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PRODUCTION OF PROTOTYPES OF “YEllOw PavINg STONES” IN BUlgaRIa

PaRT I: PhYSICal aND ChEmICal PROPERTIES

Lyuben Lakov1, Svetlana Encheva2, Peniu Tsonev3, Bojidar Jivov1, Mihaela Aleksandrova1, Krasimira Toncheva1

aBSTRaCT

Due to the growing traffic and to the increased destruction and narrowing of the perimeter of the “yellow pav-ing stones” at the center of Bulgarian capital - Sofia, as early as the 1970s attempts are made to use Bulgarian sedimentary rocks (marlstones) for the manufacturing of larger paving units. The first yellow-coloured samples are obtained at DIP “Granite” - Targovishte, but the products are not stones but paving tiles. Under laboratory condi-tions at IMSTECH “Acad. A. Balevski“, Bulgarian Academy of Sciences, and semi-industrial conditions at “Rodna Industriya” Ltd. - Popovo the first wear-resistant, high-strength yellow paving stones are produced. They are similar to the original “yellow paving stones”, imported at the beginning of the 20th century and are laid in the streets of the centre of Sofia. X-ray analysis proves the presence and quantity of crystalline phases formed in the samples - anortit CaAl2Si2O8 and fassaite diopside. The chemical analyses carried out find that the compositions of both the original and new specimens fall into the fassaite field of the triple chart diopside (CaMgSi2O6), essenite (CaFe3 + AlSiO6), and the so called imaginary molecule CaAlAlSiO6. There most of the iron is trivalent and besides there is more aluminum dioxide.

Keywords: anortit, diopside, petrugical material.

Received 20 November 2017Accepted 15 June 2018

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1 Institute of Metal Science, Equipment and Technologies with Hydro- and Aerodynamics Centre“Akad. A. Balevski” Bulgarian Academy of Sciences E-mail: silicatchim@mb.bia-bg.com, krasiton4@abv.bg2 Earth and Man National Museum, Sofia; E-mail: sencheva@abv.bg3„Rodna industria Popovo” Ltd., Popovo E-mail: bsadina@mail.bg

INTRODUCTION

A literature study on existing deposits of suitable raw materials for the production of yellow paving stones is carried out [1 - 6]. The results obtained by other authors referring to the synthesis of similar ceramic materials are analyzed [7]. The elaborated prototype technology is based on the classical methods used in the silicate industry [8 - 10]. It enables the rapid organization of production with available standard equipment. The indicators of the finished samples comply with the requirements for this type of ceramic flooring and the existing standard [11].

There are a lot of talks in the media during the last years (2014-2017) about the “yellow paving stones” at the center of Sofia. Some people discuss this problem and highlight their laboratory research without actually showing any results - an authentic pavement creation. It is worth noting that a scientific team headed by Eng. G. Gencheva has worked on the “yellow paving stones” issue back in 1969 [2]. It has diagnosed petrographic and technological clay materials from different fields, as well as compositions based on them for the production of large-sized ware. At the same time experiments are carried out under industrial conditions at DIP Granit –

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Targovishte, leading to the production of yellow tiles of a very good quality. Production of “yellow paving stones” of the authentic shape, size and colour has not yet been achieved.

These facts prompt the interest of a team of scientists and specialists two years ago, on their own initiative and self-financing, to use their experience of creating prototypes of a street flooring of an equivalent yellow colour improved performance.

The team has developed the composition of the yel-low paving stones and has participated in the elaboration of the first prototypes. The team, headed by Prof. L. Lakov, Ph.D., included a group of specialists from The

Institute of Metal Science, Equipment and Technolo-gies at Bulgarian Academy of Sciences (IMSET-BAS), “Rodna Industria” Ltd. - Popovo, “National Museam Earth and Men”, Institute of Computer and Commu-nication Technologies - BAS, Technical University of Sofia, Institute of Mechanics - BAS and The University of Mining and Geology, Sofia.

The presented work is related to a technical solution [12], on the basis of which, at the request of the authors, a preliminary novelty study is carried out. The product developed corresponds to Art. 8 of the Patent Law.

Innovative compositions and a technology for the production of prototype new “yellow paving stones” have been developed. They are equivalent in colour and superior to the reference yellow paving stones in Sofia.

The team participants have taken part and presented their rezults at: IX International Scientific Conference “Design and Construction of Buildings and Facilities” – Varna, 2016, 7th National Exhibition ITI (Inventions Technologies and Innovation), 2016, National Confer-ence with International Participation “GEONATURES 2016”.

