Rheology of the lime putties
The results of the measurement of the plastic viscosity and the yield stress of lime putties SL-A, SL-B, SL-C, and SL-B1 during their maturation at the age of 3, 30, 60, and 90 days are shown in Figs 1 and 2. The graphs show an increase in the plastic viscosity and the yield stress of the lime putties with a prolonged period of maturation. The increase in the values of the plastic viscosity and the yield stress in lime putties SL-A, SL-B, and SL-B1 is not so apparent at the ages of 3, 30, and 60 days compared to lime putty SL-C. The values of the plastic viscosity and the yield stress of lime putty SL-B increase significantly at the age of 90 days. The increase in the values of the plastic viscosity and the yield stress is slow at the age of 90 days for lime putties SL-A and SL-B1. The highest plastic viscosity and yield stress were found in the sample from lump lime C. Lime putty SL-A, prepared from lime containing grains under 90 µm, showed the lowest plastic viscosity and yield stress. Lime putty SL-B, prepared from lime containing grains under 200 µm, reached only slightly higher values of plastic viscosity and yield stress than lime putty SL-A. However, the differences in the values of yield stress and plastic viscosity were greater at the age of 90 days for lime putties SL-A and SL-B. The disruption of the particles by mixing affected the plastic viscosity and yield stress of lime putty SL-B1, which reached higher values of plastic viscosity and yield stress than lime putty SL-B, which was not exposed to mixing after hydration, and the grains of the lime are not in any way disrupted. The results of the rheological measurements are consistent with previously published results16,18,19.
Image of the microstructure of the lime putties
Figures 3–6 illustrate the microstructure of lime putties SL-A, SL-B, SL-C, and SL-B1 during their maturation at the age of 3, 30, 60, and 90 days. Figures 3I and 4I show the prismatic hexagonal crystals of calcium hydroxide created in samples SL-A and SL-B. Lime putties SL-C and SL-B1 do not contain the significant crystals of calcium hydroxide which are present in samples SL-A and SL-B. The crystals in these cases are coated with a layer of hydrogel (Ca(OH)2.aq), which is already discernible after three days of hydration (maturation) Figs 5I and 6I. During the maturation of all the lime putty samples, the regular hexagonal crystals of calcium hydroxide are transformed into smaller irregular crystals which are coated with a layer of hydrogel. The gradual transformation of the calcium hydroxide crystals is well evident in Figs 3II–IV, 4II–IV, 5II–IV, and 6II–IV. The transformation of the calcium hydroxide crystals and hydrogel formation of lime putties SL-A and SL-B is slower than in the samples SL-C and SL-B1. The micrographs in Figs 5III and 6III show the calcium hydroxide crystals coated with a layer of hydrogel which originates faster and more intensively in samples SL-C and SL-B1. Conversely, the micrographs in Figs 3III and 4III present the microstructure of lime putties SL-A and SL-B with clearly visible crystals of calcium hydroxide. The layer of hydrogel increases during the maturation of lime putties. The amount of hydrogel is greatest at the age of 90 days in sample SL-C and in sample SL-B1 Figs 5IV and 6IV. The formation of hydrogel on the surface of the crystals of calcium hydroxide proceeds best in sample SL-C, which was prepared from lump lime where the regular hexagonal crystals of calcium hydroxide were not fully developed. The process of particle disruption immediately after hydration enables the acceleration of the process of hydrogel formation on the surface of the crystals of calcium hydroxide, and, moreover, the process of transformation of the regular crystals of calcium hydroxide to the smaller irregular ones is faster. The microstructure images of lime putties confirm the results provided by the performed rheological measurements.
