Microstructural and mechanical evaluation of cement composites incorporating furniture industry wood ash and sewage sludge

Análise micro-estrutural e mecânica de compósitos cimentícios incorporando cinza e lama da indústria moveleira

Abstract

Abstract  This study evaluated the microstructural and mechanical impacts of incorporating specific furniture industry wastes (Eucalyptus wood ash and sewage treatment plant sludge) as partial Portland cement replacements. Pastes and mortars were prepared with 5%, 10%, 25%, and 50% substitution levels of either residue. A comprehensive methodology was applied, correlating macroscopic performance (workability, compressive strength) with continuous hydration kinetics (calorimetry, Ultrasonic Pulse Velocity) and microstructural evolution (SEM, XRD, FTIR). Results demonstrated that replacing 5% of cement with wood ash increased compressive strength by up to 28% compared to the reference series, driven by enhanced C-S-H formation. However, 50% ash substitution caused a 73% strength reduction, primarily attributed to a dilution effect, as the ash particles lacked the fineness required for a physical filler effect. Regarding the sewage sludge, low dosages (up to 10%) acted as a set retarder without significantly hindering overall hydration. Conversely, high sludge levels (50%) led to a severe loss of workability and an 85% drop in strength. Microstructural analyses revealed that the sludge's highly porous morphology and elevated organic content absorbed free water, critically inhibiting cement hydration.

Keywords:
sustainable technologies; circular construction; eco-efficient composites; furniture industry

Resumo

Resumo  O presente estudo avaliou os impactos micro-estruturais e mecânicos da incorporação de resíduos da indústria moveleira (cinza de eucalipto e lama de estação de tratamento de esgoto) como substitutos parciais de cimento Portland. Pastas e argamassas foram preparadas com 5%, 10%, 25% e 50% de substituição de cada resíduo. Uma metodologia compreeensiva foi aplicada, correlacionando desempenho macroscópico (trabalhabilidade, resistência à compressão) e evolução micro-estrutural (MEV, DRX, FTIR). Os resultados demonstraram que a substituição de 5% de cimento por cinza aumentou a resistência à compressão em até 28% em comparação com a série de referência, devido a um aumento na formação de C-S-H. No entanto, 50% de cinza resultou numa perda de resistência de 73%, principalmente devido ao efeito de diluição, visto que as partículas de cinza não são finas o suficiente para gerar um efeito de filler. Quanto à lama, baixos teores (até 10%) agiram como um retardador de pega sem prejudicar de forma significativa a hidratação. Por outro lado, teores altos (50%) resultaram em alta perda de trabalhabilidade e redução de 85% da resistência. A análise micro-estrutural revelou que a morfologia altamente porosa e o alto conteúdo orgânico das partículas de lama absorvem água livre, impedindo a hidratação do cimento.

Palavras-chave:
tecnologias sustentáveis, construção circular; compósitos eco-eficientes; indústria moveleira

1 INTRODUCTION

The production of Portland cement requires a large amount of energy and generates significant amounts of CO₂ as a byproduct of the calcination process, contributing to 7% of global CO₂ emissions [¹], [²]. Considering its use with aggregates in concrete and mortar, such products account for about 46% of all global raw material extraction [³]. Therefore, the construction industry has been under increasing pressure to reduce its environmental impact in recent years [⁴], [⁵]. It is crucial to develop strategies to decrease Portland cement consumption for producing cementitious composites without compromising the mechanical performance and durability of the materials [⁶]-[⁸]. One promising strategy is the partial substitution of cement with supplementary materials, which greatly reduces the environmental impact of producing the binder [⁹].

The furniture industry, as it stands, has significant social and environmental impacts due to its reliance on natural resource extraction and energy consumption. In addition, it generates waste, 88% of which is wood, with the rest consisting of hazardous chemical materials like glue, paint, varnishes, and solvents [¹⁰]. This poses health risks to industry workers, highlighting the need to improve the sustainability of the production cycle [¹¹].

Many countries around the world have significant potential for implementing a circular economy in the furniture sector. According to Furn 360 [¹²], approximately 10 million tons of different types of furniture waste are generated in European Union (EU) countries each year, the majority of which is sent to landfills or incinerated. The United States Environmental Protection Agency [¹³] estimated that 18.1 million tons of wood waste were generated in the United States, with 12.15 million tons landfilled, 2.84 million tons combusted for energy recovery, and only 3.1 million tons recycled. In Brazil, the municipality of Ubá holds significant economic importance as a key hub for furniture manufacturing. Itatiaia Móveis S/A, one of the leading Brazilian companies in this sector, generated a total of 1,137,200 kg of waste in 2020 and 967,547 kg in 2021, including ash from eucalyptus chip incineration, sludge from hook cleaning, and effluent from its sewage treatment plant (STP).

The incorporation of individual industrial wastes into cementitious matrices has been the subject of several previous investigations, primarily focusing on how these residues alter early-age properties and hydration kinetics. Generally, the results show that using small percentages of sludge does not affect the resulting material [¹⁴], [¹⁵], while wood ash can cause significant changes in the material's properties [¹⁶], [¹⁷].

