Introduction

The civil construction industry seeks construction techniques that develop materials with high-performance, value-added product, greater durability, and faster production. Thus, alternative materials have been presented as a technological, economic, and environmentally significant solution. The presence of waste in construction components, such as supplementary cementitious materials, which are by-products of steel industries, as well as reinforcement of microfibers or cellulose particles, by-products of the paper industry, are studied and generally contribute to the performance of cementitious systems (Fu et al., 2017, Lima et al., 2021; Li et al., 2020).

To minimize greenhouse gas emissions, deforestation and the consumption of natural resources resulting from the use of materials in civil construction, efforts have been made to find construction elements with low environmental impact, such as biodegradable products, non-petroleum derivatives, and materials produced from renewable resources. Another alternative to reducing environmental impacts in cementitious systems is to increase the performance and durability of construction materials and thus reduce the consumption of materials used, decreasing the demand for non-renewable raw materials (Cao et al., 2015). In this context, the combined study of renewable materials, the celluloses employed in this research, which were incorporated into cementitious boards, can minimize environmental impact.

Using cellulose microfibers in composites with matrices based on mineral binders is a technical solution with great potential for use. Still, they are characterized by a complex microstructure and significant heterogeneity, whose identification requires further advances in the current state of knowledge (Ferreira et al., 2018). The study of the interaction of cementitious matrices with cellulose microfibers is essential to advance the development of techniques for the commercial viability of these products.

The mechanism of cellulose microfibers in cement-based composites can be understood by determining the rheological, physical, and microstructural properties of these composites. In this sense, research has sought to develop cementitious composites with characteristics that meet technical requirements, evaluating the type, aspect ratio, and proportion of microfibers about the matrix, as well as methods of surface modification of the fiber to increase its performance in the fresh state, dimensional stability, and interaction with the matrix (Raabe et al., 2022; Gwon et al., 2022).

Despite the advantages provided by fibers, there are difficulties in controlling the length of cellulose fibers, resulting in inadequate fiber dispersion and, consequently, adhesion failures at the fiber-matrix interface, leading to reduced performance of cementitious systems. To overcome such limitations, using fibers with a lower aspect ratio, such as cellulose microfibers, may be a promising alternative for modifying the properties of cementitious composites (Gwon et al., 2022).

The pursuit of methods to improve the adhesion of cellulose fibers with the cementitious matrix is a research topic that has been widely studied. Chemical treatments are being investigated; however, in many situations, they may unfavorably affect the performance of the composites (Lopes et al., 2020). Generally, treatments performed on the surface of organic fiber aim to remove amorphous constituents, such as extractives, hemicellulose, and lignin, to obtain the maximum cellulose content possible with higher levels of crystallinity (Pereira et al., 2015).

Cementitious systems with the addition of cellulose microfibers exhibiting good mechanical properties and economic feasibility have been developed in the last decade. The main challenges lie in improving the long-term performance of these composites, which can be affected by the weakening of cellulose fibers due to alkaline attack and chemical incompatibility between cellulose fibers and cementitious matrix. This improvement should be achieved without increasing production costs while developing ecological technologies (Ardanuy; Claramunt; Toledo Filho, 2015).

Building upon the knowledge of the effect of cellulose microfibers in cementitious matrices, the impact of a third constituent, the use of crystalline microcellulose as a micro-additive, can be analyzed to enhance the chemical compatibility between cementitious matrices and cellulose microfibers. Using cellulose microparticles and nanoparticles is highly technological, economic, and environmentally relevant (Balea et al., 2019; Hunek et al., 2019).

Significant advancements in materials and manufacturing processes can be observed in nanotechnology across various sectors, including the construction industry. Micro and nanomaterials exhibit high specific surface area, improving cementitious composites’ mechanical properties and durability. Nanoengineering encompasses techniques for manipulating structures on micro or nanometrics to develop a new generation of multifunctional cementitious composites characterized by enhanced high-performance mechanical properties (Balea et al., 2019).

In the study by Mohammadkazemi, Aguiar and Cordeiro (2017), fibers derived from sugarcane bagasse were combined with bacterial nanocelluloses. The results revealed that the fibers tended to form hydrogen bonds, while the nanocelluloses adsorbed to the surface through hydrogen bonding, coating the fibers, enhancing dispersibility, and preventing fiber mineralization. The promising results may contribute to developing a new generation of green hybrid composites.

