From the mid-eighteenth century extensive research in Sweden and in present day Finland focused on the development of mortars with hydraulic properties, the results mainly published by the Royal Swedish Academy of Science. The aim was to replace imported Italian pozzolan and Dutch trass with a pozzolan produced in Sweden. Several products based on burned alum shale were developed. Cambrian alum shales with a high content of bitumen were fired without additional fuel to produce pozzolanic shale ash. The bituminous alum shale was used directly as a fuel in the lime burning process from the late eighteenth century. The alum shale mortars have a red-brown colour due to the high content of iron oxides. The mortars are hard, strong and generally have a good durability, both as masonry mortars and renders. Microscopic analysis shows that often only a skeleton remains of the shale particles and that large parts of the particles are consumed by the pozzolanic reaction. The field of application was initially restricted to structures in contact with water, such as locks in canals and harbours, but conventional building construction was also an early application.