MIKOVINY SÁMUEL DOCTORAL SCHOOL OF EARTH SCIENCES

Head of the doctoral school Dr. István Lakatos, Professor,

member of the Hungarian Academy of Sciences

RÉSUMÉ

of the PhD thesis

THE EFFECT OF VEGETAL PORE-FORMING ADDITIVES ON THE

COMPOSITION, MICROSTRUCTURE AND PHYSICAL PROPERTIES OF

CLAY BRICKS

by

Ferenc Kristály

Department of Mineralogy and Petrology, Institute of Mineralogy and Geology

Faculty of Earth Science and Engineering University of Miskolc

Supervisors:

Dr. Sándor Szakáll

Department of Mineralogy and Petrology, Institute of Mineralogy and Geology, Faculty of Earth Science and Engineering, University of Miskolc, Hungary

Prof. Dr. László A. Gömze

Department of Ceramics and Silicate Engineering, Faculty of Materials Sciences, University of Miskolc, Hungary

Miskolc, 2012

I. Introduction, aims of the PhD project

Brick clays are necessary, basic materials for civil and industrial constructions too. The modern

brick industry developed as a result of accommodating the fabrication to energy availability, higher

quality demand for products and competitiveness with other construction materials. Reducing

energy demand of firing is still a high priority, leading to experiments with additives that release

heat during brick firing (combustion type additives). Another crucial issue for brick clays is

improvement of heat insulation, which is effectively obtained by increasing the porosity. Thus pore

forming additives of mineral (carbonates, zeolites, perlite) and organic (vegetal materials, petrol

grade coke, synthetic materials) are applied. Organic additives are a possible solution for both heat

demand and creating porosity. But usually open porosity appears in high percent, which leads to

capillary H2O uptake and chemical alterations. Alterations are severely minimized if the bricks are

correctly fired to obtain crystallized matrix with minimal amorphous content. The aims of my PhD

project were to investigate how vegetal additives influence the preparation of clay-additive

mixtures, what are the changes in the mineralogy of fired mixtures relative to the fired clays and

what reactions are responsible for the changes.

Brick clays and their composition

Clays usually applied in brick industry are common clays, with high quartz, feldspars ± carbonate

(calcite and dolomite) content (~50%) and variable muscovite ± illite, kaolinite, chlorite and

smectite ± vermiculite. Regarding the phyllosilicate distribution, brick clays show two major

categories: illite – chlorite dominant with low carbonate content and illite – kaolinite dominant with

moderate to high carbonate content. Smectites and illite/smectite usually have significant (5-15

wt%) contributions. In my research I have used two varieties of carbonate-free illite-chlorite (from

Tiszavasvári, TV) and carbonate-rich illite-chlorite (Mátraderecske, MD) clays. The upper, oxidized

part of each deposit is usually referred to as “yellow” (Y), while the lower, still reductive part as

“blue” (B).

Pore forming additives, vegetal materials as additives

Additives with pore forming potential are intentionally used in clay bricks since the mid 1960’s and

on a larger scale since the late 1970’s. Regarding their origin, we can group them as mineral –

natural (calcite powder, expanded perlite, etc.), mineral – synthetic (rice husks ash), organic –

vegetal (sawdust, rice husks), organic – synthetic (petrol grade coke, polysynthetic aromatic

compound) and organic – mineral (lignite). Poly-cyclic synthetic aromatic products (PVC,

polystyrene, etc.) were developed specially for pore forming additives. However, to cut the high

costs of production, the natural resources e.g. vegetal materials came to use again. After their

widespread use in ancient times as tempering (slenderizing) material, in the late 20th century they

were re-discovered as engineering materials. Usually industrial by-products or waste materials are

applied, due to reduced costs of obtaining them. I have tested materials of three types for their

influence on mixture preparation and physical properties of mixtures: vegetal material (VM)

additives such as sawdust (SD), sunflower seeds hull (SSH) and rice husks (RH); petrol grade coke

(P) as organic – synthetic compound; and lignite (L) as organic – mineral material. For the

influence on fired mineralogy only vegetal additives were tested, since the L and P additives

produce firing residues in such a high and unpredictable amount, that obtaining quantitative

mineralogical data for the fired clay content is improbable.

