Joseph then went into partnership with his elder son, James, posting a notice that Aspdin had left, and that the company would not be responsible for his debts, stating "I think it right to give notice that my late agent, William Aspdin, is not now in my employment, and that he is not authorised to receive any money, nor contract any debts on my behalf or on behalf of the new firm.

[4] William had discovered that a significantly different product, with much wider applications, could be made by modifying his father's cement formulation.

By increasing the limestone content in the mixture, and burning it much hotter, a slow-setting, high-strength product suitable for use in concrete could be obtained.

[2] Although this product, referred to today as "modern" Portland cement, was entirely different mineralogically from that of his father, Aspdin did not take out a patent, or give it a new name.

Famously, he would emerge from his office when each newly loaded kiln was ready for firing, and scatter in handfuls of brightly coloured crystals over the raw mix, to give the impression that the special properties of his product were the result of an unidentified "magic ingredient".

As "Maude, Jones & Aspdin", he acquired the Parker and Wyatt plant at Northfleet creek, Kent, and transferred his manufacturing operations there in 1846.

[3]: 203 The Northfleet plant continued making Portland cement on a small scale as Robins & Co. Ltd. until it was taken over by APCM (Blue Circle) in 1900.

One of the most preposterous was William's claim that his father's cement was used by Marc Isambard Brunel in the creation of the Thames Tunnel in London.