This paper presents the study of four gypsum mixtures, focusing on the role of both inorganic and organic additives and on the micro-structural features and mechanical properties. Additives have been chosen among those most reported in historical recipes, for example magnesia, lime putty, rabbit glue. The selected mixes refer to gypsum-based materials used in artworks manufacture, such as plasters, mouldings, stuccoworks,
Gypsum is a building material that has been used over the centuries since the Egyptian civilization and still presents a major role in current building technologies (
The technology of gypseous materials allowed several working steps since very early stages, to modern industrial processes; in fact, gypsum is formed in nature in sedimentary evaporative depositional environment. To use gypsum as a binder, thermal dehydration is required. This process starts at a temperature of 42°C, but decomposition is slow. So a range of temperature between 100°–140°C is normally used in industrial processes in a wet environment. The product of the dehydration is bassanite (CaSO4 ½H2O), which rehydrates quickly when water is added. If gypsum is heated up to 200°C a soluble phase of anhydrite (γ- CaSO4) is obtained, which rehydrates in presence of water too. Heating bassanite, over 360°C leads to the formation of the insoluble anhydrite phase β-CaSO4, while α-CaSO4 is produced at 1180°C. Above 1375°C, the complete dissociation of anhydrous calcium sulphate into calcium oxide, sulphur dioxide and oxygen occurs. According to the literature there are still doubts about the transition conditions between one phase and the next (
Here we link the composition of gypsum based mixes with their final performances, discussing how this type of knowledge could be useful in conservation practice, for example when the conservator has to integrate a missing part with a new paste (
Materials used for the mixes and the laboratory model samples are summarized below:
Bassanite powder (CaSO4·0.5H2O) purchased as
lime putty aged for 48 months
MgO powder in analytical grade supplied by
Rabbit glue, supplied in pearls. The compound was soaked in demineralised water overnight, then the excess water was rinsed off and the glue was prepared by gentle heating, until the formation of a viscous solution.
Four types of mixes were prepared:
pure gypsum as a reference;
gypsum plus rabbit glue (
gypsum plus MgO (
gypsum plus lime putty (
The amount of water was considered crucial to achieve the final performances, so its weight in each mixture was measured.
MgO was selected because it is reported by crafts handbooks as an additive that improves the mechanical performances of gypsum based pastes (
The study of the final properties of gypsum based specimen has been particularly neglected in recent literature, which in fact reports only on the different type of aggregate (
The raw materials were characterized by X-ray diffraction (XRD), infrared (FTIR) and Raman spectroscopy to verify the presence of undesirable components. Bassanite is the main component of “Gesso Alabastrino”, with traces of anhydrite and dolomite, probably coming from the raw materials. The purity of MgO was confirmed, while the lime putty is designated as CL 90-Q according to EN 459-1 (
Mixtures were prepared at controlled environmental parameters (65% RH and 20 °C) and the same mixing conditions were ensured for each specimen, using the same mixer and same velocity. Steady curing conditions were maintained for 28 days (50% RH and 20°C) (
Description of the tested mixtures.
Compositional proportions for every mixtures | ||
---|---|---|
Mixture | Components | Ratio (wt/wt) |
A | bassanite + demineralized water | (100:55) |
B | bassanite + lime putty + demineralized water | (50:50:30) |
C | bassanite + rabbit glue + demineralized water | (100:15,5:37,5) |
D | bassanite + MgO + demineralized water | (100:10:55) |
A series of specimens were prepared, sized 160×40×40mm as required by current standards (
Flexural strength measurements were performed after 28 days, averaging the final value on a basis of three replicates. Compressive strength tests were performed after 28 days, averaging on the 6 replicates produced by the fracture of specimens used in flexural test.