Regarding biomass residues, a comprehensive review by Teker Ercan et al. [¹⁸] demonstrated that while wood ash can exhibit mild pozzolanic activity and provide a filler effect at optimal low replacement levels, higher substitution rates typically increase the water demand of the mixture and negatively impact the strength and durability of the composites. Ince et al. [¹⁹] analyzed the properties of mortars with partial substitution. Their results showed a significant decline in mechanical strength as the percentage of wood ash increased, with a strength loss of about 40% at a 5% substitution level. Nadeem et al. [²⁰] assessed the residual performance of mortars containing wood ash after exposure to various temperatures. They found that the compressive strength of all mortars decreased with rising temperatures. Notably, a substantial loss of strength and durability was observed in specimens exposed to 400 °C, marking it as a critical temperature for these composites.

Similarly, the use of industrial and sewage sludges in cementitious binders has been increasingly evaluated. One such study was conducted by Goyal et al. [²¹] examined the impact of different dosages of sludge from the textile industry on the mechanical strength of mortars cured for 3, 7, and 28 days. They concluded that substituting up to 5% of Portland cement with textile sludge does not significantly alter the properties of cementitious materials. Similarly, Bencardino et al. [²²] analyzed the use of wastepaper sludge-derived cellulose fibers as a partial substitute for cement in mortars up to 2% and found no significant loss of compressive strength compared to the reference mortar.

Gu et al. [²³] studied the incorporation of high-iron sewage sludge as a partial substitute in mortars. Substituting between 0 and 5% of the binder with sludge, the authors found that increasing the dosage of sludge in the material resulted in an increase in both initial and final setting times for the materials. Evaluating the compressive strength of the materials, the authors concluded that small amounts (up to 0.5%) of sewage sludge resulted in an increase in the mortar’s performance. Higher dosages, however, showed a decrease in mechanical strength.

Recent research by Zari et al. [²⁴] and Kumar and Prashant [²⁵] highlighted that the porosity, high organic content, and complex chemical composition of treated sludges can severely restrict the workability of fresh mixtures. These sludge characteristics frequently lead to a dilution effect and a reduction in the ultimate compressive strength of the hardened materials, limiting their viable replacement percentages to narrow margins.

While the use of generic municipal sludges or agricultural ashes in cementitious composites has been widely explored, there remains a significant research gap regarding the specific, complex waste streams generated by the furniture industry. This sector simultaneously produces eucalyptus wood ash from thermal energy generation and highly organic sludge from its effluent treatment plants. Therefore, the innovative strategy proposed in this article lies in the comprehensive, multi-scale evaluation of these specific, localized residues to advance circular economy practices in construction. Unlike traditional studies that rely primarily on macroscopic mechanical testing, this research adopts a correlative methodological approach. It systematically links continuous, early-age hydration kinetics—monitored via Ultrasonic Pulse Velocity (UPV) and calorimetry—with advanced microstructural phase analyses (FTIR, XRD, and SEM). By mapping the microstructural modifications and hydration inhibition mechanisms triggered by different substitution levels, this study provides a robust understanding of how furniture industry wastes interact with the cement matrix, establishing safe and scientifically justified parameters for their upcycling.

2 EXPERIMENTAL PROGRAM

A flowchart is presented in Figure 1 to summarize the methodology used in the present research, offering a structured representation of the sequence of experimental procedures followed throughout the study.

Figure 1
Summary of the conducted research methodology.

2.1 Materials

High early strength Portland cement (PC), equivalent to ASTM Type III [²⁶], was used for producing both pastes and mortars. This cement is classified as CPV-ARI according to the Brazilian NBR 16697 standard [²⁷] and was selected for its lower mineral addition content. For the mortar production, river quartz sand with particle sizes ranging from 0.6 mm (sieve #30) to 0.15 mm (sieve #100) was used.

The waste materials used in the production of pastes and mortars were collected at a furniture plant in Ubá, Brazil, and included ashes from the incineration of Eucalyptus wood chips (being produced after the burning of wood chips for energy production for the industrial processes) and sludge from the factory’s sewage treatment plant (STP), shown in Figure 2. Both wastes were oven-dried before being used, in order to avoid any water unaccounted for in the mixtures.

Figure 2
Furniture industry wastes: a) Eucalyptus wood ash; b) Sewage sludge.

The fineness index of the residues was assessed according to the NBR 11579 standard [²⁸]. The specific mass of the wastes was determined using the Le Chatelier volumetric flask method, in accordance with NBR 16605 [²⁹]. Kerosene was used as the medium in these standard test methods due to its inert nature and its specific mass (~0.731 g/cm³ at 15 °C), which is lower than that of the wastes.

The wastes were analyzed for particle size distribution using laser diffraction granulometry. The samples were dispersed in distilled water, with isopropyl alcohol used as the dispersing medium. Laser diffraction analyses were performed with a Bettersize 2000 analyzer. The particle size distributions of both sand and cement were also determined using the same methodology.

SEM analysis was conducted for morphological evaluation, while EDS analysis was used to determine the elemental composition of the residues. Samples of both waste types were mounted on SEM stubs and coated with gold using a Quorum Q150RS coater. The ash and sludge samples were dried in a laboratory oven at 100 °C for 24 and 48 hours, respectively, with the longer drying time for sludge due to its higher moisture content. Both samples were sieved through a #48 sieve (300 µm). The materials' morphologies were observed using a microscope with a voltage of 15 kV to generate secondary electron images, with magnification adjusted up to 3000×.

