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Lightweight Aggregate for Concrete Masonry
Why pumice is the aggregate of choice
Pumice is a naturally occurring, chemically inert, non-soluble, non-magnetic, non-flammable, low-density material, an amorphous blend of mainly silicon and aluminium oxide and low levels of other metal oxides.
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Pumice as a construction material has a long history of use in buildings going back to Roman times. In recent times much international research has been done on Lightweight Structural Concrete and Lightweight Concrete Masonry using pumice as a raw material. International Standards are well established for design and testing of concrete products containing pumice, and testing of raw materials.
World pumice production in 1999 was approximately 11.6 million tonne [USGS Minerals Yearbook], the majority of which was used in lightweight concrete, particularly in Europe where prefabricated panels are favoured. In the USA the major use of pumice is in the concrete masonry building block. |
Pumice has many very desirable properties when used as lightweight aggregate in concrete:
- Chemically inert [pH approx 7.0, alkali-aggregate reaction classification is non-reactive]
- Low-density raw material [600-800 kg/cubic metre dry] and product, saving transport costs and making product easier to lay by block layers.
- Excellent compressive strength to weight ratio [up to 27MPa for 1750 kg/m3 concrete]
- Excellent concrete texture, reduced block forming time [at one NZ manufacturer, reduced from 9 seconds/cycle to 7 seconds/cycle, giving significant productivity gains in block manufacture] an added benefit is much longer life for mold liners, compared to harsh aggregates like scoria.
- Superior water absorption/desorption characteristics. Moisture held in the interior of the pumice aggregate is not immediately available for chemical interaction with cement, but is extremely beneficial in maintaining longer periods of curing, giving better strength and reduced permeability in the final concrete [ASTM technical publication 169C]
- Excellent sound absorption for a given wall thickness and wall mass/m2
- Low thermal conductivity and non-flammable, giving increased fire resistance ratings [FRL's in Australia] to masonry walls. Pumice itself has an R value of 14 m2 oC/W, sand and gravel conventional concrete of 2200 kg/m3 has an R value of approx 0.55, lightweight structural pumice concrete of 1440 kg/m3 has an R value of approx 2.3 [Masonry Institute of America, NZ Standards publications]
- More attractive, whiter coloured blocks
- Able to totally replace conventional sands and aggregates in masonry formulations with a combination of pumice sands and larger pumice aggregates.
In addition to all these benefits, pumice is also:
- Easy to use in blends to obtain different concrete properties
- Pure and non-toxic, so exposure to pumice has no health implications, no special storage and handling is required.
- Environmentally friendly, with low energy extraction/preparation, lower transport costs than higher density aggregates, no degradation into soluble or volatile components with time.
- A pozzolan, i.e. it has properties when in fine particle form, where the silicon and aluminium oxides in pumice react with lime and water to form rock hard non porous material. These types of reactions are also the basis for curing of Portland cement, and were used by the Romans in the construction of most of the ruins that still exist today.
And pumice based concrete masonry is:
- Freeze/thaw resistant
- Nailable and sawable if suitably formulated
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