Ever wonder why portland cement gets so hard? Why it is so high in compressive strength? This blog post is a continuation from an earlier one from last week titled “Hot Rocks” which most people seemed to enjoy. Early lime kilns could operate only up to around 1600F — just hot enough to activate the clay, turn the limestone (calcium carbonate) to quicklime (calcium oxide) and combine them both to form belite (dicalcium oxide) to make hydraulic lime.
Joseph Aspdin (1824) had achieved a slightly higher temperature of around 2200F forming some liquid phase and combining the belite with the remaining quicklime to create alite (tricalcium silicate), the base compound for portland cement. The vertical shaft kilns could burn slightly hotter, but temperatures were mainly kept below the maximum heat achievable to limit the liquid phase and prevent the danger of a kiln blockage, a situation created when the molten mass of materials cooled within the kiln. Kiln blockage was something kiln operators historically and understandably feared; crawling down into a shaft kiln and chopping away with a hammer and chisel at a molten mass of material was to be avoided at all costs.
The rotary kiln, however, not only helped to overcome the kiln blockage challenge, but it also allowed the higher temperatures necessary to produce portland cement with a much higher compressive strength. Before the rotary kiln was invented (1889) most kilns were constructed vertically, loaded with alternating layers of limestone and fuel (wood and/or coal or both) until the materials reached the top. A fire was started at the base of the kiln and allowed to burn for a day or two depending on how large the kiln was. The entire kiln was cooled down another few days until the materials were discharged from the bottom. The process would start over again for the next batch, a very labor intensive process. The rotary kiln also allowed for continuous feeding of the kiln without the usual starting and stopping and could run 365 days per year without interruption in production.
So just how hard is portland cement? Well, in a paper presented at the American Lime Conference in Lynchburg, Virginia in March 2003, Paul Livesey of Castle Cement presented the following information about the answer to this question. When matching a “portland cement” mortar from the period between 1871 and 1920 then, we should not be confused by the terminology. Portland cement of 1871 was a different material from that of 1920 which, in turn, is totally different from portland cement today.
Portland cement products produced at the turn of the last century were fired at temperatures from 2300F to 2600F. In comparison manufacturers making portland cement in this century fire at temperatures ranging from 2800F to 3000F. The differences in mineralogy and the effect that burning at higher temperatures has on strength development are well known. For example, in 1871, portland cement tested in the 1800 psi range; today’s portland cement are in the range of 8,000 psi, an increase of 344 percent in compressive strength. Indeed, the portland cement of 1871 bears more relation to modern, higher-strength hydraulic lime (NHL 5, 1000 to 1500 psi) than it does to modern portland cement.
And when making decisions on the appropriate mortar match for historic buildings remember that the National Park Service as well as leading American experts agree that it is always better to err on the side of a lower strength mortar replacement in order to protect the historic masonry materials. Even adding a small amount of modern portland cement can have a significant affect of increasing strength when maybe you did not intend to in the first place.