X-ray fluorescence (XRF) analysis was performed to evaluate the chemical composition of the ash and sludge samples using an energy-dispersive PANalytical Epsilon3x spectrometer.

The loss on ignition of the materials was determined using two methods. The first method involved heating the samples in a laboratory furnace to 440 °C, following the NBR 13600 standard [³⁰], to eliminate volatile materials and determine the organic matter content. The second method exposed the samples to a maximum temperature of 1000 °C focusing on measuring mass losses due to decarbonation and dehydroxylation processes [³¹].

The wood ashes and sewage sludge were subjected to leaching tests and environmental classification according to the criteria established by NBR 10004 [³²]. Moreover, appropriate storage conditions were analyzed following the prescriptions of NBR 11174 [³³].

2.2 Production and testing procedures for mortars

Mortars were made by replacing Portland cement with 5%, 10%, 25%, and 50% of ash or sludge from the furniture industry, along with a reference mixture that had no substitutions (Table 1). All mixtures kept a sand-to-(cement + waste) ratio of 2.00 and a water-to-(cement + waste) ratio of 0.55 by weight. Following the NBR 13276 standard [³⁴], the mixing process was performed using a mechanical mixer. Immediately after mixing, the workability of the material was assessed based on the determination of the consistency index (Figure 3a), also in accordance with the standard.

Table 1
Identification of mortars and their mixture proportions.

Series	Constituents (g)
	Portland cement	Wood ash	Sludge	Sand	Water
REF	474.5	0.0	0.0	949	261.0
5A	450.3	23.7	0.0	948	260.7
10A	426.6	47.4	0.0	948	260.7
25A	355.5	118.5	0.0	948	260.7
50A	237.0	237.0	0.0	948	260.7
5S	450.3	0.0	23.7	948	260.7
10S	426.6	0.0	47.4	948	260.7
25S	355.5	0.0	118.5	948	260.7
50S	237.0	0.0	237.0	948	260.7

Figure 3
Testing procedures for mortars and pastes: a) Mortar workability measurement; b) Compressive strength test.

Next, the freshly mixed material was poured into cylindrical PVC molds with a diameter and height of 35 mm (the nominal dimension of the specimens was significantly higher than three and a half times the maximum aggregate size). After 24 hours, the specimens were removed from the molds, submerged in lime-saturated water, and kept at a temperature of 25 °C. These specimens were cured for different periods (1, 3, 7, 28, or 63 days) before undergoing uniaxial compression tests.

Four samples were made for each mixture and curing period, resulting in a total of 180 cylindrical specimens. Before the mechanical tests, the specimens were smoothed to ensure plane and parallel top and bottom surfaces. The compression tests were carried out with a loading rate of 0.31 MPa/s, following the NBR 13279 procedure [³⁵], in an DL20000 EMIC universal testing machine (Figure 3b).

2.3 Production and testing procedures for pastes

The hydration kinetics in cement-based materials are linked to their binding properties, physical aggregation, and improvements in the mechanical and durability properties of the hardened composite [³⁶], [³⁷]. Evaluations of hydration kinetics were performed on pastes to examine the impact of the furniture industry residues on the microstructure and hydration kinetics of Portland cement pastes.

These analyses were carried out through Ultrasonic Pulse Velocity (UPV) measurements in the fresh cementitious mixture. The methods proposed by Lee et al. [³⁸]. were applied to interpret the UPV versus time curves obtained from the tests. The experimental tests employed a Proceq Pundit Ultrasonic Pulse Velocity instrument. The transducers operated at a frequency of 54 kHz, and the velocity of longitudinal wave propagation was recorded once per minute over a 48-hour period. The minimum value measured at 10-minute intervals was used, and any outliers were excluded from the results.

The tests were performed on nine distinct types of pastes, comprising the reference paste (devoid of wastes) and pastes with varying proportions of ash or sludge, as outlined in Table 2. The water-to-(cement + waste) ratio for the pastes was maintained at 0.5 (by mass). The experimental setup suggested by Freitas [³⁹] and Machado [⁴⁰] was employed in the UPV tests (Figure 4).

Table 2
Identification of pastes and their mixture proportions.

Series	Constituents (g)
	Portland cement	Wood ash	Sludge	Water
REF	1454.2	0.0	0.0	727.1
5A	1381.5	72.7	0.0	727.1
10A	1308.8	145.4	0.0	727.1
25A	1090.6	363.5	0.0	727.1
50A	727.1	727.1	0.0	727.1
5S	1381.5	0.0	72.7	727.1
10S	1308.8	0.0	145.4	727.1
25S	1090.6	0.0	363.5	727.1
50S	727.1	0.0	727.1	727.1

Figure 4
Ultrasonic pulse velocity test setup.

The pastes were subjected to calorimetry tests during 48 hours, at a chamber temperature of 23 °C. First, the raw materials were weighed and mixed. Then, water was added to the cement and waste, and the resulting paste was mixed using a benchtop mixer at 400 rpm for 2 minutes. An ICal 2000H calorimeter, along with an A0068784 Lab Calibration calibrator, was used for these tests.

