Lightweight Insulating Concrete
PUMICE CONCRETE (not to be confused with Pumice-Crete®) is composed of Portland cement, pumice rock, pumice sand, pumice powder (pozzolan) and water. Compared to standard concrete, pumice concrete offers roughly a one-third reduction in weight. Proportioning, mixing and placing are done in a similar manner to that of standard concrete. Pumice concrete is placed and finished with typical concrete equipment and tools and is used, with one exception, for the same applications as sand and gravel concrete. That exception: due to the nature of pumice aggregate and sand, pumice concrete takes much more effort and patience to work to a standard surface finish, and for that reason is rarely used for extensive flatwork. But for any and all applications where pumice concrete is vibrated into forms, it presents no placement challenges beyond standard concrete and actually presents several advantages.
Of significant note, lightweight pumice concrete can be handled in larger volumes by lighter-duty equipment with less wear and tear on that equipment, and there is a significant reduction of dead weight on supports and formwork. Additionally, the unique lightweight quality of pumice concrete contributes to a decrease in structural steel costs, equipment and form demands, and, consequently, job costs. Also, pumice concrete has superior resistance to harsh weather conditions like freezing and thawing and an R-value some 4-times that of ordinary sand and gravel concrete—making pumice concrete ideal for colder climates and locations that experience dramatic fluctuations in the weather and temperature.
It is the use of pumice aggregate and sand that provide the lightweight advantages of pumice concrete by reducing the dead load on structural supports, trusses, girders and slabs. Pumice is a porous, naturally-occuring volcanic glass, riven with air-filled vesicles formed by the expansion of trapped gasses when the molten lava is expelled into the air and flash-cools. It is this amorphous, cell-filled nature of each stone that give pumice it’s lightweight and insulative characteristics. Pumice is also an inert material and therefore will not react with the reinforcing steel or any other concrete component.
As for how well pumice compliments Portland cement, pumice has no cementitious properties of its own, yet in the presence of hydraulic lime cement and water, pumice becomes chemically active within the concrete paste and functions as a supplementary cementing material, supercharging the process that forms Calcium Silicate Hydrate (C-S-H), the binding compound that makes concrete concrete, resulting in a stronger, denser concrete matrix. Durability increases because porosity is significantly reduced, and with that reduction come the benefits—resistance to moisture infiltration and subsequent freeze-thaw damage, resistance to destructive sulfate and chloride attacks, and mitigation of efflorescence staining.
Lightweight insulating concrete is also notable for dampening sound transmission, for elasticity that reduces brittleness under earthquake conditions, for not spalling under exposure to direct flame, and for mitigating or eliminating moisture condensation on walls and ceilings.
PROPERTIES OF HESS PUMICE AGGREGATES
» Download a PDF with the Hess Pumice Aggregates data.
PUMICE CONCRETE MIX DESIGNS
» Download a PDF with Pumice Concrete Mix Designs.
PREPARATION, MIXING, AND PLACING LIGHTWEIGHT PUMICE CONCRETE
The pumice aggregate should be uniformly prewet to reach total saturation, then allowed to sit until excess water drains out. This will fill the natural internal voids in the aggregate with water. Why? Prewetting will prevent drying out on the way to the placement site and slump will be uniform throughout the load as well as from load to load. Pumice concrete also pumps better when the aggregate and sand is prewet.
An alternative to prewetting the aggregate stockpiles, the pumice and about three-fourths of the mix water can be charged into the mixer and slowly agitated for about 10 minutes before adding the remaining ingredients.
Although handling pumice concrete is similar to normal-weight concrete, pumice concrete has about two inches less slump for the same consistency and workability. It will flow when vibrated and can be finished sooner, however, avoid over-vibrating and overfinishing pumice concrete, as this will tend to float the light coarse pumice particles to the top—not the paste, as with standard concrete.
Cure with the same methods as conventional concrete. Delay form removal for an additional 24 hours when sharp corners have been formed, as they tend to be softer for a few additional days.
A LONG HISTORY OF PUMICE CONCRETE IN CONSTRUCTION
There is nothing new about pumice concrete—it has been used successfully for over 2000 years. The Romans employed lightweight pumice concrete in many of their structures—the most famous is the dome of the Pantheon, which remains to this day the world’s largest unreinforced concrete dome. Fine-grained pumice dust was also used by the Romans as the pozzolanic catalyst that made their hydrated lime-based concrete (opus caementicium) so enduring.