cement concrete usually does not have good resistance to acids.
Some weak acids however can be tolerated, particularly if the exposure
is occasional (Table 1). There are essentially three ways to improve
concrete resistance to acids, (1) choosing the right concrete composition
to make it as impermeable as possible, (2) isolating it from the
environment by using a suitable coating or (3) modifying the environment
to make it less aggressive to the concrete.
Acids attack concrete by dissolving both hydrated and unhydrated
cement compounds as well as calcareous aggregate. In most cases,
the chemical reaction forms water-soluble calcium compounds, which
are then leached away. Siliceous aggregates are resistant to most
acids and other chemicals and are sometimes specified to improve
the chemical resistance of concrete.
Concrete deterioration increases as the pH of the acid decreases
from 6.5. In fact, no hydraulic cement concrete, regardless of its
composition, will hold up for long if exposed to a solution with
a pH of 3.0 or lower. To protect concrete from such severely acidic
environments, surface protective treatments are often used. Many
treatments are available and details are given in PCA’s Effects
of Substances and Guide to Protective Treatment.
researchers tried to improve acid resistance of concrete pipes
exposed to long-term acid attacks in a pH range between 4.5 and 6.5.
The fundamental solution of increasing chemical resistance by lowering
concrete’s permeability was used to optimize concrete mix designs.
The objective was to keep the erosion rate so low that a service life
of about 100 years could be achieved (Fig. 2). It was found that a
significant improvement in acid resistance can be achieved by carefully
controlled use of fine supplementary cementitious materials. The main
reason for the increased acid resistance of the concretes investigated
was the formation of a very dense hardened cement paste and aggregate
interface with very low porosities. The use of slag cement increased
acid resistance as well as portland cements with silica fume (up to
8% by mass of cementitious material) or fly ash. For more information
or to access the report click
|Fig. 2: Concrete surfaces with different depths
of erosion. Photo courtesy of VDZ.
Carbonation by contact with water
Natural waters usually have a pH of more than 7 and seldom less
than 6. Even waters with a pH greater than 6.5 may be aggressive
if they contain bicarbonates. Any water that contains bicarbonate
ion also contains free carbon dioxide, which can dissolve calcium
carbonate unless saturation already exists. Water with this aggressive
carbon dioxide acts by acid reaction and can attack concrete and
other portland cement products whether or not they are carbonated.
A German specification, DIN 4030,
includes both criteria and a test method for assessing the potential
of damage from carbonic acid-bearing water.
|Table 1: Acid Attack and Resistance
|Acid attack increases with
||Acid resistance increases with
|• increase in acid concentration
• constant and fast renewal of acidic solution at the
• higher temperatures
• higher pressure
|• High Ca++ content in a dense hardened cement paste
• low proportions of soluble components in concrete
• Creation of a durable protective layer of reaction products
with low diffusion coefficient (transport properties)
Effects of Substances
and Guide to Protective Treatment (IS001) Improve concrete's
durabilty by knowing what chemicals attack it and what you can do
to protect it. Nearly 250 substances (salts, acids, etc.) are listed
along with recommended protective treatments and manufacturers of
Types and Causes
of Concrete Deterioration (IS536) This practical guide
gives a clear and concise overview of each of the types and causes
of concrete deterioration, including corrosion of embedded metals,
freeze and thawing, chemical attack, alkali-aggregate reactivity,
abrasion/erosion, fire/heat, restraint to volume changes, overload
and impact, loss of support, and surface defects.