2.4 Microstructural analyses of the cementitious composites

FTIR analyses were used to identify vibration modes of different functional groups within the microstructure of the cement composites. Moreover, XRD analyses were performed to identify the crystalline phases present in these materials.

These tests were conducted on paste samples where 5% and 50% of the cement were replaced with furniture industry residues. After curing for 7, 28, or 63 days, the samples underwent hydration stoppage using isopropyl alcohol. Following the recommendations of the RILEM Technical Committee 238-SCM [⁴¹], the samples were collected from the interior of hardened cement paste, avoiding surfaces exposed to air or solutions, then crushed and sieved to pass a 1 mm aperture (Figure 5a), immediately immersed in isopropanol to stop hydration (Figure 5b), filtered under vacuum with successive rinsing using isopropanol and diethyl ether (Figure 5c), and finally dried at 40 ± 5 °C before being stored in a desiccator for subsequent analysis (Figure 5d).

Figure 5
Sample preparation procedure following the recommendations of the RILEM Technical Committee 238-SCM: (a) crushing and sieving of samples; (b) immersion in isopropanol; (c) vacuum filtration with successive rinsing using isopropanol and diethyl ether; and (d) storage in a desiccator prior to further analysis.

FTIR analyses were performed using a Thermo Fisher Nicolet iS5 spectrometer. To prepare the samples, 1% of the material was pressed into 100 mg of KBr. The materials were placed in an agate mortar and ground until fully mixed. The sample was then placed in a pelletizer and subjected to a compression load of 25 kN. The resulting pellets were identified and stored under vacuum.

The XRD analyses were conducted using a Bruker D2 Phaser diffractometer with CuKα radiation (λ = 1.789 Å). The scans covered a 2θ range from 5° to 70°, with a step size of 0.05° per second and 30 spinner rotations per minute. The sample was sieved through a #325 sieve (45 µm) and placed into the sample holder of the XRD equipment. Both the FTIR and XRD analyses were carried out at the NanoLab of the Federal University of Ouro Preto (UFOP).

3 RESULTS AND DISCUSSION

3.1 Characterization of the furniture industry wastes

The fineness index tests showed values of 60.9% for wood ash and 67.0% for STP sludge using the #200 sieve. In this study, the specific density of the ash and sludge residues were found to be 2.96 g/cm³ and 1.82 g/cm³, respectively.

Figure 6a shows the particle size distribution curves for the waste materials used in this study. The sludge had a slightly finer texture than the wood ash, with a D50 of about 55 µm. The Eucalyptus ash had a D50 just below 80 µm and exhibited a more uniform granulometric distribution than the sludge. Despite these differences, the particle size distribution curves for the wastes were quite similar, suggesting that variations in the performance of the cementitious composites are only minimally related to the particle size distribution of the wastes. The fine aggregate, which was quartz sand, passed through a #14 mesh sieve (1180 µm) and had larger particle sizes, with a D50 around 350 µm. In contrast, the cement was the finest material, with a D50 of approximately 12 µm. The finer particle size of the cement contributes to its high reactivity, a trait commonly associated with high-early-strength Portland cements. In contrast, the D50 values of the wastes are consistent with the broader literature, which reports typical average particle sizes for raw wood ashes and industrial sludges ranging from 15 to 1000 µm, depending heavily on the specific incineration and drying processes utilized [⁴²] -[⁴⁶]. Since the industrial wastes present a much coarser granulometry than the cement, substituting the binder with these residues does not provide a physical filler effect, as that typically occurs when the supplementary materials are significantly finer than the cement grains. Thus, at higher replacement levels, the inclusion of these coarser particles contributes primarily to the dilution effect, reducing the overall reactive surface area and driving the severe mechanical strength reductions discussed in Section 3.3. At low dosages (such as 5% wood ash), however, this lack of physical filler effect is overpowered by chemical interactions and increased C-S-H formation, resulting in the compressive strength gains observed for these specific mixtures.

Figure 6
Particle size results: a) Particle size distributions; b) SEM micrograph of the wood ash particles; c) SEM micrograph of the sludge particles.

SEM micrographs of the wastes showed that the larger particles of the wood ash had smooth, rounded surfaces, while the smaller particles had more angular edges and planar surfaces (Figure 6b), which agrees with previous studies [⁴⁷], [⁴⁸]. This improves the packing process, resulting in a denser and less porous matrix – which helps explain the increase in mechanical strength observed for low dosages of wood ash substitution [⁴⁹]. The sludge sample, on the other hand, had particles with rougher and less rounded surfaces (Figure 6c). The high porosity and specific surface mean that the sludge particles act as water sinks [⁴⁶], explaining the severe workability reduction observed in the sludge mixtures (with 50% replacement causing a complete loss of workability), as the particles absorb and reduce the amount of free water available to lubricate the mixture (hence the loss of workability) and hydrate the cement, contributing to the lower compressive strength observed.