II. Investigations performed during the research work

Raw materials were subjected to a series of routine mineralogical investigations prior to

experimenting. Phenomenons observed during firing were explained based on the raw properties

and additional experiments were introduced where needed. Fired products were also investigated

similarly to raw materials. Methods on investigation include:

• X-ray powder diffraction (XRD) for all materials, firing residues of VM and fired products

and also samples of clay mineral fractions

• scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) for raw

VM and fired materials

• thermal analysis (TA) by simultaneous differential thermal analysis (DTA),

thermogravimetry (TG) and derivative thermogravimetry (DTG) for raw clays, VM and

their mixtures

• evolved gas analysis (EGA) by quadrupole mass spectrometry (QMS) for raw clays and VM

samples

• optical microscopy in transmitted polarized light for raw VM and fired mixtures

Experiments of the research were:

• obtaining mixtures with 4wt% and 7wt% additives, homogenized in laboratory pan-mill

(Koller-gang) and preparing extruded test samples by laboratory screw conveyor

• obtaining the dimensional parameters and weight of test pieces in as-extruded, dried, fired

and after capillary-test states

• compressive strength determination for fired test pieces

• sequential firing of mixtures to observe VM decomposition stages

• sequential firing of additive-free clays to observe mineral transformation reactions

By the determination of dimension in as-extruded state, expansion was observed, with similar

trends in each clay characteristic for the additive types. Only VM produce expansion, the lowest

observed for RH, highest for SSH and between them for SD. Porosity was found to be made up by

open porosity due to the high capillarity. The screw-conveyor formed a highly oriented texture,

which at drying and firing (by shrinkage of clayey matrix) produced high capillarity. Compressive

strength follows a trend which is mostly the inverse of expansion seen in as-extruded state.

Characterization of clays

Quantitative mineralogical composition of clays was obtained by XRD, trough full pattern matching

(FPM) Le Bail fitting (DiffracPlus EVA) and Rietveld-refinement (DiffracPlus TOPAS).

Components were identified based on ICDD PDF-2 (2005) database by Seartch/Match procedure.

Amorphous content was estimated trough degree of crystallinity approach by Rietveld-refinement,

by applying two amorphous humps at illite/smectite (~12Å) and at silica-dominant (~3.3Å) regions.

This method was found more appropriate than application of internal standard, since random

illite/smectite can not be modeled in Rietveld-refinement. TV clays were found to contain a

muscovite-sericite-illite continouse alteration series and well crystallized chlorite. MD clays have

more important random illite/smectite content, with several wt% kaolinite contribution and ~15wt%

calcite + dolomite. All clays have albite > K-feldspar > Ca-rich plagioclase. Clay mineral

determination was performed on gravitationally separated <2µm fraction, by measuring air-dried,

ethylene-glycol solvated, 350°C and 550°C heated oriented specimens. Chlinochlore was found to

be present in a regular and a Fe-rich variety. Random illite/smectite is present in all clays, higher

ratio in MD clays. Although the contribution of kaolinite seems dominant, the 560°C heated

specimen displays a large clinochlore residual peak and the Rietveld-refinement returned

clinochlore in higher amount.

Thermal characterization was done on all clays, calculating organic content from TG in the 250°C –

450°C region, ~1wt% for TV clays and ~3wt% for MD clays. Illite shows two important OH-loss

reaction in TV clays, at ~540°C and ~800°C, these reactions are overlapped by carbonate

decomposition trough solid-state reactions in MD clays. MD clays also show a more distinguishable

clinochlore (± kaolinite) OH-loss between 580-600°C.

EGA-QMS revealed CO2 emission in the 250°C – 450°C region and important H2O and OH

emission starting from ~300°C, with peak values in 750°C – 790°C region, corresponding to the

second illite OH loss. Carbonates were decomposed <850°C as evidenced by CO2 peaks, and solid-

state reactions are made responsible for lowering the decomposition T(°C).

Based on XRD, TA and EGA results, a sequential firing scheme was set up, by heating ~1g of

material to 560°C, 740°C, 780°C and 950°C of TV clays and 560°C, 720°C and 920°C for MD

clays (with soaking times of 30min). XRD patters were recorded after each firing cycle.

Characterization of additives

Vegetal material grains were embedded in epoxy-resin to obtain cut and polished surfaces. The

grain structure and chemical composition were observed by SEM and EDS. Three different material

were revealed: SD is fibrous with Ca, Mg and K content, SSH is sponge-like with K-enriched

external shell and RH is hollow type with high Si content. Optical microscopy revealed the

crystalline nature of fibers, arranged in polycrystalline structures.

XRD on raw VM also showed the poorly crystalline cellulose in each sample. Comparing the

patterns and peak position I have obtained to those from literature specific for cellulose research,

the presence of α-cellulose is with certainty proved.