One promising construction material for the application of the combined system of cellulose microfibers and crystalline microcelluloses is cementitious boards, which are typically produced using various wood-based materials (particles, fibers, microfibers). Cementitious boards are used for internal and external cladding systems due to their high fire resistance, good thermal and acoustic insulation, and dimensional stability (Ashour; Heiko; Wu, 2011).

Composites can contain additions at various scales, allowing for the combined effect of two or more materials employed at different scales (micro and macro). Multi-scale composites have attracted attention in advanced materials (Kim et al., 2009). The knowledge generated in this research will advance the understanding of the combined effect of cellulose microfiber and crystalline microcellulose in cementitious matrices for consolidation in cementitious boards produced by pressing.

Materials e methods

Materials

The materials employed in the research (crystalline microcelluloses, cellulose microfibers, and cement), as shown in Figure 1, are commercial products. The process of obtaining FC-MCC celluloses was provided by the company that donated the inputs for the research.

The cellulose microfiber was obtained through the reprocessing of industrial paper waste. The material was ground, separated by air separators, passed through magnetic and heavy particle filters, and moistened to form pulp. Subsequently, it was washed, sieved, and dried in an oven. It underwent separation again before being ground to a size of 400 micrometers. Finally, calcium carbonate was added to facilitate mixing the microfiber to apply cementitious materials. In Table 1 the semi-qualitative analysis of cellulose microfiber is shown.

With the chemical analysis by X-ray fluorescence, in which the results are expressed in percentage oxides, it is observed that the ashes of cellulose microfibers are mainly composed of calcium oxide, silica, and alumina (85.7%). The high contents of silica (22.9%), alumina (13.6%), and calcium oxide (49.2%) found in the ashes of cellulose microfibers result from the treatment performed on the microfibers. The chemical compositions of natural cellulose microfibers and cement are different, and the interaction between them is generally complex, causing incompatibility along the interface of cementitious systems (Kabir; Lau; Cardona, 2012).

The crystalline microcellulose was obtained by subjecting purified cellulose fiber to acid hydrolysis under controlled conditions. The cellulose fiber pulp was treated with a diluted acid solution in the first phase. During hydrolysis, the acid molecules acted on the amorphous regions, breaking the β-bonds. The resulting water-soluble cellulose oligosaccharides and glucose were removed in the subsequent process. Subsequently, the paste underwent washing, filtration, and grinding to obtain the particle. In Table 2, the bulk density and mean particle size of the FC-MCC celluloses are shown, which were provided by the manufacturer.

The type of cement employed in the research is CP V ARI, justified by the absence of physical and chemical interference of the FC-MCC celluloses in the production of cementitious boards.

The evaluated contents of cellulose microfiber (0.5% and 1.0%) and crystalline microcellulose (0.4% e 0.6%) used in this research were based on previous studies (Fan et al., 2012; Bezerra et al., 2006; Ardanuy; Claramunt and Toledo Filho, 2015; Silva et al., 2018; Moraes et al., 2018). To select combined formulations for producing cementitious boards, the criteria of calorimetry and squeeze-flow previously evaluated by Bilcati, Costa and Tamura (2024) were defined:

1. calorimetry assesses the hydration process during the first 24 hours of cementitious paste production. FC-MCC celluloses may delay the setting time of the boards, rendering the production process unfeasible (Okino et al., 2004); and

2. the squeeze-flow test simulates the condition existing in the practical situation of molding in the production process of compression-molded cementitious boards. During the squeeze-flow test, the material in its fresh state is subjected to requests involving compression and geometric restriction, similar to cementitious boards.

Based on this study, the formulations FC 0.5-MCC 0.4; FC 0.5-MCC 0.6 and FC 1.0-MCC 0.4 were validated for producing cementitious boards.

Research Methodology

The cementitious boards, combined with cellulose microfiber and crystalline microcellulose, were molded on a laboratory scale using the pressing molding technique, based on the method developed by Bison (1978). The formulations of the cementitious boards with the addition of FC-MCC celluloses are shown in Table 3.

