Zprávy památkové péče 2019, 79(3):347-352 | DOI: 10.56112/zpp.2019.3.13

The Influence of frost and anaerobic aging on particle size of lime slurry prepared by different procedures

Dagmar Michoinová¹, Radovan Nečas

¹ NPÚ GnŘ

The article presents selected results of research on the properties of lime slurries prepared using different methods but from the same raw material, in order to determine whether a traditionally prepared slurry can be replaced by a slurry created using an alternative process. The research also included a partial experiment that studied the effect of frost on the properties of lime slurry.
The investigated lime slurries were prepared by 1) traditional slaking of lump lime (Traditional slurry) and by 2) dry blending a slaked lime hydrate with water (Hydrated slurry). The raw materials used to prepare the slurries came from the same producer (Vápenka Čertovy schody a.s.). This reduced the impact of raw material quality and the firing regime on the evaluated parameters of the slurry in the experiment. The slurry was evaluated after 6 months, 1 year, and 2 years, while the effect of frost on the slurries was studied after the slurry had anaerobically aged for 30 months. The evaluation was performed by comparing the data measured under the same conditions. This reduced the possibility of error due to measurement conditions.
The results of change in the granulometry of the slurry particles during the aging process are described. For both slurries prepared in a different way, the anaerobic aging, i.e. long-term contact of the lime with water in the absence of air, is a process that leads to a refinement of the binder particles; this can be interpreted as an improvement in the slurry properties and is important for construction purposes, particularly for the modern renovation of historical architecture (plasticity, processability, binding, and the like).
Traditional slurry prepared by wet slaking showed slightly better properties in the particle refinement process when aged, compared to the Hydrated slurry. This occurred more intensively for the Traditional slurry, especially after the first year of aging and also spontaneously after the frozen slurry was thawed and the thawed Traditional slurry was intensively mixed. These results were obtained by extending the experiment to include an assessment of the properties of the slurries exposed to frost. After freezing and thawing, the slurries lose their characteristic properties (binding, consistency, plasticity). The size distribution of the lime particles shifts towards the coarser particles after freezing and thawing the slurries, which can be explained by the formation of agglomerates of portlandite (Ca(OH)2) crystals in the slurries. Already after a month after the slurries were thawed, there was a tendency of both slurries towards a spontaneous refinement of the slurry structure after freezing. This was more pronounced in the Traditional slurry than in the Hydrate slurry, as already mentioned. Intensive mixing (mechanical reactivation) of the slurries exposed to frost resulted in both slurries gradually returning to the particle granulometry they had before freezing. The likely cause of this is the breakage of particles (agglomerates of portlandite crystals) by mechanical action during stirring. The Traditional slurry also showed more intense texture refinement than the Hydrate slurry in this experiment.
These conclusions apply to the investigated type of lime. In order to generalize the measured results for other types of lime, it would be necessary to carry out the same experiment for them.
Although granulometry is a precisely evaluable material property, as yet there are no ranges of values to determine which particle size or size distribution clearly indicates the suitability of a slurry for construction purposes. However, it can be generalized that the user properties of the slurries generally improve with the increased fineness of the particles.
Since lime particle size refinement occurs intensely with lime slurry aging, the use of aerated lime in the form of an aged lime slurry can be preferred over the direct use of lime hydrate for the preparation and the subsequent direct use of mortars for heritage preservation.
The research has contributed to the understanding that the properties of building materials prepared by traditional methods exhibit unsurpassed properties so far proven by empirical excellence. The disappearance of traditional building skills, as well as traditional crafts or arts, represents an irreplaceable cultural loss. With the loss of crafts, we lose more than we presently realize.

Keywords: building technology, properties of lime slurry, lime, traditional preparation of building materials

Published: September 1, 2019  Show citation