Glass bottles powder’s impact on the technological characteristics of clay mixtures used for porcelain stoneware tiles

Abstract

This study aims to enhance porcelain’s physicochemical properties by adding glass powder as flux. Kaolinitic clay, sand, and glass powder were used to formulate six grades, fired between 1140-1260ºC for 2 h. Specimens were tested for shrinkage, mass loss, water absorption, density, porosity, and mechanical strength (three-point bending). Phases were identified by X-ray diffraction and compared with industrial porcelain. The incorporation of glass powder has enabled the reduction of the firing temperature, the enhancement of densification, the augmentation of mechanical properties, and the valorizations of glass waste. The grade with 20 wt% glass (P20) fired at 1220ºC (P20-122) showed the best results: 13% shrinkage, 3.93wt% mass loss, 0.2wt% water absorption, 0.4% porosity, density 2.4 g/cm³ (46.6%). Using glass powder not only improves porcelain quality but also contributes to recycling glass waste on an industrial scale.

Keywords:
mineral glass; waste; environment; clay minerals; porcelain

INTRODUCTION

In a world increasingly conscious of environmental challenges, waste management and recycling have become global priorities. Among many types of waste that pose a problem, glass stands out for its non-degradability, with a lifespan of around one million years, and its versatility. Although glass can be recycled ad infinitum with no loss of quality, much of it still ends up in landfill sites, contributing to pollution and the depletion of natural resources¹. According to a World Bank report in 2018, global waste production will increase by 70% by 2050². As a result of rapid urbanization and population growth, the amount of waste produced each year is growing at a remarkable rate. According to World Bank forecasts, by 2050, annual global waste production could reach 3.4 billion tons^{2), (3}. Around two hundred million tons of glass are landfilled every year, 80% of which is soda-lime glass, much of it used in beverage bottles⁴. This alarming situation highlights the urgent need to adopt more sustainable practices. Ceramic materials, especially porcelain, are indispensable to the daily life of the human race. Porcelain is used in many fields for its beauty, durability, stain resistance, and resistance to thermal shocks. Tableware (plates, cups, ...), sanitaryware (toilets, bathtubs, ...), bioceramics (dental prostheses, ...), and laboratory porcelain (crucibles, mortar, ...) are just some of the areas of porcelain applications^{5), (6}. The term “porcelain” designates a particular ceramic material, which is vitrified, translucent, and has a white paste, fine, closed, and homogeneous. The firing process is conducted at a temperature range of 1250-1460ºC, resulting in specific technological properties, including linear shrinkage (≤15 %), water absorption (<0.5 wt%), apparent density (>2.3 g/cm³) and flexural strength (>35 MPa)^{7), (8), (9}. Conventionally, the process of heat treatment is carried out with a heating rate of 2-10ºC/min and with sintering durations of 60-120 min¹⁰. Although these materials are used in large quantities, they are still imported into many developing countries. This is due to a number of factors. Firstly, the relatively high firing temperature is a barrier, given the energy problems¹¹. Secondly, the specificity of the raw materials required for this type of ceramic makes its production even more complex in these countries¹². With a view to finding a solution, ceramic researchers have taken up the cause. In the case of Burkina Faso, there is some formulated porcelain with local raw materials^{13), (14}. The energy challenge is being addressed by research efforts aimed at reducing sintering temperatures through optimized formulations. In ceramic systems, a range of waste types have been the focus of recent studies to enhance product properties while minimizing environmental impact. A notable example is the study by Maryam Adamu Sunusi et al. (2025) on the utilization of palm oil fuel ash as a substitute for silica in porcelain glaze formulations¹⁵. Another pertinent example is the study by Manoela da Silva Carvalho et al. (2025), which explored the incorporation of blast furnace flue dust as a partial substitute for sand in mortars¹⁶. Martini et al. in 2017, in a project called “sanister,” were able to reduce the firing temperature of porcelain from 1280ºC to 1170ºC by substituting industrial quartz and feldspar with a mixture of waste glass, granite, and fired broken ceramic pieces¹⁷. However, the use of glass powder (GP) in conventional porcelain formulations remains limited to ≤10wt%, due to the harmful properties (swelling) it may cause. The fight against glass waste in our environment requires recycling or use in the formulation of ceramics, particularly porcelain. In ceramics, glass powder could play an essential role due to its relatively low melting point¹⁸. Environmental issues call for the consistent use of unused glass to make a significant contribution to the fight against glass waste, but although many researchers have invested in this area, its use in porcelain formulation remains minimal. A. Djemli et al. (2023) used 25wt% powder of glass in place of feldspar to assess the effect of temperature on the different phases in porcelain, without evaluating the mechanical and physical properties of the materials¹⁰; furthermore, the glass used is derived from automotive glass, not bottled glass, which is the real danger to our environment. Dunia K. M. Al-Nasrawy (2009) research focused on the replacement of 25 wt% of feldspar with soda lime glass in a standard porcelain composition. The study systematically evaluated the differences in their physical properties at various firing temperatures. The author obtained new porcelains at a firing temperature of 1250ºC, exhibiting 0.391% water absorption, 8.8% shrinkage, and a density of 2.257 g/cm³⁽¹⁹. The research conducted by O.R. Njindam et al. (2018) on the combination of clay with quartz sand, feldspar, and occasionally waste glass demonstrated that a mixture containing 30% glass powder can be utilised as porcelain stoneware tile when subjected to a temperature of 1150ºC. This material exhibits a water absorption value of 0.4% and a flexural strength of 39.06 MPa, thereby conforming to the ISO 13006 standard requirement²⁰.