The raw materials and the mixtures were characterized by XRD, FTIR, and Raman Spectroscopy. The mineralogical composition after the specimen setting was analysed too, in order to understand the effects induced by the added compounds. For each specimen three replicates were carried out. X ray diffraction was carried out on powdered samples by PANalytical X’Pert PRO X-ray diffractometer, with geometry goniometer θ-θ. The diffractograms were recorded between 3° and 75° 2θ with a scanning speed of 0.21 θ/sec, using a Cu Kα radiation, a PW generator 3040/60 in the conditions of 40kV and 40mA, and a solid-state multi-detector X’Celerator PW3015/20, with Ni filter. The powdered samples were deposited on a ground glass support. Results were interpreted by the use of the X’Pert HighScore software.
A Micro-Raman spectrometer Horiba Jobin-Yvon LabRam was used, coupled to a confocal microscope Olympus BH-4 equipped with motorized XY stage. The instrument works with two laser lines: 632.8 nm (He-Ne, 20 mW) and 473.1 nm (Nd: YAG laser, 100mW). The Raman signal is dispersed by a holographic grating with 1800 lines/mm on a Peltier cooled CCD detector (256 × 1024 pixels). The Rayleigh radiation is blocked by notch filters BraggGrateTM (OptiGrate), which allow collecting signals from 10 cm−1 from laser line. The laser power reaching the sample can be limited through the use of neutral density filters. The entrance slit was set at 150 microns. Spectral resolution is ~1.5 cm−1 using the 632.8 nm. Spectra were acquired in the range from 100 to 4000 cm−1 using the line at 473.1 nm with the objective 50x ULWD. Laser power of the order of tens of mW was used, averaging 3-4 analysis with acquisition times of 20–160 seconds each, using the filters D1 and D0.6.
Fourier Transform Infrared spectra (FTIR) were recorded with a Thermo Scientific Nicolet iS10 instrument in the 4000–600 cm−1 range, equipped with an ATR accessory with diamond crystal, with a resolution of 4 cm−1, collecting 32 scans. Previously to each sample a background analysis was recorded, and automatically used to correct the measurement.
In depth morphological and microstructural observations were carried out using a stereomicroscope Leitz Wild M420 at different magnifications in order to observe the surface morphology in detail and a Scanning electron microscope JEOL 5910 LV, source tungsten filament, coupled with X-ray spectrometer (EDS) in dispersion of IXRF-2000 energy. Analyses were conducted in low vacuum. The compositional nature and distribution of elements on sample was investigated by the EDS spectra and maps from 0 to 20 keV. The measurements were carried out on calibrated images to study the dimensions of gypsum crystals in the samples by image analysis. The software ImageJ was used for this purpose.
A NETZSCH STA 409 PC/PG was used for TG-DSC analysis, with N2 fluxes of 20 and 40 mL/min respectively with a temperature ratio of 20/10.0(K/min)/900.
The flexural and compressive strength were measured according to European Standard (UNI EN 13279-2 2004) (
For each mixture, the open porosity and the pore size distribution were measured. The pressure applied was between 0.1 and 200 MPa; the resolution (pressure) is: 0.01MPa until 100 Mpa and 0.1Mpa from 100 to 200 MPa; the accuracy is >0.2%; the resolution in volume is 0.1 mm3 and the range of measure (radius) is 7.5 – 3.7 × 10−3 µm. The sample dimension was about 0.4×0.4×0.8mm (
The characterization of the raw materials and of the mixtures was carried out combining mineralogical (X ray diffraction) and spectroscopic analyses (Raman Spectroscopy and FTIR). Each analysis was performed on three replicates. The mixtures B, C and D were analysed after the curing time (1month) too in order to verify the effect of the addition of MgO, rabbit glue and lime putty on hardening: gypsum, anhydrite, and brucite were identified in mixture D, while gypsum, calcite, anhydrite and portlandite were found in the mixture B. The presence of brucite Mg(OH)2 is due to the hydration of MgO during the preparation of the mixture D. A high content of bassanite was put in evidence, in mixture C, as well as the presence of gypsum and anhydrite. According to Elert et al. (
The presence of calcite and portlandite in mixture B is linked to the process of lime putty hardening. The determination of brucite (mix D) and portlandite (mix B) suggests the incomplete carbonation of lime putty in the samples. This phenomenon could also be present in building materials, for the very low kinetics of carbonation reaction depending on relative humidity, size and geometry of the artefact. In fact, in the case of bulky architectural elements, it is hard to ensure the effective CO2 diffusion from the atmosphere (
Thermogravimetry analyses (
The table shows the weight loss during the heat flow.