Three boards were made for each formulation with dimensions of 47 cm x 35 cm x 1.5 cm (length, width, and thickness). The mixing method combined the dry materials (cement + cellulose microfiber) inside plastic bags until homogenization was achieved. To promote the dispersion of crystalline microcelluloses in the cementitious system, a more straightforward and less intensive technique was developed based on the methodology of Bilcati, Costa and Tamura (2024). The crystalline microcelluloses were added to water, and the solution was then stored for 24 hours to hydrate the microparticles. Subsequently, the aqueous solution was manually stirred for 5 minutes and immediately added to the dry materials to produce the cementitious boards. After preparation, the mixture was weighed and distributed into a wooden forming box (Figure 3a). Separated by aluminum sheets (Figure 3b), a second and a third panel were overlaid (Figure 3c). Stapling was carried out, which involved keeping the board under constant pressure for 24 hours after opening the press. For this purpose, screws and threads were placed connecting the entire panel (Figura 3d).

The cementitious boards were pressed at room temperature (20º ± 2 ºC) and relative humidity of 65 ± 5٪ using a hydraulic press for molding with a capacity of 40 tons and a constant pressure of 40 kgf/cm² for approximately 5 minutes (Figure 4a). After this stage, the boards were kept in closed form for 24 hours to ensure the necessary pressure on the newly molded board. After pressing, the boards were removed from the press (Figure 4b) and immediately taken for curing, remaining in this state for seven days. The standard curing procedure involves storing the cementitious boards in a humid chamber with control over temperature and relative air humidity. However, the cement board production laboratory has no humid chamber. Therefore, the curing procedure adopted by Rossetto (2007) was chosen. The curing procedure involved immersion in alkaline water, saturated explicitly with calcium hydroxide (2g Ca(OH)₂/liter of water). After curing, the prepared boards were taken to the internal chamber at room temperature for 21 days for cement maturation. Tests on the cementitious boards were conducted at 28 days of age.

The cementitious boards were cut into specimens using a diamond saw blade. For static bending tests (SS, 1993a), three specimens per panel were prepared with nominal dimensions of 50 mm in width, 15 mm in thickness, and 370 mm in length. For apparent density (ME) testing EN (SS, 1993c), water absorption (AA), and thickness swelling (IE) (SS, 1993c), five specimens were prepared with dimensions of 50 mm in width, 50 mm in length, and15 mm in thickness. Figure 5 shows the specimens obtained by cutting the cementitious boards.

Physical-mechanical characterization of the cement boards

The cementitious boards produced by pressing were characterized through static bending tests (elastic modulus and rupture modulus), as well as apparent density, thickness swelling, and water absorption at 2 and 24 hours of immersion. For the static bending test (SS, 1993a), the Emic universal testing machine, 2000 kgf (or 20 kN) with a digital data collection system, was used, from which the values of modulus of elasticity at rupture and modulus of elasticity in bending were obtained (Figure 6a). For the tests of apparent density, water absorption, and thickness swelling (SS, 1993b), the thickness of the specimens was measured with a micrometer with a division of 0.001 mm, working range of 0-25 mm, and weighed on a precision balance. Then, the specimens were immersed in water for two hours and measured and weighed again. After the measurement, they were engaged in water again for 24 hours and measured and weighed.

Results and discussion

The combined additions of cellulose microfiber and crystalline microcellulose were investigated for their influence on the physical and mechanical properties of cementitious boards produced through the pressing process. To evaluate the results, ANOVA statistical analysis and comparison between means using the Tukey test were performed, considering confidence intervals of 95%.

Physical properties of cement boards

To analyze the combined formulations of cellulose microfiber and crystalline microcellulose selected to produce cementitious boards, the results of average values of physical properties such as apparent density, water absorption, and thickness swelling were obtained.

Apparent density, water absorption, and thickness swelling

In Figure 7, the average values (Figure 7(a)) and statistical analysis (Figure 7(b)) of the apparent density of cementitious boards with and without the addition of FC-MCC celluloses are presented after 2 and 24 hours of immersion, respectively. Based on the statistical analysis, it was possible to verify that some formulations showed significant differences between the means of the apparent density, as shown in Figure 7(b). Notably, non-significant differences were not plotted on the graph (Figure 7(b)).

The data from Figure 6(b) shows that there was a significant effect on the type and content of FC-MCC celluloses in the cementitious boards. The formulation FC 0.5-MCC 0.6 showed higher average apparent density values, significantly differing from the other formulations. It is also noted that the lowest average apparent density values were recorded when a higher content of cellulose microfiber was used, which were statistically different. It is possible to conclude that the high content of microfiber may have contributed to great air incorporation, as mentioned by Pescarolo et al. (2022). In contrast, the high content of crystalline microcellulose tends to produce denser materials. Figure 8 shows the average water absorption values at 2 and 24 hours of immersion (Figure 8(a)) and the statistical analysis where differences between the significant means considering the 95% confidence interval are presented in graph (Figure 8(b)).