In this work, the amount of glass bottles powder is optimized for ordinary porcelain formulation while ensuring physical and mechanical properties. The field of porcelain and ceramics in general could therefore be one of the ways to solve the environmental problem of waste from glass bottles. Burkina Faso is not immune to these problems. Specifically, collected glass is characterized, and then formulated test specimens are created by combining these glass powders with local clays of known characteristics. Finally, the formulated and sintered specimens will be characterized using physico-chemical and mineralogical techniques with a discussion.

MATERIALS AND METHODS

Two clay raw materials are used in this work. These minerals are kaolinitic clay and pegmatite, respectively referenced as SA and PEG. These clays come respectively from the “Centre-Nord” and “Haut Bassin” regions of Burkina Faso, with geographic coordinates of 13.23º North Latitude and 1.54º West Longitude for SA and 4º4’ West Longitude, 11º7’ North Latitude for PEG. Sand was used as a degreaser and referenced SAB. It was collected in Borodougou in the Hauts Bassins region at the geographical coordinates 11º10’ North Latitude, 4º17’ West Longitude.

These raw materials, SA, PEG and SAB have already been characterized in previous works^{13), (21}. Here, we recall some of the results of their characterization, in particular their chemical and mineralogical characteristics. Silica and alumina are the major oxides in all three samples. The SiO₂/Al₂O₃ mass ratio of SA is around 3.36. The diffractogram of SA shows that it contains Kaolinite, Illite, and Quartz²¹. PEG contains Albite, Muscovite, and Quartz. The sand (SAB), on the other hand, contains only quartz²². For SA, Y. Sawadogo et al (2022) evaluated its mineralogical composition, which indicates that SA contains 45% quartz (SiO₂), 40% kaolinite (Al₂Si₂O₅(OH)₄), and 13% illite ((K, H₃O)Al₂Si₃AlO₁₀(OH)₂)¹³.

Waste glass bottles are collected in the public landfills of Ouagadougou. The bottles were sorted from three different types of colors commonly found in public landfills, washed, and crushed before ground (Figure 1). The clay raw materials used are shown in Figure 2. The glass bottle powder particles are referenced as GP. The clay materials were crushed, ground using a jar-turner, and sieved to 100 µm. Proportions of raw materials used to formulate different swatches are shown in Table I. The different swatches were obtained by substituting PEG with glass powder.

Figure 1
Photograph of glass bottles: (a) Collected glass bottles; (b) Crushed glass bottles; (c) GP sample.

Figure 2
raw materials.

Table I
Composition of the formulated mixtures.

	P0	P10	P15	P20
SA (wt%)	50	50	50	50
SAB (wt%)	25	25	25	25
PEG (wt%)	25	15	10	5
GP (wt%)	0	10	15	20
Water (wt%)	50

Slips of different shades were shaped by casting. The slips were poured into porous plaster molds to obtain the raw specimens. Each slip was prepared with 50 wt% water relative to the dry mass of the materials, following literature recommendations¹². This water content is a judicious compromise between plasticity, homogeneity, and control of the material’s final properties. To reduce particle agglomeration and improve slip fluidity, ≈0.5% of sodium carbonate was used as deflocculant. Sodium carbonate acts as a deflocculant and provides a buffering effect on pH, thereby preventing undesirable precipitation of other components. The mixture was run through a jar-turner for 10 min at a speed of 75 rpm to obtain a homogeneous paste. Casting was carried out in plaster molds of dimensions (1x1x7) cm³ and (1x5x7) cm³. After demolding, Figure 3, specimens were kept in air at laboratory temperature, about 30ºC, for 48 h before being placed in a DHG-9023A oven set at 105 ºC for 24 h to evaporate residual water. Sintering was carried out using a NABERTHEM P330 oven, with 2 h step and a set rate of rise of 5ºC/min. Sintering was carried out at temperatures ranging from 1140ºC to 1260ºC, in steps of 40ºC. Figure 4 summarizes the different mixtures of the experimental process. Test pieces formulated according to the glass powder content and fired at different temperatures are codified in accordance with Table II.