The chemical profiles obtained from the XRF and EDS analyses (Tables 3 and 4, respectively) are highly consistent with the specific industrial origins of these residues. For the wood ash, the high concentrations of CaO (24.29%) and SiO₂ (17.11%), alongside potassium levels, are intrinsic characteristics of eucalyptus biomass [⁴⁵], [⁴⁸]. Conversely, the chemical signature of the sewage sludge is heavily influenced by the industrial effluent treatment process. The substantial presence of Al₂O₃ (11.51%) and Fe₂O₃ (3.41%) can be directly attributed to the chemical coagulants, such as aluminum and iron-based salts, typically employed in the plant to promote the flocculation of suspended solids [⁵⁰], [⁵¹].

Table 3
Chemical composition of the wastes.

Chemical element (%)	Waste
	Wood ash	Sludge
Carbon (C)	14.81	19.81
Oxygen (O)	34.27	39.64
Sodium (Na)	2.81	1.41
Aluminum (Al)	4.34	10.16
Silicon (Si)	6.26	4.82
Sulfur (S)	4.33	3.29
Potassium (K)	5.98	1.78
Calcium (Ca)	22.79	13.03
Manganese (Mn)	0.69	0.19
Iron (Fe)	3.71	5.86

Table 4
Results of the XRF analysis.

Chemical element (%)	Waste
	Wood ash	Sludge
MgO	4.276	0.364
Al2O3	9.837	11.511
SiO2	17.110	6.857
SO3	5.727	4.450
K2O	3.332	0.993
CaO	24.297	12.753
TiO2	4.762	5.900
Fe2O3	4.715	3.414
Other oxides	5.864	3.078
LOI	20.080	50.680

The mass loss percentages from the LOI tests (Table 5) indicated the presence of organic matter in both types of waste. Since organic matter absorbs a significant amount of water, incorporating these wastes is expected to affect the workability and mechanical performance of the cementitious mixtures. After heating to 1000 °C, the LOI values for wood ash and sludge (26.69% and 50.00%, respectively) were consistent to those obtained from the XRF analyses (20.08% and 50.68%, respectively). The high values obtained in the sludge are expected consequences of accumulating organic matter, fine suspended wood dust, and residual organic chemicals (e.g., glues and varnishes) that are washed into the effluent during the furniture manufacturing cycle [⁵²].

Table 5
Results of the loss on ignition tests.

Waste	Maximum temperature (°C)	Loss on ignition (%)
Wood ash	440	11.78
	1000	26.69
Sludge	440	38.77
	1000	50.00

The environmental classification results show that the ashes used in this study meet all the parameter ranges specified in Annex F of NBR 10004 [³²], based on quantitative data from leaching tests. However, the analysis of the solubilized extract from the ashes revealed concentrations of chloride, sodium, sulfates, and surfactants that exceed the limits set in Annex G of the same standard.

The wood ash was classified as “Non-Hazardous Waste Class II A – Non-Inert”. In this case, proper waste disposal must follow the criteria outlined in NBR 11174 [³³]. This standard considers various factors, including the quantity and physical characteristics of the residues, with a focus on the potential for disposal in industrial landfills.

The sludge showed no issues with flammability, corrosivity, or reactivity. Moreover, the leachate extract from this waste did not reveal concentrations exceeding the limits specified in Annex F of NBR 10004, according to the leaching tests.

The NBR 10006 standard does not apply to liquid wastes. Since the sludge residue didn't exceed the concentration limits set in Annex F of NBR 10004, it can be classified as “Non-Hazardous Waste – Class II”. For temporary disposal, it must follow the criteria outlined in NBR 11174. These criteria consider factors such as the quantity and physical properties of the waste, with a particular focus on the potential for disposal in industrial landfills.

3.2 Workability of mortars

In this study, mortars were made with a sand/(cement + waste) ratio of 2.00 and a water/(cement + waste) ratio of 0.55 (by weight). As a result, the consistency of the mortar was not controlled but measured, leading to the values shown in Table 6.

Table 6
Consistency indices for the mortars.

Series	Spreading diameter (mm)
REF	300.0
5A	283.0
10A	279.7
25A	238.3
50A	-
5S	296.3
10S	295.3
25S	244.0
50S	-

Increasing the waste content reduced the workability of the mortars. For example, replacing 25% of Portland cement with wood ash led to a 21% decrease in the spreading diameter. The decrease in spreading diameter was less noticeable with mortars containing sewage sludge. Replacing 5% and 10% of cement with sludge resulted in spreading diameters like those of the reference mortar. However, replacing 25% of cement with sludge caused a 19% reduction in spreading diameter. Mortars with a 50% cement replacement for either type of waste showed poor workability, lacking proper cohesion and experiencing mass collapse, which hindered reliable measurement.

Goyal et al. [²¹] investigated the standard consistency of pastes by substituting cement with different percentages of sludge from the textile industry. They found that paste consistency increased by about 7.0% with 20% sludge content. In another study, Rais et al. [⁵³] conducted experimental and analytical investigations on ash-based mortars, revealing consistency reduction with the addition of ash across a range of 0% to 50%. Specifically, replacing 50% of cement with ash led to an 8.3% drop in consistency in cement pastes. Substituting 10%, 25%, and 40% of cement with ash resulted in consistency reductions of 1.7%, 3.3%, and 6.7%, respectively. These findings highlight the influence of ash content on water demand, attributed to modifications in the early-age hydration rate of mortars, which decreased as ash replaced cement.

