Effect of drying method on the specific surface area of hydrated lime: A statistical approach

Lime putty is a traditional binder, experiencing a new advent in the preservation of historical buildings. Recently it was shown that lime putty microstructure evolves with ageing time, generally resulting in a continuous quality improvement, but possibly also passing a minima/maxima. Hence, periodical quality checks during ageing are needed to optimize quality and avoid excessive storage. The specific surface area (SSA) of lime putty is a potentially valuable parameter for quality control as it influences the workability and setting of lime mortars. Gas adsorption and the Brunauer–Emmet–Teller (BET) theory is a popular method for its determination, requiring a dry powder. Generally, freeze-drying is used for powder preparation as this method is assumed to diminish particle aggregation. However, no systematic investigation of the effect of powder preparation method on BET SSA has previously been reported. In addition, reproducibility evaluations of such methods are also lacking. This work was aimed to fulfil these gaps, using both calcitic and dolomitic lime putties. Freeze-drying was compared to heat-induced drying (105 °C) under air as well as at low pressure. In addition, sample microstructure was evaluated using X-ray Powder Diffraction data and Rietveld refinements as well as Electron Microscopy techniques (SEM, TEM). It was statistically proven that freeze-drying, compared to the other dehydration methods, resulted in a 20–35% higher BET SSA for calcitic lime putties consisting mainly of nanoparticles. Instead, BET SSA of a dolomitic lime putty containing micrometre-sized hexagonal platelet crystals was not influenced by drying method. No statistically significant difference in phase composition was found between the samples dried by the different methods, excluding carbonation of the hydroxides as influencing factor. Finally, high reproducibility of BET specific surface area was obtained regardless of drying method which is an important characteristic of a standard test method for quality control.