The results of water absorption after two hours of immersion for the FC 1.0-MCC 0.4 and FC 0.5-MCC 0.6 formulations increased the average values compared to the reference, differing statistically. The average values found after 24 hours of immersion in water for formulations containing cellulose microfiber and crystalline microcellulose differed statistically from the reference. This phenomenon occurs because cellulose-based materials are hydrophilic, providing more hydroxyl groups to the cementitious system (Çavdar; Yel; Torun, 2022). The average values obtained for water absorption after 2 and 24 hours of immersion in this study ranged from 12.42% to 18.38% at 2 hours of immersion and from 14.27% to 19.75% at 24 hours of immersion, and are considered relatively low, especially when compared to Iwakiri et al. (2012) where treatments ranged from 17.72% to 25.27% at 24 hours of immersion, as well as Castro et al. (2014) ranging from 21.89% to 45.85% at 2 hours of immersion and 29.11% to 48.66% at 24 hours of immersion. Figure 8 shows the average values and statistical analysis of thickness swelling of the cementitious boards with and without the addition of FC-MCC celluloses after 2 and 24 hours of immersion, respectively. Through statistical analysis performed by the Tukey test and considering 95% confidence intervals, it was found that the formulations did not present significant differences between the means, as shown in Figure 9(b).

The thickness swelling variables at 2 and 24 hours were unaffected by the combined addition of cellulose microfiber and crystalline microcellulose. This can be justified by the cementitious matrix preventing volumetric and linear movement of the cementitious boards through pressing. The results of water immersion thickness swelling indicated minimal dimensional variation, where at 2 hours of immersion, all studied formulations ranged from 0.67% to 0.82%, and at 24 hours ranged from 1.33% to 1.79%. This factor may be related to the uniformity of the cementitious boards due to pressing. In the treatments performed, it is indicated that the formulations do not promote an increase in dimensional variation. All cementitious boards evaluated in this study showed thickness swelling below the maximum reference value of 0.8% at 2 hours of immersion and 1.8% at 24 hours of immersion, indicated as requirements for cementitious boards by the Bison method (1978) and ISO standard 8355 (ISO, 1987).

Mechanical properties of cement boards

To evaluate the effect of the combined addition of cellulose microfiber and crystalline microcellulose on mechanical performance (FC 0.5-MCC 0.6, FC 1.0-MCC 0.4, and FC 0.5-MCC 0.4), the flexural modulus of elasticity (MOE) and modulus of rupture (MOR) were assessed.

Flexural modulus of elasticity and modulus of rupture

In Figure 9, the average values (Figure 9(a)) and statistical analysis (Figure 9(b)) of MOE and MOR of all studied formulations. Through statistical analysis using the Tukey test with 95% confidence intervals, it was observed that only the addition of FC 0.5-MCC 0.6 showed a significant difference between the means for both MOE and MOR results, as depicted in Figure 10(b). Non-significant differences between the means were not plotted on the graph.

The average values of the MOE of the cementitious boards with and without the combined addition of FC-MCC celluloses show no significant difference between the studied formulations, except for the FC 0.5-MCC 0.6 formulation. The modulus of elasticity ranged from 2313 to 5428 MPa. The values obtained in this research were like those found by Çavdar, Yel and Torun (2022) for cementitious boards reinforced with crystalline microcelluloses ranging from 2294 to 6106 MPa. On the other hand, when compared to cement panels reinforced with wood particles, the maximum MOE values obtained by Iwakiri et al. (2012) were 3586 MPa for paricá particles and 3539 MPa for embaúba particles. Mendes et al. (2011) found a maximum value of 1665.73 MPa for eucalyptus clones, which are lower than the maximum values found in this research. Based on the results obtained, it is possible to conclude that the combined addition of FC 0.5-MCC 0.6 promoted an increase in the MOE. The maximum content of crystalline microcellulose used in this study resulted in a significant increase in MOE, which can be attribute to the enhanced hydration provided by the crystalline microcelluloses to the cementitious system (Bilcati; Costa; Tamura, 2024; Hoyos; Cristia; Vázquez, 2013). According to ISO 8355 (ISO, 1987) and Bison (1978), cementitious boards must have standardized minimum values of 3000 MPa. All additions of FC-MCC celluloses combined in the cementitious boards met the minimum requirements of ISO 8355 (ISO, 1987).

