RILEM TC 277-LHS report: How hot are hot-lime-mixed mortars? A review

Abstract

It is believed that many historic mortars were made using hot-lime mixing techniques. They are back in use today, and their good qualities are often praised, including being more compatible and a better match with historic fabrics. This paper studies the methods of producing hot-lime mortars and putties. It discusses the variables that determine the properties of the resultant mortars such as slaking and calcination, and compares hot-lime mortars with their equivalent putties, and with factory-produced calcium lime and hydraulic lime mortars. The paper concludes that the most important variable that governs the properties of hot-lime mixed mortars is the quantity of water used for slaking, because it determines the temperature reached during slaking which makes the resultant Ca(OH)2 vary from a fairly large size to extremely small, hence producing mortars with different properties. Based on scientific and historic evidence, it is concluded that the best method for hot-lime mixing is dry-slaking (sand-slaking) with long storage, because it combines a high slaking temperature (that reduces particle size and increases the surface area of the hydrate), with gradual slaking (that lowers volume expansion and crack development) and long storage (to ensure complete slaking hence no expansion cracks). Many historic mortars were probably hot-lime mixed. However, it is practically impossible to recreate them today due to the different limestones, kilns, calcination regimes and slaking/storage methods used in the past. Hydraulic and magnesian quicklimes were used historically for hot-mixing. In contrast, most of the factory quicklimes used today are purer limes with higher free lime content and a greater reactivity. Therefore, a hot-lime mix made with a factory-produced quicklime may not be more authentic or compatible than a natural hydraulic lime –NHL– mortar designed to suit a specific fabric and application. To ensure quality mortars that can be consistently repeated, a hot-lime mixing specification should contain both the process and the materials including: type of slaking (dry/wet); amount of water used; mixing details and the time at which it takes place; storage time and at what stage does it occur. To control the slaking temperature, the right amount of water should be established (according to free lime content) by trial which will also inform on the amount of yield and hence allow proportioning. With careful site work and specification, high-quality, compatible mortars can be made with both NHLs and hot-lime mixing. However hot-lime mixing requires more time and logistics, closer care and a more complicated specification.

The Use of Lime over the Centuries: The Complexity of the Apulian Built Heritage

“…When the slaking involves quicklime also containing MgO (periclase) in turn produced by the calcination of magnesian limestones, the binder includes Mg(OH) 2 (brucite) and excess insoluble MgO together with Ca(OH) 2 (portlandite). Magnesian limes slake slower and require longer periods, have a less energetic slaking reaction, and expand less due to their higher magnesium content, compared to fat limes which double their volume during the process [63].…”

“…The dry mix is sieved to remove lime particles, and then mixed with water to create a mortar. The mortar is covered with waterproof sheeting and left to mature before use [63].…”

“…It would not have been possible to make such a rich combination using lime putty since the mortar would have been slinky. Several experimental works showed the variability of B/A ratios between 1:5 and 1:13 in hot-lime mortars [63,66].…”

“…A temperature increase of approximately 60 • C, or even higher, together with the high pH of the mix promote the pozzolanic reaction between lime and hydraulic materials, when present. Steam generated during the slaking process entrains air and improves pore interconnectivity, which facilitate moisture transfer and accommodate freeze/thaw cycles, enhancing durability [63,65,66,73]. Dolomitic lime in mortars typically increases mixing water demand, influenced by lime surface area and micropore volume.…”

“…Compared to cold mixtures, the hot-lime mix stiffens more quickly due to heat. As a result, construction can proceed more quickly and walls can be raised higher without mortar leaking out due to the weight above [63].…”