Synergistic effects of açaí seed ash and thermal processing on the physico-mechanical properties of ceramic blocks

ABSTRACT

This study evaluated the influence of firing temperature on the physical and mechanical behavior of red ceramic blocks produced with partial replacement of clayey soil by açaí seed ash (ASA) at contents of 0%, 5%, 10%, and 15%. Raw materials were characterized through chemical, mineralogical, thermal, and physical analyses. The blocks were fired at 950 °C, 1050 °C, and 1150 °C. At 950 °C, ASA incorporation reduced compressive strength and bulk density while increasing water absorption, porosity, and initial suction. However, from 1050 °C onward, these properties improved, indicating greater densification of the ceramic matrix. The mixture containing 15% ASA fired at 1150 °C showed the best performance, with higher compressive strength and lower porosity and water absorption. The formation of a liquid phase at higher temperatures contributed to pore filling and grain bonding. ASA incorporation did not affect efflorescence. The results demonstrate that the performance of the ceramic blocks depends on the combined effect of residue content and firing temperature. Additionally, the reuse of ASA reduces clay extraction and promotes the valorization of an abundant agro-industrial residue from the Amazon region, contributing to more sustainable ceramic production.

Keywords:
Açai seed ash; Agro-industrial waste; Mechanical properties; Porosity.

1. INTRODUCTION

The states of Pará and Amazonas are Brazil’s leading producers of açaí fruit (Euterpe Oleracea). According to the IBGE [¹], around 1.7 million tons of fruit were produced in 2022. After pulp extraction, the seed, which represents about 85% of the fruit’s volume, remains as a by-product and lacks an adequate destination or disposal after processing the fruit. The seed is generally released irregularly on roads, in public places, at fairs and vacant lots [²,³,⁴].

Because of the high volume generated, açaí seed waste requires proper disposal and utilization in various sectors. Some studies have investigated the use of açaí seeds in ethanol production, giving their lignocellulosic biomass properties [⁵, ⁶]. The calorific value of açaí seeds makes them an appealing biomass source, offering a significant supply of thermal energy that can be further increased through drying [⁷]. This utilization represents an alternative energy source that helps mitigate the effects of fossil fuel consumption. CORDEIRO et al. [⁸] researched the potential utilization of açaí seed ash in the construction industry and affirmed that the residual ash has physical properties adequate for its use as a non-reactive additive in cementitious matrices.

Studies on agro-industrial residues indicate that their physicochemical characteristics, such as chemical composition and reactivity, are fundamental in defining their interaction with surrounding matrices and their influence on microstructural development and environmental performance, especially in systems subjected to thermal or chemical activation. This behavior has been widely investigated in alkali-activated materials, as reported by [⁹]; however, similar principles can be considered when evaluating the role of ash in ceramic matrices subjected to high firing temperatures.

The açaí seed can be used as a source of heat (or biomass) in ceramic manufacturing plants. The seed has a high calorific value and is widely employed as an energy source. Ash is the final residue of the process. The ash presents a high pH value and cannot be directly incorporated into the cementitious matrix, as it tends to reduce the pH of the mixture.

Ceramic block masonry, on the other hand, is prominent due to the large number of materials involved and is the most common technique used in Brazil for masonry. The manufacture of ceramic blocks results in significant environmental impacts due to the use of clayey soils. Although soil extraction is not classified as a polluting activity, it leads to sedimentation in rivers and environmental degradation through topographical modifications and deforestation in extraction zones [¹⁰, ¹¹].

Regarding the quality of ceramic blocks, PINHEIRO and HOLANDA [¹²] observed that firing red ceramics is a fundamental step that significantly affects the product quality. SALEIRO and HOLANDA [¹³] affirmed that the burning process and the composition of the raw materials are the main factors to consider during the production process. Consequently, research aimed at minimizing the environmental impacts of industrial waste has been consolidated over the years [¹⁴,¹⁵,¹⁶]. At the same time, the red ceramic industry is a promising segment for incorporating waste materials due to its capacity to absorb discarded materials. Ceramic masses are heterogeneous by nature and, therefore, tolerant to adding various wastes before burning [¹⁷, ¹⁸].