Figure 3
Image of specimens: (a) specimens in a plaster mould; (b) specimens before sintering, and (c) specimens after sintering.

Figure 4
Experimental process.

Table II
Codification of formulated samples

	Temperature (ºC)
Samples	1140ºC	1180ºC	1220ºC	1260ºC
P0	P0-114	P0-118	P0-122	P0-126
P10	P10-114	P10-118	P10-122	P10-126
P15	P15-114	P15-118	P15-122	P15-126
P20	P20-114	P20-118	P20-122	P20-126

Chemical and mineralogical analyses of the glass powder and materials were carried out by X-ray fluorescence and X-ray diffraction, respectively. Diffractograms were obtained using a Panalycal Empyrean diffractometer, equipped with a Kα-1.5406Å copper anode and controlled by a computer with a data collector. Infrared (IR) spectroscopy of the glass powder was carried out using an IRTracer-100.

The glass powder was produced using a LAARMANN LM2000 Pulveriser ring mill, equipped with a steel grinding bowl, ring, and roller. The optimal filling volume of the grinding bowl is 1.3 L. The milling process operates through a three-dimensional vibration system at a rotational speed of 700 rpm, yielding a particle size distribution with an average fineness of approximately 75 µm.

Following milling, the powder was subjected to dry sieving using a certified AFNOR sieve with a mesh size of 100 µm, ensuring that the resulting fraction consisted of particles below this threshold.

Particle size analysis was performed in accordance with ISO 13320:2020 (Laser diffraction methods) to ensure accuracy and reproducibility of the measurements. Furthermore, PEG, SAB, and GP powders were sieved using a 100 μm mesh sieve, certified according to ISO 3310-1:2016/AFNOR standards, ensuring compliance with international requirements for test sieves and guaranteeing the removal of oversized particles.

The density of the slip is determined by the ratio between the mass of the slip and the volume it occupies.

Viscosity was evaluated using a flow cup. The Marsh funnel, standardized under ASTM D6910/D6910M and API Recommended Practice 13B-1, is a viscometer employed for the rapid determination of the apparent viscosity of suspensions. The instrument consists of a conical body with an upper diameter of approximately 152-155 mm and a total height of about 305 mm, designed to contain a nominal volume of 946 mL. At the inlet, a circular sieve of 63.5 mm diameter with a mesh size of 75 µm retains coarse particles, while the outflow occurs through a brass orifice measuring 4.76 mm in internal diameter and 50.8 mm in length. The principle of the method is based on recording the time required for a fixed volume of fluid (typically 946 mL) to flow through the funnel under gravity, which provides an index of its apparent viscosity. Measurements were conducted at controlled laboratory temperature to ensure reproducibility²³.

To ensure reproducibility and consequently statistical reliability, four samples were used for each characterization test.

Physico-chemical and mechanical properties evaluated in this work include linear shrinkage (R), mass loss (LOI), water absorption (E), porosity (n), density (d), and stress at break. The difference between the dimensions of the material before and after sintering gives the linear shrinkage. In the framework of this study, dimensional measurements were performed using a Fischer Darex caliper, offering a measuring capacity up to 200 mm, with a resolution of 1/50 mm (0.02 mm) and a tolerance of ±0.02 mm. This is determined using equation A²⁴.

$R\left(\%\right)=\frac{L_v-L_v}{L_v}\times 100$ (A)

where R is the measured shrinkage, L_v is the length measured after drying at a given temperature of 105ºC, and L_C is the length of the specimen after firing.

The specimens produced undergo mass loss during sintering. This ignition loss is determined using a NABERTHEM P330 furnace at 1000ºC. Loss on ignition (LOI) was calculated using equation B¹²:

$LOI\left(WT\%\right)=\frac{m_1-m_2}{m_1}\times 100$ (B)

m₁ is the mass of the dried sample at 105 ºC before firing, and m₂ is the mass of the sample after firing

Water absorption (E(%)) is characterized by the boiling water impregnation method in accordance with ISO 10545²⁵. Mass measurements were performed using an HZT-A300 electronic balance, a precision instrument with a maximum capacity of 1,000 g and a readability of 0.001 g. The device, powered by AC 220 V/50 Hz, is equipped with functions for tare (T/C), piece counting (PCS), percentage weighing (%), and data output/printing (PRT). A mass m₁ of each sintered specimen is immersed in boiling water for two hours. After four hours of cooling in the water at room temperature, the new mass m₂ of the wet material is measured. Mass differences give the mass of water absorbed by the material during this time. Water absorption corresponding to coefficient E is calculated using equation C.

