving bricks to replace broken ones on a
chimney. I shall make a wooden mould first. Is it practical to speed
curing by gentle baking in an electric oven or even in a microwave?

Heat dramatically speeds up setting and curing, so yes as long as its
at low enough power that it doesnt raise it too near boiling and dry
it out. Maybe plastic wrap it to prevent drying, and heat several
bricks at once on 10% nuke, or 60C oven.


NT

peejos wrote:

I want to make a few special coving bricks to replace broken ones on a
chimney. I shall make a wooden mould first. Is it practical to speed
curing by gentle baking in an electric oven or even in a microwave?


No, all that will do is result in excessive shrinkage and a very weak
product.

You could cure them underwater in warm water, which would help, but
personally I would use high alumina cement or "ciment fondu". This
cures much more quickly than Portland cement and should be perfectly
satisfactory for your application.

High alumina cement was a controversial material because designers
omitted to allow for the fact that concrete made with HAC suffers a
loss of strength if its environment is damp. School swimming pool
roofs were an example of improper use of the material, and several
failed because the designers worked on the basis of the "unconverted"
strength. But that won't affect you because the "converted" strength
should still be more than sufficient for your needs.

You can get high alumina cement on special order from good builders'
merchants. They will probably warn you about the "conversion" issue,
but if you tell them what you will be using it for, there shouldn't be
a problem.

wrote:
Thus spake Phil L ) unto the assembled multitudes:
peejos wrote:
I want to make a few special coving bricks to replace broken ones on a
chimney. I shall make a wooden mould first. Is it practical to speed
curing by gentle baking in an electric oven or even in a microwave?

Definately not.
Concrete cures better when wet, lots of people have the mistaken notion that
'dry' means hard, but it's the opposite that is true, the longer it stays
wet, the harder it becomes, drying out too quickly causes cracks and stops
the cement from hardening properly, resulting in a crumbly texture

Agreed: years ago when I laid a 6" thick self-mixed concrete slab for a
new 'kit' garage, I covered the freshly poured concrete with plastic
sheeting and kept it moist for about a week. Result, one very hard tough
slab.



You are correct. Many people believe it goes hard by drying. It does
not. There is a chemical reaction that requires water. Too little and
it will not cure properly, too wet and little bubbles of water form in
the mix which will never be absorbed so become little holes in the mix.
Not so strong.