Roman Formula
The secret of Roman success in making cement was traced to the mixing of slaked lime with pozzolana, a volcanic ash from Mount Vesuvius. This process produced a cement capable of hardening under water. During the Middle Ages this art was lost and it was not until the scientific spirit of inquiry revived that we rediscovered the secret of hydraulic cement -- cement that will harden under water.
Repeated structural failure of the Eddystone Lighthouse off the coast of Cornwall, England, led John Smeaton, a British engineer, to conduct experiments with mortars in both fresh and salt water. In 1756, these tests led to the discovery that cement made from limestone containing a considerable proportion of clay would harden under water.
Making use of this discovery, he rebuilt the Eddystone Lighthouse in 1759. It stood for 126 years before replacement was necessary.

We can distinguish two different ways of producing slow setting natural cements. Firstly, they can be
prepared by grinding a natural rock after it has been burned at a temperature sufficient to reach the
sintering point. In this case, almost all the ‘cement rock’ is burned at high temperature and is intended
for the production of one type of slow setting binder. Most of these products are sold as ‘natural
Portland cement’ or simply as ‘Portland cement’ as in the Grenoble area (France). The latter
designation should not be confused with ‘artificial Portland cement’. Another way to produce this type
of natural cement is to pick out the overburnt lumps from the heated rocks initially intended for the
production of quick setting natural cement. The selection is mainly done by hand-picking. The
overburnt lumps are also called ‘grappiers’ and the resulting binder used to be sold as ‘Grappier
cement’. Overburned lumps may also occur in kilns intended for the production of hydraulic lime.
These grappiers can also be used for the production of ‘grappier cement’. This type of cement was
invented at Teil (France) in 1869 (Boero, 1925).