TG-DSC Results | |||||
---|---|---|---|---|---|
Sample | Weight loss per temperature range (mg) | ||||
< 120 | 120–200 | 200–400 | 400–600 | > 650 | |
Mixture A | - | 9.37 (56.41%) | - | - | 1.09 (6.62%) |
Mixture B | - | 5.94 (40.63%) | - | 1.05 (7.16%) | 2.31 (15.83%) |
Mixture C |
- | 6.01 (43.79%) | - | - | 1.08 (7.85%) |
Mixture D | - | 8.02 (50.43%) | 0.65 (4.08%) | - | 1.09 (6.91%) |
Compound identified | - | Gypsum | Brucite | Portlandite | Calcite |
In the mixture C an endothermic peak at 364°C was recorded, probably due to the organic additive
SEM observations (
Images of the four gypsum-based mixtures observed by scanning electron microscope (SEM). Bar = 10µm. a) gypsum, b) gypsum and lime putty, c) rabbit glue and d) gypsum and MgO.
EDS maps of the surface of the mixture B: it is possible to observe an area with a greater abundance of calcium and the relative absence of sulphur, suggesting the presence of a lime lump.
EDS maps of the surface of the mixture C, showing the accumulation of Mg in some small areas (max. width of about 50 microns), probably as impurity.
EDS maps of the surface of mixture D: some small magnesian lump are evident, probably due a non-uniform mixing.
Thank to SEM images the crystal size distribution was evaluated. In mixtures A, B and D (
Length/width ratio of gypsum crystals for each mixture type.
A | B | C | D | |
---|---|---|---|---|
Length/Width | 6.8 | 5.7 | 1.6 | 4.1 |
Average values of the length and width of gypsum crystals.
The initial setting time and final setting time were measured by Vicat needle using standard UNI EN 13279-2 (
Initial setting time and final setting time of the mixtures A, B and C.
Vicat needle results: | ||
---|---|---|
Initial setting time | Final setting time | |
Mixture A | 9.57 min | 14.27 min |
Mixture B | 17.06 min | 24.06 min |
Mixture D | 13.09 min | 24.32 min |
Vicat graph shows the initial setting time and the final setting time values for the mixture A, B and D.
Lime putty and MgO acted as a retardant in gypsum initial setting time. It was also verified that the mixture containing rabbit glue did not provide useful values in Vicat measures because it had not hardened in the measuring range (15h). Anyway, the measure confirmed the role of rabbit glue as strong retardant. This topic will deserve in depth study in the future, with an on purpose procedure overlooking the standard used. It is important to remark that currently no reference values are available for these mixtures, except for pure gypsum used as binder in plastering. In order to comply with the standard EN 13279-1,2, the initial setting time of more than 20 minutes was established (
The mixture A showed (
The results of workability tests, carried out according to the procedure described in UNI EN 13279-2 2004 (
Flow tables results.
Flow table results | |||
---|---|---|---|
Flow table | Measured values [mm] | Medium diameter (Ø) [mm] | |
Gypsum | 230×225 | 227.5 | |
Gypsum and lime putty | 178×180 | 179 | |
Gypsum and rabbit glue | 160×180 | 170 | |
Gypsum and MgO | 200×200 | 200 |
These mixtures could be used in material integration, re-pointing and other restoration practices. Hence, the workability test proved to provide fruitful results, although they should be performed specifically on a case-by-case basis, in order to better contextualize the needs of each circumstance.
The porosity values and the pore size distribution are reported in
Average MIP results.