$E\left(w\%\right)=\frac{m_2-m_1}{m_2}\times 100$ (C)

Porosity of a material is the total number of pores it contains. Pores are voids contained in materials. Open pores, which communicate with the surface of the material, interconnected pores, which communicate with each other, and closed pores, which are isolated within the material. Total porosity of the material is the sum of these types. Porosity is given by the following equation D. This equation gives the volume of the solid, which is expressed in equation E.²⁶:

$n\left(\%\right)=\frac{V_V}{V_t}\times 100$ (D)

V_v is the void volume, and V_t is the total volume of the sample. The total volume is measured directly, and the void volume is calculated according to V_v=V_t-V_s, where Vs is the solid volume:

$V_S=\frac{M_t}{{\rho }_s\left(1=w\right)}\times 100$ (E)

where M_t is the total mass of the sample, ρ_s is the density of solid particles, and W is the water content.

According to ISO 10545-5, the density of consolidated materials can be determined by hydrostatic weighing¹⁹. Material of known initial mass is weighed and then fully immersed in water. The volume of the material is deduced from the weights.

Density is determined by equation F²⁷.

$d=\frac{m_s}{m_{s+p}-m_{h\left(s+p\right)}-{\displaystyle \frac{m_{s+p}-m_s}{{\rho }_p}}}$ (F)

m_h(s+p)is the mass of the paraffin-coated sample in water, m_s+p is the mass of the paraffin-coated sample in air, r_p is paraffin density (880 kg. m^-3), m_s is the sample mass.

Three-point bending tests were carried out using a CBR-2 electrohydraulic loading press at multi-speeds of 1 mm/min and 1.27 mm/min and a power of AC220V±10% 50 Hz.

Mechanical bending strength (σ) is calculated by the following equation G²⁸:

$\sigma =\frac{3\left(F_{max}xL\right)}{2xbx{h}^2}$ (G)

where F_max is the maximum breaking force, in Newtons (N);

L is the maximum distance between supports, in millimeters (mm);

b is specimen width, in millimeters (mm);

h is specimen height, in millimeters (mm).

σ is the mechanical bending strength, in mega Pascal (MPa)

RESULTS AND DISCUSSION

Clay SA displays a fine and homogeneous particle size distribution, predominantly composed of the silt fraction. The mean diameter of the particles is 5.63 μm, with a median diameter (d₅₀) of 6.51 μm. The granulometric profile is characterized by a d₁₀ value of approximately 2 μm (2.24 μm), indicating the presence of a limited proportion of ultrafine particles. In addition, a moderate d₉₀ value of 21.17 μm is observed, confirming the absence of coarse grains (Table III).

Table III
Particle size parameters of SA.

Sample (SA)	d10 (µm)	d50 (µm)	d90 (µm)
	2.24	6.51	21.17

The results of the chemical analysis of glass powder reported in Table IV show silica (74wt%), Na₂O (15wt%), and CaO (5wt%). N. Saliou et al. (2025) reached the same conclusion, indicating that glass powder contains a high proportion of amorphous SiO₂²⁹. This composition indicates that the glass used is a soda-lime glass. Soda-lime glass is more susceptible to thermal, mechanical, and chemical damage. The presence of alkali (15wt%) and alkaline-earth (9wt%) oxides makes this glass powder a potential flux in ceramic formulation.

Table IV
Elemental chemical composition of GP in mass percentage (wt%).

Oxides	SiO2	Al2O3	CaO	K2O	MgO	Na2O	TiO2	Fe2O3	Total
Percentage (wt%)	74	1	5	-	4	15	-	-	99

It is suitable for use as a substitute for commercial or natural fluxes (feldspars, pegmatites...) in the production of ceramics in general and porcelain in particular.

Figure 5 shows the diffractogram of GP. It indicates the presence of a dome between 20º and 40º, which is characteristic of amorphous materials. Some peaks characteristics of quartz or crystallized silica can also be seen on the dome. Thus, glass powder would not exhibit any long-term order in its mineral’s atomic arrangement. An apparent background can be observed throughout the diffractogram. This could be due to factors such as environmental interactions, impurities in the material, or even background from devices. The FT-IR spectrum of the glass used (Figure 6) shows an absorption band associated with Si-O valence vibrations around 800 cm^-1 and 1000 cm^-1. This band is characteristic of the presence of silicon and oxygen in the glass structure, which would justify the chemical composition of the glass, which consists almost entirely of silica (SiO₂)²⁸. This silica represents the amorphous silica revealed by the diffractogram of the glass powder. This amorphous phase, in addition to the melting elements revealed by the chemical composition, would accentuate the amount of the melting phase in the glass powder.

Figure 5
Diffractogram of GP.

Figure 6
Infrared spectrum of the glass powder use.