Research on the use of waste materials in ceramic blocks has gained prominence in recent years [¹⁹,²⁰,²¹,²²]. For example, sugarcane bagasse ash (SCBA) has been shown to produce red ceramics with acceptable physical and mechanical properties when used in moderate proportions (≈5–10 wt%), contributing to resource conservation and waste valorization, although higher ash contents may compromise performance. Similar findings with rice husk ash (RHA) indicate beneficial effects on properties such as shrinkage, density, and water absorption, suggesting that such residues can diversify raw material sources for ceramics.

Research on malt bagasse ash highlights technical challenges, as ash additions beyond limited thresholds tend to reduce mechanical strength and alter porosity and dimensional stability, indicating narrow windows of practical incorporation. Additionally, recent work on ceramic tiles incorporating SCBA suggests that controlled ash content can maintain or even enhance microstructural features without degrading key properties. Collectively, these studies reinforce that while moderate inclusion of agro-industrial ashes can enhance sustainability and retain functional properties, optimization of ash content and processing conditions remains critical to avoid negative effects on mechanical integrity and durability [²³,²⁴,²⁵,²⁶].

It is noted that the incorporation of ASA residue into ceramic blocks mitigates the problem of residue disposal and the extraction of natural resources at the same time. The ceramic matrix can encapsulate the ASA, which can partially replace natural clayey soil, and stabilize the waste under high temperatures.

This study aimed to evaluate the performance of ceramic blocks produced using açaí seed residual ash (ASA) as a partial replacement for clayey soil, at different incorporation levels. The research involved the physico-chemical characterization of the ash, the analysis of the technological behavior of the ceramic blocks during the shaping and firing stages, and the evaluation of the physical and mechanical properties of the resulting products. In addition, the work seeks to reduce the environmental impacts associated with clayey soil extraction and the improper disposal of ASA, while also exploring the physico-mechanical characteristics of the red ceramics produced with the incorporation of ash under thermal treatment, across variations in firing temperature.

It is important to highlight that the use of ash as an alternative material for red ceramic production occurs at a delicate moment for the Brazilian construction industry, whose production chain still relies heavily on non-renewable natural resources and presents high energy consumption, both in the extraction and transportation of raw materials and in manufacturing processes. Therefore, incorporating agro-industrial residues emerges as a viable strategy to reduce the environmental impacts associated with the production of traditional ceramic materials.

In a broader context of environmental protection and sustainable development, the United Nations established 17 Sustainable Development Goals (SDGs) to be achieved by 2030. Among these goals, the present work contributes directly to:

• SDG 9 (Industry, Innovation and Infrastructure): by promoting technological innovation in the ceramic sector through the use of an abundant agro-industrial residue from the Amazon region;

• SDG 11 (Sustainable Cities and Communities): by encouraging the production of sustainable materials with lower consumption of natural resources through the use of residues;

• SDG 12 (Responsible Consumption and Production): by fostering the circular economy, transforming an environmental liability into a productive input for the industry;

• SDG 13 (Climate Action): by proposing the replacement of natural raw materials and encouraging sustainable production practices in the construction sector.

Thus, this research not only evaluates the technical feasibility of incorporating açaí seed ash into the production of ceramic blocks but also highlights the potential for valorising this regional residue, thereby reducing environmental impacts and strengthening the local economy through socioeconomic benefits.

2. MATERIALS AND METHODS

To obtain the expected results, this study was divided into two stages: 1) Collection and characterization of the materials; 2) Production of ceramic blocks; and 3) Analysis and discussion of results.

2.1. Collection and characterization of the materials

The following materials were used to develop this work: clayey soil and açaí seed ash (ASA), which were collected in the municipalities of Ourém and Castanhal, in the state of Pará, respectively.

The ASA samples were collected from a fruit pulp industry, that generates one ton of ASA monthly. In this company, part of the seed is reused as biomass for boiler fuel. Before burning, the residue was spread on tarps and undergoes an open-air drying process. After drying, the residue is burned at an average temperature of 600°C, generating heat for the boilers. However, the temperature and calcination time are not controlled during the process, so the material is referred to as residual ash.