3.3 Compressive strength of mortars

Figure 7 shows the resulting compressive strengths of each mortar mixture at different curing periods. The reference mortars displayed a 66% average increase in compressive strength from 1 to 7 days. Afterward, from 7 to 63 days, the increase was smaller, at 32%. The microstructural mechanisms behind these variations in compressive strength are discussed in the following subsections.

Figure 7
Mortar compressive strength results: a) Reference and series A (ash); b) Reference and series S (sludge).

Mortars containing wood ash (Figure 7a) showed significant increases in compressive strength from 1 to 7 days, with increments of 81% for the 5A series, 73% for the 10A series, and 65% for the 25A series. From 7 to 63 days, the improvements were 45% for the 5A series, 30% for the 10A series, and 76% for the 25A series. The greatest strength gains over the entire curing period were observed in the 5A series mixtures.

An increase in average compressive strength was observed when 5% of the cement was replaced with ash. For example, the 63-day compressive strength of the 5A series was 28% higher than that of the reference mortar. However, as the ash content increased from 5% to 50%, the 63-day compressive strength of the mortars decreased progressively, with a reduction of up to 73%.

The results from Figure 7b show a decrease in mechanical strength with the replacement of cement by sludge. The strength reductions due to sludge were more significant compared to those from high ash contents. For example, the 50% S series had a 63-day compressive strength that was 85% lower than the reference series. While replacing 5% of cement with ash increased compressive strength, incorporating any amount of sludge led to reductions in mechanical strength.

Previous studies have also documented strength reductions due to the incorporation of sludge from various industrial activities into cementitious composites. For example, Goyal et al. [²¹] found that replacing up to 5% of cement with textile sludge had no adverse effects on strength. However, higher replacement levels led to a significant loss of strength. Yan et al. [⁵⁴] tested mortars containing sludge from the paper industry and found that the 91-day compressive strengths of mortars with 2.5% and 20% sludge were 27.6 MPa and 20.5 MPa, respectively. These values were 83% and 62% of the compressive strength of their reference mortars (33.2 MPa).

Ikotun and Raheem [¹⁷] conducted a comparative study on mortars incorporating fly ash and wood ash from a bread bakery. Mortars with 10% and 25% cement replacement by wood ash had average compressive strengths of 26.2 MPa and 25.4 MPa, respectively. Notably, mortars containing fly ash showed higher strengths than those with wood ash at a 10% cement replacement level. Specifically, the mortars with fly ash had a 90-day compressive strength that exceeded that of the reference mortars.

3.4 Ultrasonic pulse velocity

The UPV versus time curves exhibit three distinct stages. In Stage I, after the dormant induction period, hydrates form and ultrasonic waves pass through a viscous suspension akin to water. Stage II involves the gradual formation of hydration products, which leads to increased UPV values. As hydration products fill the pores, the water-saturated porous structure of the composite becomes more interconnected, resulting in rapid stiffness improvements [⁵⁵]. In Stage III, the formation of hydration products reduces pore volume. The rate of UPV increase slows, and the velocity values reflect the solid structure that has developed [³⁶], [³⁸]. The UPV measurement results are shown in Figure 8.

Figure 8
Ultrasonic pulse velocity (UPV) measurements of a) REF and series A pastes, b) REF and series S pastes.

The UPV measurements show the initial 48 hours of the curing period for various pastes. In the reference paste, the highest UPV value recorded was approximately 3000 m/s. The addition of different amounts of ash to the cement mixture did not significantly alter the maximum UPV, which consistently ranged between 2500 and 3000 m/s. When 5% or 10% of the cement was replaced with sludge, the maximum UPV stayed around 3000 m/s. However, replacing 25% of the cement with sludge resulted in the maximum UPV dropping to 2300 m/s.

Shorter delays were found in the initial setting time for pastes containing ash (A-series) compared to reference pastes. Small amounts of sludge (5S and 10S series) did not delay the initial setting time relative to the reference paste. However, incorporating a high amount of sludge (25%) significantly delayed the initial setting time.

The reference curve and the A-series curves showed similar behavior: a sudden increase in stiffness immediately after the initial setting time, followed by a more gradual increase over the first 24 hours, with smaller gains thereafter. This pattern resembles a 4-stage process, indicating that the stiffness gains in both the reference paste and the pastes containing ash are likely similar in the early stages.

However, the introduction of sludge changed this behavior. Small amounts of sludge smoothened the initial stiffness gains, leading to a transition from a 4-stage to a 3-stage pattern. In contrast, high amounts of sludge resulted in a significant reduction in UPV values throughout the measurement period, with slower gains in the final stage compared to the 10S paste.

Notably, there were significant reductions in strength in mortars with higher cement replacement levels (Figure 7). These observations closely match the low UPV values seen in pastes with a 25% cement replacement.

3.5 Calorimetry

Figure 9 shows the results of calorimetry analyses conducted on various pastes examined in this study. The results are presented per unit of cement mass to assess the degree of hydration. During the initial 48 hours, the highest heat release per cement mass was observed in the REF, S5, S10, and A series. These findings indicate significant progress in hydration reactions for samples with ash and lower sludge contents. In addition, the calorimetry results for these series correspond with the higher UPV values observed in the UPV measurements for samples with minimal waste dosage (Figure 8).

