ROMANS' "SECRET"

Evening Post, Volume CXII, Issue 121, 18 November 1931, Page 7

ROMANS' "SECRET"

MORTAR THAT STUCK

USE OF POZZOLANA

DOMINION SUPPLIES

"Pozzolana," or <<pozzuolana".as it is altornativcly spelt, is not the name of somo fearsome skin disease or anything of that kind. To that wide branch of humanity, usually designated under the heading of "the man-in-tho-street;" pozzolana is probably unknown; to architects and builders it is known, and by them it is esteemed. Webster's Dictionary defines pozzolana as "volcanic ashes from Pozzuoli in Italy; used in a kind of mortar which hardens under water." Beccnt tests conducted under the auspices of the Department of Scientific and Industrial Research indicate that certain of the soft vitnc tuffs of the Dominion's central North Island volcanic plateau make excellent pozzolana, better even than most of thoso in Italy, Greece, and other countries. This discovery cannot but be of far-reaching importance in connection with the' construction of buildings, roads, ana engineering structures generally, especially in the North Island. The Romans knew how to make mortar, using pozzolana, which would hold, together and survive the majority of earthquakes to which Italy has always been subject. And now, in view of the recent discoveries about tho Dominion's pozzolanas, there does not seem any reason why brick and mortar buildings in New Zealand should disintegrate to the same extent as they did in tho recent Hawkes Bay earthquake. In a memorandum on tho subject of pozzolanas, tho Department of Scientific and Industrial Research remarks that the history of building shows that certain types of lime mortar, particu r larlv those used by the Romans in some of their large public works, have proved to possess extraordinary hardness end durability, as well as a degree of impermeability to water that compares well with that of first-class Portla™ cement mortar. In many eases the strength ana adhesive power of the mortar is such that, when a piece v of hardening, are in weak friatlo.condition, and fall to dust when tho buMinK is disturbed. In general, the difference is most striking in mortars in tho interior of thick walls, e.g., in the rough rubble construction frequently used to fill piers and woUs m ancient buildings with ashlar or: Brick. . Tho hasty conclusion is so. met,im^ drawn from such observations that the ancient builders possessed some secret for tho mating of lime mortar, which has been lost, so that now only Portland and other modern "tificial cements can vie with.the old- Roman lime mortars; the secret, however, lies< merely in the successful use of a pozzolana in conjunction with properly slaked lime, either ."fat" or "hydraulic," and there are now available not only the identical materials used throughout centuries by the ■ Bomaiv builders, but also a range- of both natural and (more" recently developed) artificial pozzolanas from many different and widely distributed sources. ' It is possiblo to-day to specify lime-pozzo-lana mortars for use under. exacting conditions, which will certainly »qual the old Roman mortars in endurance and strength. ~..,., r .' ...,'. ...j ' EFFECT WITH CEMENT. j

A substance is said to bo pozzolanic when, while not necessarily cementitious by itself, it possesses constituents which will combine with hydrated lime at ordinary temperatures in the presence of moisture to form stable insoluble compounds of cementitious value. When mixed with lime mortar in addition to, or in partial substitutna for, sand, it will impart hydraulic properties and greater strength. Additions of pozzolanic substances to ■Portland cement mortar and concrete are of great importance. Set Portland cement contains free slaked lime, and this compound is vory easily attacked by many chemical substances and is the cause of most of the efflorescences on cement products. The addition to cement of a material capable of combining with the free lime renders it much less affected by the action of various chemically destructive agencies; also, the resistance of Portland cement to injury by severe heating can be materially increased by a pozzolanic addition.

The pozzolanic activity of ono.sample of the vitric tuff referred _ to, as measured by its rato of reaction with1 the lime solution, was 3.8 units|' compared with Italian pozzolsinas, 2.4, and Santorin earth from Grnece^ about the same value. Pozzolanip cements conlain a much lower proportion of lime than does Portland cement, the specific gravity, is less than 2.8, they are .wellsetting, the rates increasing as the proportions of lime and alumina increase, and they usually act better under water ♦ban in, air.