The ASA samples were transported to the laboratory, where they were ground in a 120-liter porcelain ball mill (model SL-36/120). The milling methodology proposed by POUEY [²⁷] consists of using a ball-to-ASA ratio of 5 kg of balls per 1 kg of ASA, with 1/3 large balls and 2/3 small balls. This proportion is necessary to obtain a material with a grain size passing through a #200 sieve (0.075 mm mesh). The grinding time was fixed at three hours, and obtaining about 50 kilos of ash was possible by this process. The clayey soil samples were collected near the ceramic block factory and subjected to seasoning, drying, mixing, and crushing.

X-ray fluorescence spectrometry (XFS) was used to determine the chemical compositions of clayey soil and ASA. Regarding physical characteristics, the laser granulometry test was carried out, and the Atterberg limits of the mixtures were determined based on NBR 6459 [²⁸] and NBR 7180 [²⁹] standards. The mineralogical compositions of the ASA samples were determined using X-ray diffraction method (XRD). Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses were carried out to be possible to observe the behavior of ASA and clayey samples upon increasing temperature, in addition to differential scanning calorimetry (DSC).

2.2. Production and characterization of ceramic blocks

The blocks production were performed in a red ceramic industry in the municipality of Ourém-PA. The replacement contents of 0%, 5%, 10%, and 15% of clayey soil by ASA in the compositions of ceramic blocks were defined in this study. These contents were chosen based on research by MARINS et al. [³⁰] and FERREIRA et al. [³¹], as well as to cover a representative range from low to relatively high incorporation levels, allowing the evaluation of the progressive influence of the residue on the technological properties of the material without compromising its workability and structural integrity.

Ceramic blocks were manufactured in the dimensions of 9 × 14 × 19 cm, in accordance with the NBR 15270 – 1 [³²]. Initially, the clayey soil was transported through conveyor belts to the crusher, where the ASA samples were added. Then, the mixtures were passed through the mixer and laminator to homogenise the materials. Finally, the material was pressed and molded by extrusion (Figure 1).

Figure 1
Block manufacturing - (a) Mixing of materials; (b) process of manufacturing and obtaining the blocks; (c) and (d) visual appearance of an example of each class of formulated blocks, in the raw state.

Based on the research by MENDES et al. [³³] and PEDROTI et al. [³⁴], the amount of water in the order of 10% was defined for all samples, due to ASA’s hydrophilic characteristics. The blocks were produced with a mix of clayey soil and ASA at different temperature levels 950 °C, 1050 °C and 1150 °C, so it was possible to verify the effect of the firing temperature on the physical-mechanical behavior of the blocks. These temperatures were selected to represent typical industrial firing conditions for red ceramics and to cover different stages of the sintering process.

Each block group was burned at the proposed temperatures for 48 hours in mobile ovens. The method was based on SANTOS [³⁵]. NBR 15270-1 [³²] and NBR 15270-2 [³⁶] establish geometric, physical, and mechanical parameters to evaluate the main parameters of ceramic blocks. Among the analyses performed, the following were evaluated: geometric characteristics, water absorption rates, initial water absorption, apparent specific mass, compressive strength, and the pathological manifestation of efflorescence. Tests were also carried out to determine the apparent porosity of the blocks following the ASTM 373:88 [³⁷] standard and scanning electron microscopy (SEM) analyses were carried out to identify and examine the pores present in each sample.

3. RESULTS AND DISCUSSIONS

3.1. Materials characterization

Table 1 shows the results of the chemical analysis of the materials used in this study. It is possible to see that both clayey soil and ASA exhibit chemical compositions similar to those reported in the literature [⁸, ³⁸,³⁹,⁴⁰,⁴¹]. Regarding the LOI value of ASA, a value exceeding 30% was found, which can be attributed to residual elements that decomposed, such as organic matter. This is expected behavior when analyzing residual ashes from an uncontrolled burning process. The oxide contents presented in Table 1 were obtained by X-ray fluorescence (XRF) analysis and were not normalized to 100%. The lower total percentage observed for ASA is mainly attributable to its high loss on ignition and the presence of residual, unquantified phases, which are typical of ashes produced by uncontrolled burning processes.

