*Article* **Bamboo Sawdust as a Partial Replacement of Cement for the Production of Sustainable Cementitious Materials**

**Yunyun Tong <sup>1</sup> , Abdel-Okash Seibou <sup>1</sup> , Mengya Li 2,\* , Abdelhak Kaci <sup>2</sup> and Jinjian Ye <sup>1</sup>**


**Abstract:** This paper reports on the utilization of recycled moso bamboo sawdust (BS) as a substitute in a new bio-based cementitious material. In order to improve the incompatibility between biomass and cement matrix, the study firstly investigated the effect of pretreatment methods on the BS. Cold water, hot water, and alkaline solution were used. The SEM images and mechanical results showed that alkali-treated BS presented a more favorable bonding interface in the cementitious matrix, while both compressive and flexural strength were higher than for the other two treatments. Hence, the alkaline treatment method was adopted for additional studies on the effect of BS content on the microstructural, physical, rheological, and mechanical properties of composite mortar. Cement was replaced by alkali-treated BS at 1%, 3%, 5%, and 7% by mass in the mortar mixture. An increased proportion of BS led to a delayed cement setting and a reduction in workability, but a lighter and more porous structure compared to the conventional mortar. Meanwhile, the mechanical performance of composite decreased with BS content, while the compressive and flexural strength ranged between 14.1 and 37.8 MPa and 2.4 and 4.5 MPa, respectively, but still met the minimum strength requirements of masonry construction. The cement matrix incorporated 3% and 5% BS can be classified as loadbearing lightweight concrete. This result confirms that recycled BS can be a sustainable component to produce a lightweight and structural bio-based cementitious material.

**Keywords:** bamboo; sawdust; pretreatment; bio-based material; mechanical property

### **1. Introduction**

The Paris 2024 Olympics have committed to reducing their carbon footprint, with the Olympic and Paralympic Villages being built using 100% bio-based materials. This example illustrates that the trend in the construction industry in the twenty-first century is towards sustainable and environmentally friendly building materials. To date, concrete has been the most widely used building material in the world, with ordinary Portland cement being the key ingredient in concrete. Cement production exceeds 4 billion tons per year and leads to ~8% of the world's carbon dioxide emissions [1], which is the main driver of the greenhouse effect. According to the Paris Agreement on climate change, emissions from the cement sector should decrease by at least 16% by 2030. These factors explain our passion for investigating a natural and sustainable substitute for cementitious materials.

China contains 5.4 million hectares of bamboo planting area where the environment and climate are suitable for bamboo growing [2]. Bamboo belongs to the grass family, which consists of more than 1600 species. As the fastest-growing plant in the world, bamboo can grow up to 90 cm per day and reach maturity in only 2–3 years. Mature bamboo has almost the same strength and hardness as hardwood, which take more than 50 years to mature [3]. Its rapid maturity and sustainability have made bamboo the main rival for wood in the construction industry over the past 20 years. Applications of bamboo in

**Citation:** Tong, Y.; Seibou, A.-O.; Li, M.; Kaci, A.; Ye, J. Bamboo Sawdust as a Partial Replacement of Cement for the Production of Sustainable Cementitious Materials. *Crystals* **2021**, *11*, 1593. https:// doi.org/10.3390/cryst11121593

Academic Editors: Chongchong Qi, Yifeng Ling, Chuanqing Fu, Peng Zhang and Peter Taylor

Received: 25 November 2021 Accepted: 17 December 2021 Published: 20 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

various fields are increasing, and include framework, cladding, posts, and furniture. The main environmental problem in the booming bamboo industry is solid waste disposal. According to a report from the Cultural and Tourism Office of Anji, China, in 2015, the city produced more than 0.16 million tons of bamboo waste per year. Most bamboo waste is generated as clean biomass energy, but limited amounts of bamboo are reused. The efficient recycling of these wastes and their reuse as an ingredient in bio-based materials was the initial objective of this research.

