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Re: [ATM] portland cement; grind. etc.
Copy to all ATMers to further the discussion...
Hi ,
I only use Ordinary Portland Cement (OPC). The drying times of this mix
reduces the chances of minute cracks in a thick block. No drying agents are
needed for a small block like these with OPC but if you use a small squirt
of washing up liquid in the water you will need less water and the mix will
be more malleable. This is a cheap version of an Add Mix known as a
plasticiser. Not good for buildings but adequate for grinding tools. If you
wish you could by a water proofer add mix (£10/5litres) this will stop water
soaking into the dry concrete when in use.
If you use a lot of water the internal structure of the disc may have cracks
that run the width of the tool. This could lead to a catastrophic crack
while grinding.
The mix should be a 'thick' mix with very little slump. The way to test this
is to fill a food can with both ends cut off. When the can is full and
tamped down remove the can to make a sand castle (this is the best way I can
describe it at the moment) and it should hold a very similar shape to the
can but bulging slightly at the bottom, reducing the height by about 1".
Without going into the technicalities of cement mixes I think this would be
the best way to assess your mix, professionally we use a 400mm high cone.
Once this slump factor is found it can be used to assess all your other
projects.
Aggregates whether they are in fibre or stone form will help with the
structure and usability of the tool. I have used stone aggregate in the form
of granolithic dust (grano dust here in the UK). These particles are small
stone chippings usually 2 to 4 mm in size ( I sieve to get smaller particles
and discard the larger), used to screed the top of rough concrete fills . A
mix with this aggregate does allow a more robust tool to be made. I have
also used Mersey Grit (I wonder where I come from?) This sand aggregate has
small stones in the sand.
The process I described of soaking he concrete in water after setting allows
full hydration of the cement and is used in labs to test concrete cubes.
These cubes come from the concrete taken from building sites.
I also forgot to mention that when you have cast your tool place a layer of
polythene over the top to reduce the evaporation of water from the mix.
Cheers
Glynne
----- Original Message -----
From: "dune" <dune@shadow.freeserve.co.uk>
To: "Malcolm Macdonald" <toliman@wave.co.nz>; "atm" <atm@atmlist.net>
Sent: Sunday, June 25, 2006 7:11 PM
Subject: Re: [ATM] portland cement; grind. etc.
> The perfect recipe for making any concrete structure whether it be a
> grinding disc or building is control of water. Too much or too little will
> reduce the strength of the unit.
>
> As a guide the concrete should have enough water to allow the full
> hydration
> of the cement within the unit but not enough so water capsules are left
> entrained. If larger discs are required you may place chicken wire in the
> uppermost surface which will become tension members.
>
> Compaction of the concrete should be undertaken to expel all air. A
> reasonable tamping rod would be a 1" x 1" flat timber stick. I would
> suggest
> that after a minimum of three minutes compaction all air entrainment
> should
> be expelled.
>
> As the concrete will be cast on top of the mirror with reasonable formwork
> about the disc the top surface should then be floated smooth.
>
> After 24 hours drying in a warm room dislodge the disc and submerge it in
> water. Leave it in the bath for a minimum of seven days. This will ensure
> that the concrete will attain its maximum 7 day strength. The disc will
> not
> gain its 98.8 % strength until 30 days, so its up to you whether you leave
> it in the water till then, but if your like me you will want to use it
> sooner rather than later.
>
> If anyone is interested I can teach you how to calculate your concrete mix
> by weight so you can end up with a known quantity.
>
> Cheers
> Glynne
>
>
> ----- Original Message -----
> From: "Malcolm Macdonald" <toliman@wave.co.nz>
> To: "atm" <atm@atmlist.net>
> Sent: Sunday, June 25, 2006 12:35 PM
> Subject: Re: [ATM] portland cement; grind. etc.
>
>
>>
>> ----- Original Message -----
>> From: "KR" <kreid@primus.ca>
>> To: <atm@atmlist.net>
>> Sent: Sunday, June 25, 2006 9:24 AM
>> Subject: [ATM] portland cement; grind. etc.
>>
>>
>>> Greetings,
>>> The cheap grinding, polishing tool made from portland cement has a
>>> limited life.
>>> I've had two break unto pie shaped wedges after a years time. Just a
>>> note
>>> to
>>> those who
>>> like to spend eternity perfecting.
>>> regardes
>>> KR
>>>
>>>
>> Sounds like they are shrinking as they dry out. How do you make them? eg
>> Do
>> you bake them to dry them or use some kind of rapid drying agent.
>>
>>
>> I have been making and using concrete tools exclusively for about
>> ten
>> years now and never had any problems. I have so many in the shed now
>> that
>> I
>> contemplated making a garden path with them, but are too shiny and
>> slippery
>> for that.
>>
>>
>> My method :- Put a piece or paper/plastic on the glass blank, make a dam
>> around the edge with Duct tape or anything water proof. Arrange your
>> Stones/glass offcuts/tiles, then pour over your cement. Tap the edge of
>> the
>> class, vigorously, all round with a piece of wood to remove air and make
>> a
>> nice smooth back. Then leave a day or two to dry. After a year they
>> Should
>> be hard and ping like glass when tapped with a hammer. I use them time
>> after
>> time and they seem to last forever.
>>
>> Cheers Malcolm.
>>
>>
>>
>>
>>
>> --
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>
>
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