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Returning user. Request Username Can't sign in? Forgot your username? The early age stage of grinding provided the best conditions to optimize the onset time for effective mechanochemical reactions [ 4 , 9 , 10 ]. Various types of mills were used for the MCA, however, the application of a planetary ball mill proved to ensure sufficiently short processing times, safe handling and good reproducibility [ 1 , 8 ].
Planetary ball mills produced high acceleration forces, acting on the processed material through a combination of a rotational motion around the jar axis and a planetary motion around the main axis of the mill [ 11 ]. Several undesirable effects can occur during the MCA process, including for example agglomeration, caking effect or rolling movement of the balls [ 10 , 12 , 13 ]. An increased kinematic energy can lead to a more efficient process but at the same time, it can also result in caking, abrasion or contamination.
Increased impact forces tend to produce finer powders but also increase the temperature of the processed material, which causes adhesion of the material to the inner surface of the jar. With the ongoing grinding process, the subsequent layers will start to build up and promote the caking effect, which eventually can decrease the efficiency of the entire process [ 14 ]. Using water or alcohol as a grinding medium can reduce the agglomeration and increase the efficiency of the process.
At the same time, wet grinding can intensify shrinkage, cracking and deformations of the material exposed to the subsequent drying [ 4 , 10 ]. The dry grinding appeared to be more effective in imposing significant structural changes, even despite problems related to high temperature or agglomeration [ 4 , 15 ]. One example of a potentially wide application of the MCA process is the activation of clay minerals.
The clay minerals are sustainable, commonly occurring and widely used in various applications including for example the construction sector. The calcinated kaolin, known as metakaolin, is used as a supplementary cementitious material SCM in the production of concrete or as a precursor for geopolymers [ 16 , 17 , 18 ]. High costs of the final product and requirements of high processing temperatures during the calcination have brought the attention to alternative processes. The MCA appeared as an environmentally friendly alternative excluding the need of high temperature, or chemical additives [ 7 , 8 , 19 , 20 ].
Earlier studies confirmed that the MCA is able to affect the structural order and the reactivity of such raw clay minerals as kaolinite, montmorillonite, illite and pyrophyllite [ 21 , 22 , 23 ]. Interestingly, the calcination process does not sufficiently affect the structural order of illite and montmorillonite but the MCA does [ 24 ]. The present study focused on determination of effects of various MCA process parameters on amorphization degree and chemical activation of a Swedish raw clay. A special emphasis was put on shortening of the processing time, minimization of contamination due to wearing of the grinding equipment and limiting agglomeration of the processed material.
Additionally, preliminary tests were performed to verify the applicability of the processed clay as cementitious binders for geopolymer concrete. The raw material used in this study was collected near Stockholm in Sweden, Figure 1. Geotechnical properties of the collected material were determined using the Atterberg limits. The chemical composition of the used clay is shown in Table 1.
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The unprocessed reference sample S0 was also analyzed. The used milling stainless steel balls had a diameter of 3 mm. The used process parameters are summarized in Table 2. The step size was 0. The XRD samples were not pre-treated. Back-loading sample holders was used to avoid preferred orientation of the particles.
The main identified phases include kaolinite Kln , muscovite Ms , illite Ilt , montmorillonite Mnt , quartz Qz , and calcite Cal. Lorentz fitting is used to assess the FWHM indexes [ 25 , 26 ]. High vacuum mode and a secondary electron detector SED were used. Images were taken from at least three. No conductive coating was applied. The particle size distribution and agglomeration were determined using a low vacuum mode and the backscattered electron detector BEC.
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Nine images per sample were obtained at a magnification of times. Subsequently, images were combined together with the use of the ImageJ software [ 27 ]. The images were preprocessed with median filter with 2 pixel kernel to remove the noise. Afterwards, the threshold was adjusted manually to binarise the images, i. The analysis of the binarised images included determination of the basic morphological parameters, i. Both image processing and calculation were performed with the use of ImageJ software [ 27 , 28 , 29 , 30 ].
