Portland cement as widely used today was originally formulated by Englishman William Aspdin, building on the work of other Englishmen, including his father Joseph Aspdin and, ultimately, John Smeaton:
Smeaton investigated the cementing properties of various mortars, made from lime obtained from various locations, and discovered that the best mortars were made from the calcination of limes that contained considerable proportions of clay minerals (argillaceous lime). This was the first occasion that the importance of clay mixed with the lime had been recognized in the formation of a hydraulic setting cement. It was found that limes that did not dissolve completely in nitric acid (clay being insoluble in the acid) possessed good hydraulic properties (Kohlhaas, 1983). The cementitious agent Smeaton finally used was made from such a clay containing lime which was mixed with an equal quantity of pozzolana (Lea, 1970). The lighthouse that he constructed stood for 123 years until 1879 and only failed when its foundations were undermined by the sea. Smeaton's conclusions about the importance of the presence of clay were not published until after his death in 1792. Smeaton was the first to call himself a civil engineer (as distinct from a military engineer). In the preface to his book, Hydraulischen Moertel, W Michaelis stated in 1869 (in translation):A century has elapsed since the famous Smeaton completed the building of the Eddystone Lighthouse. Not only the seafaring but for all humanity stands as a true signal of blessed work, a light in the dark night. From the scientific point of view it illuminated the darkness of nearly 2000 years.The cement was called 'Roman cement' although it in no way resembled the true Roman cement, except for its hydraulic setting reactions.
The errors which came to us from the Romans and which were shared even by the excellent Belidor, were dispersed.
The Eddystone Lighthouse is the foundation upon which our knowledge of hydraulic mortars has been built and is the chief pillar of modern construction. Smeaton freed us from the shackles of tradition by showing us that the purest and hardest limestone is not the best, at least for hydraulic purposes, and that the source of the hydraulicity of lime mortar must be sought in the argillacrious admixtures (Draffin, 1976).
[Mary S. J. Gani; Cement and Concrete; p. 5]
The Romans ceased building with high-quality concrete by around A.D. 300. Our modern knowledge of cement owes nothing to them.
Cement was used from the decline of the Empire and through the Middle Ages, but none of it was any good until comparatively recent times (Davey, 1961). [. . .]
In this case, it was not that a Roman secret was lost, rather that the Romans, who did no testing, never learned what they had. The very idea of testing is comparatively recent, and the engineer John Smeaton, who tested samples for the construction of the Eddystone Light in the years 1756– 1759 (Davey, 1961) is, I suspect, the first man on the planet deliberately to test cements of differing compositions.
[Thomas Nelson Winter. Roman Concrete: The Ascent, Summit, and Decline of an Art. Transactions of the Nebraska Academy of Sciences 7 (1979), pp. 137-143.]