ExPERImENTal

The essence of the experimental part was to create an exemplary composition of a petrugical material of the patent application comprising the preparation of the molding parts: molding, drying, curing and examination of the colour and properties of the resulting product. The plastic mass was mainly produced on the basis of sedimentary marble rocks (Svetlen village, Targovishte region), and 10 % building waste (from the production of bricks and tiles) in the form of a chamotte of a cer-

tain granulometry. The dosed materials were mixed on a collegling and placed in a watertight container where they were wetted to humidity 22 - 24 %. Processed raw materials matured for 24 hours. The resulting mixture was treated mechanically three times with smooth rolls and a screw press with a mouthpiece. The pieces cut were pressed in a metal mold of a plastic molding press where the prototype paving blocks were obtained. The pavement obtained corresponded to the requirements of BDS EN 1344 2013 with the status of Bulgarian standard of ceramic paving from 17.03.2014. The prepared sam-ples were subjected to drying at 50oC - 60oC to a residual humidity 5 % - 6 %. The high temperature synthesis was carried out in a special mode at a homogeneous tempera-ture field and in a muffle furnace environment with a temperature regulator allowing high temperature liquid phase synthesis of four paving stones. The established heat treatment regime and the furnace atmosphere at different temperatures represented an important step in the patented technology for the maximum isothermal retention at 1100 ± 5oC for up to 6 hours.

The qualitative and quantitative composition of the materials was determined by X-ray diffraction analysis of the Bruker D8 Advance automatic X-ray powder dif-fractometer with Cu K radiation and a CynxEye solid state detector. The quantitative analysis was performed using the Rottweil method with Topaz 4.2. Study ac-curacy of ± 1 %.

Together with the prepared samples (conditionally referred to as “new”, Fig. 1), the samples of original paving stones, imported from Austro Hungary (conven-tionally referred to as “old”), were also tested.

The chemical analyses of the individual phases were performed on a scanning electron microscope JEOL JSM 6010 Plus / LA & EDS System at the University of Mining and Geology “St. Ivan Rilski”, Sofia. In order to solve the Fe2 +/ Fe3 + separation problem, wet chemical analysis was performed at the Chemical Laboratory of Sofia University “St. Kl. Ohridski “.

Microscopic observations and micrographs were performed on the Amplival Pol D polarizing microscope with the ProgRes CT3 digital camera in the museum “Earth and Man” and a scanning electron microscope JEOL JSM 6010 Plus / LA & EDS System at the Uni-versity of Mining and Geology “St. Ivan Rilski”, Sofia.

A Munsell system was identified and compared to the prototype designs and available original standards.

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Equivalent colour characteristics were found by compar-ing the “new” and “old” samples.

The air and fire compaction of the samples were determined. The preliminary technical and economic calculations were carried out to determine the cost of production in the future.

RESUlTS aND DISCUSSION

The presence of predominantly crystalline phases - CaAl2Si2O8 anions and CaMgSi2O6 diopside, which are also found in the samples of the original flooring of the prepared prototypes, has been proved by the X-ray analysis (Figs. 2 and 3, Table 1). From the qualitative and quantitative X-ray analyses of the prototypes (“new” sample) and the standards (“old” sample) a difference was found in the content of the basic phases anorthite and fassaite diopside. In the created prototype the phase of the annotation prevails. This explains the lower density

Fig. 1. “Yellow bricks” produced in Bulgaria with an equivalent yellow colour, shape and dimensions compared to the original standards produced in Aus-tro-Hungary at the beginning of the 20th century.

Fig. 2. X-ray diffractogram of a “new” specimen of marl in the village of Svetlen, Targovishte region.

Table 1. Quantitative phase composition of “new” and “old” samples of synthesized material.

The kind of material Phase fomation Content wt. %

Degree of crystallinity

%

A "new" sample

Anorthite Diopside Quartz

Traces of Fayalite

59.0 39.0 2.0

79 %

An “old “sample”

Anorthite Diopside Quartz

Traces of Vollastonite

37.0 62.0 1.0

86 %

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and better physico-mechanical performance.The data are: 3.95 wt. % Fe2O3 and 0.4 wt. % FeO for the

old paving stones; 5.47 wt. % Fe2O3 and 0.28 wt. % FeO for the new ones. With this data, corresponding recalculations of the formulas of diopside compositions were obtained. The results are presented on a triangular diagram with coordi-

nates: diopside, esseneite, and the Ca-Tschermak molecule (Fig. 4). The fields of the chart are according to [18] and correspond to the dominant caution in position M1 - Fe3+ (esseneite), Al3+ (CaTs) and Mg (diopside. Fig. 4 shows that the analyses of the old and the new paving stones fall unambiguously in the field of the “fassaite”.

Fig. 3. X-ray diffractogram of an “old” sintered specimen of Austro-Hungary material.