Table 1
ASA and clayey soil chemical composition.

SAMPLE	COMPOSITION
	SiO2	Al2O3	Fe2O3	CaO	MgO	K2O	Na2O	TiO2	MnO	P2O5	SO3	Others	LOI
Clayey Soil	71.84	15.77	2.16	0.04	0.37	1.15	0.26	1.06	0.02	0.05	0.00	0.00	6.93
ASA	8.31	0.43	1.12	1.04	0.97	4.51	0.00	0.10	0.21	2.80	0.65	< 0.1	34.45

• *LOI – loss on ignition.

The ASA and clayey soil samples were analysed using laser granulometry, and it was observed that the soil has an average diameter of approximately 5 µm, indicating a high degree of fineness. The clayey soil consists of 43% clay (Ø < 2 µm), 29% silt (2 µm < Ø < 20 µm), and 27% sand (20 µm < Ø < 200 µm). Due to the clay and sand content in the sample, the raw material was considered ideal for producing ceramic blocks. The percentages are within the limits proposed by SANTOS [⁴²]: 15% to 45% for clay and 15% to 30% for sand.

It was also possible to note that the grinding processing produced a coarse material. It might be considered a coarse material, compared to clayey soil, with a small content (4%) of fine particles (Ø < 2 µm), 63% of medium particles (2 µm < Ø < 20 µm) and 32% of coarse particles (20 µm < Ø). The waste was classified as non-plastic, and, according to PRACIDELLI and MELCHIADES [⁴³], when moderately incorporated into ceramic compositions, it may contribute to reducing the effects of drying and firing shrinkage.

Regarding the Atterberg limits (Table 2), it was possible to observe that the samples containing only soil presented the highest plasticity index (24.02%) and liquidity and plasticity indexes. The sample containing 15% ASA and 85% clayey soil showed the lowest plasticity index (19.26%). FERREIRA et al. [³¹] noted that the higher the Plasticity Index, the greater the need to add water to the material to achieve the desired plasticity. The decrease in Plasticity Index does not affect the workability, but it is satisfactory since lower water contents required contribute to reducing the ceramic pieces’ shrinkage.

Table 2
Atterberg limits of samples.

SOIL CONTENT (% VOLUME)	ASA CONTENT (% VOLUME)	ATTERBERG LIMITS (%)
		LL	PL	PI*
100.00	0.00	45.19	21.17	24.02
95.00	5.00	42.57	20.49	22.08
90.00	10.00	40.84	20.08	20.76
85.00	15.00	39.63	20.37	19.26

• *PI=LL-PL; PI - plasticity index; LL - liquid limit; LP - plasticity limit.

Figure 2 shows the X-ray diffractograms (XRD) of the clayey soil and the ASA samples, respectively. It is possible to observe the presence of quartz (SiO2), kaolinite (Al₂Si₂O₅(OH)₄), mica, and feldspar in soil samples. Quartz is a non-plastic raw material that remains inert during sintering [⁴⁴]. Kaolinite may help in the plasticity of clayey soil when in contact with water. Mica and alkaline feldspar, on the other hand, act as fluxes since their structures contain alkaline oxides. The ASA diffractogram shows an amorphous halo and crystalline peaks of quartz, qualitatively indicating the material’s amorphous nature.

Figure 2
Clayey soil (left) and ASA (right) – XRD.

The thermograms (TG), the second derivative (SDTG), and the differential scanning calorimetry (DSC) curves of the clayey soil and ASA are shown in Figure 3. The DSC curve indicates that the soil exhibits kaolin characteristics, consistent with OLIVEIRA and BARBOSA [⁴⁵] and SANTOS [⁴²]. Between 50 °C and 137 °C, there is a slight endothermic peak, possibly referring to the loss of surface moisture. Another small endothermic peak is observed between 450 °C and 620 °C, which may be associated with the transformation of kaolinite to metakaolinite [⁴⁶]. Around 500 °C, the elimination of organic matter and loss of interstitial water (hydroxyls) occurs, and from the temperature of 620 °C onwards, the mass loss curve is relatively stable, which indicates little mass loss from temperatures close to 700 °C. Up to 1200 °C, the total mass loss of the sample was 12.20%. Around 950 °C, there is an exothermic peak, which SANTOS [⁴²] assumes results from the transformation of metakaolinite into mullite.