TA also revealed similar reactions for VM, with strong endothermic in the adsorbed H2O domain

and several overlapping strong exothermic reactions in the 220°C – 500°C region. The exothermic

reactions are attributed to oxidation of cellulose polymorphs and lignin. Deconvolution based peak

separation of DTA curve revealed the existence of 3 exothermic peaks associated with 3 or more

endothermic processes. Two major exothermic peaks are observed: in the 310°C – 350°C domain

the cellulose depolymerisation and lignin oxidation, while in the 450°C – 490°C domain the total

organic oxidation. Important endothermic peak immediately after the second oxidation is due to

crystallization – reorganization of inorganic residues (pyrolysis is not considered possible, due to

low – 5mg – sample quantity and air atmosphere).

XRD on TA solid residues showed the nature of inorganic remnants: amorphous silica for each VM

is present, while RH remnants are exclusively amorphous SiO2. SD remnants have important quartz

content formed, with a K-Ca-Al silicate, while SSH has no quartz but the K-(Ca-Al) silicate.

EGA revealed CO2 release at T(°C) of observed DTA peaks, with H2O and OH associated mainly to

the first exothermic process. No gaseous sulfur compounds were detected.

Sequential firing of mixtures at the T(°C) of detected reactions was performed, and thin sections for

optical microscopy prepared. The transformation of VM has different stages for the different

material types. RH has remnants in amorphous state at ~250°C with stable morphology and

becoming glassy (still amorphous) at higher T(°C). SD is oxidized at >350°C forming thin pipes as

remnants, preserving partial crystallinity which is inherited by the inorganic remnants. SSH is also

oxidized >300°C, the annular remnants preserve the outer grain shape and shrink up to ~80% in

diameter and >90% in volume.

SEM and EDS on mixture samples fired at 900°C revealed the remnants that were formed, specific

for each VM. The cations observed in raw VM are preserved in the remnants. SD grains formed

remnants by preserving the original fibers trough thin-walled (<0.1µm) tubular forms. SSH formed

thin foils in the pores, while the surface of hulls left powerful imprints seen on the pores wall. RH

forms reticular, hollow network of SiO2 walls (preserving the original cell walls) covered by thin

foils.

Characterization of fired products

Fired mixtures samples were characterized by XRD, SEM and EDS investigations. XRD was

performed in two stages: a first stage of reconnaissance to observe possible differences in the

mineralogy of samples than a second detailed investigation on mixtures with VM. The first stage

showed that mineralogy – the crystalline components present – are not dependent on the types of

additives. In the second stage samples with α-Al2O3 as internal standard were prepared and

investigated. Raw clays were investigated in 3 specimens from 3 different test pieces, to test the

homogeneity of samples. Mixture samples were investigated for TV-B and MD-B clays with 7wt%

of VM. Rietveld-refinement in TOPAS was applied to extract quantitative compositions. Existence

of crystalline components in the samples determined by XRD was tested by SEM and EDS, finding

compositions assignable mostly to mixtures of expected components. Reaction coronas, evidence

for diffusion and recrystallization were observed. TV clays were found to produce K-feldspar by

firing, in accordance with sequential-firing experiments. Additionally, spinel-type components were

also identified. MD clays formed gehlenite – diopside – anorthite mixture by firing. Significant

amount of amorphous content was also detected (~25wt% for TV-B, ~30wt% for MD-B), lower in

mixture samples than in additive-free clays.

III. New scientific results of the PhD project

Mineralogy of clays and fired products

My investigations lead to the conclusion, that the existence of a muscovite-sericite-illite weathering

series in clays substantially influences the crystallization of components formed by firing. My

results considered as new to the field of raw and fired brick clay mineralogy, as well as to the firing

reactions, are summarized in the following part.

1. XRD investigation of >45µm, 45-2µm and <2µm fractions of clays, as well as TA and EGA

results sustain the presence of micaceous material with decreasing crystallite size in decreasing

grain size fractions – which I termed muscovite-sericite-illite alteration series – (XRD) that

produces continuous H2O release >350°C to ~800°C (EGA) and produces endothermic reactions

(DTA)

a. If calcite or dolomite not present in raw clays, such as in TV clays, this micaceous material

transforms into K-feldspar, spinel-type minerals and amorphous material; if Ca and Mg (e.g.

from carbonates) is available, gehlenite-akermanite, diopside and anorthitic plagioclases are formed

(Fig 1).

