Removing Stains from Concrete

Simple stains and everyday grime can be removed by washing and
scrubbing.Water jet washing may also be successful. Stains from
oil, rust or paint leave ugly marks on concrete, ruining its
appearance. These stains soak into the concrete surface and
can often be very difficult to remove.
A stain may be removed using a special chemical stain
remover, or a specially prepared chemical mix. In
extreme cases if a stain cannot be removed chemically,
it may be sand blasted. This removes the surface of
the concrete and may expose the aggregates.
Some types of stains can be covered by simply painting
over with a concrete paint. However oil based stains may
soak through a paint and must be removed first.
SAFETY
When using any chemicals mentioned in this chapter always wear
protective clothing, gloves and shoes. Protective eye goggles and face
masks may also be necessary.
Don’t breathe in fumes from any of these chemicals.
If chemicals come in contact with skin or eyes, wash the area with
plenty of cold water, and seek doctor’s advice.
Always read the safety directions on the label of any chemical
container prior to starting work.
Ways to remove specific stains are:
OIL AND GREASE STAINS These can be difficult to remove completely
because they tend to soak into the concrete surface. If the oil or grease has
hardened it can simply be scraped off. If an oil or grease spill has just
occurred, stop it spreading by encircling it with sand, dirt, sawdust or cat
litter. These can also be used to assist in soaking up or removing as
much of the oil and grease as possible.


Cover residue stain with a poultice made of 1 part lime to 2 parts
mineral turpentine. Spread a 5 mm layer of the paste over the
stained area ensuring the spread is 50–100 mm
beyond the edge of the stained area. Cover with
plastic sheeting and leave for 24 hours.
Remove the cover and scrape off
the powder. It may be necessary to
repeat this process again within a
day or so to remove any deeply
ingrained oil or grease. Scrub with
warm water and laundry detergent
then rinse with clean water at
the end of the treatment.
RUST External rust, from objects placed on the concrete,
may be removed with a detergent based concrete
cleaner or a weak solution (1:25) of hydrochloric acid
(if this is not successful please seek professional advice).
Prior to applying the acid solution, wet the concrete first
and always wash down the surface with clean water
afterwards. Be careful where the run off goes as it may
create problems on other concrete surfaces or gardens.
A poultice method may also be used.
Stains from rusting of the embedded steel reinforcement,
if this type of stain is present, seek professional advice.
TIMBER Timber stains wash off with a domestic
chlorine bleach.
Scrub the area with bleach.
Wash with water.
If this does not work well, mix 120 grams of oxalic acid
with 4 litres of hot water. Apply, wash off and neutralise
with a solution of bicarbonate of soda and water.
PAINT Paint spillage stains come off best with a paint remover.


ALGAL AND FUNGAL GROWTH Algal and fungal stains
are removed with domestic chlorine bleach.
Wash and scrub the area with bleach.
Leave for a few days.
Scrub or scrape growth off.
Wash with water.