Historic Preservation - Technical Procedures
Removing Efflorescence From Concrete
Hstrc Concrete: Investigation & Rpr/Pre-Conf Training - 1989
Removing Efflorescence From Concrete
REMOVING EFFLORESCENCE FROM CONCRETE
THE CLEANING OR REMOVAL OF STAINS FROM CONCRETE MAY INVOLVE THE
USE OF LIQUIDS, DETERGENTS OR SOLVENTS WHICH MAY RUN OFF ON
ADJACENT MATERIAL, DISCOLOR THE CONCRETE OR DRIVE THE STAINS DEEPER
INTO POROUS CONCRETE. USE THE PRODUCTS AND TECHNIQUES DESCRIBED
HERE ONLY FOR THE COMBINATIONS OF DIRT/STAIN AND CONCRETE SPECIFIED.
A. This procedure includes guidance on removing
efflorescence from concrete using chemical solvents.
B. Efflorescence is a condition where white deposits form on
the surface of the concrete. These deposits often
contain calcium, sodium and potassium hydroxides or
carbonates, bicarbonates, chlorides and sulfates of
calcium and magnesium.
C. The surface deposits may originate as soluble compounds
within the concrete or in the soil. These compounds
combined with the presence of water gradually migrate in
solution to the wall surface, where they remain when the
water evaporates. Surface deposits may also result from
acid etching with hydrochloric acid, sometimes used to
roughen the concrete surface.
D. Surface deposits originating from within the concrete are
usually soluble and may be removed by scrubbing with
water alone or hosing with water under high pressure.
E. Surface deposits composed mainly of calcium acid
carbonate and magnesium acid carbonate from the soil or
calcium hydroxide should be washed off as soon as
possible using water alone. These deposits are water-soluble
for only a brief period of time after reaching
the atmosphere, when carbon dioxide converts them to
water-insoluble calcium carbonate and magnesium
carbonate, which are impossible to remove without the use
F. Safety Precautions:
1. DO NOT save unused portions of stain-removal
2. DO NOT store any chemicals in unmarked containers.
3. EXCELLENT VENTILATION MUST BE PROVIDED WHEREVER ANY
SOLVENT IS USED. USE RESPIRATORS WITH SOLVENT
4. No use of organic solvents indoors should be
allowed without substantial air movement. Use only
spark-proof fans near operations involving
5. Provide adequate clothing and protective gear where
the chemicals are indicated to be dangerous.
6. Have available antidote and accident treatment
chemicals where noted.
G. See 01100-07-S for general project guidelines to be
reviewed along with this procedure. These guidelines
cover the following sections:
1. Safety Precautions
2. Historic Structures Precautions
4. Quality Assurance
5. Delivery, Storage and Handling
6. Project/Site Conditions
7. Sequencing and Scheduling
8. General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performing
this procedure and should be followed, when applicable,
along with recommendations from the Regional Historic
Preservation Officer (RHPO).
NOTE: Chemical products are sometimes sold under a common
name. This usually means that the substance is not as pure as
the same chemical sold under its chemical name. The grade of
purity of common name substances, however, is usually adequate
for stain removal work, and these products should be purchased
when available, as they tend to be less expensive. Common
names are indicated below by an asterisk (*).
A. Use one of the following solvents (see Section 3.02 A.
below for mixing proportions):
Acetic Acid (C2H4O2):
1. A colorless pungent liquid acid that is the chief
acid of vinegar and that is used especially in
synthesis (as of plastics).
2. Other chemical or common names include Vinegar
acid*. (Vinegar itself, which contains about 4%
acetic acid, may be suitable for some purposes
requiring acetic acid.)
3. Potential hazards: CORROSIVE TO FLESH AND
CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS.
4. Available from chemical supply house (both
commercial and scientific), drugstore or
pharmaceutical supply distributor, grocery store or
supermarket, or hardware store.
Hydrochloric Acid (30-35%):
1. A strong corrosive irritating acid.
2. Other chemical or common names include Chlorhydric
acid; Hydrogen chloride; Muriatic acid* (generally
available in 18 degree and 20 degree Baume
solutions); Marine acid*; Spirit of salt*; Spirit
of sea salt*.
3. Potential Hazards: TOXIC, CORROSIVE TO FLESH;
CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS,
4. Available from chemical supply house, drugstore or
pharmaceutical supply distributor, or hardware
Phosphoric Acid (H3PO4):
1. A syrupy or deliquescent tribasic acid used
especially in preparing phosphates (as for
fertilizers), in rust-proofing metals, and as a
flavoring in soft drinks.
2. Other chemical or common names include
3. Potential Hazards: CORROSIVE TO FLESH; CORROSIVE
TO CONCRETE, STEEL, WOOD OR GLASS.
4. Available from chemical supply house or hardware
B. Calcium Hydroxide:
1. Other chemical or common names include Calcium
hydrate*; Hydrated lime*; Lime hydrate*; Slaked
2. Potential Hazards: SKIN IRRITANT, AVOID INHALATION
OF THE DRY POWDER.
3. Available from chemical supply house, construction
materials yard, construction specialties
distributor, garden and lawn supply center, or
C. Filler material such as paper pulp
D. Mineral water
E. Plastic sheeting
F. Clean dry towels for blotting the area after treatment
G. Masking tape
H. Accessible source of water, soap and towels for washing
and rinsing in case of emergencies associated with the
use of chemicals
A. Glass or ceramic container for mixing the poultice
B. Rubber of plastic pale for mixing the acid and water
C. Wooden utensil for stirring the ingredients
D. Wood or plastic spatula
1. Provide adequate wash solutions (i.e. water, soap
and towels) before starting the job.
2. Whenever acid is used, the surface should be
thoroughly rinsed with water as soon as its action
has been adequate. Otherwise it will continue
etching the concrete even though the stain is gone.
3.02 ERECTION, INSTALLATION, APPLICATION
NOTE: DO NOT TRY MORE THAN ONE TREATMENT ON A GIVEN AREA
UNLESS THE CHEMICALS USED FROM PRIOR TREATMENT HAVE BEEN
A. Mix in a glass or ceramic bowl one of the following:
1 part hydrochloric acid in 9 to 19 parts water, OR
1 part phosphoric acid in 9 parts water, OR
1 part phosphoric acid plus 1 part acetic acid in 19
CAUTION: ACID IS ALWAYS ADDED TO WATER; NEVER ADD WATER
TO CONCENTRATED ACID BECAUSE THE WATER CAN BECOME
SUPER-HEATED AND TURN TO STEAM, SPLASHING ACID ON THE USER.
B. Saturate the concrete with clean, clear water.
C. Begin by using the first mixture listed above and apply
to the affected concrete surface with a stiff, non-metallic bristle brush.
D. Thoroughly rinse the area with clean, clear water and
allow to dry.
E. If the first mixture is unsuccessful in adequately
removing the efflorescence, repeat the treatment using
the other mixtures listed in the order displayed until
successful results are achieved.
F. For concrete heavily laden with potential efflorescence:
1. Remove all visible surface salts following A-E
2. Follow by applying a poultice of paper pulp
saturated in water and allow to dry.
3. Remove the dried poultice using a wood or plastic
4. Thoroughly rinse the surface with clean, clear
water and allow to dry.
5. Repeat as necessary to achieve the desired level of
A. At the end of the job, if there is a supply of dilute
acid to be disposed of, neutralize it by stirring in 3
pounds of powdered calcium hydroxide for every gallon of
the dilute (1-3) acid. The resulting solution is a
harmless mixture of calcium hydroxide and calcium
END OF SECTION