An approach for a rapid determination of the aging time of lime putty
Graphical abstract
Total mineralogical composition of lime putties (wt%) obtained from QL1 (♦), QL2 (▲) and QL3 (■) quicklimes as a function of the slaking time in excess water.
Introduction
Lime putty has been used in mortars for masonry construction for millennia reaching the maximum level of quality during the Roman Empire. It is prepared by slaking quicklime under an excess of water. In recent years, there has been an increasing use of hydrated lime, Ca(OH)2, as a binder for restoration and conservation of historic and architectural structures [1]. The main reason is due to the fact that lime mortars are more compatible with materials of ancient buildings [2], [3]. Lime putty aged for long time under an excess water manifests improved quality of portlandite in lime mortar or plasters [4]. High plasticity, easy workability, reduced shrinkage and increased speed of portlandite carbonation favour, in fact, better quality of corresponding lime-based mortar or plaster [5], [6], [7]. The long exposure of lime putty to water determines on the whole a reduction of portlandite crystals size. Simultaneously plate-like crystals are produced from starting prismatic crystals due to the higher dissolution of prism faces [7], [8]. A continuous change in the microstructure was detected for extended periods of aging time which favours broader particles size distribution of portlandite crystals [9] of the more aged lime putty. Such features account for the quality improvement in terms of plasticity and workability of lime putty based mortars [10], [11], [12], [13].
How old should lime putty be? As established by an ancient Roman law, an aging time longer than 36 months is necessary [14]. From the commercial point of view, actually there is not a method to determine the effective aging time of lime putty. On the other hand numerous and uncontrollable features affect the characteristics of lime putty with the aging time [5], [15], [16].
This communication concerns a rapid method for verifying an effective long aging of lime putty using the thermo-gravimetric analysis as main test.
Section snippets
Experimental
Three different quicklimes, QL1, QL2 and QL3 with starting composition reported in Table 1, were slaked under an excess of water for times ranging from 3 to 66 months so obtaining numerous lime putties. To avoid contact with the supernatant water, each stored sample was taken from inside the stock of the corresponding aged lime putty and was dehydrated by lyophilization and subsequently characterized.
The water retention content was determined on sample directly dehydrated under vacuum at −46 °C
Results and discussion
Fig. 1 shows the XRD patterns of the 3-month and 36-month-old aged lime putties, respectively obtained slaking quicklime QL1 reported in Table 1. Both samples contain portlandite, Ca(OH)2, as main crystalline phase and some calcite, CaCO3, as secondary phase. Some brucite, Mg(OH)2, was also identified in the XRD pattern of lime putty (here not reported) of the slaked QL3 precursor which is richer in MgO content compared to QL1 and QL2 quicklimes, respectively (Table 1). The (0001) diffraction
Conclusion
The long slaking time under excess water of quicklime favors the formation of lime putty of better quality to prepare lime mortars of high performance. The increasing aging time affects some features of portlandite such as the morphology from prisms to platelike crystals, the reduction of the average crystal size, an increase of the surface area, and finally the formation of intra-aggregated pores smaller in size responsible of a high interaction with water retention of lime putty. A strong
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