Considering the results presented in Figure 9, it can be observed that the combined additions did not lead to an increase in MOR when compared to the reference, except for the FC 0.5-MCC 0.6 formulation, which showed a statistically significant increase. The average MOR values obtained in this study ranged from 4.90 to 9.32 MPa. In comparison with the data presented in the literature, the MOR values of this research were similar to those found by Çavdar, Yel, and Torun (2022), where the average values of cementitious boards with the addition of crystalline microcellulose ranged from 3.40 to 10.55 MPa at 28 days. Tonoli et al. (2010) obtained a modulus of rupture at 28 days of 9.9 MPa from cellulose pulp, a value like the maximum modulus of rupture obtained in this study. Mendes et al. (2011), Iwakiri et al. (2012), and Castro et al. (2014) obtained maximum MOR values lower than those found in this research.

Regarding the mechanical properties presented, it was possible to observe that the combined addition of FC 0.5-MCC 0.6 meets the minimum requirement set by ISO 8355 (ISO, 1987) and Bison (1978) for cementitious boards, which establish values higher than 9.0 MPa for MOR. However, the other combined additions of FC-MCC celluloses tested did not meet the minimum requirement standardized by ISO 8335 (1987). Notably, the MOR was influenced by the high content of crystalline microcellulose, indicating that MCCs can increase the rupture resistance of cementitious boards. Çavdar, Yel and Torun (2022) also observed an improvement in the mechanical strength in the modulus of rupture of cementitious boards with the addition of crystalline microcellulose. The authors attributed the good performance of MCCs in cementitious boards to increased hydration. The cementitious boards with the addition of FC-MCC celluloses are heterogeneous products composed of domains of different materials (phases), so the characteristics obtained from the boards are the result of combined effects of different microstructural parameters, such as specific surface area (crystalline microcellulose) and packing density (cement), which contribute to the refinement of the matrix pores. The presented results indicated the ability of crystalline microcellulose at a 0.6% content to enhance the mechanical performance of cementitious boards with the addition of cellulose microfiber at 0.5%, highlighting the potential of combined additions of crystalline microcellulose and cellulose microfiber to produce cementitious boards.

The results obtained showed that the high content of crystalline microcellulose at 0.6% employed in this research resulted in an increase in both apparent density and rupture and elasticity moduli. This suggests that these materials act as nucleators for hydration products in Portland cement pastes at the microscopic level, filling the nanopores of cementitious plates. This phenomenon was also observed by authors He et al. (2023) and Wu et al. (2021).

Conclusions

This study aimed to evaluate the influence of combined FC-MCC cellulose additions on the physical and mechanical properties of cementitious boards produced by pressing. The following conclusions can be outlined:

1. regarding the physical properties of cementitious boards, thickness swelling was not affected by the combined addition of cellulose microfiber and crystalline microcellulose, promoting thickness stability. The average thickness swelling values are in accordance with the standards established by ISO 8355 (ISO, 1987) and Bison (1978);

2. concerning the mechanical properties of the boards, the formulation combined with a higher content of crystalline microcellulose FC 0.5-MCC 0.6 showed an increase in the average values of modulus of rupture and elasticity, which was statistically significant, unlike the other formulations;

3. all results of thickness swelling, and flexural modulus of elasticity obtained for cementitious boards with combined FC-MCC addition meet the minimum requirements established by ISO 8335 (ISO, 1987); and

4. the presented results highlighted the potential of crystalline microcellulose at a 0.6% content to optimize application in cementitious boards when combined with 0.5% cellulose microfiber. This formulation was identified as the best performance for the development of cementitious boards.

The results obtained in this research are useful for understanding the interaction of two types of cellulose combined in the performance mechanisms of cement boards in both fresh and hardened states. For further studies, it is possible to subject the cementitious boards with FC-MCC addition to natural and accelerated aging to verify the combined effect of FC-MCC on the durability of cementitious boards, as well as to assess the correlation with the increase in rupture and elasticity modulus due to porosity reduction.

References

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