TV-B (Bragg-Brentano geometry) TV-Y (Bragg-Brentano geometry)

MD-B (parallel-beam geometry) MD-Y (parallel-beam geometry)

Fig 1. XRD results of sequential firing experiments on raw additive free clays.

b. In the presence of this micaceous material (similarly to dehydrated smectites and kaolinite)

carbonates are decomposed by solid-state reactions at lower than expected T(°C), as evidenced by

the EGA and sequential firing experiments on MD clays, forming amorphous or nanocrystalline

silicates which crystallizes at higher T(°C) (based on sequential firing experiments)

Regarding the composition of VM, the direct observation of texture and composition changes, as

well as the detailed thermal characterization of oxidation reactions is for the first time deciphered in

brick industry related research.

2. I have evidenced by XRD, TA and microscopy, that the different VM additives all have

crystalline cellulose as main component. The oxidation reactions are the same, but their T(°C) is

influenced by their inorganic content. I have used the DTA curves to calculate temperature

difference curves for several mixture samples, by measuring the DTA value in fixed point for raw

clay DTA and mixture DTA. By this calculation I have evidenced that the temperature in the

mixture samples during firing is considerably higher than in raw clays. As one of the main

new results I have evidenced that VM additives do not form pores of 100% volume of raw

grains, but they produce remnants, depending on VM type (Fig 2).

Sawdust grain in fired sample Sunflower seeds hull grain in fired sample Rice husks grain in fired sample

Fig 2. BSE images of vegetal material remnants in the fired samples

a. Deconvolution based DTA peak separation revealed also the overlapping reactions, leading to

further new observations: after the oxidation of organic compounds, the inorganic remnants

reorganize and crystallizes, producing endothermic effects, overlapped by the exothermic peaks; the

heat release by DTA is decreasing in SSH>SD>RH order

b. By microscopy techniques I have evidenced that VMs have inorganic content, moderate Ca, Mg

and K in SD, high K in SSH, and high Si in RH; I also could locate these enrichments in the VM

grains

c. I have evidenced that solid residues observed at TA and characterized by XRD are also formed in

the bricks during firing, and they form a replacement of the original vegetal texture, thus their pore

forming is inversely related to inorganic content, which is readily determined by TG.

3. TA on mixture samples evidenced that SSH in TV-B clay acts differently than the other VM: the

exothermal peaks of organic matter oxidation are lower in intensity than expected and so is the

second peak of illite OH loss (~780°C). I have attributed this observation to the sponge-like

structure of SSH grains that act as catalyst, suction media for H2O (and OH) from the

mineral lamellas, this way provoking a large weight loss in the first illite OH loss, which reduces

the intensity of exothermal peaks in turn. Since in MD-B clay the illite-sericite (micaceous)

component has lower contribution, this effect is not observed, thus clinochlore OH loss is not

influenced as severely as illite (and similar minerals).

4. The behavior of mixtures at shaping could be related to the mineralogy of clays and structure –

composition of VM. The most important among detected processes is expansion: a stress-release

reaction of a plastically deformed system. Cellulose content, inferred from DTA peak intensity,

was found to be directly related to the degree of expansion, which also increased with the

amount of added VM (Fig 3). Besides cellulose content, three more factors were determined

contributing to expansion:

Fig 3. The relation between organic additive types, added amount, cellulose content and expansion after

extruding (4=4wt% additive, 7=7wt% additive

a. Total non-plastic (tnp) content of clays – including muscovite – based on XRD was found to

significantly contribute to expansion: highest expansion TV-Y with 68wt% tnp, than TV-B with

62wt% tnp and MD-Y with 63wt% tnp, finally MD-B with 60wt% tnp

b. The texture and morphology of VM grains determines the elasticity for them, beyond cellulose

content. The sponge-like SSH grains, with closed micro pores, triggers the largest expansion,

while the fibrous – elongated SD grains produce lower expansion and finally the platy, hollow

grains produce the lowest expansion (Fig 3).

c. The high Si-content of RH is also reinforcing the organic structures, thus the grains are less

susceptible for compression and expansion. This conclusion is supported by microscopy

observations, where only RH grains were found in pores with minimal micro crack system around

the pores

5. Properties of fired products like compressive strength and porosity were also found to be

influenced by the nature and quantity of additives. Porosity was found to be only open or

capillary type, by H2O uptake experiments. Compressive strength has more determinant factors:

a. As the effect of additives, the smallest compressive strength values were found for the mixtures

with largest expansion.

b. As the effect of raw clays mineralogy, the improvement of compressive strength was found

were Ca and Mg was available – in the case of MD clays – to form a strong crystalline matrix

of gehlenite-akermanite, diopside and anorthite.

c. From the above results I have inferred that for conventional clay bricks the Ca- and Mg-

silicate matrix is stronger/harder than the K-feldspar and spinel-type oxides matrix (Fig 4).

