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Hypertufa is an anthropic rock made from various aggregates bonded together using Portland cement.
Contents
Hypertufa is intended as a manufactured substitute for natural tufa, which is a slowly precipitated limestone rock; being very porous, it is favorable for plant growth.
Hypertufa is popular for making garden ornaments, pots and land forms. Hypertufa is relatively light compared with terracotta or traditional concrete and can withstand harsh winters, at least down to −30 °C (−22 °F).
Hypertufa was invented for use in alpine gardens. Alpine gardeners formerly used antique animal watering troughs, which became rare and expensive.
Composition
Aggregates are generally Sphagnum (peat moss), sand, and perlite or vermiculite. Coconut coir is coming to take the place of sphagnum moss, as the latter is a very slowly renewing natural resource and the former is a ready byproduct of the coconut industry— it has all of the advantages of the moss but without the environmental costs. Hypertufa made with the classic proportions for mortar (1 part cement: 3 parts aggregate) has a composition of
or 3 parts cement to 9 parts aggregate ("parts" is by volume, not by weight, and is only a very rough measure).
To increase structural strength and longevity, polymer fibers, liquid acrylic, and fiberglass may be incorporated into the mixture, along with various grades of sand, pebbles, and crushed rock which add to the final object's overall strength and stone-like appearance though they increase its weight. Powdered concrete dyes (in small amounts) also tint the hypertufa to resemble natural rock.
Manufacture
After water is added to the mixture, it is packed into a previously constructed mold, then sealed or covered in plastic and allowed to cure for up to two months. The object may be temporarily removed from its wrapping after 24 hours for trimming and/or distressing, after which it is re-wrapped. After the hypertufa object is completely cured, it is removed from the plastic, rinsed thoroughly, and allowed to sit exposed to the elements for several more weeks to reduce its otherwise-toxic surface alkalinity. It can then be used to hold plants.