Figure 9
Calorimetry results: a) and b) thermal power per unit weight of cement for A-series and S-series, respectively; c) and d) heat per unit weight of cement for A-series and S-series, respectively.

The introduction of higher concentrations of sludge hindered the progress of hydration reactions. Calorimetry tests showed that the S25 and S50 series released the least amount of heat per gram of cement. Since these series also exhibited the lowest mechanical strength values (Figure 7), it is reasonable to conclude that the calorimetry results are consistent with the compression test findings. Importantly, while substantial amounts of ash did not significantly affect the degree of hydration, the primary factor contributing to the low strength of mixtures with high ash content was the dilution effect.

Figure 9d identifies two primary groups: the mixtures REF, 5S, and 10S show similar cumulative heat values over 48 hours, indicating a comparable degree of cement hydration. In other words, these replacement levels did not significantly hinder the hydration of the cement grains. In contrast, the curves for 25A and 50A (Figure 9c) demonstrate a lower degree of cement hydration, which correlates with poorer mechanical performance and lower UPVs. Moreover, Figure 9b reveals a retardation effect of the sludge, evident in the delayed peak of the acceleration period (related to the formation of C-S-H and CH). Despite this delay, the intensity remained close to that of the REF sample. This retardation effect of sludge might not be a disadvantage, as it can act as a retarding admixture. Mixtures 25S and 50S clearly inhibit the formation of C-S-H and CH, a finding supported by XRD diffractograms. In contrast, ash did not exhibit a retarding effect (Figure 9a).

3.6 X-ray diffraction

The XRD diffractograms of the wastes (Figure 10) exhibited peaks corresponding to calcite, syngenite, ettringite, and quartz in the wood ash, and calcite, ettringite, and quartz in the sludge. These crystalline phases were also identified by Areias et al. [⁵⁶] and Goyal et al. [²¹] in sludge from by the textile industry. In addition, Bayuseno and Schmahl [⁵⁷] and Silva et al. [⁴⁴] observed typical signatures of syngenite in the characterization of fly ashes and wood biomass ashes, respectively.

Figure 10
X-ray diffraction (XRD) spectra of pure wastes.

Figure 11 presents the XRD analysis results of pastes with different waste contents. High-intensity peaks of CH were observed in the spectra of the REF, 5A, 50A, and 5S series for all curing periods. In contrast, these peaks were absent in the spectra of the 50S series throughout all curing times. Since CH is a by-product of the cement hydration process, these XRD results indicate reduced hydration reactions in pastes with high sludge content. In other words, the XRD findings confirm the significant decreases in compressive strength observed in Figure 7 and align with the calorimetry analysis.

Figure 11
X-ray diffraction (XRD) spectra: a) reference mortars; b) mortars with wood ash; c) mortars with sludge.

In addition, XRD signatures of ettringite were observed in the 50S series. Since ettringite is one of the initial hydration products in the cementitious matrix, this finding suggests a delay in the hydration kinetics of pastes with high sludge content, along with an inhibition of C-S-H and CH formation. This behavior was also discussed in the previous section.

3.7 Fourier-transform infrared spectroscopy (FTIR)

Figure 12 displays the FTIR spectra obtained for the wastes. These spectra revealed high-intensity bands at 880 cm^-1 and 1300-1600 cm^-1, which can be attributed to symmetric (ν2 and ν4) and asymmetric (ν3) vibration modes of calcium carbonate structures [⁵⁸]-[⁶⁰]. This observation aligns well with the XRD results of the pure wastes.

Figure 12
Fourier-transform infrared spectroscopy (FTIR) spectra of pure wastes.

Moreover, bands associated with Si-O vibration modes related to quartz particles of the sludge were identified at approximately 450 cm^-1, 692 cm^-1, and 810 cm^-1 [⁵⁹]-[⁶²]. The bands at 3400-3500 cm^-1 and 1600-1685 cm^-1 indicate the water chemically bound to the waste compounds [⁶⁰], [⁶³], [⁶⁴].

The vibration bands associated with different compounds of the pastes were assessed based on the FTIR spectra illustrated in Figure 13. According to previous research [⁵⁸], [⁵⁹], [⁶¹], the band at 900-1050 cm^-1 has been associated with the asymmetric stretching vibrations of Si-O in the calcium silicate hydrates (C-S-H). The results presented in Figure 13 clearly indicate the presence of this band in the reference paste and pastes produced with the replacement of 5% of cement by ash or sludge. In contrast, this band was not observed in pastes with a cement replacement level of 50%. These results corroborate the observations in the calorimetry measurements and XRD diffractograms and align with the compressive strength results of mortars containing those waste concentrations. Increased formation of C-S-H structures contributes to enhancements in the mechanical strength of the cementitious matrix.

Figure 13
Fourier-transform infrared spectroscopy (FTIR) spectra: a) reference mortars; b) mortars with wood ash; c) mortars with sludge.

The stretching of CH structures led to slight absorption bands at 3640 cm^-1 [⁶⁵]-[⁶⁷]. These bands were observed in reference pastes and pastes produced with a cement replacement level of 5%. However, this band was not detected in the spectra of pastes with a high residue dosage. These results are also consistent with the calorimetry measurements, XRD diffractograms, and compressive strength results of mortars, as CH is one of the hydration products closely related to the formation of C-S-H, reflecting the increases in compressive strength [⁶⁸].