The results were compared by fitting the Gaussian probability distributions to the histograms of selected morphological parameters. Mortar mixes were used to determine the mechanical properties of alkali-activated processed clay samples. The sand to clay ratio was in all tested mixes. The maximum particles size of the used sand was 0.
The modulus of the sodium silicate solution was adjusted by the addition of sodium hydroxide pellets. The used amount of alkali activator was 10 wt. Mix composition of the alkali-activated raw clay is shown in Table 3.
Mechanochemistry in nanoscience and minerals engineering
KG, Senden, Gremany , with rotations per minute. The mixing procedure included 1 min of mixing of all dry components followed by 2 min of wet mixing. Samples were removed from molds after 24 hours and were kept in sealed conditions until the compressive strength tests, which were done after 7-days. The compressive strength values were determined in a mechanical testing machine, type Instron model The tests were done on samples made using three types of clay; the untreated clay, S0 , the dry ground clay sample DG-3R , and the most amorphized sample DGR. The XRD analysis were done for dry and wet ground materials, Figure 2.
The dry grinding process indicated a decrease in the crystallinity of the processed samples, Figure 2 a. Intensities of the peaks related to kaolinite Kln , muscovite Ms or calcite Cal almost disappeared. Lorentz fitting of FWHM of the kaolinite peak  has shown a changing value from 0.
Decreased intensities and increased FWHM values suggested a structural disorder and initial amorphization of these phases. FWHM indexes of kaolinite peaks  higher than 0. The intensity of peaks related to quartz Qz did not show significant changes due to MCA process. It can be related to its greater hardness in comparison with kaolinite 7 versus 2—3 in the Mohs scale which would require application of a significantly longer and more intensive grinding process to induce a visible structural disorder [ 31 ]. Less powder enabled a more rapid comminution process and, consequently, less time was needed to initiate the MCA.
The effect was more pronounced for samples containing less powder, e. Using alcohol or performing brief peptisation helped to avoid these undesirable effects. The morphology of the untreated sample consisted of a mixture of platy clay particles and quartz particles. On the contrary, the morphology of the activated sample DGR was uniform and dominated by spherulitic and quasi-regular particles. Several studies have reported that MCA of kaolin has generated structural changes, as e.
The grinding time was 15 min. The XRD peaks did not reveal as significant changes as observed in the dry grinding process, Figure 5. FWHM values did not exceed 0. This behaviour indicated that a higher amount of water could also lead to an ultimate amorphization but it would require significantly longer grinding times. The presence of water in the system appeared to dilute the suspension and to lower the viscosity.
As a result, the generated impact forces were significantly lower. Earlier results showed also that progressing morphological changes tend to affect the effectiveness of the grinding. The dry grinding process imposed a rapid decrease of the kaolinite Kln peaks intensities, while peaks related to quartz Qz were barely affected, Figure 2 a. The wet grinding, on the other hand, appeared to produce strong changes of XRD peaks related to quartz peaks, Figure 2 b. It complies with earlier studies showing more efficient grinding of quartz in the presence of water [ 31 , 36 , 37 ].
Present results showed traces of chromium Cr in samples ground in the presence of water. This contamination can be related to wearing and corrosion of grinding balls and of the inner surface of the jar. In the dry process, the powder surrounding balls and filling the jar tended to act as a protection layer preventing direct impacts.
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On the contrary, in the wet grinding process, water supported the movement of the powder and prevented the occurrence of the so-called caking effect. It also favored direct impacts between balls and the jar thus increasing the wearing rate. In general, additives, including also water, enable to change the viscosity of the ground material and thus its movement. Consequently, the resistance to mechanical forces will be decreased [ 4 , 38 , 39 ]. Furthermore, according to A.
Westwood [ 40 ], water can also influence the movement of dislocations, their accumulation nearby the surface, and thus it can modify the hardness of the material [ 4 , 10 ]. The effects of the grinding duration were investigated for both dry and wet processes. In general, based on the intensities of the XRD spectra and on the FWHM values of the kaolinite , it can indicate that the amorphization degree was enhanced with longer grinding duration Figure 6.
