Concrete Chemistry and Protective Coatings
July 2017 www.coatingsworld.com Coatings World | 97
Although cement is relatively expensive,
the total amount of cement in concrete can
be reduced by the addition of some recycled
and low cost ingredients like fly ash formed
in the coal fired power plants. Fly ash has
a pozzolanic activity (when finely divided
it reacts with water and calcium from lime
to form a compound with cementitious
properties) and can reduce the amount of
expensive cement in the mixture by up to
30%. Fly ash is formed during the burning
of coal at temperatures of 2,700°F – it is a
finely divided alumino silicate glass that is
carried off by flue gasses and collected electrostatically. Because the particles are melted and reformed in the gas phase, they are
spherical. The shape and composition have
a lot to do with the beneficial properties
fly ash brings to the concrete. Because of
the spherical shape, fly ash can reduce the
amount of water required for good workability which contributes to higher strength
when cured.2 In large pours of concrete
where heat dissipation is a potential problem (temperature differentials can lead to
concrete cracking), the fly ash can significantly reduce the heat generated. Most
significantly, however, fly ash also replaces
part of the expensive cement (typically up
to 30%) which reduces the cost of the concrete, while also giving improved performance. There is one significant drawback,
however. The solubility of the fly ash is less
than some of the other components of concrete and it reacts more slowly. As a result,
fly ash will significantly slow the cure time
of the concrete, which means that contractors have to wait longer to finish a job.
Despite this downside, the overall benefits
of fly ash are significant enough that it is in
common use in concrete.
The amount of water is also key to
the properties of concrete. The strength
of the concrete depends to a large extent
on the amount of water relative to the ce-
ment (w/c ratio). This is a balance that
has an optimum w/c ratio, however. Free
water must be present for the hydrates to
form and the concrete to gain maximum
strength. Although the water is essen-
tial to the formation of the hydrates, too
much water can detract from the strength
because it can create unfilled voids in the
concrete. There is a set amount of wa-
ter which is required to form the calcium
silicate hydrates, but extra water is needed
to make the concrete workable. Too little
water will give a mixture which is diffi-
cult to pour out and work; if it is difficult
to compact, then the strength will also
be lower because of air voids. A quality
concrete is made by using as little water
as possible but still maintaining enough
water for good workability. If too much
water is added at the job site to make the
concrete easier to work when it is being
poured, the concrete will have voids in it
which are not filled by the hydrates, and
the strength will be lower.
Cement Components
To understand the formation of hydrates
which are the key to the strength of
concrete, we need to look at the different components of cement individually,
although the actual reactions result in a
mixed product of each of the following
chemicals. Cement chemists use a different notation to describe the materials.
Alite
The alite is the most abundant mineral in
portland cement at 40-60% of the total.
It is the hydration reaction of the alite
with water which gives concrete its initial
strength. Alite can form several crystal
structures, however when it is cooled rapidly after calcining, it forms an awkward
structure in which the calcium and oxygen
do not fit together well in the crystal matrix. This gives alite its high reactivity and
solubility. The alite rapidly dissolves in
the water phase and, after the solution becomes super saturated, it precipitates out
as a calcium silicate hydrate gel (C-S-H).1
The C-S-H hydrate gel which forms is
not intrinsically strong, but it forms a layer
around other particles in the concrete and
this binds the individual particles together.
As the CSH gel forms it contains millions
of tiny water filled voids in the gel matrix
(much smaller than the water filled pores
in the concrete). This CSH gel with the micro voids has a much larger volume than
the original tricalcium oxide silicate particles. As a result the CSH gel expands outward and encapsulates the other aggregate
particles in the concrete. This causes the
concrete to set and eventually to harden
to a strong solid. During this process the
water filled capillary pores in the concrete
get smaller or close up. This reduction in
the volume of the pores decreases the permeability of the concrete.
The calcium hydroxide (Portlandite)
which forms during this hydration reaction contributes only slightly to the
strength of the concrete. It reacts with
liquid water and helps close the pores and
it forms individual crystals which are resistant to shrinkage during drying. The
CH is water soluble, however, and if the
concrete is exposed to fresh water, the
CH will leach out, leading to larger pores.
Belite
Belite is the name of the dicalcium oxide
silicate which is also present in cement after the calcination. It is formed at lower
temperatures than the alite and early concrete (created in lower temperature kilns)
was almost exclusively belite. Belite is
much less soluble than alite and doesn’t
react as fast. Because of the slower hydration rate, belite contributes little to
TABLE 1:
Nomenclature of components in cement (From The Science of Concrete1)
Chemical Name Chemical Formula Oxide Formula Cement Notation Mineral Name
Tricalcium Silicate Ca3SiO5 3CaO.SiO2 C3S Alite
Dicalcium Silicate Ca2SiO4 2CaO.SiO2 C2S Belite
Tricalcium Aluminate Ca3Al2O6 3CaO.Al2O3 C3A Aluminate
Tetracalcium
Aluminoferrite Ca2AlFeO5 4CaO.Al2O3.Fe2O3 C4AF Ferrite
Calcium hydroxide Ca(OH)2 CaO.H2O CH Portlandite
Calcium sulfate
dihydrate CaSO4.2H2O CaO.SO3.2H2O CSH2 Gypsum
Calciumoxide CaO CaO C Lime