The FTIR spectra of the pastes also revealed bands at 713 cm^-1, 874 cm^-1, and 1300-1600 cm^-1 related to the symmetric (ν2 and ν4) and asymmetric (ν3) vibration modes of calcium carbonate, as observed by other authors [⁵⁸]-[⁶⁰]. Since the high early strength cement used in the paste production contains up to 5% of limestone filler, the inclusion of residues may have reduced the calcite contents by dilution effect and, consequently, the strength of the cementitious matrix, as calcium carbonate also contributes to its mechanical performance.

Bands at 3400–3500 cm^-1 and 1600–1685 cm^-1 were also observed in the pastes, which can be attributed to the presence of water chemically bound to the hydration products of the cementitious matrix [⁶⁰], [⁶³], [⁶⁴].

According to previous studies [⁶⁰]-[⁶⁴], the FTIR band that eventually appeared at 1200-1100 cm^-1 was associated with the S-O stretching vibration mode of acicular crystals of ettringite (AFt). The results in Figure 13 indicated the presence of this band in both the reference samples and pastes produced with a replacement of 5% of cement by the furniture industry wastes. However, this band was not observed in samples produced with high residues concentrations. This result aligns with the compressive strength results, as increases in AFt concentration may contribute to the mechanical strength of cementitious materials at early ages [⁶⁹].

4 CONCLUSION

This study presents an innovative strategy for recycling of wood ash and sewage sludge, based on the application of these residues to produce eco-efficient composites intended for the construction industry. The following conclusions were obtained from the presented results:

• Increasing the amount of wood ash and sludge reduced the workability of the mortars, with 50% replacement resulting in inadequate workability. Lower dosages, however, still maintain workability comparable to that of the reference mortar – with 19% and 21% decreases for 25% sludge and ash, respectively.

• The partial substitution of Portland cement with 5% ash resulted in a 28% increase in compressive strength when compared to the reference mortar. Higher percentages, however, resulted in worse mechanical performance, up to a 73% decrease for 50% substitution. Using sludge resulted in significantly lower compressive strength for all evaluated amounts – up to an 85% loss of mechanical strength with 50% substitution.

• Adding ash did not significantly change the setting time of cement pastes, suggesting that the hardening process of both reference pastes and those with ash is likely similar in the early stages.

• Small additions of sludge (5 and 10%) delayed the acceleration period of the hydration kinetics without impacting the final heat output (and, thus, the gel formation). Higher dosages (25 and 50%) shortened the setting time and slowed down the hydration reactions, severely inhibiting the formation of C-S-H and CH.

This study indicates that small amounts of furniture waste can be incorporated into cementitious composites as recycled materials with minimal impact on performance. This result is promising for improving sustainability in both the furniture and construction industries. A better understanding of how industrial waste affects cement-based composites opens the door to developing material formulations that advance circular economy principles in both sectors.

Future studies are recommended to investigate the combined use of these wastes and to employ techniques such as mercury intrusion porosimetry (MIP) to further elucidate the pore structure and its relationship with mechanical performance. The influence of specimen size and shape on the measured mechanical properties should also be investigated, in order to assess possible scale and geometry effects on the experimental results. Moreover, future research should identify the specific production stages at which different types of furniture waste are generated and compare residues from these stages to better understand their distinct effects on cement-based materials.

ACKNOWLEDGEMENTS

This research was funded by Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG, Finance Code 5.12/2022) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, grant number PQ 308943/2022-8). This work was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001. The authors appreciate the collaboration and support from the Laboratory of Construction Materials at UFV, the Laboratory of Composite Materials at UFV, the Laboratory of Construction Materials at UFOP, and the Laboratory of Construction Materials at UENF, including their equipment and technical assistance. Thanks also to the Brazilian research groups SICon-CNPq/UFV, ATIVE/UFOP, and RECICLOS-CNPq/UFOP, part of the Rede Mineira de Pesquisa, Desenvolvimento Científico, Tecnológico e Inovação research network (FAPEMIG, grant number RED-00191-23). Finally, we are grateful to the Itatiaia furniture industry for providing the materials for this study and to Matheus Henrique Dela Costa Ferreira for his essential support with FTIR and XRD analyses.

• FINANCIAL SUPPORT:
  This research was funded by Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG, Finance Code 5.12/2022) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, grant number PQ 308943/2022-8). This work was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.
• Data Availability:
  The data that support the findings of this study are available from the corresponding author, P. H. R. Fialho, upon reasonable request.
• How to cite:
  R. M. Gomes et al., “Microstructural and mechanical evaluation of cement composites incorporating furniture industry wood ash and sewage sludge”, Rev. IBRACON Estrut. Mater., vol. 19, no. 3, e19119, 2026, https://doi.org/10.1590/S1983-41952026000100019

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Edited by

• Editors:
  Edna Possan, Leandro Trautwein.

Data availability

The data that support the findings of this study are available from the corresponding author, P. H. R. Fialho, upon reasonable request.

Publication Dates

• Publication in this collection
  07 Aug 2026
• Date of issue
  2026

History

• Received
  17 Oct 2025
• Reviewed
  19 May 2026
• Accepted
  24 June 2026