As concrete ages, it will begin to deteriorate and, if the cement is too alkaline, the ageing process causes star-shaped cracks to appear which will allow rainwater to penetrate. In wintertime, in cool temperate and in cold climates, the deterioration is accelerated due to freeze-thawing of the water within the cracks. This phenomenon is known as 'concrete cancer'. In order to understand how this occurs, it is necessary to understand something about the chemistry of concrete manufacture.
Manufacture of Concrete
Cement itself is made by roasting together crushed chalk (calcium carbonate) and clay slurry in giant steel kilns to a temperature of 1,450C. During this process water and carbon dioxide escape, and the dry residue is crushed to form cement powder. This consists mainly of a mixture of calcium and aluminium silicates:
3CaCO3(s) + Al4Si4O10(OH)8(s)→ Ca3SiO5(s) + Al4Si3O12 + 4H2O(g) + 3CO2(g)
During the roasting process, some calcium carbonate decomposes to calcium oxide, but does not then become neutralised by the clay, so the cement, and the subsequent concrete, contain unreacted calcium oxide, a strong base:
CaCO3(s)→ CaO(s) + CO2(g)
Concrete is quite a porous material and, as rain water soaks through it, the calcium oxide dissolves to form calcium hydroxide solution (limewater):
CaO(s) + H2O(l)→ Ca(OH)2(aq)
Calcium hydroxide is also alkaline. The level of alkali in the powder determines the levels of additives to mix in to make viable concrete.
Silica, SiO2, being the oxide of a non-metallic element, is acidic and, in the presence of water, reacts with the alkali in the cement in what is called an 'Alkali-Silica Reaction' (ASR) - sometimes 'Alkali-Aggregate Reaction' (AAR).
Mechanism of ASR
During the Alkali-Silica Reaction, a gel1 is formed that swells as it draws water from the surrounding cement paste.
Alkali hydroxide + reactive silica gel → reaction product (alkali-silica gel)
The amount of gel formed in the concrete depends on the amount and type of silica and alkali hydroxide concentration. In absorbing water, these gels expand, thus inducing pressure and subsequent cracking of the aggregate and surrounding paste:
Gel reaction product + moisture → expansion
Very often, the cracks appear in a star formation. Once this has occurred, water is able to penetrate the concrete to a deeper level. In winter freeze/thawing actions can cause the concrete to break up even more.
The chemistry of concrete is quite complex, and the visible deterioration of concrete, known as spalling, may be due to Alkali-Silica Reaction, or to a number of other causes.
It is difficult to tell the difference between cracks caused by Alkali Silica Reaction and things like tension marks or admixtures which cause the evolution of heat to speed up curing time. The only way to tell the difference is by taking samples of the concrete and testing them. However, for some causes, such as tension cracks, it is possible to identify where they are and thus determine the cause.
What is Affected by Concrete Cancer
Not surprisingly, every structure built from concrete could be affected by concrete cancer; including bridges, highways and housing. Indeed, lots of structures in England's south-west are affected by concrete cancer, due to the 'South West Alkali Incident'.
Curing Concrete Cancer
Much concrete nowadays is used as steel reinforced concrete and, when spalling occurs one will observe crumbling concrete falling away and, perhaps, rusty reinforcement steel showing through. Once the steel bar is exposed to the elements it becomes subject to corrosion2, resulting in a further loss of structural integrity. This is because, as the reinforcing steel corrodes, the volume of the oxidation products are greater than the original and in effect 'blows the concrete apart'.
In order to effect repairs, the spalled concrete must be removed and any exposed steel must either be replaced or cleaned and treated. The area is then repaired to the original concrete profile using cement mortar, epoxy mortar or concrete, depending on the size of the damage and the structural requirements. Cracks are repaired using suitable epoxy resins, special mortars and injection techniques.