Nanostructure and Irreversible Colloid Behavior of Lime (Calcium Hydroxide) as Observed in Slaked Lime and Hydrated Lime

Work Completed

This work was carried out at the University of Granada and published as “Nanostructure and Irreversible Colloidal Behavior of Ca(OH)2: Implications in Cultural Heritage Conservation,” Langmuir, 2005, 21, 10948-10957, by C. Rodriguez-Navarro, E. Ruiz-Agudo, M. Ortega Huertas, and E. Hansen. It was shown that the irreversible aggregation associated with drying of either slaked lime putty or the industrial process resulting in the formation of dry hydrated lime is detrimental to the colloidal behavior of Ca(OH)2. It strongly affects the rheological properties of hydrated lime putty and the resulting mortar/plaster performance. In particular, it affects the plasticity of lime putties.

Authors Abstract for AATA (Eric Hansen)

"Hydrated lime has been a building material since 8,000 BC and is still one of the most important components in the formulation of many mortars and plasters. Today, high-calcium lime mortars, whose binder is derived primarily from calcium hydroxide (Ca(OH)2), are formulated with putties made with the traditional method of slaking calcium oxide (quicklime) or by the addition of water to commercial dry hydrated lime. Lime putties have been known since the 1920’s to behave as an “irreversible” colloid; i.e., once dried, the reactivity and rheology of the lime putty dispersions are drastically altered. This irreversible colloidal behavior is explained for the first time to be the result of an oriented aggregation mechanism triggered by drying. Oven drying affects the kinetic stability and particle size distribution of slaked lime putty, increasing the settling speed and particle (cluster) size."

"The same process occurs in the production of the dry hydrated lime as a result of the removal of water through drying in the production process. Using electron microscopy analyses, it was demonstrated that porous, randomly oriented, micron-sized clusters are dominant in the dispersions before and after drying. However, oriented aggregation of the nanocrystals of Ca(OH)2, those around 60 nm (which are more prevalent in slaked lime putty), is also observed. This oriented aggregation occurs to a limited extent before drying but it is extensive during drying. The random aggregation is to some extent reversible upon reintroduction of the aggregated crystals to water, while the oriented aggregation is not. It was shown, using FESEM, TEM and HRTEM, that nanocrystals self assemble in a crystallographically oriented manner either along the (100) or equivalent (110) directions, or along the Ca(OH)2 basal planes. The oriented nanoparticles form strong coherent bonds that do not disaggregate upon redispersion in water. Overall, the irreversible aggregation associated with drying of either slaked lime putty or As part of the industrial process that results in the formation of dry hydrated lime strongly affects the rheological properties of lime putty and the resulting properties of the mortars made with them. The plasticity of slaked lime putty is particularly affected, as plasticity is strongly dependent upon the abundance of colloidal (submicrometer) tabular-shaped Ca(OH)2 particles. These observations on irreversible colloidal behavior associated with the drying of Ca(OH)2 dispersions have important implications in heritage conservation and, additionally, suggest that slaked lime is one of the oldest materials known to mankind."

Further comments

It is important to note that the observations on crystal coarsening required characterization of the lime putty both before and after drying, and the hydrated lime before and after constituting a putty by mixing in water. Special sample preparation is required for tests sensitive to drying, including scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen adsorption (by BET method); these methods required freeze-drying as opposed to oven drying if the crystal shape and size of calcium hydroxide in lime putty are to be characterized. In this respect, previous results in the literature that have not used freeze-drying might not provide an accurate representation of the microtexture of portlandite crystals in a wet putty.

Page updated: October 2007