PETROGRAPHIC EXAMINATION OF HISTORIC CONCRETE

William Aspdin was the son of Joseph Aspdin, the inventor of portland cement. William founded a separate company that also manufactured portland cement. He planned a grand house which was to be called Portland Hall and was to be located in the Leith Park Road - Windmill Hill area of Gravesend, England. William Aspdin encountered financial difficulties and the house was never completed. However, a wall that surrounded the property was built between 1855 and 1860 and stood for many years. The wall was reported to have had pilasters that were topped with elaborate capitols made of brick with a thick coating of cement and aggregate.

A sample of intact concrete known to have been produced in England in the mid nineteenth century and that represents a decorative portion of the Portland Hall complex was received by one of the authors (W.G.H.). Significant features of the concrete detected petrographically include: 1) The presence of slag that was judged to be the by-product of copper smelting and that had been ground to the fineness of the portland cement. 2) The presence of spherical air voids that are consistent with air-entrainment. 3) The lack of complete cement paste carbonation. Use of copper slag as a pozzolan is consistent with innovations in production of concrete that took place during the nineteenth and twentieth centuries and with changes in cement manufacture that have occurred since the concrete was cast. Since detection of copper slag in either unused cement or in the hardened concrete would have been difficult or impossible without conducting a microscopic examination, its presence may properly be called a “trade secret” of the time. The current studies could not determine whether the spherical air voids that are characteristic of entrained air represent a purposeful addition of a surfactant, or the presence of a serendipitous “air-entraining” agent or “workability” aid. The portland cement contained in the concrete was coarsely ground by modern standards. The lack of complete carbonation of the cementitious matrix that was also detected during the studies is likely due to the presence the thin carbonated layer beneath the surface in combination with continued hydration of the coarsely ground portland cement contained in the concrete.