Implications of The Patent
Aspdin called the product Portland cement because set mortar made from it resembled “the best Portland stone”. Portland stone was the most prestigious building stone in use in England at the time. The patent clearly does not describe the product recognized as Portland cement today. The product was aimed at the market for stuccos and architectural pre-cast moldings, for which a fast-setting, low-strength cement was required (see cement). It was fired at low temperature (below 1250°C) and therefore contained no alite.
The product belongs to the category of “artificial cements” that were developed to compete with Parker’s Roman cement, and was similar to that developed much earlier by James Frost. The process described is a “double burning” process in which the limestone is burned on its own first, then slaked, mixed with clay, and burned again. This was a common practice for manufacturers of both Artificial and Portland cements when only hard limestones were available. The grinding technology of the time consisted only of flat millstones, and it was more economic to comminute the limestone by burning and slaking than by grinding.
The limestone he used was the Pennine Carboniferous limestone of the area, which was used for paving in the towns and on the turnpike roads. The characteristic practise of the patent (and of his lime patent) is the use of “road sweepings” as a raw material. He says that if the sweepings are not available he obtains ‘the limestone itself”. It is significant that Aspdin was twice prosecuted for digging up whole paving blocks from the local roads. Limestone supply was clearly a major headache for Aspdin in the days before stone could be brought in by rail. This provides context for the friction that developed with his son William. William’s innovation was to make a mix with a higher limestone content, to burn it at a higher temperature using more fuel, and to grind the hitherto-discarded hard clinkered material, hence increasing wear-and-tear in the grinding process. William subsequently moved south to north-east Kent, where inexhaustible supplies of soft chalk were available, and launched the “modern” Portland cement industry.
Read more about this topic: Joseph Aspdin
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