A dense slip tends to flow more slowly, due to its higher viscosity. The addition of glass powder had a slight influence on slip parameters. In fact, GP acts as a degreaser in ceramic paste and slip. In tests carried out at room temperature, slips formulated with different proportions of glass powder produced the results shown in Table V. Kaolinite is composed of layered aluminum silicate sheets, which give the clay its plastic properties and its ability to bond with other materials such as glass powder grains in slip or raw material^{30), (31), (32}. These layers also play a crucial role in water retention, which is essential for maintaining the consistency of the slip³². The addition of glass powder causes the powder grains to occupy the sites of these layers, releasing the retained water³³. This leads to a drop in viscosity, making the slip less dense³⁴. The drop in viscosity is probably linked to the reduction in interactions between solid particles and the introduction of a more fluid phase, thanks to the glass³⁵. As a result, an increase in glass powder could lead to a decrease in the density and viscosity of the paste at the same time, which would justify the results obtained. However, the density and viscosity obtained are consistent with those of slips used to formulate industrial porcelain.

Table V
Density, viscosity, and water content of different nuance slips

Nuance	P0	P10	P15	P20	Literature36), (37
Density (g/cm3)	1.748±0.14	1.733±0.09	1.729±0.11	1.727±0.09	1.72-1.75
Viscosity (cSt)	15.17±0.12	13.70±0.66	12.53±0.42	9.8±0.58	7-42
Water content (wt%)	(45-50)				(40-42)

Kinematic viscosity is expressed in centistokes (cSt). 1 cSt corresponds to 10^-6 m²/s.

Firing shrinkage was calculated using equation A, and its evolution as a function of temperature and GP content is shown in Figure 7. For all grades, shrinkage increases with temperature and GP content. It is lowest at 1140ºC and highest at 1260ºC. This increase is attributed to the removal of residual and structural water from the clay and to particle densification, with or without partial fusion. All the specimens show shrinkage levels that are normal for porcelain (less than 15%).

Figure 7
Evolution of linear shrinkage with temperature and GP content.

Figure 8 shows the results of mass loss of the various specimens as a function of temperature and glass powder content. Mass losses increase with temperature and GP content for all grades except at 1260ºC, from P10 to P20, when a decrease is observed. However, a slight generalized mass decrease is observed on all specimens with glass powder addition. The greatest loss (4.25±0.48wt%) was observed on specimens without added GP (P0). Conversely, the lowest loss (3.6±0.34wt%) is obtained with the addition of a large quantity of GP (P20). The evaporation of chemically and physically bound water, together with the decomposition of certain clay components, accounts for the higher mass loss observed in specimens with low glass powder content. For mass loss, GP is not concerned as temperature increases; an increase in the glass powder content is equivalent to a decrease in the clay content in the specimen, and this result leads to a reduction in mass loss. Thus, depending on the temperature and the quantity of GP, for the control, P0, except 1140ºC and 1220ºC, the losses obtained are in line with the literature, which requires a value between 4wt% and 4.5wt%³⁸. Following the introduction of GP, it was established that only P20-122 falls within the required range.

Figure 8
Evolution of mass loss with temperature and GP content.

Figure 9 shows the evolution of water absorption as a function of temperature and glass powder content. Absorption decreases sharply with increasing temperature. It tends to cancel out at temperatures above 1220ºC when the glass powder content exceeds 15wt%. As GP content increases, water absorption also decreases. This could be explained by the reduction in internal pores following pegmatite melting. For P0, the value required by the literature (0.5wt%) is obtained from 1260ºC. This same value is obtained with the addition of GP at 1220ºC, which is a relatively low temperature. This means firing uses less energy, saving money (time and production costs), and reducing carbon emissions. This phenomenon can be explained by the melting of the amorphous phase of the glass powder, which begins to soften at around 700ºC. This softening is followed by melting, which brings the particles closer together and reduces the number of pores. A reduction in open pore volume means less water can be absorbed by the material.

Figure 9
Evolution of water absorption with temperature and glass powder content.

The evolution of specimen density and porosity with GP content and temperature is shown in Figures 10 and 11, respectively. Increasing temperature leads to an increase in density with or without the addition of GP. The addition of GP also densifies the material. P0 has a higher density (2.36±0.16 g/cm³) at 1260ºC, and with the addition of 20wt% GP at 1220ºC, a density of 2.46±0.2 g/cm³ is obtained. Porcelain literature calls for a density greater than 2.3 g/cm³⁽⁹; thus, P20-122 would be in line with the literature. In fact, feldspar melting and GP lead to pore closure and densification of the material. Porosity evolves inversely to density, decreasing with both increasing temperature and GP addition. This decrease could be justified by the densification of the specimens and the reorganization of the particles following the rapid melting of fluxes to occupy the pores and voids between particles. P0-126 has a porosity of less than 0.5±0.02%, while the addition of GP enables porosities below 0.5±0.01% (the maximum value required in the literature)²⁰ to be achieved at 1220ºC with the addition of 20% GP (P20-122).

Figure 10
Evolution of density with temperature and glass powder content.