The HR-ESEM images of lime putty SL-A without preparation of the sample and sputtering at the age of 3 and 30 days are shown in Fig. 7. Owing to the sophisticated and, in this work, experimentally proven method for the preparation of samples for HR-SEM, the images from the HR ESEM provide very similar information: significant hexagonal crystals of calcium hydroxide see Fig. 7I. The regular hexagonal crystals of calcium hydroxide are gradually transformed into smaller irregular crystals coated with a layer of hydrogel during the maturation of lime putty Fig. 7II. The HR-ESEM is a preferable method for observing micro-morphological changes during the maturation of lime putties because it is not necessary to use complicated sample preparation methods. Incorrectly sputter coated samples can lead to the formation of artefacts which can distort the micro-morphological interpretation. The HR-SEM is a suitable method for the evaluation of morphological changes during the maturation of lime putties; however, this method places heightened demands on the sample preparation, and, therefore, it can introduce inaccuracies into the characterization of the microstructure. The HR-ESEM is a more accurate, faster, and easier method for the evaluation of the morphology lime putties, as the samples are not treated at all before their observation20,21. Thus, there are no artefacts distorting the microstructural interpretation.
The same specific differences in the sample morphology may also be caused due to the samples drying and coating of a metal layer for SEM observation. Conversely, observation in ESEM does not require the above processes and the samples are observed naturally wet and without any modifications. The surface morphology of the crystals observed using the ESEM are somewhat smooth and free of sharp edges and micro-cracks see Fig. 7I and II vs. Fig. 3I and II.
Microstructure of carbonated lime putties
Figure 8 shows the development of calcite crystals in carbonated lime putties C-SL-A, C-SL-B, C-SL-C, and C-CL-B1 after 14 days of storage in a box with a high concentration (20%) of CO2 and humidity (70%). The length of the side of the calcite crystals was measured. The regular crystals which grew separately were selected for measurement. The images in Fig. 8 and Table 1 show the different development of calcite crystals in the carbonated lime putties. The size of calcite crystals is affected by the preparation method of the lime putties and the granulometry of lime. The smallest calcite crystals were developed in lime putty C-SL-A, which was prepared from lime with a grain size of under 90 μm Fig. 8I; whereas the larger crystals originated in lime putty C-SL-B Fig. 8II. The largest calcite crystals were found in lime putty C-SL-C Fig. 8III. The method of preparation, i. e. the mixing of lime putty SL-B1, affected the size of the calcite crystals C-SL-B1 Fig. 8IV. The mixing process leads on to the easier dissolution of the grains of quicklime, subsequently to the formation of larger crystals of calcium hydroxide.
Differential thermal analysis of the carbonated lime putties
The results of differential thermal analysis of carbonated lime putties C-SL-A, C-SL-B, C-SL-C, and C-CL-B1 after 14 days of storage in a CO2 environment are demonstrated in Fig. 9. The curves TG and DTG show that the carbonation process for the individual lime putties is different. The carbonation of lime putty samples C-SL-A and C-SL-B proceeded similarly. The smaller grains in lime A lead to a lower content of calcium hydroxide and a higher content of calcium carbonate (25.2% Ca(OH)2 and 65.7% CaCO3) compared to lime B (20.0% Ca(OH)2 and 68.7% CaCO3). Sample C-SL-C, prepared from lump lime, carbonates very quickly (2.8% Ca(OH)2 and 85.8% CaCO3). Sample C-SL-B1 contained less calcium hydroxide and, actually, more calcium carbonate (8.9% Ca(OH)2 and 80.0% CaCO3) than lime putty C-SL-B due to the process of activation.
Strengths of lime mortars
The effect of lime putty preparation on the carbonation process was also assessed by determining the strengths of lime mortars. The results of the tensile and compressive strengths measurements of lime mortars M-A, M-B, M-C, and M-B1 at the age of 28 days are illustrated in Table 2. The strengths of the lime mortars vary depending on the granulometry of the original quicklime from which the lime putties and the mortars were prepared. Mortar M-B, prepared from lime putty SL-B, reached higher strengths than mortar M-A, which was prepared from lime putty SL-A. Mortar M-C, which was prepared from the putty made from lump lime, reached the highest strengths.
The effect of the method of preparation of lime putty is also noticeable because mortar M-B1 achieved higher strengths than mortar M-B. Both of these lime mortars were prepared from lime with the same granulometry, but lime putty SL-B1 was mixed, unlike lime putty SL-B.