Mercury intrusion porosimetry results | |||||
---|---|---|---|---|---|
Mixture | Total cumulative volume (mm3/g) | Bulk density (g/cm3) | Average pore radius (μm) | Total specific surface area (m2/g) | Total (open) porosity (%) |
369.7 | 1.2 | 0.8 | 1.5 | 43.7 | |
452.6 | 1.0 | 1.7 | 1.8 | 46.9 | |
245.6 | 1.3 | 0.7 | 1.6 | 32.7 | |
297.0 | 1.2 | 0.8 | 2.8 | 36.7 |
The plot shows the average pore radius vs the total open porosity, the plot highlight the differences between mix B and the rest of the mixes.
The plot shows the average pore radius compared to the total cumulative volume, the plot highlight the differences between mix B and the rest of the mixes.
Pore size distribution in relation to the cumulative volume and the relative pore volume of the mixture A, B, C and D. Mixes A and B present similar pore size distribution compared to the other mixtures.
Mechanical tests have been carried out on the hardened mortars after 28 days of curing time, as suggested by standards (
Flexural strength results (MPa).
Flexural strength results | |||||
---|---|---|---|---|---|
Specimens | Flexural Strength | Average | Std.dv | ||
Mixture A | 6.62 | 5.64 | 6.06 | 6.11 | 0.49 |
Mixture B | 2.92 | 2.84 | 2.50 | 2.75 | 0.22 |
Mixture C | 5.03 | 5.27 | 5.10 | 5.14 | 0.13 |
Mixture D | 5.19 | 8.24 | 3.68 | 5.70 | 2.32 |
Compressive strength analysis results (MPa).
Compressive strength results | ||||||||
---|---|---|---|---|---|---|---|---|
Specimens | Compression Strength | Average | Std.dv | |||||
Mixture A | 18.46 | 17.84 | 19.56 | 19.93 | 19.99 | 19.62 | 19.23 | 0.88 |
Mixture B | 9.50 | 11.40 | 9.32 | 9.87 | 14.41 | 11.96 | 11.08 | 1.95 |
Mixture C | 16.55 | 16.55 | 14.41 | 16.55 | 15.88 | 16.62 | 16.09 | 0.87 |
Mixture D | 31.53 | 30.04 | 31.58 | 29.78 | 28.20 | 29.74 | 30.15 | 1.27 |
Comparison of the flexural strength measuremenst after 28 days for mixture A (blue bar), B (orange bar), C (grey bar) and D (yellow bar). The average is calculated on 3 samples.
Comparison of the compressive strength measurements after 28gg for mixture A, B, C and D; the average is calculated on 6 samples.
The measures were carried out after 3, 7, 14, 28 days (
|∆| Linear shrinkage [mm] for the four mixtures analysed.
Linear shrinkage [mm] | ||||
---|---|---|---|---|
Days | Mixture A | Mixture B | Mixture C | Mixture D |
3 | −0.15 | 0.12 | −3.17 | −0.33 |
7 | 0.07 | 0.30 | −3.44 | 0.04 |
14 | 0.11 | 0.46 | −2.45 | 0.23 |
28 | 0.18 | 0.51 | −2.41 | 0.25 |
The contact angle is given by the encounter of the tangent of liquid-vapour interface with a solid liquid interface. By convention, a surface material having a contact angle with water greater than 90° is defined as hydrophobic, while a surface with angles minor than 90° is hydrophilic. The measurement were performed according to UNI EN 15802:2010 (
The results have clarified the characteristics of the gypsum mixtures prepared in this work. The compositional analyses (after 28 days) allowed identifying the chemical and mineralogical composition of the mixtures. The mixture made of gypsum and MgO showed the presence of brucite, due to hydration of MgO. It is not possible to hypothesize that this contributes to change the final properties of gypsum, as we have deduced from the analysis of this mixture. XRD analysis has been repeated after 12 months, without detecting any significant change, suggesting that the formation of magnesium carbonate or hydromagnesite displays very low kinetics. Atzeni et al. shows that in 18 months aged lime mortars containing magnesium some magnesium hydroxide and other minerals, for example MgCO3, MgCO3 3H2O and Mg4(OH)2(CO3))H2O, are present. They also underlined that the formation of hydromagnesite is more favoured compared to magnesium carbonate (
As for the microstructure, the mixture gypsum/lime putty showed the highest average values of total porosity and of pore radius. These results could be related to the low mechanical properties measured, even if further analyses are necessary. The presence of portlandite in this mixture is due to the incomplete carbonation of the raw materials, which probably led to an overall weakness, as resulted from mechanical tests.
