The possibility of thermal activation of bentonitic clays: impact of temperature

This article deals with the possibility of thermal activation of bentonitic clays. Specifically, the impact of temperature on the overall performance of two varying thermally activated bentonitic clays was investigated and compared with conventional kaolinitic clay. Firstly, the raw materials were characterized; their phase composition, granulometry and thermal analysis were measured. Then studied clays were fired at different temperatures from 500°C to 650°C. Afterwards, the overall performance of thermally activated clays was assessed through the view of their characterization as well as their applicability as Portland cement replacement. The main outcomes were as follows: Bentonites were more susceptible to agglomeration compared to kaolinite. They showed higher mass changes during the heat treatment, but lower increases in specific surface areas. On the other hand, for proper thermal activation, they needed a lower temperature. 600°C seemed to be optimal for the bentonitic clays, while for kaolinite the better results were observed in the case of 650°C. Moreover, thermally activated bentonites reached comparable mechanical strengths of blended cement pastes as the one with kaolinitic clay.


Introduction
Concrete is indisputably one of the most widely used and most common building materials in the world.About 30 billion tonnes were produced in 2021 (1), and the rate of its consumption is still gradually increasing.The basic components of concrete are cement, aggregate and admixtures and additives.Cement is an essential component, as it plays a binder role in concrete.Unfortunately, the production of cement is extremely energy-demanding, and it is also an extensive producer of emissions.On that account, there are currently many attempts to lower the consumption of cement by the application of suitable supplementary cementitious materials (SCM).Among the most common substitutes for cement in concrete are blast furnace slag, silica fume, or thermally activated clays (2).Commonly used clays contain a high amount of kaolinite, which when heated is converted into metakaolin.Metakaolin is very advantageous mainly because of its high pozzolanic activity, which proposes higher compressive strength, and increased durability of concrete (3).However, kaolinite, kaolinitic clays and rocks respectively are still quite rare raw materials frequently used also in other production industries.On the other hand, there are other types of clays, which could be thermally activated (4).This study is focused on bentonitic clays and the possibility of their thermal activation.Two clays with a higher amount of montmorillonite were chosen and thermally activated at different temperatures.The impact of temperature was assessed by virtue of varying granulometry, specific surface area and thermal analysis.Afterwards, the pozzolanic activity of studied materials was assessed by means of the compressive strength of blended cement pastes.

Experimental methods
Particle size distribution of studied clays was measured by the employment of a laser particle size analyser.Specifically, Bettersizer S3 Plus was used.This instrument has a measurement range from 0.01 µm to 3500 µm.The specific surface area was determined with the help of air permeability, by automatic Blaine apparatus UTEST UTCM-0280.The measurement is based on comparative methods and it was done in accordance with the standard (5).Thermal analysis was performed by the equipment Labsys Evo STA.It allows simultaneous thermogravimetric analysis from ambient temperature up to 1600°C.In this study the experimental setting was as follows: the temperature range of 10°C /mint, the maximal temperature of 1000°C and the argon atmosphere.For the determination of compressive strength, the loading press ED60 with the Compression Frame Jig Assembly (ELE International) was used.Because cement blended pastes were examined in this paper, smaller samples were examined in order to prevent cracks due to shrinkage.The dimensions were 100 x 20 x 20 mm.Except for the dimensions of the sample, the measurement procedure was adopted from the standard (6).

Studied clays
In this paper, three varying clays are examined.Two bentonitic clays and as a reference kaolinitic clays were chosen.Kaolinitic clay K is from the quarry Rokle and it originates from the company Keramost a.s.It composes of 86% of kaolinite subsequent by a minor amount of illite and quartz.The first bentonitic clay B is commercially labelled as Bentonite 75, and it is also quarried by Keramost a.s., According to the chemical composition it is calcium-magnesium bentonite.It contains about 41% montmorillonite accompanied by 10% of Illite and 4% of Kaolinite.Regarding the impurities, quartz in the amount of 6% and 3% of calcite was observed.The last bentonitic clay G is commercially available as GEM and it is quarried by the company LB Minerals, s.r.o., specifically, it is from the area of the Cheb basin.This bentonitic clay is classified as magnesium-potassium bentonite.From the point of view of mineralogical composition, it is composed of 36% of montmorillonite, 12% of Illite and 15% of kaolinite.Contrary to the previous clay B, it shows only a minor amount of impurities, specifically 3% of quartz was detected by XRD analysis.The basic characterisation of studied clays is summarized in Table 1.It is visible that bentonitic clays show about 4% lower density than kaolinitic clay.When focused on the granulometry respectively the specific surface areas, this time used clays vary considerably.The finest particles were observed in the case of bentonitic clay B, while the second bentonite G showed a somehow coarser distribution.

Thermal treatment of studied raw-materials
For thermal activation, it was firstly important to set suitable temperatures for thermal treatment.These were chosen on the basis of the thermal analysis.The results of raw clays are delineated in Figure 1a.The first peak up to about 300°C is commonly attributed to the dehydration of water in the interlayers (7,8).Comparing the performance of bentonite clays with kaolinite, they show a significantly higher peak in this temperature range.On that account, it can be derived that bentonitic clays contained a remarkably higher amount of physically bounded water.However, for thermal activation, it is the second peak (in the temperature range from 500°C to 700°C) which is of the greatest importance.In these temperatures, dehydroxylation of clay minerals takes place.This time it was kaolinitic clays which shows higher peaks, which means that they contained a higher amount of hydroxyls.Regarding the remaining peaks, at about 800°C calcite in the bentonitic clay B decarbonized.In the case of kaolinite K mullite arose above 950°C.Based on the observed temperature range of dehydroxylation, four temperatures were chosen for thermal activation.Namely, the performance of clays after exposure to 500°C, 550°C, 600°C and 650°C was examined.The treatments were performed in the electric oven.The temperature rate was 10°C/min and after reaching the target temperature, clays were exposed to it for three hours.Afterwards, they were left to spontaneous cooling.

