Benefits - Durability
At least three mechanisms must be considered to understand how BASF's MetaMax® high reactivity metakaolin (HRM) improves the durability of concrete:
- The addition of MetaMax yields denser and less porous concrete to prevent the intrusion of water, gases, chloride ions, and other chemicals that can attack concrete. Read more »
- MetaMax “consumes” free lime in concrete so it cannot contribute to detrimental processes such as efflorescence and alkali silica reaction (ASR). Read more »
- Because concrete with MetaMax is stronger, it is less likely to be damaged due to wear and tear or excessive loads. Read more »
Avoiding problems due to porosity
We think of concrete as a solid material, but in reality, it is full of holes. Some of the holes, such as the controlled air cells formed by air entraining admixtures, are beneficial to concrete. But most holes just result in gaps that weaken and increase porosity.
Some of the holes are caused by voids between pieces of aggregate. Even fine aggregates such as sand leave small crevices between adjoining pieces, creating voids so small that cement particles cannot fill the spaces. But MetaMax particles are less than a tenth the size of portland cement particles and act as micro filler to plug the gaps and create denser concrete.
Another factor responsible for holes is the excess water necessary to make workable concrete. In theory, a water to cementitious materials ratio (w/cm) of only 0.22 is necessary to fully hydrate portland cement. This assumes, however, that cementitious materials can be completely dispersed without clumping, the concrete can be mixed so that every particle is exposed to sufficient water, and that no water evaporates or is absorbed by forming materials during curing. These conditions seldom exist in practice, and extra “water of convenience” is almost always required to create a concrete mixture that can be mixed, placed and finished. It is not uncommon for ready-mixed concrete to have a w/cm as high as 0.45, double the amount required for hydration.
This extra water creates interconnected pores in the concrete matrix. Some of the moisture evaporates while the concrete is still green, forming microscopic channels at the concrete’s surface. These pores and channels form pathways through which corrosive liquids and gases penetrate concrete.
MetaMax fills these holes due to its pozzolanic nature. That’s because metakaolin consumes the lime formed as a byproduct of cement hydration to produce additional crystals of calcium silicate hydrate, CSH, the interlocking crystals that act as the glue to hold concrete together. As the additional CSH crystals form, they plug many of the pores and channels to reduce porosity and resist moisture migration and gas diffusion within the concrete.
Furthermore, lime is soluble and can leach out of concrete and create even more voids. Consuming lime before this happens is another way MetaMax reduces porosity.
Corrosion
Moisture penetration into concrete helps to set up galvanic cells that result in corrosion of steel reinforcing other steel embedded in concrete. Chloride ions – from salt water, deicing salts, or atmospheric pollution – accelerate corrosion and can be carried into concrete by moisture penetration. When steel turns to rust, it expands and exerts an internal force that leads to concrete spalling and cracking.
Moisture and chloride ion penetration is an especially important consideration in parking ramps, bridges, water treatment facilities, coastal and marine structures, and industrial facilities where harsh chemicals are handled, and in areas prone to freeze-thaw conditions.
Laboratory tests and field performance trials confirm that MetaMax® metakaolin effectively reduces permeability and chloride penetration, offering assurance of longer service life.
Avoiding problems due to excess lime
Efflorescence: Moisture migration can leach soluble lime and other mineral salts out of concrete and deposit them as efflorescence on the surface. If not removed promptly, efflorescence converts to calcium carbonate (similar to limestone) when exposed to air and can permanently disfigure concrete.
MetaMax reduces efflorescence by both consuming free lime and plugging pores.
Alkali Silica Reaction
Alkali silica reaction (ASR) takes place when free lime (alkali) in concrete interacts with reactive silica present in many types of aggregates. The reaction forms a gel that absorbs moisture, and expands and creates tensile stresses that can crack concrete. MetaMax mitigates ASR by consuming the free lime essential for ASR to occur and by reducing penetration of the moisture necessary for ASR.
In this stained petrographic micro photo, the light bluish-white material is the gel formed by ASR at the interface between aggregate particles and the cement paste. When the gel expands in the presence of moisture, it forms stresses that can crack concrete.
ASR typically creates a distinctive pattern of perpendicular cracks.
ASR can occur in almost any part of the country. It is of increasing concern since many of the best sources of non-reactive aggregates have been exhausted. When this happens, the use of MetaMax in a mixture can become financially attractive because it may permit the use of lower cost or locally available aggregates that comply with the LEED credit for locally extracted materials. Read more »
Another type of reactive aggregate that is receiving much attention is recycled glass. Silica in glass is highly reactive in concrete. New research shows that adding metakaolin enables glass to be used as aggregate. Thousands of tons of glass can now be reecycled annually, and concrete producers across the country continue to find new uses for the material. Read more »
Avoiding problems due to wear and tear
MetaMax helps produce concrete with greater strength and density that is more resistant to wear and tear. This makes MetaMax high reactivity metakaolin the concrete additive of choice whether you are building an industrial floor, a major highway, or even a maritime structure.