CHANGES IN THE MECHANICAL STRENGTH OF CLAY PRODUCTS IN RELATION TO STRUCTURAL CHANGES IN CLAY MINERALS DURING FIRING

P. G. Usov, E. A. Guber

(Presented by the scientific seminar of the Department of Silicate Technology)

The mechanical strength of clay products during firing is associated mainly with the phenomenon of melting and is explained by the cementation of the structure by a solidified melt. In studying the behavior of loams during firing, it has been established that, in the formation of a strong structure, melting phenomena are not the principal factor, but represent the final stage of a rather complex process of strength formation. Strength is built up at an earlier stage, at lower temperatures. The ability of clays to form a strong structure in products during drying and firing at low temperatures is an individual property and has been termed by us the reactivity of clays. For a quantitative evaluation of this property, the accepted index is the mechanical strength of products after firing at a temperature of 800°, i.e., below the temperature at which the primary melt appears.

Studies of changes in the mechanical strength of products in connection with structural changes in clay minerals during firing were intended to provide certain information that would make it possible to determine the relative role of structure and reactivity in clays of different mineralogical composition.

It is known that raw materials of different mineralogical composition undergo structural changes to different degrees at different temperatures. It should be assumed that the pattern of increase in mechanical strength should also be different in clays of different mineralogical composition, probably depending to some extent on the structure of the clay-forming mineral. In this connection, the objects of the study were samples of different mineralogical composition, which were subjected to firing.

The samples selected for study are approximately polymineral and differ not only in the structure of the clay-forming mineral, but also in the quantitative ratio of the crystallized clay mineral and the colloidal component. The rocks investigated (fractions finer than 5 μm) are characterized by the data given in Table 1.

We have indicated [1] that reactivity is related to the amount and composition of colloids, since with their removal from the rock the ability of clays to build up the mechanical strength of products decreases very sharply.

The increase in strength at low firing temperatures (up to 800°C) occurs as a result of solid-phase reactions in the hardened colloidal films, which are qualitatively different in different minerals. Thus,

in kaolinite and in Chasov-Yar clay the colloidal films enveloping the crystalline clay mineral are close in composition to the clay mineral itself; in montmorillonite the composition of the colloids is considerably more complex. Since the colloidal films are qualitatively different,

Table 1

the reactions occurring in them during drying and firing determine differences in the activities of clays of different mineralogical composition.

What, then, is the role of the structure of the crystalline clay minerals themselves, the amount of which exceeds the content of the colloidal part?

Fig. 1. Increase in mechanical strength in clays of different mineralogical composition: 1—1—Elenino kaolin; 2—2—Prokop’evsk montmorillonite; 3—3—Chasov-Yar clay; 4—4—Omutninsk clay.

The literature contains an indication that, for three-layer minerals of the talc type, the change in properties during firing [2] can be explained by the shifts that occur in the structure of talc, in contrast to kaolinite, where shifts, in the opinion of A. I. Avgustinik, lead to the splitting of the packet into its elementary parts.

There is sufficient information in the literature on changes in structures during firing and on temperature phases [3, 4, 5]. Determining the relationship between structural changes and the strength characteristics of products was the subject of the investigation. Firing was carried out from 100 to 1200°C at intervals of 100° on specimens of each mineral variety (obtained from a plastic paste). The strength index was determined as the average reading from data for three specimens tested in the dry state and after saturation with water. For each mineral variety, x-ray diffraction patterns were taken and interpreted for unfired specimens and for specimens fired to 400, 600, 800, and 1000°C.

The change in mechanical strength with increasing firing temperature for each mineral variety is shown in Fig. 1. As can be seen

As can be seen from the figure, the mechanical strength at 800° of clays of different mineralogical composition is different. Thus, the activity is very low in a sample of kaolinite and sericite clay; it is considerably higher in Chasov-Yar clay and still higher in a montmorillonite sample, and in the last two specimens the mechanical strength is fairly high even at low temperatures. The process of strength increase over the entire temperature range is not the same for specimens of different mineralogical composition.

In kaolinite and sericite, the compressive strength \(\sigma\) does not change from 100 to 1100°; in montmorillonite and Chasov-Yar clay, the compressive strength \(\sigma\) begins to rise sharply at 300°. No sharp decrease in mechanical strength is observed in any segment of the strength-change curve. At the same time, firing up to 1200° is accompanied by profound changes in structure.

The phenomena occurring during heating of clay minerals at a definite stage of heating cannot be considered separately from the dehydration process; upon heating to relatively high temperatures, the changes may be associated with other processes.

Dehydration in kaolinite occurs in the temperature interval 500–550° and leads to the formation of metakaolin.

Studies of the structure of metakaolin [4] have established that metakaolin retains a well-ordered structure, despite the fact that it does not possess three-dimensional regularity; periodicity is retained in two directions in the plane of \(a\) and \(b\), and there is no periodicity in the \(c\) direction; i.e., when kaolin is heated to 550° the layered structure is retained (the structure of kaolin is inherited), but the reduction of the interplanar spacing to 6.3 Å (instead of 7.15 Å), caused by dehydration, disrupts the periodicity in the direction perpendicular to this layering. These processes do not influence the change in strength.