Figure 11
Evolution of porosity with temperature and glass powder content.

Measuring the stress at the break of sintered materials is of great importance in assessing their mechanical behavior in use. The tests involved only specimens fired at 1220ºC, except the control specimen, which was fired at 1260ºC in accordance with the literature³⁹. 1220ºC is the optimum temperature, which gives convincing results considering previous tests. The results obtained are shown in Figure 12. These results show an increase in material strength with the addition of glass powder. For materials fired at 1260ºC and 1220ºC, the strength increases from 39±0.38 MPa for P0 to 76±1.4 MPa for P20. The addition of glass powder influences the final structure and properties of the material. Glass powder strengthens the matrix, improving cohesion and stress distribution. Its effect on the microstructure is evident in the reduction of microporosity and the increase in bulk density, consistent with the observed trends in density and porosity. The ceramics industry typically requires a flexural strength exceeding 35 MPa⁹. All specimens developed in this study meet or surpass this requirement, in agreement with values reported in the literature.

Figure 12
Variation of three-point flexural strength with GP content: Fired at 1260ºC for 0% and 1220ºC for others.

X-ray diffraction of the P0 sample sintered at 1260ºC, marked P0-126, and P10, P15, and P20 samples sintered at 1220ºC, marked P10-122, P15-122, and P20-122, respectively, allowed the diffractogram of Figure 13 to be obtained. P0-126 is made up of mullite and quartz. In addition to mullite and quartz, we see the appearance of new phases after the addition of glass powder, such as anorthite. Mullite, anorthite, and residual quartz, which are the main phases, play a crucial role in the strength of the material. Indeed, although the formation of anorthite following recrystallization of the liquid phase partially reduces the crystallization of mullite, the vitrification of anorthite compensates for mullite, increasing the mechanical strength of the ceramic³⁸. As for residual quartz, it improves rigidity. The presence of anorthite in a ceramic matrix has been demonstrated to regulate crack propagation and the nature of failure mechanisms, thereby functioning as crack deflectors and mechanical bridges. It has been posited by numerous researchers that anorthite would result in a redistribution of the stress field under load and an augmentation of mechanical strength^{39), (40), (41), (42}.

Figure 13
Powder diffractograms of specimens P0-126, P10-122, P15-122, and P20-122.

The more abundant the viscous flux, the faster the dissolution of the phases and the subsequent recrystallization with vitrification and densification of the material.

At temperatures above 1260ºC, the microstructure consists of quartz and mullite for 0% glass powder. These crystalline phases are obtained when the raw material mixture consists of kaolinitic clay and industrial feldspar⁹. Kaolinite passes through intermediate phases before reaching mullite. Kaolinite is transformed into metakaolinite at around 700ºC, after which mullite crystals begin to appear at 1000ºC⁴³. The addition of powder results in the addition of new chemical elements (CaO; MgO) to the initial mixture. As a result, new phases may form during sintering due to reactions between the new elements introduced by the glass powder and the chemical elements in the other raw materials, or in situ.

Analysis of the diffractograms reveals a notable change in the peaks located around 16.39º and 21.64º, corresponding respectively to the crystalline phases of mullite and anorthite. A gradual decrease in the intensity of the mullite peak is observed, while that of anorthite intensifies with the increase in glass powder content in the mixture.

This trend would explain a competitive interaction between the two phases during heat treatment. Mullite, which is generally formed at high temperatures from aluminosilicates, appears to be inhibited by the increasing presence of glass components, such as alkali oxides (Na₂O) and alkaline earth oxides (CaO), which promote the crystallization of anorthite.

The increased appearance of anorthite at the expense of mullite, therefore, reflects a reorganization of the mineral phases caused by the modified chemical composition.

Quantitative analysis of phases by Rietveld refinements applied to the diffractograms of samples P0-126, P10-122, P15-122, and P20-122 was summarized in Table VI. The control sample P0-126 reveals two phase compositions dominated by mullite and quartz, while the samples modified by the addition of glass powder (P10-122, P15-122, P20-122) show an additional phase: anorthite, in addition to mullite and quartz. Results of this analysis highlight a significant change in mineralogical composition depending on the content of the glass powder. The gradual introduction of the latter leads to a decrease in the mass fractions of quartz and mullite, accompanied by a notable increase in the anorthite content. This behavior can be attributed to the high content of alkali and alkaline earth oxides (particularly CaO and Na₂O) in the glass powder, which act as high-temperature fluxes. These oxides promote the formation of the anorthite phase (CaAl₂Si₂O₈) through a reaction between silica, alumina, and calcium components, to the detriment of mullite and residual quartz.