Table 2. Chemical composition of the diopsy and annotation of the prototypes (“new” model) and reference standards (“old” model)

Diopside „new” sample

Diopside „old”sample

Anorthite „new”sample

Anorthite „old” sample

SiO2 47.25 48.28 48.88 53.41 52.98 54.09 49.99 50.86 52.87 54.74 TiO2 1.03 0.98 1.01 0.47 0.59 0.67 0.47 0.34 0.00 0.29 Al2O3 13.61 13.97 13.34 7.11 6.57 11.83 26.16 26.48 27.97 25.75 FeO 9.33 8.46 8.33 5.69 5.95 5.62 3.22 2.88 2.04 2.44 MnO 0.00 0.00 0.00 0.22 0.18 0.00 0.00 0.00 0.00 0.00 MgO 7.43 7.75 8.04 11.17 13.27 9.20 1.59 0.99 0.50 0.50 CaO 19.96 19.23 19.47 20.62 19.07 16.23 16.88 16.47 13.25 12.81 Na2O 0.52 0.49 0.00 0.58 0.49 0.93 0.84 0.86 2.35 1.87 K2O 0.88 0.84 0.93 0.74 0.91 1.43 0.85 1.12 1.02 1.60

TOTAL 100.01 100.00 100.00 100.01 100.01 100.00 100.00 100.00 100.00 100.00

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Only in view of the retorto-structural analysis data and the greatest increase of the microscope it can be distinguished the diopside phase (high relief and short-prism habitus) and the anorthite phase (low relief and xenomorphic form). Optically, however, the pores, which in both samples are in the range of 0.01 to 0.2 mm and in an amount of about 5 - 6 % of the surface of the microscopic preparations. In both samples a teriogenic fraction of less than 1 % is presented, mainly from quartz beads.

On the scanning electron microscope, minerals are well-known. The diopside forms of the idiomorphic cryopresmatic crystals, and the anorthite are in the form of irregular platelet constructions. In addition, the two samples are presented with a certain amount of intermediate phase (non-crystallized glass with a modal eutectic composition of 62 % diopside and 38 % anorthite, obtained by petrochemical calculations using the pyrocen method).

The results from the chemical analysis of the miner-als confirm the similarity in the contents of the “new” and the “old” species. Lower content of SiO2, MgO and

CaO and higher quantities of Al2O3 and FeO in diopside phase in the “new” samples were determined when com-pared to the “old” bricks. At the same time in the case of anorthite in the “new” samples a little lower content of SiO2 and Al2O3 and higher amounts of FeO, MgO and CaO were determined.

The question concerning the yellow colour of the paving stones is answered. It is related to the valence of the iron ions in the crystal phases, which is generally true. However, the most accurate answer is mineralogi-cal - the reason for the yellow colour is that one of the main phases is not simply a diopside, but a variant of it - “fasatoite diopside.” The chemical analysis carried out shows that the compositions of both prototypes fall into the field of faeces in the triple diaphysis chart of CaMgSi2O6, CaFe3 + AlSiO6 esenite, etc., the imaginary molecule CaAlAlSiO6, where most of the iron is triva-lent. Besides, it contains more aluminum oxide. The standardization and the colour characterization are also carried out by the American company “Mancel, Colory Company” system. The colour characteristics of the standard and prototype are identical on the surface and

Fig. 4. Clinopyroxen compositions in the system: CaMgSi2O6-CaFeAlSi2O6-CaAlAlSiO6 (fields of the diagram ac-cording to Cosca and Peacor, 1987); squares – original bricks, rhombs – new bricks.

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in depth. They refer to 2.5 Y 5/6, which in turn demon-strates their colour equivalence.

CONClUSIONS

Prototypes of the “yellow paving stones” were pro-duced on the ground of sedimentary rocks from the village of Svetlen, Targovishte, in Bulgaria, after a complex semi-industrial research at “Rodna Industriya” Ltd. in Popovo, Bulgaria. A plastic mass thermally treated at 1100 ± 5°C was also used. The products and the standard from the street floor at the center of Sofia were of an identical yel-low colour, but they were characterized by a higher coef-ficient of static friction with respect to different counter bodies under a dry and a wet environment and a lower abrasion wear compared to those of the reference pavers.

The X-ray analysis of the obtained prototypes re-vealed the presence of the crystalline phases - an anomeric CaAl2Si2O8 and diopside CaMg(Si2O6), which are also proven in the standards. Through the chemical analysis of the individual minerals a significant similarity was found in the compositions of the new samples obtained and the samples from the authentic pavement. In the diopside of the produced prototypes, a lower SiO2, MgO and CaO content was recorded and higher amounts of Al2O3 and FeO compared to the standards. At the same time, the an-orthite of the prepared samples has slightly lower amounts of SiO2 and Al2O3 and higher amounts of FeO, MgO and CaO, compared to the original samples. It is the form of a fasato-type diopside the reason for the yellow colour.

REFERENCES

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