Fig 4. Uniaxial compressive strength of fired mixture samples (blue color for “B” clays, orange color for “Y”)

clays).

6. The mineralogy of fired mixtures was influenced by VM additives by the reduction of

amorphous content, a more evident process in the case of TV clays.

a. as a result of RH high Si content which leads to high amount of remnants of amorphous SiO2, the

amorphous content of RH containing mixtures is higher, than for other VM

b. By the Rietveld-refinement method a quartz content formed during firing was identified, since no

instrumental or sample convolutions, nor texture corrections made possible the fitting of quartz

peaks by one regular quartz structure. I have termed this quartz as nanoquartz due to <100nm

crystallite sizes, and in the fired mineralogy of bricks it can be assigned the “Quartz2” code

(due to its secondary origin as related to the raw mineralogy of clays).

IV. Publications related to the PhD research

Conference and extended abstracts

Kristály F., Zajzon N. (2007): Behaviour of organic pore-forming additives in traditional clay-

bricks: effects on mineralogy and microstructure. 6th International Conference of PhD

Students, Natural Sciences section. pp. 65-70

Kristály F., Zajzon N. (2007): Clay-brick fabrication and metasomatic processes, a comparison of

natural to synthetic mineral transformation reactions. 6th International Conference of PhD

Students, Natural Sciences section. pp. 71-76

Gömze, A..L. & Kristály, F. (2009): Stages of transformation of vegetal additive materials upon

clay mixture firing. XIV International Clay Conference Book of Abstracts, Vol 1. p 116

Kocserha, I. Gömze, A. L. & Kristály, F. (2009): Effects of Extruder Head’s Geometry on

Properties of Extruded Clay Products. http://fold1.ftt.uni-miskolc.hu/pdf/091011.pdf

Kristály, F. & Gömze, A..L. (2009): The transformation of added vegetal waste materials during

clay brick firing. http://fold1.ftt.uni-miskolc.hu/pdf/091012.pdf

Kristály, F. & Kocserha, I. (2009): Correlations between combustion type additives and expansion

after extrusion of clay bricks. http://fold1.ftt.uni-miskolc.hu/pdf/091013.pdf

Scientific papers

Kristály F. & Gömze A. L. (2008): Remnants of organic pore-forming additives in conventional

clay brick materials: Optical Microscopy and Scanning Electron Microscopy study.

Építőanyag, 2008/2, pp. 34-38.

Kocserha I. Gömze A. L. Kristály F. (2010): Effects of Extruder Head’s Geometry on Properties

of Extruded Clay Products. Materials Science Forum Vol. 659, pp 499-504

Kristály, F., Gömze, A..L. & Papp, I. (2010): The transformation of added vegetal waste materials

during clay brick firing. Materials Science Forum Vol. 659, pp 37-42

Kristály, F. & Kocserha, I. (2010): Correlations between combustion type additives and expansion

after extrusion of clay bricks. Materials Science Forum Vol. 659, pp 43-48

Kristály F., Kelemen É., Rózsa P., Nyilas I. & Papp I. (2011): Mineralogical investigations of

medieval brick samples from Békés county (SE Hungary). Archeometr, 54/2, pp 250-266.

(IF: 1.53)

Kristály F (2011) Agyagos kőzetek ásványtani összetétele (Szenterzsébet, tégla- és cserépgyár

agyagbányája valamint Doborka, tempesztit szelvény). Kincsünk, a környezetünk-Föld- és

Környezettudományi Terepi-Tavaszi Egyetem Dél-Erdélybe 2O11. Április 28-Május 3.

Terepgyakorlat vezető & Oktatási Segédanyag. ISSN 1843-3368. Top Invest Kft,

Székelyudvarhely, pp.55-6O

Kristály F (2011) Investigations on the Pottery Firing Kiln and some Pottery Samples from

Odorheiu Secuiesc-Alsólok . Drehscheibentöpferei im Barbaricum: Technologietransfer und

Professionalisierung eines Handwerks am Rande des Römischen Imperiums Akten der

Internationalen Tagung in Bonn vom 11. bis 14. Juni 2009Vor- und Frühgeschichtliche

Archäologie Rheinische Friedrich-Wilhelms-Universität Bonn, ISBN 3-936490-13-9 pp.

457-462 (Appendix to Zsolt Körösfői, András Sófalvi, Zsolt Nyárádi: Töpferöfen in der

Siedlung der Sântana de Mureş-Černjachov-Kultur in Odorheiu Secuiesc-Alsólok,

Siebenbürgen, same volume, pp. 445-456)