Table VI
Mineralogical compositions of samples by quantitative Rietveld refinements

samples	Quartz (wt%)	Mullite (wt%)	Anorthite (wt%)
P0-126	76.2	23.8	00
P10-122	60.5	8.72	30.8
P15-122	51.70	3.25	45
P20-122	46.6	3.58	49.80

Furthermore, the reduction in mullite can also be explained by a relative dilution of alumina in the matrix, due to the addition of amorphous silica from the glass powder. This chemical redistribution modifies crystallization equilibria and directs formation toward feldspathic phases such as anorthite, which are thermodynamically favored in environments rich in CaO and SiO₂^{44), (45}.

Thus, the addition of glass powder is not limited to a simple vitrification effect; it causes a mineralogical restructuring of the system, directly influencing the nature and proportion of the crystalline phases formed. This structural evolution is probably responsible for the modification of the physicochemical, mechanical, thermal, and microstructural properties of the porcelains obtained.

According to Table IV, the chemical composition of glass powder revealed the presence of elements such as Na, Ca, and Mg. These elements therefore affect the sintering parameters, i.e., densification rate, eutectic point, and crystallization rate³⁸. With a melting temperature estimated at around 700ºC, the availability of glass powder makes it an asset for ceramic materials⁴⁶.

SA, consisting of kaolinite, quartz, and illite, could provide plasticity to the paste and the slip as well as mechanical strength to raw pieces (preforms). According to Zahide Bayer Ozturk et al (2024), the production of quality porcelain requires the use of kaolin to ensure the plasticity of the mixture, feldspar as a flux to lower the firing temperature, and quartz to reinforce the structure of the final product⁴⁷. Given the physico-chemical properties of SA, SAB, and GP, they could be favorable to the production of quality porcelain.

According to O. R. Njindam et al (2018), quartz is a residual mineral in raw clay and glass powder²⁰. During firing, kaolin undergoes dehydroxylation to form metakaolin from 700ºC, which transforms into unstable spinel that subsequently becomes mullite around 1000ºC. These transformations are obtained in accordance with the reaction equations H and I⁴⁸. The change in color of the specimens (Figure 3) is certainly linked to the physico-chemical transformations that occur during firing, particularly the dehydroxylation of the kaolinite, while the fusion of the silica contained in the glass darkens the surface of the material.

The melting of the amorphous phase of the glass powder, which begins at relatively low temperatures, gives rise to a viscous liquid that densifies the material, reducing open porosity and improving certain parameters (density, flexural strength, water absorption) of the final material. This liquid phase is further intensified by the melting of the Na₂O and CaO oxides contained in the GP. The presence of CaO does not go unnoticed during sintering. (O. R. Njindam et al.; 2024) revealed the appearance of certain phases other than mullite and quartz on the finished product. These include anorthite (Na, Ca),(Si, Al)₄O₈; wollastonite (CaSiO₃), and plagioclase, a group of minerals that form a series of solid solutions ranging from pure albite (NaAlSi₃O₈) to pure anorthite (CaAl₂Si₂O₈)⁴⁸. While the transformation from metakaolinite to mullite in industrial ceramics is virtually complete, this is not the case when glass powder is added. In fact, part of this metakaolinite reacts with the heat (CaO) to produce anorthite according to equation J. This anorthite can also be obtained by the reaction between the mullite formed and the hot (CaO) according to reaction equation K²⁰.

$A{l}_2{O}_3.2Si{O}_2.2H_{2}O\left(kaolin\right)\to A{l}_2{O}_3.2Si{O}_2\left(metakaolin\right)+2H_{2}O$ (H)

$A{l}_2{O}_3.2Si{O}_2\left(metakaolin\right)\to 1/3\left(3A{l}_2{O}_3.2SiO\right)\left(Mullite\right)+4/3Si{O}_2$ (I)

$A{l}_2{O}_3.2Si{O}_2+CaO\to CaO.A{l}_2{O}_3.2Si{O}_2$ (J)

$3A{l}_2{O}_3.2Si{O}_2+3CaO+4Si{O}_2\to 3\left(CaO.A{l}_2{O}_3.2Si{O}_2\right)\left(Anorthite\right)$ (K)

The P0-126 control, formulated with only clay, pegmatite, and sand, confirmed that these local materials are suitable for porcelain formulation³⁶. However, these elements give satisfactory results from 1260ºC upwards; well before the use of glass powder, the density of materials increases with temperature. At industrial porcelain firing temperatures (T ≥ 1260ºC), the densities reach the value of 2.36±0.2 g/cm³, superior 0.2 g/cm³, thus lying within the range of the value required in the literature³⁹. This growth is due to the elimination of open porosity as heating progresses. This elimination of porosity is explained by the formation of liquid phases during sintering above 1050ºC, promoting the densification process⁴⁹. These phases block the interconnected pores, making the material increasingly dense. From 1260ºC onwards, porosity is essentially equal to zero, and the apparent density of the material is around 2.46 g/cm^3, making it suitable for porcelain²². Densification is due to the formation of viscous flux from the melting elements or minerals of the raw materials.