Regarding the pore size distribution, mixture B presents pore size distribution % and porosity values similar to mixture A (gypsum), even if the average radius is different. However, the results confirmed the retardant role of lime in setting and hardening mechanisms.
Moreover, the average pore radius in mixtures C and D is closer to the one measured for gypsum in mixture A, although mixture C seems to have a bimodal pore radius distribution (
SEM allowed an in depth microstructural survey, which put in evidence the changes induced by the additives, for example those regarding the dimensions and the shape of the gypsum crystals in the mixtures.
The setting and hardening measures clarified that all the additives here considered acted as retardants. An exception to this is the rabbit glue mix, which did not harden at the time of the measurements. Coming to mechanical tests, the mixture including MgO gave an important result: it showed a resistance to flexural strength similar to the gypsum mixture and compression strength higher than each of the other mixtures. As for this mixture, the compressive strength values are higher than the values measured by Sing and Garg in gypsum mortars retardants increasing it to pH 7 (
The mixture of MgO and gypsum showed excellent results in the shrinking test too, obtaining a dimensional variation similar to that of pure gypsum. This should be considered a very important result, if we bear in mind the possible application of this material in integration and repointing operations during the restoration practice, because of possible detachment problems. A variation of linear shrinkage slightly greater than that recorded for gypsum was presented by mixture B, while the worst results were obtained in mixture C.
Actually, mixture C is not suitable for injection operations because of its high shrinkage: if used on fragile ancient materials, it could in fact damage or cause cracks in the plaster during the setting and the hardening, especially where the integration is called at filling bulky volumes. Currently it is not possible to compare our results to (
A significant difference in water transport phenomena can be assumed between mixture C and the rest of the inorganic mixes here studied.
When the conservator is using gypsum mixes in integration, embedding or re-plastering operations, very often the ready made products are chosen among those offered by the market. Unfortunately, their composition is not known in most cases. For example, more than one synthetic polymeric addition could be present. Nevertheless, the habit of preparing on site “on purpose” mixes according to the traditional recipes was lost over the XX century. Hence, empiric knowledge of the properties of these mixes is not very often part of the skills of many conservators. Moreover, even if the general empirical properties are esteemed, normally they have not been measured. In this paper a first approach is attempted and proposed.
This work investigates the effect of three very common additives added in gypsum-based mixtures that acted as retardants. The results obtained from the experimental studies can be summarized as follows:
The addition of MgO, lime putty and rabbit glue acted as retardant; any of these additives gives time to the conservator of plastering and modelling in relief integration works, proving rabbit glue to be the most retardant compound;
The mixture made with MgO could provide promising results in restoration practice when a general strengthening is required; anyway, some in-depth studies are needed in order to highlight the possible transformation of Mg compounds in the mix, taking into account that some Mg sulphates may form very dangerous efflorescences.
The rabbit glue changes the microstructure and the pore network. Most probably, the water transport phenomena could be reduced and a small increase in water repellency could be expected as well, even if some condensation problems on the surface should be considered. In fact, mixture C showed a contact angle of 41° and the material should be considered hydrophilic, but the results interestingly proved the increase of water repellency caused by the addition of rabbit glue. However, more in depth studies are in progress to better understand this aspect.
A study of the properties of the mixtures after a longer period of curing time could give more information on the characteristic of these materials and their possible applications.
The authors would like to acknowledge Alessandra Botteon, Mariagiovanna Taccia, Riccardo Negrotti, Cristina Corti and Francisco Jesús Carmona Fernández for their precious contribute in acquiring the data and translation.
The raw/processed data required to reproduce these findings can be shared on request.