Soluble Iron Staining
Some time ago we were asked to in-
spect and pass opinion on an exterior
limestone installation where the stone
had changed colour, deposits of efflo-
rescence had formed and a
“mysterious” deposit of a “red sub-
stance” (as described by the customer
and installer) had appeared over much
of the stones surface. The job turned
out to be an interesting case study into Fig 1.
how natural materials interact with their White deposits easily visible on surface.
environment including the installation
system. Perhaps more importantly how The second and third issues are the
the installation system needs to be most important and are definitely
carefully designed to best manage these linked.
natural characteristics. Let’s look at the red deposit first.
The red deposit (fig 2) was in our opin-
The job had three main issues. ion a soluble iron compound (most likely
Issue 1: a ferrous carbonate) leaching out of the
There was the more common problem stone and oxidizing to the ferric state on
of efflorescence. This was easily ex- the stone surface, visible as red or rust
plained to the client as the soluble salts like. A quick test with an oxalic acid so-
we are all familiar with. lution also pointed to this theory that
Issue 2: was confirmed some time later with lab
There were the red deposits that were testing done at a local university. Like
very visible in areas like on the stair ris- the issue of the soluble salts the cata-
ers and treads. lyst for the soluble iron compound to
Issue 3: precipitate on the surface is water.
Lastly the biggest issue was the overall
change in colour of the stone from its
natural light beige to an overall pink
It was our opinion that the red deposit
and overall colour change of the stone
were linked and that in fact all three is-
sues were ultimately triggered by a
common factor, water.
So let’s look at each issue in detail
The deposits of white efflorescence (fig
1) were the easiest to explain and we
won’t go into the mechanism here be-
cause we are all familiar with how it
works. The most important thing to note
is the catalyst for the soluble salts is
Fig 2. Heavy red iron deposits on stairs
September 2008 Page 1
The last issue is the complete discoloration of the lated to the amount of Calcium Oxide present in
stone (Fig 3). the formula. We looked at the physicals of the 4
The mechanism involved here is more complex. adhesives and bearing in mind that 2 out of the 3
Before we were asked to inspect the job the adhe- adhesives that discoloured the stone were white,
sive company was asked for their opinion. The cli- we found that all 3 had very similar amounts of
ent believed the sole source and reason for all the Calcium Oxide and alkalinity. The fourth that did
job problems was the adhesive used to install the not discolour the stone had significantly lower lev-
limestone. In our opinion this was both true and els of both, explained by the fact that it was the
false. The adhesive was not the source or reason only one to use Danish White cement. This cement
for the heavy red iron deposits but could well be a is world-renowned for it’s quality, consistency and
major contributor to the overall change in the relatively low alkalinity.
stone’s colour. The conclusion here was that the adhesive did in-
deed affect the discoloration of the stone, but did
not cause the large deposits of soluble iron. It may
well have helped facilitate the reaction though. We
can tell you that all of this speculation was later
confirmed by in depth testing done by the same
So what is there to learn in this instance?
What could have been done when designing
this installation to minimize and perhaps
manage these problems bearing in mind the
client did not want to change to another type
The primary answer is to design and specify the
correct method of installation and use of products
to manage water.
Fig 3. Original stone on top.
Bottom right: The only adhesive not to change the If the water that moves in and out of the system
colour of the stone. can be kept to a minimum then so can the adverse
effects that water triggers, in this case that of
It was decided to do a test off site (Fig 3.) compar- efflorescence and oxidization of soluble iron.
ing the adhesive used to install the stone with
three other products from different manufacturers. The list of considerations is:
The photo Fig 3 shows four pieces of stone installed Load bearing waterproof membrane.
with thinset adhesives from different companies. Placement of a waterproof membrane as close to
Three of the adhesives use white cement and one the surface (or source of water) as possible is im-
grey. The stone lying across the top is an uncon- perative. This greatly reduces the amount of water
taminated piece also from the original batch. It is in the system because the substrate, potentially
easy to see that 3 of the 4 adhesives have discol- the greatest reservoir, is kept dry. Our case study
oured the stone. One of these adhesives was used had a sand and cement mortar bed (which the
on the job itself. The adhesive in the lower right stone was thinset to) and underneath a bituminous
hand corner however did not discolour the lime- waterproof membrane, allowing a lot of water to
stone and is perhaps the clue to what was happen- collect in the 50mm bed. The more water that sits
ing on site. and evaporates out through the stone surface the
higher the risk of the catalytic effects. Waterproof
Apart from the fact that traditionally grey cement membranes like allow stone or tile to be adhered
contains higher iron content than white it is the al- directly (as opposed to bituminous varieties) sit-
kalinity of the cement (the catalyst again is water) ting on top of the mortar bed, keeping everything
that can most affect the oxidation of any iron in a dry.