FWHM indexes for kaolinite  were higher than 0,40 for both wet and dry ground samples after 20 min. Additionally, a contamination of the processed material originated from the mechanical wearing of the jar and grinding balls appeared to be present during wet grinding. The wet processed powders had a darker color in comparison with the reference sample S0 and samples activated with the dry process.
Observations on particle size distribution showed changes in their trend suggesting a correlation with the grinding duration. Prolonged grinding of raw clay produced smaller particles, Figure 8 a. In the clay particles size range 0.
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Grinding modified the agglomerates size distribution, as shown in the Figure 8 b. Earlier studies also confirmed that particle size distribution changed mostly in the first hour of grinding and depends on the grinding equipment parameters [ 35 ]. Effects of the grinding speed were evaluated for both dry and wet processes, including , and rpm, Figure 9 and Figure In all the cases, a higher rotation speed increased the temperature of the grinded material. In addition, higher speeds applied in the wet grinding produced finer particles and created pastes having a higher viscosity. As a result, grinding balls were fully immobilized and the activation process was completely hindered after a short time.
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The removal of the grinded material was extremely difficult. The amorphization evaluated by the decreased peaks and FWHM indexes from the XRD diagrams Figure 10 , showed a generally enhanced amorphization with the increased grinding speed for both processes. However, the maximum used speed of rpm, which induced extensive amorphization, also caused a strong caking effect.
The amorphization of kaolinite, muscovite and calcite was significantly increased at higher speeds in the case of both wet and dry processes. SEM analysis detected morphological changes, Figure Particle size distribution was investigated also for different grinding speeds. Fewer agglomerates and a particle size distribution with increased amount of finer particles were observed with increasing grinding speeds Figure 11 a. Ground clay with higher rotation speed produced smaller agglomerates Figure 11 c.
The dis-agglomeration seemed to be affected by the rotation speed parameter of the grinding equipment. Regarding the smaller particles present in the clay, there was no evident change in their size Figure 11 b. Mortar samples produced with untreated clay did not show any mechanical strength indicating a lack of chemical activity. These results clearly indicate induced chemical reactivity during the MCA process, which increased with higher degree of amorphization as determined by the XRD analysis.
Future research will be continued on optimization of the MCA process and on the alkali activation. The effects of grinding media and grinding duration on the amorphization and morphological changes of the raw Swedish clay were studied. The following main conclusions were formulated:. Dry grinding process promoted more extensive amorphization when using a higher number of grinding balls versus the amount of the processed clay powder. Dry grinding was more effective in amorphization of kaolinite than wet grinding process. Effectiveness of the wet grinding increased with a higher water to processed powder ratio but required longer processing times in comparison with the dry grinding process.
Longer grinding enabled a more extensive amorphization in both wet and dry processes, produced finer particles with spherulitic morphology, but also tended to increase agglomeration when outside of the Rittinger zone. Higher rotational speed enhanced amorphization efficiency of clay minerals in both wet and dry processes. The trend was similar in the case of quartz but only when the wet grinding was used.
This process also caused less agglomeration, caking effect and wearing. The processed raw clay, using the defined parameters, should enable its usage as pozzolanic material and as cementitious binder for production of concrete. Preliminary strength tests validated the concept that the MCA can enhance the reactivity of the raw clay to a degree enabling its utilization as a binder in alkali-activated systems for e.
Mechanochemistry in Nanoscience and Minerals Engineering
The authors would like to thank Jan Laue and his team from the Department of Soil Mechanics for providing the Swedish raw clay samples and the soil characterization tests. Conceptualization, A. R; Formal Analysis, I. National Center for Biotechnology Information , U. Journal List Materials Basel v.
cszplayers.com/163.php Materials Basel. Published online Sep Author information Article notes Copyright and License information Disclaimer. Received Aug 14; Accepted Sep