Young’s modulus of illitic clay and CaCO3 mixtures during thermal treatment up to 1100 °C

The dynamic Young’s modulus of samples, made from a mixture of illitic clay and laboratory grade CaCO3 with 6% addition of 3% solution of polyvinyl alcohol in anorthite stoichiometry is studied. The samples with different CaCO3 contents, namely 17.6%, 21.6%, and 25.6% are made by dry pressing. The measurements of the dimensions, mass, and resonance frequency of the samples are performed for the determination of the dynamic Young’s modulus in the temperature interval from 25 °C to 1100 °C. The influence of the amount of CaCO3 on the Young’s modulus of the samples is investigated. During the firing of the studied samples, the values of Young’s modulus increase due to the liberation of physically bound water (up to 250 °C) and sintering (above 850 °C). The values of Young’s modulus after a firing reach 16.7 – 18.4 GPa, which is about 6 – 9 times higher as the values for the raw samples.


Introduction
Traditional or building ceramics have a dominant position in the market.Their production is based mainly on the usage of illitic clays and the maximum firing temperature rarely exceeds 1100 °C.However, the production does not focus on supporting the crystallization of anorthite (compared to kaolinitic clays).Anorthite (CaO•Al2O3•2SiO2) has a triclinic crystalline system.It is predominantly white (reddish or gray).The melting temperature is 1553 °C.High melting point of the anorthite causes a delayed formation of the glassy phase in the ceramics.It is known that the crystallization of anorthite is connected to the addition of a CaO source.Mainly, this is true in the production of kaolin-based porcelain, where anorthite porcelain can be produced at low temperatures, and in addition with low shrinkage during a firing.This small shrinkage allows higher heating rates to be applied without damaging of the ceramic body.Therefore, anorthite formation can speed up production and help to reduce its costs.
Several studies [1][2][3][4][5][6][7][8][9][10][11][12] conducted on calcareous illitic clays and their mixtures with CaO-containing waste materials suggest that increasing formation of anorthite leads to improved performance of ceramic materials (such as a mechanical strength, a porosity, and a shrinkage during a firing).The most commonly used waste materials, which content CaO are: ash, granite dust, marble dust, and waste limestone [1][2][3][4][5][6][7][8][9][10].The idea of adding CaO to ceramic materials is not new.CaO-containing ceramics have been produced for centuries, but CaO was not added intentionally, but as a natural component of illitic clays [1].The aim of this paper is to study the effect of the amount of calcium carbonate (CaCO3) on the Young's modulus of illitic samples in the temperature interval from 25 °C to 1100 °C.

Samples
Illitic clay (Füzérradvány, Hungary), lab-grade calcium carbonate (CaCO3), and polyvinyl alcohol (PVA) were used to produce the studied samples.The chemical composition of illitic clay, which was determined with XRF analysis is in table 1.At first, illitic clay was dried, then ground using a planetary ball mill (Retsch PM 100) and sieved below 100 µm using an analytical vibrating sieve (Retsch AM 200).Subsequently, 3% solution of PVA was prepared.Illitic clay and CaCO3 were mixed in three different proportions (17.6%, 21.6%, and 25.6%).The exact stoichiometric ratio of illitic clay and CaCO3 for the anorthite crystallization is 78.4% of illitic clay and 21.6% of CaCO3.The lower and the higher values of the CaCO3 content was chosen for a comparison.PVA was used to reduce the brittleness of the raw samples.The amount of 6% (ratio to the total weight of the mixture) of 3% solution of PVA was added to the prepared dry mixture.The solution was gradually added dropwise to the dry mixture in a grinding mortar.After every tenth drop, the mixture was mixed.Each mixture was dry pressed with a pressure of 21 MPa to obtain prism-shaped bars (7.5  8.5  100.5 mm 3 ).

Measurement methods
For the correct determination of Young's modulus of the samples during thermal treatment, dimension changes, mass changes, and resonance frequencies of the samples must be determined.The thermal expansion of samples was studied by thermodilatometry using a push rod horizontal dilatometer Netzsch DIL402 from 30 °C up to 1100 °C with a heating rate of 5 °C•min -1 and in a nitrogen atmosphere with a flow rate of 50 mL•min -1 .The initial length of the studied samples was about 25 mm.
The mass change was performed by apparatus Mettler Toledo TGA/SDTA851 e from 25 °C up to 1100 °C with a heating rate of 5 °C•min -1 and in an air atmosphere with a flow rate of 50 mL•min -1 .The initial mass of the studied samples was about 50 mg.
The measurements of the resonance frequency were performed by the impulse excitation technique (IET) [14] in the temperature interval from 25 °C to 1100 °C with a heating rate of 5 °C•min -1 and in static air atmosphere.The initial length of the studied samples was about 95 mm.Two samples from each mixture were measured in order to compare the reproducibility of the measurements.
When the dimensions, mass, and resonance frequency are determined, Young's modulus of elasticity E can be calculated as: where m is the mass of the sample, l is the length of the sample, d is the thickness of the sample, b is the width of the sample, f is the resonance frequency, and T is the correction coefficient, which must be used if the ratio of thickness and length of the sample d/l is less than 20.The value of T can be calculated from the relation given in [15].

