Technical data:
A pozzolan is broadly defined as an amorphous or glassy silicate or aluminosilicate material that reacts with
calcium hydroxide formed during the hydration of Portland cement in concrete to create additional cementitious
material in the form of calcium silicate and calcium silicoaluminate hydrates. The first pozzolans were used by the
Romans to make cement from burned limestone and Santorum earth from volcanic eruptions. These ancient
concrete mixes were extremely durable and many architectural elements survive today. They underline the fact
that one of the compelling reasons for incorporating pozzolans in concrete today is to improve quality and to
extend service life by enhancing the durability of this ubiquitous construction material.
To function properly, pozzolans must be amorphous or glassy and generally finer than 325 mesh (45 microns) in
particle size. Finer particle sizes generally have greater reactivity, meaning they more quickly convert to
supplementary cementitious material, helping in early strength development as measured by standardized tests
such as ASTM C618/C109.
Pozzolans can continue to react in concrete for many years, further strengthening the concrete and making it
harder and more durable during its service life. Pozzolans also serve to densify and reduce the permeability of
concrete, which helps to make it more resistant to deterioration and swelling associated with various exposure
conditions.
Types of Pozzolan
Pozzolans commonly used in modern concrete construction include coal fly ash (aka pulverized fuel ash or PFA),
ground granulated blast furnace slag, silica fume, and metakaolin (calcined clay). The common feature of all these
pozzolans is that they are silicates or aluminosilicates that have been converted to amorphous or glass phases in a
high temperature furnace or combustion chamber, followed by rapid cooling or quenching under various
conditions. The amorphous or glassy form allows the silicates to react readily as the concrete cures. For use in
modern cement and concrete applications, pozzolans must be low in alkalis (Na2O and K2O), which cause longterm
durability problems in concrete by expansion due to the alkali-silica reaction (ASR).
Mix Design with Pozzolans
Over the last 50 or more years, engineers and scientists have developed a number of strategies for effectively
designing concrete with pozzolans. Depending on the type of pozzolan used and the design goals for the concrete,
adjustments will be made to at least the cement replacement factor and the w/c ratio. Pozzolans such as VCAS,
blast furnace slag, and fly ash have relatively low water demands and can be used to replace up to 40% of cement
in some concrete mixes. In contrast, silica fume has much finer particles with very high water demand,
necessitating the use of high range water reducers or superplasticizers to even achieve a 10% cement replacement.
At the same time, the fine particle size renders this pozzolan more reactive, allowing concretes to achieve higher
strengths more quickly. Like silica fume, metakaolin also has high water demand and is rarely used at replacement
levels above 10%.
Another important consideration is that VCAS pozzolan has a specific gravity (SG) of 2.6 which is significantly
lower than the SG 3.15 of Portland cement. This means that when VCAS is substituted for cement on an equal
weight basis, there will be an increase in the paste volume. For a nominal 20wt% VCAS replacement of cement,
this amounts to about a 4.2% increase in paste volume at fixed w/c ratio. This difference translates into an
improved yield of cementious product and a mixture that is perceived to be more creamy and cohesive in
consistency.
Depending on the reactivity of the pozzolan, the cement replacement factor, and the w/c ratio, the initial strength
(up to 3 days) of concrete with pozzolans may be reduced by up to 20% compared to the strength of a control
concrete without pozzolan. Between 14 and 28 days, however, the pozzolanic concretes typically have similar or
higher strengths than the control concrete, with great benefit to the long-term durability, hardness, and strength of
the concrete structure or component. High performance pozzolans, such as silica fume and metakaolin, typically
have the least impact on the early strength. VCAS pozzolans fall into this high performance category and can
reach or exceed the control strength by 3 days. Specific strength development data on the performance of VCAS
pozzolans can be found in the VCAS Technology Data Sheets, and Technical Bulletins.
VCAS pozzolans are comparable in reactivity with silica fume and metakaolin, and are engineered to achieve their
performance both by uniform materials chemistry and quenching and by fine particle size. As such, VCAS
pozzolan have low surface area and smooth surfaces that have 10% less water demand than silica fume or metakaolin. This allows VCAS pozzolans to be used at cement replacement rates up to 30% or higher. Low water
demand, high reactivity, and white color after cure are the hallmarks of an excellent pozzolan for white Portland
cement concrete.
Benefits of Using Pozzolans
The following are benefits that are generally obtained from using pozzolans in concrete.
Workability: As they are replacing various percentages of Portland cement, pozzolans can have a significant
impact — both positive and negative — on the workability of a concrete mix. Low water demand pozzolans, such
as VCAS, blast furnace slag and fly ash, generally increase workability at a given w/c ratio. In contrast, high
surface area pozzolans such as silica fume and metakaolin cause a large reduction in the fluidity of the concrete,
which necessitates the use of costly chemical admixtures (high range water reducers, superplasticizers) to make
them workable. Even then, these types of concrete tend to be sticky and difficult to finish. VCAS pozzolans fall
into the category of low water demand pozzolans.
Reduced Heat of Hydration: Historically, one of the most important reasons for including pozzolans in concrete
was to lower the heat of hydration of the Portland cement in order to reduce thermally induced stress cracking.
This is an especially important factor in massive concrete structures such as dams, tunnels, bridges, and large
precast elements.
Increased Durability: Properly designed concretes made with pozzolans are generally stronger, harder and more
durable. The physical densification of the cement matrix, coupled with the consumption of calcium hydroxide and
cement alkalis brings about the following improvements in durability in pozzolanic concretes: greater resistance to
sulfate attack; reduced alkali aggregate reactivity; less susceptibility to corrosion of embedded steel by chloride
ion intrusion; and reduced efflorescence.
Reduced Efflorescence: Pozzolans react with excess calcium hydroxide and cement alkalis, helping to prevent
primary efflorescence. Once the concrete cures, the pozzolanic reaction described earlier causes the concrete to
become denser and less permeable. This limits egress of moisture, and reduces the lime and alkalis available to
migrate to the surface causing primary and secondary efflorescence. VCAS pozzolans are similarly very effective
at controlling efflorescence, particularly in white or colored concretes where this phenomenon can be a serious
problem.
Improved Form Detail: A high fineness, high performance pozzolan such as VCAS-micronHS is effective at
reducing bug holes and better reproducing form detail in precast gray concrete. VCAS pozzolans are very
effective at improving the surface quality and detailing of precast and molded white cement concrete products
such as artificial stone, stucco, cladding panels, pool mixes, and terrazzo.
Pozzolan Color: The vast majority of pozzolans, such as coal fly ash, iron ore blast furnace slag, and silica fume,
are dark colored and will usually strongly discolor white cement. Over 14-million tons of these pozzolans are used
in gray Portland cement. VCAS and metakaolin pozzolans can be used with white cement. Upon the initial mixing
of the white concrete paste, VCAS pozzolan will be slightly blue tint, and metakaolin will be slightly buff tint.
During the first seven days of curing, the white concrete will brighten as the pozzolan reacts, such that both
pozzolan will be about as white as the white cement control. The VCAS blue tint, measured as lower B value, will
often make the VCAS pozzolan appear whiter to the human eye than metakaolin.
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Related Articles:
- Vitro
- VCAS
- Metakaolin
- Coal fly ash