Impact of particular temperatures on studied clays
Due to the thermal treatment, the granulometry of the studied clays was significantly changed.The values of median diameter (summarized in Table 2) showed that during the treatment grains were agglomerated.It was the most significant in the case of bentonitic clay G, where the values of the median diameter of particles went up by almost 18 times.The lowest susceptibility to agglomerate was observed in the case of kaolinitic clay K.However, also this material showed more than 2 times higher value after exposure to temperature.Despite the negative agglomeration of grains, specific surface area went up due to the thermal treatment.This was caused by dehydration and dehydroxylation of the thermally treated clays.However, some linkage between the agglomeration and specific surface area can be derived as well.Kaolinitic clay K, which showed the lowest increase of median diameter, showed the highest increase of specific surface area (by 67%), while both bentonitic clays showed much lower growth.Interestingly, both bentonitic clays reached the highest values after the temperature exposure to 600°C.In Figure 1 there are also summarized results of the thermal analysis.The first peak up to 300°C was substantially lowered, but it did not disappear.After the exposure to 650°C, there were still observed about 20% of the initial heat flow.This can be attributed to hygroscopic properties o studied clays.When focused on the peak of dehydroxylation, it is visible that at the temperature of 500°C the thermal activation was not sufficient.Studied clays exposed to 500°C showed somewhat lower but still a significant peak at the temperature range 500-650°C.On that account, this temperature was excluded from further investigation.The other clays exposed to 550°C, 600°C and 650°C showed quite similar performance of heat flow release, thus they were used in blended cement pastes for assess they applicability as SCM.

Composition of studied pastes
Thermally activated clays were used as a cement replacement in cement-based pastes in order to prove their pozzolanic activity.As a reference cement, CEM I 42.5R from the Českomoravský cement, a.splant Mokrá was used.Except for the reference pure Portland cement paste (PC), 12 blended mixtures were designed and produced.They always contained 10% of thermally activated clay and their labelling is derived from the used clay and particular thermal treatment from 550°C to 650°C.The mixture sets also include materials, where clays without any temperature exposure were used.This was done to complement the study of the possible impact of temperature.Regarding the water dosage, the approach of equal value was adopted; specifically, w/c was 0.33 in all cases of studied pastes.
From the fresh mixtures, 3 prismatic specimens were produced.There were cured in the climatic chamber, with a controlled temperature of 20°C and relative humidity of 80%.After 28 days of curing, compressive strengths were examined.

Properties of cement pastes
Reached results of compressive strength at 28 days are summarized in Figure 2.There is also a delineated strength activity index, which was counted as a ratio of the compressive strengths of particular blended cement paste to Portland cement paste.In this study, the target performance of suitable admixture was set to have a strength activity index higher than 100%.It is visible that the thermal activation of bentonite is not only possible but the particular temperature is of great importance.In both cases of bentonitic clays the treatment by 600°C proposed the best performance, while higher temperatures led to the deterioration of pozzolanic properties.Contrary, kaolinitic clays reached the best value after exposure to the highest temperature of 650°C.Such performance can be attributed also to the negative agglomeration, which was much more significant in the case of bentonitic clays.When comparing the performance of compressive strengths of blended cement pastes, kaolinitic clay showed the highest value.However, this outcome was assumed, as it is the kaolinite which has the highest reactivity.Nevertheless, the bentonitic clay B and G showed only by 5% and 10% respectively lower values than kaolinite.Taking into account the equal temperature, the bentonitic clays performed comparably as thermally activated kaolinite after exposure 600°C.bentonitic clay B showed even by 4% higher value, while G reached only by 2% lower value of compressive strength.In general, bentonite clay B despite the higher amount of not clay minerals seemed to have better potential to be used as SCM.Considering the it can be deduced, that the more important for proper thermal activation of bentonitic granulometry precisely the finesses of used clay) than the amount of impurities.

Conclusion
This paper was aimed at the thermal activation of bentonitic clays.Two representatives were selected, and their performance was compared with kaolinitic clay.Studied clays differed in mineralogical composition as well as in granulometry.After their characterization, clays were exposed to four different temperatures treatment, namely temperatures of 500°C, 550°C, 600°C and 650°C was chosen.It was proved that due to the temperature exposure, clays tended to agglomerate.Bentonitic clays showed higher growth of particle diameter.On the other hand, the specific surface area was significantly increased as well.Afterwards, thermally activated clays were used to produce blended cement pastes.
As was assumed, the kaolinitic clay reached the highest values of compressive strength.But it was necessary to use higher temperature, specifically exposure to 650°C.The bentonitic clays showed the best performance after exposure only to 600°C.Moreover, when comparing the values of compressive strengths after 600°C, bentonitic clays reached even comparable values as the kaolinitic clay.Regarding the particular bentonitic clays, bentonite with a higher amount of impurities but with finer particle size distribution reached better value of compressive strength.

Figure 1
Figure 1 Results of thermal analysis: a) raw clays, b) kaolinitic clay K, c) bentonitic clay B, d) bentonitic clay G

Table 1
Characterization of studied clays

Table 2
Impact of temperature on the particle size distribution

Table 3
Impact of temperature on the specific surface are