stone. The higher the cements alkalinity the more Polymer Modified Sand and Cement Adhesives.
it facilitates oxidization. Generally it can be consid- Our case studies sand and cement mortar bed is an
ered that the alkalinity of the cement is directly re- ideal potential source of soluble salt or even …..
possibly iron. The use of washed sand is an impor- Finally how were the issues on our case study re-
tant precaution in minimizing this and the easiest solved?
way to guarantee washed sand is to use a trusted The heavy deposits of iron and efflorescence were
brand of factory prepared adhesive. Using a good removed using acids (oxalic and phosphoric) - with
polymer modified adhesive is also helpful as it hy- the stone then being re-honed. This however did
drates the cement with a lower ratio of water again not totally resolve the stone’s discolouration. For
keeping the system dryer and of course reducing this Mother Nature took charge with natural UV
shrinkage. This has the effect of creating a denser light fading the stone almost back to normal over
mortar that absorbs less moisture, all good for 18 months. The stone was sealed using Aqua Mix
overall water management. Sealers Choice. The job has remained in good or-
However our case study also revealed three factory der with no reoccurrence of the iron deposits. How-
prepared adhesives that used grades of cement ever small traces of soluble salts have returned on
(one grey and two white) that discoloured the lime- several occasions each time successfully cleaned
stone by way of high alkalinity. On closer inspec- with Aqua Mix Eff-Ex.
tion two of the three companies did print cautions
on their packaging regarding the use of these par-
ticular adhesives on some Limestone.
Perhaps more importantly both had other products
in their line that used higher quality white cement
to guard against such problems.
The bottom line is to not only use quality factory Another initiative by Aqua Mix.
prepared adhesives but also to make sure you use Can you afford to be left behind?
the correct adhesive for the job.
These can be used to further minimize the moisture
that gets into the stone. It is important to recog-
nize that a sealer will not stop water from penetrat-
ing (as it must be able to allow moisture vapour
transmission), but it can greatly reduce a stone or
tile’s overall water absorption. The ability of a
sealer to perform this function can be greatly im-
proved by applying it to the back and sides of the
stone or tile as well as the surface. This has tradi-
tionally been difficult because most sealers are
bond breakers adversely affecting the bond of the
mortar or adhesive. However Aqua Mix has several
penetrating type sealers that are not bond breakers
with one of them specifically formulated for appli-
cation on the back and sides of stone and tile called
Supporting these three main areas are other details
that collectively assist in reducing water penetra-
tion into the bonded system. Chief among these
are: correct falls so water can flush away, correct
sealant placement especially at changes of plane
where water can collect externally, and dense poly-
mer modified grouts.
In summary problems such as those encountered
in our case study can be greatly minimized or man-
aged by designing and specifying the correct
method and products for the installation. This in-
volves using a complete system of integrated tech-
Job Site News: New Zealand Job Site News: Australia
Steam Kiwi is a small New Zealand company that is rela- The main Apple Store in Sydney Australia opened re-
tively new to Aqua Mix products. Their main business cently and is another Aqua Mix success story. The floor is
was steam cleaning which they felt needed some diversi- a limestone paver and the grout installation was left
fication. They identified tile and stone as a major part of looking very uneven in colour. Aqua Mix Grout Colorant
this diversification and approached us to see if they could (Pewter, Silver & Black) saved the day with installation of
attend an Aqua Mix Applicators training course. The rest the colorant occurring right up to the grand opening. All
as they say is history and they are now operating as a floors areas including toilet areas had Aqua Mix Grout
recognised and recommended Aqua Mix applicator. Colorant applied in order to give a stain proof uniform
The Case Study advertorial below is an example of some look with easy maintenance capable of taking the wear
of the work they are doing, this one using Ex-Treme. and tear of a busy commercial environment. Apple being
The job is a public swimming pool complex where the an internationally recognised premium brand gives Aqua
anti-slip porcelain in the shower and changing areas had Mix another premium job reference.
become heavily soiled. The reason this type of area is so
hard to clean is that the build up is a mix of body fat,
shampoo, soluble minerals from the water, and bleach
based cleaners that are used for routine maintenance.
The anti-slip tiles do a very good job of holding on to this
chemical concoction and without a proper regular heavy
duty clean (which must include agitation and extraction)
this type of maintenance can only end up like this floor.
As you can see from the photo Ex-Treme did a great job
working primarily on the build up of bleach and also hard
water minerals that were acting as a binder for the fats
and oils. Ex-Treme is not designed to act as a degreaser
however this is an example where the removal of the
harsher chemicals (bleach, minerals, including iron from
the local water) unlocked the other contaminants.
In summary a good result and it is especially nice to see
a company doing so well with our products after attend-
ing one of our applicator courses.