Many studies have been carried out on mortar or concrete that incorporates lignocellulose aggregates, including hemp [4–6], wood [7], sisal [8], jute [9], coconut [10], palm oil [11], and sugarcane bagasse [12]. Cellulose, hemicellulose, and lignin are the primary biochemical compositions of these natural fibers. Other components as impurities, such as pectin and wax substances, also exist on the fiber surface. Cellulose is the main composition of cell wall, which provides the strength of fiber. It is insoluble in water, organic solvents and alkaline solution [13]. Hemicellulose comprises the sugar component, which bridges cellulose fibers with hydrogen bonds. Lignin is a cementing material. Bamboo fibers contain 40–55% cellulose, 18–20 hemicellulose, and 15–32% lignin [14]. The high proportion of cellulose makes bamboo a potential component for building materials. However, limited literature has focused on the valorization of bamboo in cementitious materials. Fias et al. [15] studied the replacement of 10–20 wt.% cement by Brazilian bamboo leaf ash (BLA) calcined at 600 ◦C. The same compressive strength resulted for the control mortar and BLA blended mortar, which confirmed the use of BLA as a substitute for cement. Xie et al. [16] observed that the flexural strength and impact resistance of mortar could be improved by reinforcement with 4–16 wt.% bamboo fibers (BF). Shrinkage could be inhibited by 12% because of the BF reinforcement [17]. However, efficient incorporation of vegetable products in the cementitious matrix requires the overcoming of problems of workability, compatibility, and durability. The workability problem relates to the high water absorption of vegetable fibers [18]; the compatibility problem relates to a weak bonding between natural fibers and the cement matrix [19]; and the durability problem relates to biodeterioration and a resistance to freezing and thawing [20]. To overcome these problems, the fibers are pretreated before fabricating the composite. The pretreatment methods, such as cold water, hot water, and alkali solution, have been investigated in the literature in order to modify the structure and morphology of natural fibers and improve the matrix interface, as well as increasing the durability of fibers in the alkaline environment of the cement matrix [21–24].

Sawdust is a by-product generated during the process of manufacturing. The recycling of such waste provides the benefits of reducing the need to extract new raw materials and limiting the air pollution due to incineration. Ahmed et al. investigated the potential of wood sawdust as a replacement of fine aggregates in concrete [25], and confirmed the utilization of this material for structure application. Besides, lightweight concrete incorporated sawdust presented a thermal conductivity 23% lower than conventional concrete [26].

To the best of our knowledge, few studies have reported incorporating bamboo sawdust (BS) in a cementitious matrix. This study contributes to the design of a new cementitious material containing local BS. The effect of different pretreatment methods for BS on the composite were firstly assessed through their morphological, physical, and mechanical behaviors, which aimed to choose a more efficient treatment. Furthermore, the characteristics of composites incorporating different proportions of alkali-treated BS were analyzed.

## **2. Materials and Methods**

### *2.1. Raw Materials*

The bamboo particles that were used in this study were recycled sawdust from a local bamboo furniture manufacturing industry (Zhejiang, China). The species of bamboo was moso (Phyllostachys edulis), which is most common in China.

The particle size distribution of the recycled BS is presented in Figure 1. The average particle size (D50) of the BS was 0.09 mm. A BS particle size of between 0.037 and 0.16 mm was used. The basic characteristics of the moso BS are summarized in Table 1. The particle density was determined by the Archimedes method, where the apparent mass of BS was measured in air and after immersion in distilled water with a pycnometer. The bulk density was calculated from the BS mass divided by the volume occupied.

**Figure 1.** Recycled bamboo particle size distribution.

**Table 1.** Characteristics of moso BS.


Sawdust treatment modifies the rheological, physical, and mechanical properties of mortar that is reinforced with vegetable particles. Three types of treatment were used to remove excessive lignin and hemicellulose, and to improve the wettability [22]:


After treatment, the BS was dried in an oven at 60 ◦C for 48 h, and then stored in a desiccator.

### *2.2. Mixture Proportion*

Ordinary Portland cement CEMI 42.5 and standard sand with a proportion of 1:3 were used to prepare the control mortar with no added BS in accordance with EN 196 [27]. The relative specific gravity of cement and sand was 3.12 and 2.64, respectively. The water to binder (W/C) ratio was fixed at 0.5 for mortar and 0.25 for cement paste. Similar to most cellulosic fibers, the BS adsorbs large amounts of water. The BS was pre-wetted to prevent the inner structure from absorbing water from the mixture. The quantity of pre-wetting water (PW) was calculated from:

$$\text{Mass of pre-wetting water} = \frac{\text{mass at saturation state } - \text{dry mass}}{\text{dry mass}} \times \text{BS doseage} \tag{1}$$

The pre-wetting time was determined by the saturation state of the bamboo particles, as shown in Figure 2. The water absorption increased to 86.7% saturation in 1 min, the

growth rate slowed in the following 4 min, and then the growth was stable until complete saturation. The study of Monreal et al. [28] showed that the pre-wetting water should be below complete saturation, otherwise excessive water may result in the mixture. From a perspective of mixing time and energy saving, the pre-wetting time was set to 3 min, at 95% particle saturation, where the pre-wetting water quantity was three times the BS. With the same cement and sand ratio, 1 wt.%, 3 wt.%, 5 wt.%, and 7 wt.% cement was substituted by BS, and the new biomaterials were designated as bamboo sawdust cement mortar BSC1, BSC3, BSC5, and BSC7, respectively. The limit of substitution was set at 7 wt.% due to the very low workability of composite at fresh state (nearly 0), which is not suitable for construction. The results of the slump test are discussed in Section 3.2.1. Details of mixture proportions for formulations are recapitulated in Table 2.