Compared with P0-126, P20-122 shows similar evolution or even better results from 1220ºC onwards. Stress at break is 76±1.4 MPa for P20-122 versus 39±0.38 MPa for P0-126. Density and porosity are proportional and show similar results to P₀, which presents results in line with industrial porcelain. Porosity of 0.5±0.02 % for P0-126 and 0.3±0.01 % for P20-122; density of 2.36±0.16 g/cm³ for P0-126 and 2.46 g/cm³ for P20-122. The addition of glass powder resulted in an increase in density and a decrease in porosity. This phenomenon is mainly due to the glassy phase generated by the melting of the GP, which fills the voids between the particles, making the material denser and less porous. Reducing the porosity of the materials has contributed to a significant reduction in the amount of water absorbed. Indeed, for 5 wt% glass powders, the material absorbs up to 12±0.21wt% water for materials fired at 1220ºC. On the other hand, at the same temperature, the addition of 20wt% glass powder reduces the amount of water absorbed to 0.3±0.01 wt%. Rietveld refinement confirmed the crystalline phases revealed by X-ray diffraction and their quantifications. It shows a change in the balance between mullite, quartz, and anorthite due to an increase in the glass fraction. This corroborates the evolution of physical and mechanical properties.

The results obtained in this study are confirmed by previous work on the beneficial role of glass in ceramic formulations, while also providing significant improvements. Njindam et al. (2024) reported densities in the range of 2.32-2.36 g/cm³ and residual porosities of 1.5-2.5 % for porcelains incorporating recycled glass⁴⁸, while Darweesh (2019) observed water absorption values between 0.8wt% and 1.5 wt% for compositions containing 20-25 wt% glass⁵⁰. Coşkun et al. (2023) further indicated flexural strengths of 40-55 MPa in optimized porcelains⁵¹. In comparison, the present work achieved a density of 2.46±0.2 g/cm^3, a porosity reduced to 0.5±0.01%, a nearly negligible water absorption of 0.3±0.01wt%, and a flexural strength of 76±1.4 MPa, almost double the minimum values generally reported. These excellent results demonstrate that the incorporation of glass at 20 wt% promotes enhanced densification and provides exceptional mechanical reinforcement to the ceramic matrix.

The incorporation of glass powder resulted in a reduction of the firing temperature by approximately 40ºC. This reduction is of great significance in an industrial context characterized by mounting energy challenges. The utilization of glass powders in the production of porcelain has been demonstrated to result in a reduction in energy consumption during the formulation process. This has been shown to engender significant economic and environmental benefits.

The originality of this study lies in the specific choice of glass and the integration with local raw materials. Whereas most recent investigations have focused on industrial glass sources such as automotive or flat glass, this work explores the valorization of bottle glass, an abundant, growing, and problematic waste stream. This approach not only improves the technological properties of porcelain and reduces energy consumption by enabling firing at lower temperatures compared to conventional formulations but also contributes to a strategy of environmental sustainability and circular economy. By demonstrating that it is possible to exceed the traditional limits of glass incorporation in porcelains while simultaneously exploiting local resources and abundant waste materials.

CONCLUSION

The incorporation of glass powder, rich in fluxes such as calcium oxide (CaO) and sodium oxide (Na₂O) and presenting an amorphous phase, makes it possible to lower the sintering temperature of porcelain. The specimens show a gradual improvement in physicochemical and mechanical properties as the glass powder content increases. In addition to the traditional phases (mullite, quartz), the formation of anorthite enhances the density and strength of the material.

Indeed, grade P20-122 shows results in line with the literature (13±0.36% shrinkage, 3.93±0.27wt% mass loss, 0.3±0.01wt% water absorption, 0.5±0.01% porosity, 2.46±0.2 g/cm³ density and 76±1.4 MPa flexural strength). Using 20wt% (which is a relatively high content) of glass powder combined with clay raw materials from Burkina Faso makes it possible to obtain porcelain at relatively low temperatures, and above all, helps to combat bottle waste in the environment.

The formulated porcelains exhibit 0.2% water absorption and 76 MPa mechanical resistance, rendering them suitable for a range of applications, including tableware, sanitary ware, and electrical insulators. However, a microstructural study could be carried out to understand the behavior of the different phases within the porcelain. A study of the behavior of porcelain in the face of aggressive agents would also be necessary.

ACKNOWLEDGMENTS

The authors would like to express their gratitude to the “Bureau des Mines et de la Géologie du Burkina Faso (BUMIGEB)” for grinding our raw materials.

DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Edited by

• AE:
  Daniel Zanetti de Florio

Publication Dates

• Publication in this collection
  20 July 2026
• Date of issue
  2026

History

• Received
  18 Jan 2026
• Reviewed
  13 Mar 2026
• Accepted
  15 Apr 2026