Posts Tagged portland cement

Hot Rocks and Compressive Strength

Looking down into a rotary kiln - the limestone can be seen on the right

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.

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Historic Portland Cement

Roadside Marker in Lehigh Valley, PA

It was David O. Saylor back in 1871 that produced the first portland cement in the United States in Lehigh Valley, Pennsylvania.  Between 1871 and 1920, American portland cement production skyrocketed, due in part to the increasing demand for automobiles and the attendant need for roadways during this period.  The years 1871 to 1920 also saw a major change from “traditional” manufacture of cement to a more technically aware, science-based industry.

As a consequence, it was inevitable that cement products that reflected both traditional and modern scientific production methods were on the market at the same time. Thus, anyone looking to match a historic portland cement mortar from the time period between 1871 and 1920 will benefit from considering the evolution of cement production technology during this period.

As popularity of portland cement grew over the period, so did its compressive strength, from 1800 psi in 1871 to 3000 psi by 1920, an astounding 110 percent increase. An examination of the limestone and clay used to produce portland cement, however, shows that they changed very little after 1871. What, then, had changed? Mainly it was the production process and the ability to fire the raw materials to consistently higher temperature.

The main technological breakthrough came with the invention of the rotary kiln, which was first used commercially in Lehigh Valley, Pennsylvania in 1889. Initial trials by Ransome and Stoke in England had lacked the necessary financial backing to succeed.  In the end, it was left to Seaman and Hurry in the United States to make the final technical refinements that could produce sufficient temperatures and efficiency to unleash the massive portland cement industry we know today.

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Hot Rocks!

Rotary Kiln Firing Limestone in the Production of Cement

In writing a recent guide specification I was asked to describe the difference in all the basic historic mortar binders. I thought for a moment and came up with the following description and short explanation (without the chemistry).

Background – Historic mortars can represent four (4) different binder types, or combination of them, depending on the time-period of construction. For example, a building constructed in 1810 might be built with a straight lime putty binder type because the discovery of natural cement binder types had not occurred yet until the early 1820s. A building constructed in 1940 might be built with portland cement (1871) and hydrated lime (1930s).

The historic binder types include: non-hydraulic lime (fat lime, lime putty or hydrated lime); hydraulic lime (feebly, NHL 2, moderately NHL 3.5, imminently and NHL 5.0); natural cement; and portland cement. The binder types are all derived from limestone. Each successive type is fired at higher temperatures in a kiln to the point of vitrification or liquid phase (2200-2800F) when portland cement is developed. Lime can be slaked into a hydrate powder or putty form by adding water due to the lower firing temperatures (1650-2000F), while cement products must be crushed mechanically into a powder form before use.

Each binder type has its own unique performance properties in relation to historic masonry units and the building wall design. For example, a mortar formula made from lime putty (low compressive strength) will accommodate building movement in load-bearing masonry much more effectively than a portland cement formula of much higher compressive strength.

Performance characteristics of the replacement mortar should be identified carefully based upon evaluation of the existing historic mortar. Each binder type or mixture of mortar shall have a cement, lime, or combination thereof consistent with the original existing mortar content in order to provide uniform durability, weathering characteristics, and the same, or better, life-cycle performance expectations.

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