Figure 3
Clayey soil (left) and ASA (right) - Thermal analysis.

When analyzing the ASA sample thermogram, a mass loss of around 7% is observed, between the range of 25 to 112 °C, probably due to the elimination of water in the sample. The ASA TG curve 4 may represent the slow degradation of the remaining lignin (still present) in the sample. A small endothermic peak between 850 °C and 950 °C is presented as observed by SEYE et al. [⁴⁷], where they assume that the fact may be related to the melting of the ashes. The TG curve indicates that up to 1200 °C, the total mass loss of the sample was 37.2%.

3.2. Physical and mechanical analysis of blocks

Table 3 presents the geometric characteristics of blocks with and without ASA. The results indicate that the blocks’ geometric characteristics follow NBR 15270-1 [³²] and neither the ASA replacement nor the temperature affected the geometric parameters measured.

Table 3
Blocks – geometric characteristics.

TEMP. (°C)	BLOCKS	MEASUREMENT OF THE SIDES (mm)			DEVIATION RELATION TO THE SIDES (mm)	STRAIGHTNESS OF THE SIDES (mm)	MEASUREMENTS OF EXTERNAL SIDES AND HOLLOWS (mm)
		WIDTH	HEIGHT	LENGTH			EXTERNAL SIDES	HOLLOWS	AVERAGE*
950	BREF	89.83	138.28	187.95	1.81	1.52	7.22	6.01	21.61
	B5C	88.96	138.72	187.67	2.13	1.35	7.15	6.14	21.71
	B10C	88.84	138.41	187.52	1.81	1.38	7.17	6.02	20.90
	B15C	89.73	138.63	187.84	1.66	1.44	7.24	6.15	21.23
1050	BREF	89.32	138.16	187.65	1.71	1.41	7.36	6.20	21.35
	B5C	88.92	138.55	187.81	1.82	1.42	7.23	6.17	20.88
	B10C	89.13	138.30	187.97	1.64	1.51	7.19	6.27	21.15
	B15C	88.55	138.42	187.71	1.68	1.33	6.84	6.16	20.72
1150	BREF	88.40	137.36	187.24	1.67	1.44	7.25	6.21	20.62
	B5C	88.34	137.32	187.13	1.73	1.36	7.22	6.14	20.76
	B10C	88.12	137.52	187.24	1.73	1.20	6.91	6.15	20.93
	B15C	88.11	137.43	187.16	1.51	1.52	7.05	6.02	20.81

• *Average of the minimum sum of external and internal thicknesses in the cross-section.

Table 4 presents data on the physical characterization of the blocks with and without ASA. The results indicate a difference between the behavior of the blocks burned at 950 °C and compared with the others. The lower the sintering temperature, the higher the apparent porosity. The use of ASA decreases the apparent porosity in blocks burned at higher temperatures.

Table 4
Blocks – physical characteristics.