Metakaolin is an intermediate stage in the regular transition from kaolinite to the cubic phase—spinel. This transition occurs in a certain temperature interval (550–925°C) and is characterized by the following changes in structure.

The structure formed at 550° becomes considerably compacted upon further heating (the density increases continuously), which is explained [5] by the progressive entry of two oxygens of the elementary cell into the voids of the Si—O layer, forming a more compressed sequence. Such a system, in which layers of 6 and 8 oxygens are arranged alternately, is unstable and leads to the separation of \(\mathrm{SiO_2}\), forming a new sequence that is now interpreted as a phase of the spinel type—aluminosilicon spinel [5].

Schematically, all the changes may be represented as shown in Table 2.

Table 2

Despite changes in the sequence of layers, the mechanical strength of the product fired to 900° does not increase.

The spinel phase exists over a short temperature interval, from 925 to 1075°; i.e., it is thermally unstable, allowing migration from the SiO₂ structure.

A rapid, sharp increase in strength occurs at 1000°, at the time when the spinel phase exists. Mullite appears at 1050°; with further firing it increases crystallinity, while at the same time the liberation of SiO₂ continues, and at 1400° the crystal lattice acquires parameters corresponding to the composition of mullite—$3\mathrm{Al}_2\mathrm{O}_3 \cdot 2\mathrm{SiO}_2$ (sillimanite structure).

Together with these changes, there is also an increase in strength caused by profound structural changes, probably not without the participation of a melt.

The phase changes occurring during heating of clay minerals are determined mainly by the X-ray method. Analysis of X-ray patterns of kaolinite (Fig. 2) confirms the stability of the kaolinite structure up to 600°. In the X-ray pattern of the specimen fired at 600°, there are very few reflections and they are of low intensity. On firing at 800°, reflections characteristic of quartz appear, as well as some new ones with $d = 4.71$ and $d = 3.70$ of medium intensity, evidently belonging to Al—Si spinel. At 1000°, the quartz lines remain and the spinel lines become stronger. Thus, X-ray analysis establishes kaolinite, quartz, and spinel; the phase formed at about 600°, apparently because of lattice disorder, gives barely perceptible reflections.

Fig. 2. X-ray patterns of kaolinite specimens fired to different temperatures

Dehydration of montmorillonite begins in the temperature range 100–200°, which is accompanied by a decrease in the cell dimensions along the $c$ axis from 10.0 Å to 9.4 Å [2]. The loss of interlayer water does not affect the mechanical strength of the specimen. Loss of hydroxyl water begins at about 500° and ends at about 800°, reaching a maximum at about 700°. According to Thilo, Grim, and Bradley [2], in montmorillonite, upon removal of hydroxyl water, the layered type is retained up to temperatures of 800–900°. In this case only a slight change is observed in the relative arrangement of the layers; the identity period along the $c$ axis increases by 0.1–0.3 Å, causing a regrouping of the octahedral layer. The loss of hydroxyl water and the regroupings that take place in the structure do not have a noticeable effect on mechanical strength. In montmorillonite the compressive strength begins to increase at about 300°C and continues to rise up to 600°, after which the specimens swelled.

On firing to temperatures above 800°, various primary phases may form in different montmorillonites: either spinel or quartz. In the first case, the new phase is formed from octahedral-

layers of the clay structure, in the second—from tetrahedral ones. Quartz will develop in montmorillonite in this case if substitution is absent in the tetrahedral layer, and spinel—when there is significant substitution in the tetrahedral layers of silicon by aluminum and at a low iron content. Studies by Brindley and Grim [2] established that the high-temperature phases depend on the structural features of the montmorillonite itself. The initial phases should be hereditary with respect to the original structure; the later phases, developing at higher temperatures, depend on the bulk composition, and “heredity” plays an insignificant role.

Fig. 3. X-ray diffraction patterns of montmorillonite samples fired to different temperatures

In the X-ray diffraction patterns of fired samples of montmorillonite (Fig. 3), at 400° montmorillonite and quartz are detected (the latter is also present in the original specimen); subsequently the intensity of the montmorillonite lines decreases, while that of quartz increases. At 800° the lines are for the most part very diffuse because of the forming spinel phase, which evidently has no definite composition. The quartz lines are preserved. In the diffraction pattern of the sample fired to 1000°, those reflections that were diffuse at 800° become stronger, i.e., spinel particles grow. It should be noted that, unlike kaolinite, montmorillonite in the unfired state gives a diffraction pattern with reflections of comparatively low intensity because of dilution by colloids by one third. The presence of iron in montmorillonite significantly lowers refractoriness; therefore the samples deform at 700°. Thus, in montmorillonite, X-ray analysis reveals changes in the crystalline phases during firing and, consequently, structural changes occur that do not affect \(\sigma\) in compression (it increases continuously with increasing temperature).

For the third group of clay minerals—hydromicas—there are very few data in the literature [2] on structural changes during firing. According to Roy [2], in muscovite appreciable changes in structure occur between 940 and 980°. The release of the greater part of the hydroxyl water is accompanied by a slight expansion of the lattice along the \(c\) axis and by its incomplete destruction.

