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Re: [ATM] portland cement; grind. etc.
I have been having a little think about the problems posed by this thread
and have written some theoretical guide lines for producing a stable
grinding disc. It is a little long but bear with me.
To explain:
Hydration of Ordinary Portland Cement is a purely chemical reaction between
the four main components Tricalcium Silicate (C3S), Dicalcium Silicate
(C2S), Tricalcium Aluminate (C3A) and Tetracalcium Aluminoferrite (C4AF).
The composition by weight is approximately 48% C3S, 24% C2S, 13% C3A and 9%
C4AF dependant on the manufacturer and topographical area the elements are
mined from. When water is added to this concoction the chemical reaction can
cause flash setting, therefore gypsum is added controlling the setting time.
Injection of carbon to override the gypsum and cause flash settings will
inherently cause cracking through heat propagation. The internal temperature
will be a great deal warmer than the surface. This may not be obvious to the
naked eyed but the cracks will be there. Whether this will adversely affect
the end use of the grinding disc is unknown to me as I have never tested the
process.
Water to Cement Ratio: An ideal water/cement ratio by weight would be 0.38.
If this mixture is not pliable enough an admixture can be used
(waterproofer, see later or washing up liquid). Only a small squirt is
required in a bucket of water.
Plastic Shrinkage: To reduce plastic shrinkage during initial setting a
water proof membrane should be placed over the cast. Plastic shrinkage is
generally due to water being lost from the open surface during curing. The
loss can be from temperature differentials, wind and over compaction.
Shrinkage will cause strain in the form of tension to be created in the top
surface leading to cracks as concrete has no strength in tension.
Over compaction: Compaction will force larger particles to the bottom and
the finer particles to rise. During compaction water will also be displace
to the top surface, known as bleeding, causing further loss of water through
evaporation and compounding shrinkage. Your mix will also be separated.
Temperature: Through consultation of several text books I have found that
for OPC a temperature of approximately 130C is the optimum ambient
temperature for curing concrete. I suppose that's why it works so well in
England! A higher initial strength (7 days) is found if this temperature is
maintained. The water bath that samples are dried in is kept around this
temperature.
Tension Proofing: The forces placed upon the disc whilst in use will cause
tension in the front face; concrete has no strength in tension. This may
cause spalling of the concrete and or tiles. Placing of chicken wire some
10mm from the front face and edges will act as reinforcement wire and become
the tension controlling member. The rear face will always be in compression
and concrete is very good at accepting this force. Adding of fibres as
already suggested in previous posts will also reduce tension transmitted to
the concrete. Pour a small amount of concrete into the cast first and place
the wire on top. Cover to required depth.
You will notice that all of the above are interlinked: If these guidelines
are followed a stable substrate should be forthcoming. I have not
experimented upon physical units as yet but will do so during my next
project.
The concrete will expand or contract dependant on the moisture content of
the block. Once the initial curing has taken place say about seven days (if
your impatient), the block can be coated with something that will prevent
the take up or release of moisture. You should dry off the unit first, say
place in an oven for an hour or two at a low temperature. Warning if the
temperature is too hot it could cause the concrete to explode! I would paint
an epoxy resin to all faces. This should stop further contraction or
expansion and allow tiles to remain firmly adhered to the grinding surface.
I have assumed that a waterproof tile adhesive and grout is being used. I
always use a grout between the tiles as even water proof adhesive will
become re-hydrated if left in contact for long enough. Try putting an old
tile with paste in a bucket of water for 30 minutes.
Incorporation of a waterproofer admix should also control the amount of
moisture within the dry unit, this will also help with the pliability during
moulding.
I have built a grinding machine for mirrors and have found if I take a
little extra care during construction of the grinding disc and can attach a
central pinion to the unit via rawl plugs (dont know what they are called
over the pond but I use them for holding up shelves!). I did attempt to cast
into the block a pinion but found it difficult to level. My discs are also
quite deep to increase loading on the mirror, I tend to have a minimum depth
of 125mm (5").
I would be happy to help if you have any questions (even if it concerns a
different matter regarding concrete or building i.e. foundations for piers,
etc.) and to learn of problem that you may encounter. With this information
I would be able to refine these guidelines. I hope this helps.
Glynne
----- Original Message -----
From: <atm@hudler.org>
To: "'ATM List'" <atm@atmlist.net>
Sent: Monday, June 26, 2006 6:23 AM
Subject: Re: [ATM] portland cement; grind. etc.
> Repost: (sorry dune kept sending it to you)
>
> Look at QUIKRETE Non-Shrink Precision Grout
> http://www.quikrete.com/catalog/Non-ShrinkPrecisionGrout.html
>
> I use it for telescope piers, filling machine tool bases and making
> concrete
> counter tops.
>
> If you have a vacuum pump then pull a vacuum on it and let it foam up on
> the
> top. When the foam breaks you can remove the vacuum OR you can pressurize
> with
> CO2 to say 15 or 20 PSI. The CO2 will cause the Portland cement to cure in
> a few
> days and will be very HARD and non-porous.
>
> For an instant cure, the CO2 needs to be pressurized to 250 PSI, this is
> called
> Supercritical CO2 Carbonation. The cure will be complete in a few minutes
> no
> matter what the thickness. Yep that's right; something that would take
> centuries
> to cure will cure in minutes.
>
> http://www.pnl.gov/er_news/08_97/ws.htm
>
>
>
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