TEMP. (°C)	BLOCKS	APPARENT POROSITY [%]	APPARENT SPECIFIC MASS [g/cm3]	WATER ABSORPTION INDEX [%]	INITIAL WATER ABSORPTION (IWA) [g/194cm2/min]
950	BREF	28.68 ± 0.228	1.82 ± 0.082	18.22 ± 0.089	20.82 ± 1.204
	B5C	29.54 ± 0.207	1.81 ± 0.074	19.37 ± 0.175	23.05 ± 1.520
	B10C	30.26 ± 0.250	1.78 ± 0.096	20.12 ± 0.292	23.18 ± 1.549
	B15C	31.66 ± 0.270	1.73 ± 0.107	21.88 ± 0.271	26.52 ± 1.665
1050	BREF	27.50 ± 0.187	1.84 ± 0.089	18.13 ± 0.121	21.08 ± 1.563
	B5C	26.08 ± 0.192	1.87 ± 0.091	16.75 ± 0.242	22.78 ± 2.040
	B10C	24.22 ± 0.192	1.88 ± 0.073	17.43 ± 0.307	22.77 ± 2.079
	B15C	24.92 ± 0.327	1.93 ± 0.092	15.20 ± 0.261	23.13 ± 1.881
1150	BREF	23.48 ± 0.164	1.85 ± 0.067	14.37 ± 0.121	18.25 ± 1.539
	B5C	22.26 ± 0.230	1.92 ± 0.135	13.72 ± 0.354	17.72 ± 2.003
	B10C	20.38 ± 0.303	1.94 ± 0.108	12.75 ± 0.266	16.70 ± 1.905
	B15C	16.70 ± 0.721	1.92 ± 0.098	11.62 ± 0.534	12.32 ± 2.040

It is noted that the higher the ash content, the greater the apparent porosity of the material at a temperature of 950 °C. Water absorption values are also higher, as ASA modifies the matrix microstructure. If we assume that increasing the ASA content leads to an increase in apparent pores, in this case, these tend to interconnect and generate more voids, directly affecting the specific gravity and water absorption capacity of the blocks.

The increase in ASA content leads to a decrease in apparent porosity at burning temperatures of 1050 °C and 1150 °C, which shows the positive influence of ASA on the ceramic matrix. PUREZA [⁴⁸] points out that reactions between alumina and silica with other elements occur in the temperature range of 900–1000 °C in clay-based ceramic matrices, potentially producing complex silicoaluminates. When the temperature is close to or above 1000 °C, the glassy phase forms from the softening and fusion of these silicoaluminates, thereby providing mechanical strength, compaction, hardness, and greater impermeability to the ceramic blocks.

Figure 4 presents the compressive strength results for the blocks. All blocks presented average compressive strength values greater than 1.5 MPa, the minimum value established by NBR 15270-1 [³²] for ceramic sealing blocks. Tables 5 and 6 present the results of the statistical analysis, which indicate a significant influence of both ASA content and the temperature.

Figure 4
Blocks – compressive strength.

Table 5
ANOVA - Effects of firing temperature and ASA content on compressive strength.

	SQ	GDL	MQ	F	P-VALUE	SIGNIFICANCE
Temperature	5.031	2	2.516	56.182	0.000	yes
ASA %	0.315	3	0.105	2.342	0.076	no
Temp. x % ASA	1.967	6	0.328	7.321	0.000	yes
Error	6.448	144	0.045

• SQ – Sum quadratic; GDL – Degrees of freedom; MQ – Mean square; F – Calculated value of F; yes – Significant; no – Not significant.

Table 6
Tukey test - effect of temperature and ASA content in compressive strength values.

INTERACTIONS	DIFFERENCE BETWEEN AVERAGES	STANDARD ERROR OF DIFFERENCE	95% CONFIDENCE INTERVAL FOR THE DIFFERENCE	P-VALUE
Bref [950 e 1050]	–0.115	0.083	(–0.316; 0.085)	0.421
Bref [950 e 1150]	–0.115	0.083	(–0.316; 0.085)	0.421
Bref [1050 e 1150]	0.075	0.083	(–0.201; 0.201)	1.000
B5C [950 e 1050]	–0.185	0.083	(–0.385; 0.016)	0.081
B5C [950 e 1150]	–0.392	0.083	(–0.593; –0.192)	0.000
B5C [1050 e 1150]	–0.208	0.083	(–0.008; 0.393)	0.040
B10C [950 e 1050]	–0.400	0.083	(–0.601; –0.199)	0.000
B10C [950 e 1150]	–0.608	0.083	(–0.808; –0.407)	0.000
B10C [1050 e 1150]	–0.208	0.083	(–0.408; –0.007)	0.040
B15C [950 e 1050]	–0.700	0.083	(–0.901; –0.499)	0.000
B15C [950 e 1150]	–0.508	0.083	(–0.708; –0.307)	0.000
B15C [1050 e 1150]	0.192	0.083	(–0.008; 0.393)	0.064