The structure of biotite, according to Roy, is retained up to 1100°. Grim and Brindley investigated illites [2] and showed that the loss of hydroxyl water in illite is not accompanied by destruction of the structure, but only by small changes, as in montmorillonite. Disruption of the structure of illites occurs at 850°, when spinel is formed, the amount of which increases with rising temperature up to 1200°.

Grim and Brindley assume that the octahedral layer, which includes \(\mathrm{Al_2O_3}\), magnesium, and iron, participates in the formation of spinel, while alkalis and \(\mathrm{SiO_2}\) from the tetrahedral layers participate in the formation of glass. At 1100° mullite is formed, which persists up to 1400°,

and spinel at 1300° dissolves in the glass. The quartz phase is not detected when illites are fired to high temperatures.

From this group of minerals we studied the Chasov-Yarsk clay, represented by a monotermite, and the Omutninsk clay, of sericitic composition. Judging from the strength-growth curves, these clays behave differently during firing: the Omutninsk clay behaves like kaolinite, the Chasov-Yarsk clay like montmorillonite. The difference between the two clays lies not so much in the structure of the principal clay mineral as in the particle-size distribution and mineralogical composition. Thus, the Chasov-Yarsk clay contains 80% of fractions finer than 1 µm, including 8% colloids, whereas in the Omutninsk clay the fraction finer than 1 µm is 3%, and there are no colloids at all.

Fig. 4. X-ray diffraction patterns of specimens from Chasov-Yarsk clay fired to different temperatures

In the X-ray diffraction pattern of the unfired Chasov-Yarsk clay (Fig. 4), reflections belonging to illite are observed, but at the same time strong kaolinite lines are also noted \((d = 7.13,\ d = 3.58)\). On firing at 600°, the intensity of the illite lines increases, while the kaolinite lines disappear. In the thermogram for firing at 800°, no new reflections are observed. The decrease in line intensity occurs at 1000°, apparently because of destruction of the illite structure.

Fig. 5. X-ray diffraction patterns of specimens from Omutninsk clay fired to different temperatures

The second specimen from this same group of minerals gives X-ray diffraction patterns whose changes are analogous to those of the first (Fig. 5). Sericite (possibly hydromuscovite) is detected in the diffraction patterns at all firing temperatures, up to 1000°; moreover, at 600° the kaolin lines disappear (with \(d = 7.12\) and \(d = 3.55\)), the admixture of which was present in the specimen, and an intense line appears with \(d = 3.71\), characteristic of sericite. In their structure both specimens are dioctahedral; they differ, however, in the degree of crystallinity: the Chasov-Yarsk clay is finer, while the Omutninsk clay has more perfect crystals [6].

Thus, in this group of minerals as well, the increase in strength is not caused by structural changes, since even within a single group, in one case $\sigma$ in compression begins to rise sharply at 400°, whereas in another it does so only at 1000°C.

Thus, if one speaks of the relative importance, in the development of mechanical strength, of the melting stage or of the stage characterized by the activity of clays (up to 800°C), then for clays of different mineralogical composition their role is not the same. For clays in which the principal mineral is kaolin and coarse-crystalline micas, strength increases at the temperatures at which melt appears, and the melting stage is decisive; at the same time, fundamental changes in the structure occur at temperatures above 1000°C. For clays of montmorillonite and illite composition, the decisive role belongs to reactions at low temperatures, i.e., strength is dictated by the reaction capacity of these clays. It is known that firing cannot be completed at this stage because of the reversibility of many processes at low temperatures (the compression $\sigma$ of specimens tested after saturation with water decreases considerably). High-temperature firing is therefore needed to consolidate the results obtained at the earlier stage.

If one speaks of the role of structural changes in the development of mechanical strength during firing, judging from the results of the study of the specimens cited above, then these changes affect mechanical strength either very weakly or, possibly, do not manifest themselves at all. Unlike talc, which belongs to the trioctahedral minerals with a closed packet and in which shifts in the structure during firing determine many of its properties, the samples studied are dioctahedral, with an open packet, while kaolin has a two-layer packet; therefore shifts in them lead to changes of a different order.

It should be noted that for a more categorical statement, purer monomineral clays and thorough X-ray structural investigations of single crystals are required.

LITERATURE

1. P. G. Usov, E. A. Guber. Processes in low-melting clays during firing and their significance in the manufacture of products. In: Results of Research in Chemistry, 1917–1967. Tomsk State University Press, Tomsk, 1966.

2. A. I. Avgustinik. Change of Talc upon Heating. Promstroiizdat, Moscow, 1952.

3. R. E. Grim. Clay Mineralogy. IL, Moscow, 1956.

4. X-ray Methods for Determining Clay Minerals. Collection of articles. IL, Moscow, 1955.

5. Problems of Clay Mineralogy. Collection of articles. IL, Moscow, 1962.

6. Kolbin Mineral Deposit of Clay. Report, Tomsk, 1961.

7. G. N. Dudarev. Firing of Sintering Ceramic Bodies. Promstroiizdat, Moscow, 1957.