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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