The statistical analysis using Tukey’s test (Table 6) indicated that only the reference blocks (Bref) did not show statistically significant differences (p > 0.05) between firing temperatures, suggesting that the clay matrix had already reached a stable densification stage. Although the blocks with 5% replacement (B5C) showed similar behavior between 950 °C and 1050 °C, significant differences were observed at higher temperatures, indicating the onset of the residue’s influence. In this context, strength is primarily governed by the densification of the base clay at low replacement levels, while the contribution of the residue becomes more pronounced as its content increases [⁴⁹]. Furthermore, the results reveal a clear dependence of mechanical behavior on residue content. While intermediate levels (B10C) exhibited significant differences across all firing conditions, indicating a strong interaction between temperature and residue, higher substitution levels (B15C) showed a stabilization trend at elevated temperatures. In this case, no significant differences were observed between 1050 °C and 1150 °C, suggesting the onset of a saturated vitrification stage.

Temperatures between 1050 and 1150 °C are generally more suitable for producing ceramic blocks. It is worth noting that it may be useful to set the temperature to 1050 °C rather than 950 °C to obtain blocks with 15% ASA and a compressive strength up to 40% higher than the reference value.

3.3. Analysis of efflorescence

The efflorescence analyses showed no salt spots in any specimens. Thus, the results indicated that ASA, as a partial substitute for clayey soils used in this study, did not cause the pathology to appear.

FERREIRA AND BERGMANN [⁵⁰] mentioned that the most common salts that cause the phenomenon of efflorescence are potassium nitrate (KNO₃), magnesium sulfate heptahydrate (MgSO₄.7H₂O), calcium carbonate (CaCO₃), sodium nitrate (NaNO₃), potassium sulfate (K₂SO₄), potassium chloride (KCl), gypsum (CaSO₄.2H₂O) and hematite (Fe₂O₃). They pointed out that soluble salts in ceramic products can originate from the raw materials that constitute the ceramic mass or be formed during drying through the reaction between ambient sulfurous gases and the ceramic mass. As analyzed by the chemical analysis of this research, both clayey soil and ASA have hematite in their compositions, 2.16% and 1.12%, respectively. However, the low content of this salt in the ceramic masses was not decisive for the appearance of efflorescence in the blocks analyzed with the naked eye.

3.4. Scanning electron microscopy (SEM)

Figure 5 shows micrographs of the blocks of each class produced at different firing temperatures. At 950 °C, the blocks’ surfaces are highly rough and structurally porous, consistent with the apparent porosity measurements. PINHEIRO and HOLANDA [¹²] state that these interparticle open pores with irregular morphology are characteristic of the initial sintering stage.

Figure 5
Blocks – SEM.

However, the increase in ASA content modifies the surfaces at higher temperatures, making them smoother and with fewer dark spots (pores). From 1050 °C, the formation of a liquid phase is evident since, after cooling, glassy filaments are observed, which reduce porosity by partially filling the pores. SOUZA et al. [17] reiterate that particles tend to merge within this temperature range, leading to grain compaction and union and consequently reducing pore volume. This effect is more pronounced at higher ASA contents and at higher temperatures.

The results demonstrate a clear synergistic effect between the incorporation of açaí seed ash (ASA) and the applied thermal processing. The increase in firing temperature intensified the interaction between the amorphous and crystalline phases of ASA and the clayey matrix, promoting better densification of the ceramic structure. As a consequence, the mixtures containing ASA exhibited reduced apparent porosity and improved mechanical strength when compared to the reference composition without ash, particularly at firing temperatures of 1050 °C and 1150 °C. This behavior indicates that the thermal treatment enhances the encapsulation of the residue within the ceramic matrix, allowing ASA to act simultaneously as a filler and as a fluxing component, contributing to the formation of more stable silicoaluminate phases.

