Bonding to concrete
using cement paste

Ask the pros-September, 2004
By Reggie Bullard

Several years ago my wife & I began building a small vacation home in Honduras.

I read your website recommendation against concrete glue as a bonding agent. Although I didnt use form oil, need I rough-up the face of poured concrete to get a good bond ?

Altho I can buy good portland cement, un-screened river sand & lime in Honduras, specialty materials (admix, trim etc) are not available. I must ship down everything else I will need, which requires advance knowledge of my needs. To this end, I will appreciate and recommendations you can provide me.

Please click here to see pictures of this beautiful project.
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This is part one: Bonding stucco to concrete using portland cement
No need to rough up the concrete, as long as it is clean.

An excellent method of bonding cement mortar to concrete
is an Acrylic additive such as Flex-con (what we use) or Acryl-60.

I want to explain another method which works, however old-fashioned
and only requires portland cement. This may be a lot more practical
considering the effort and expense required transporting Additives to
the Island.

First of all, don't even dream of putting cement mortar directly
on concrete because it won't permanantly bond. It may stay
there 2 or three monthes but will eventually fail.

A method that I learned from an old plasterer is this:

Mix a portland cement paste in a bucket, with no sand and
no lime. It should be stiff enough to paint on with a paint brush.
Wet your concrete and when enough water runs off or soaks in,
you can paint on the paste.

 You need to mix up some paste in
a bucket (we have used a cup for patch work) and have
some portland sand mortar mixed and ready.
You paint on the paste with a stiff brush or broom 
leaving the surface rough. You will notice the paste sets
hard immediately after touching the concrete. The paste then
starts to dry quickly because the concrete sucks the water out of the mix.
THIS IS IMPORTANT: You want to put mortar on the paste BEFORE it dries.
This could be done with two people, one painting and the other scratching on
mortar, or one person, working in small sections, applying mortar before it 
dries out.

Scarify your scratch coat good and let it set at least over night, better 2 days,
before applying the brown coat. If your concrete is nice and flat you
may get by with just a scratch coat, evened out and roughed up with a float,
a brush, or  a wood straight edge.

Does this work ? Swimming pools were plastered this way for years
before acrylics were invented. This is still done a lot
in Latin America on concrete. I saw this method of cement paste 
used when I was in  Mexico on exterior ceilings. Just think about
it when you walk under ceilings on office buildings in Mexico
that 3/4" of stucco is bonded using cement paste.

Click here for bonding to concrete using acrylic additives.

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