When grinding hard concrete your
cupwheel diamond segment bond needs to match the
concrete hardness.
The confusing
thing is that soft bonds are for hard
concrete and hard bonds are for soft
concrete.
The diamonds which
are embedded in a metal called the
bond (or matrix) only
cut well if they are exposed.

Diamond
segment showing exposed diamonds and
"rocket trails" behind each diamond
The final product is called a segment
or diamond block and can be made in a variety of sizes
and shapes then silver soldered onto a plate, cupwheel, or cutting blade for final
use.
The size of the diamonds can vary and
so can the hardness of the metal bond around
them which determines how fast they wear.
How diamonds cut
concrete
When
grinding concrete the diamonds
cut into it and scratch it away breaking it
down to sands and dust. During
this process the diamonds fracture and wear
down, but at the same time the bond
metal is worn away by the sands of the
concrete which exposes more of the
embedded diamonds so that cutting can continue.
When grinding hard concrete the
diamonds cut, blunt and break with
much less cutting action than medium
hardness or soft concrete and as a
consequence there is less sand and dust
produced. The hard metal matrix is then not worn away
sufficiently by the small
amount of sands and dust so the new
diamonds are not exposed. This is when the
cupwheel stops
working and rubs on the floor instead of
cutting into it because the diamonds
have worn down to the metal, but the
metal matrix is not wearing away to
expose new diamonds.
The diamond block is then flattened,
polished and becomes very hot with
continued use. At this stage you need to press
very hard and stay in the same
spot longer for the cupwheel to grind
any concrete.

Diamond
segments where the diamonds have
flattened after being used on hard
concrete. Note the absence of "rocket
trails".
To recognize all the signs of
polished (glazed) cupwheels go to
Glazed Diamond Segments.
Dry Grinding hard concrete
To dry grind hard concrete use a soft
bond cupwheel with larger diamonds
around 20 grit size if possible so that the bond can
be worn away at a similar rate to the
diamonds. Concrete grinding will still
be slow, but at least grinding will be
happening. When that fails to
work there is another trick that often does the job.
Grinding trick
Wetting the concrete before grinding
helps to expose the diamonds a little
more. Try to scratch open the surface
first, then apply water and give it a
little time to soak in. Broom off the
puddles and remaining water leaving the
surface wet. This
process makes the dust a little heavier
which cuts the matrix more to expose
more diamonds.
Don't
be fooled
It will
seem that the water has done nothing
when you grind because the dust is the
same as if you never wet the floor, but
grinding will be more productive. Repeat
this process for each new cut.
To find out how to renew your polished cupwheels
go to
Glazed Diamond Segments.
Sands and bonds are
the key elements for grinding
productivity
Sands produced by concrete grinding
will
wear away the metal matrix/bond which
keeps the diamonds exposed. Without
these sands the matrix will not be cut
back to keep some diamonds exposed. When
the diamonds are no longer exposed they
will not cut the concrete.
The
bond, or matrix is made of metal powders
that are heated in a furnace until they
melt into a hard block and these metals
can be made to wear longer or faster
when abraded by sands.
So for hard concrete you need a
softer bond so that it wears away a
little.
What about soft concrete?
Just the opposite.
You need a hard
bond for soft concrete because it
produces plenty of sands and they can wear
the bond very quickly.
What makes concrete
hard to grind?
This is the million dollar question.
Two main things contribute to
concrete hardness - the ratio of cement
& water
in the mix which determines the hardness
of the the concrete paste and the type of aggregate
used.

Exposed
aggregate showing the stones cut down
level with the surrounding paste
Hard concrete means a hard paste (the
grey matter that we recognize as
concrete).
Hard aggregate means the hardness of
the stones used in the concrete mix and
there are very hard ones and quite soft
ones used depending on the local supply.
Grinding aggregate
The unexpected variation comes when
you have medium strength concrete (with
very hard aggregate) which you grind
easily at first, but as you get into the
aggregate your speed slows down
dramatically because there is possibly
only 15% concrete paste now and 85%
aggregate which may be considerably harder than the
concrete.
So then you may have to change to a
softer bond (cupwheel) than the
one you started with.
Diamond size
Generally speaking you use a larger
diamond grit (low number) for faster
removal of concrete and for harder
concrete.
Because larger diamond grits are more
aggressive they are not as smooth to
grind with which you will notice at
30/40 grit and below.
Polished concrete
To polish concrete you start with a
coarse diamond and approximately double
the grit number (halve the diamond size)
with each new pass until the floor is
polished which may take several steps.
ie. 30/40, 50/60, 80/100, 200, 500,
1000, 2000 and then 5000.
For example, with Situp N Grind use
30/40+40/50 Coarse for first cut then change
to 100/80 Fine for second cut.
The concrete will not be polished but
it will be very smooth, ready for resin
bond pads to complete the polishing work
using low speed hand grinders.

Resin
bond polishing pads
Note that Situp N Grind fitted to a
9 in (230mm) angle grinder runs too fast
for resin pads and grits above 80 mesh
will quickly heat up the segments if the
concrete is medium to hard strength. You
then need to use a slow speed grinder or
grind it wet.
Polished concrete look
using a clear sealer
After using the fine cupwheel
you can get a polished concrete look by
applying a couple of coats of clear
urethane or clear epoxy without the need to
actually polish the surface. In between
the grinding it will be necessary to
fill in the thousands of tiny holes in
the concrete to stop bubbles and holes
appearing in you final coat.
Next page "Glazed
Diamond Segments"