Such behaviour has also been observed in other studies, such as MOSTAFA et al. [⁵¹], in which firing at 1100 °C increased linear shrinkage, bulk density, and compressive strength. This indicates an improvement in the material’s structure upon exposure to higher temperatures, which may be associated with the removal of water and air during fire. These factors contribute to densification of the ceramic matrix and, consequently, to improved material durability.

Similar trends related to the existence of optimal residue incorporation levels have been widely reported in the literature. In cementitious systems, the incorporation of açaí seed ash has been associated with limitations in the replacement level, since improvements in microstructure are observed up to certain levels, whereas higher contents tend to compromise mechanical performance due to changes in particle packing and matrix continuity, as reported in [⁵²], which identified an optimal content between 10% and 20% of ASA.

Consistently, studies on agro-industrial ashes indicate that material performance is directly related to the incorporation level and processing conditions, with an optimal range commonly observed in which a balance between porosity and mechanical strength is achieved [⁵³]. In this context, the superior performance observed for 15% ASA at higher firing temperatures reinforces the existence of an optimal incorporation range for ceramic systems.

From a sustainability perspective, the incorporation of agro-industrial residues in construction materials has been recognized as an effective strategy to reduce environmental impacts associated with conventional production processes. Studies indicate that the use of waste-derived materials can help reduce carbon emissions and promote circular economy practices in the construction sector [⁵⁴]. In this context, the use of açaí seed ash in ceramic production contributes not only to reducing clay extraction but also to the valorization of an abundant regional residue in the Amazon, reinforcing the environmental relevance of the proposed approach.

Thus, this work demonstrates that the efficient reuse of ASA in red ceramics requires an integrated approach that simultaneously considers the replacement level and thermal processing parameters, and confirms the technical feasibility of incorporating ASA into red ceramic manufacturing, yielding promising results.

4. CONCLUSIONS

The partial replacement of clayey soil with açaí seed residual ash (ASA) was technically feasible for the manufacture of ceramic sealing blocks. The incorporation of the ash did not compromise the workability of the ceramic blocks; on the contrary, it contributed to increased plasticity and reduced the amount of water required for molding, resulting in lower shrinkage of the blocks after firing.

Variations in firing temperature directly affected the technological properties of the blocks. It was observed that increasing the temperature led to a reduction in the initial water absorption (capillary suction), as well as in the overall water absorption and apparent porosity. This effect was enhanced by the presence of ASA in the mixtures, indicating a favorable interaction between the residue and the clayey matrix under higher thermal conditions. The mechanical results showed that the incorporation of 15% ASA, combined with firing temperatures of 1050 °C and 1150 °C, yielded the highest compressive strength values, indicating that the residue can serve as a functional material in densifying the ceramic structure. Based on these results, a processing window can be defined, in which the use of 15% ASA at firing temperatures equal to or above 1050 °C represents an optimal condition, balancing mechanical performance and microstructural densification. Regarding visual behavior, the presence of ASA did not influence the occurrence of efflorescence, and no changes were observed in the surface appearance of the blocks after the test. In addition, the incorporation of ASA does not require modifications to the conventional manufacturing process, indicating that its use can be implemented without disrupting existing production workflows.

From a practical and environmental perspective, the use of ASA as a partial replacement for clayey soil represents a sustainable alternative for reusing an abundant residue from the Amazon region, contributing to the reduction of environmental liabilities and to the valorization of local agro-industrial wastes.

The combined evaluation of replacement levels and different firing temperatures provides scientific support for the safe application of ASA in red ceramic production without compromising the final material quality.

5. ACKNOWLEDGMENTS

The authors thank the CAPES Foundation, the Interdisciplinary Laboratory for the Development of Nanostructures, and the Laboratory of Analysis of Minerals and Rocks for their support. The authors received no financial support for the research but did receive financial support for the publication of this article from PROPESP/PAPQ/UFPA. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.

6. DATA AVAILABILITY

All data generated or analyzed during this study are included in this published article. Additional data supporting the findings of this study are available from the corresponding author upon reasonable request.

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Publication Dates

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

History

• Received
  22 Nov 2025
• Accepted
  26 May 2026