Results and discussion
During the thermal treatment five endothermic reactions occurred in the samples, which correspond with the mass loss shown in figure 1.The first two mass losses in the temperature interval from a room temperature up to 170 °C correspond to the liberation of physically bound water from pores and surface of crystals.This release of water took place in the two stages.The mass loss reached about 2%.The next two mass losses in the temperature interval from 450 °C to 730 °C are due to the dehydroxylation of illite/smectite, where the chemically bound water was removed from the structure of illite/smectite.This process runs in two steps: the first step is the trans-vacant illite layers dehydroxylation (at 570 °C) and the second is the cis-vacant illite layers dehydroxylation (at 680 °C) [16].The dehydroxylation was also overlapping with the decomposition of CaCO3 in the temperature interval from 600 °C to 800 °C [13].
The final mass loss at 1100 °C reached values from 12.9% to 15.8%.The mass loss was higher with a higher content of CaCO3.In the samples also one exothermic reaction occurs which is not accompanied with mass loss and therefore it is not visible in figure 1.This reaction corresponds to the crystallization of anorthite and gehlenite [13].All reactions described above (figure 1) influence also thermal expansion of samples.The results of thermal expansion are in figure 2. The liberation of physically bound water (up to 200 °C) caused small contraction of all studied samples.Then, normal expansion occurred up to 450 °C.Above 450 °C, the deydroxylation of illite/smectite started what caused the increase in the expansion.This expansion is typical for illitic clays [3].In figure 1, there is visible that dehydroxylation had two steps.The studied samples contained also small amount of quartz, therefore in this temperature interval also → transformation of quartz occurred [16].The expansion of studied samples in this interval was caused by two overlapping processes.After dehydroxylation, the contraction of samples started, which was caused by decomposition of CaCO3, and also solid-state sintering.The thermal expansion of studied samples is almost the same up to 950 °C.Next differences are due to the crystallization of gehlenite and anorthite.The higher content of CaCO3 in the samples was the higher expansion occurred.The final thermal expansion was 0.69% for ISC17.6,1.11% for ISC21.6, and 1.53% for ISC25.6.
Finally, the results of Young's modulus of the studied samples are presented in figure 3. Young's modulus of raw samples was from 1.7 GPa to 2.4 GPa and there was no influence with the amount of CaCO3 in the samples.After the liberation of physically bound water (200 °C), Young's modulus increased and reached values in the range from 3.5 GPa to 4.5 GPa.Now, the values of Young's modulus increased with the amount of CaCO3.Then, Young's modulus slightly decreased up to 700 °C.After dehydroxylation and decomposition of CaCO3 (800 °C), the values of Young's modulus was almost the same, and then Young's modulus increased rapidly due to the sintering.The influence of the crystallization of anorthite and gehlenite is not visible.At the final firing temperature (1100 °C), the value of Young's modulus was 18.3 GPa for ISC17.6,19.2 GPa for ISC21.6, and 21 GPa for ISC25.6.Here, it was clearly visible the influence of CaCO3.The values of Young's modulus after the firing are presented in table 2. As it is visible from table 2, Young's modulus after the firing increased from 6 to 9 times.The increase in the Young's modulus of the sample with CaCO3 content of 17.6% was 779%, in the case of a sample with a CaCO3 content of 21.6%, it was 894%, and in the case of a sample with a CaCO3 content of 25.6% it was 666%.This means that the highest increase of Young's modulus was recorded for the sample with CaCO3 content of 21.6%, which is the exact stoichiometric ratio for the formation of anorthite.

Conclusions
The effect of the amount of calcium carbonate in illitic clay on the dynamic Young's modulus in the temperature interval from 25 °C to 1100 °C was studied and following findings were observed: • The mass loss increased with the amount of CaCO3 in the samples linearly.
• The thermal expansion was almost the same up to 900 °C, and then depended on the amount of CaCO3 in the samples.• Young's modulus of raw samples was in the interval of 1.7 GPa to 2.4 GPa and did not depend on the amount of CaCO3 in the samples.• After the firing of the studied samples, a significant increase in the Young's modulus was observed (from 6 to 9 times).
Based on the obtained results, it can be concluded that the highest increase of Young's modulus (894%) was recorded for the sample with a CaCO3 content of 21.6%, which is the exact stoichiometric ratio for the formation of anorthite.

Figure 1 .
Figure 1.The mass change of studied samples

Figure 2 .
Figure 2. The thermal expansion of studied samples

Table 2 .
Young's modulus of raw samples E1 and after the firing E2