**Figure 2.** BS particle water-absorption rate.


### *2.3. Mixture Preparation*

The BSC composite was prepared by using a mortar mixer. The bamboo particles were pre-wetted for 3 min, and then mixed at a low speed of 140 <sup>±</sup> 5 r/min−<sup>1</sup> for 3 min. Cement and sand were added and mixed for 2 min at a low speed. Mixing continued for 1 min 30 s, after which water was added. After 1 min 30 s, the mixer was stopped as it was scraping the bowl. The last step involved restarting the mixer and running at 285 <sup>±</sup> 10 r/min−<sup>1</sup> for 1 min. The mixing procedure is summarized in Table 3.



All molds that contained specimens were kept in a humid atmosphere (50 ± 5 RH%) for 24 h at 20 ± 2 ◦C before being demolded, and the demolded specimens were kept under water in a temperature-controlled wet preservation cabinet until the tests. A general view of the BSC specimens is presented in Figure 3.

**Figure 3.** BSC mortar specimens.

### *2.4. Morphology*

The BS microstructure with and without treatment, and the composite mortar at 28 days were investigated by scanning electron microscopy (SEM, HITACHI Model TM3000). The BS samples were placed directly on aluminum stub with a diameter of 25 mm. The BSC samples were sputter coated with Au alloy in order to reduce white regions on the image, which were caused by electron charging. The observation condition mode 15kV was applied.

## *2.5. Rheological Properties*

The cement paste and mortar that contained BS were evaluated according to their setting time and workability in the fresh state, respectively. The setting time of the cement paste was measured by using a Vicat apparatus in accordance with NF P15-431 [29]. Slump tests were performed by using a mini slump cone with diameters of 50 mm and 100 mm at the top and base, respectively, and a height of 150 mm, in accordance with MBE (method to design mortar concrete containing admixture) [30] to evaluate the workability of the composite mortar.

### *2.6. Physical and Mechanical Properties*

The density, porosity, and compressive and flexural strength in the hardened state were determined for all specimens. The specimen density was calculated from the measured dry mass at 28 days and the volume. The porosity was characterized according to the AFPC-AFREM testing protocol [31]. The specimens that were cured at 28 days were placed in a desiccator, where a maximum internal constant pressure of 25 mbar was maintained by a vacuum pump. After 4 h, water was introduced to immerse the specimens, and the same pressure was maintained in the desiccator for 68 h. The water-accessible porosity was calculated from:

$$\text{Porosity} = \frac{M\_{\text{air}} - M\_{\text{dry}}}{M\_{\text{air}} - M\_{\text{sat}}} \times 100\% \tag{2}$$

where *Mair* and *Msat* are the mass of vacuum-saturated specimens measured in air and water, respectively, and *Mdry* is the mass of specimen that was oven-dried at 105 ◦C for 24 h.

The compressive and flexural strengths were measured according to EN 196 [27] at 3, 7, 14, and 28 d curing. Cubic specimens (40 × 40 × 40 mm) were prepared for a study of their compressive behavior. Tests were carried out using a compressive machine model STYE-1000, with a force-controlled rate of 2500 N/s. The flexural behavior was evaluated using an electric three-point bending testing machine, model DKZ-6000. The set-up was force controlled with a rate of 50 N/s. Specimens of 40 × 40 × 160 mm were characterized for flexural strength.

### **3. Results and Discussion**

### *3.1. Effect of BS Treatment on BSC Composite*

### 3.1.1. Mass Loss of BS

A remarkable mass loss was observed during the BS treatment. The percentage mass loss because of the treatment was calculated from:

$$\text{Percentage weight loss} = \frac{W\_1 - W\_2}{W\_1} \times 100\% \tag{3}$$

where *W<sup>1</sup>* (g) and *W<sup>2</sup>* (g) represent the BS dry mass before and after treatment, respectively.

Figure 4 shows the BS mass variation between the different treatments. The mass loss was rapid in 1 min, and then slowed to a plateau. The BS lost 13.7% of its mass after aqueous treatment. The mass stabilized after 8 h. However, the hot aqueous accelerated the component removal, and improved the efficiency with 17.5% of the components eliminated in 4 h. The mass variation of alkali-treated particles was most important, with an earlier plateau and a mass loss of 28.2% higher than in the aqueous treatment and 23.6% higher than in the hot aqueous treatment. Das et al. [22] reported the similar significant mass loss of bamboo fibers occurred by alkali attack. It can be noted that the fiber cell wall contains an amount of hydroxyl groups (-OH), which can react with alkaline solution [32]:

$$\text{Cell-OH} + \text{NaOH} \rightarrow \text{Cell-O-NA}^{+} + \text{H}\_{2}\text{O} + \text{Surface impurities} \tag{4}$$

**Figure 4.** BS mass variation during different treatments (**a**) in 24 h (**b**) in the first 15 min.

The removal of alkali-sensitive components (hemicellulose and lignin) results in the mass loss of fiber. The good side is to provide the mechanical and thermal stability of fiber in the composite matrix. The alkali concentration and immersion time both affect mass loss [33].
