1. Introduction
Rice cultivation is predominantly concentrated in Asian countries, and rice serves as a primary staple food for Taiwanese people. In 2023, the rice cultivation area in Taiwan reached 222,000 ha, yielding 1.46 million tons of paddy rice. During the rice production process, large amounts of crop residues, such as rice husk (RH) and straw, are generated. In Taiwan, 292,074 tons of RH, which accounts for ~20 wt.% of paddy rice, were generated in 2023 [
1]. The main components of RH are approximately 80% organic substances (e.g., cellulose, hemicellulose, and lignin) and 20% inorganic substances, most of which are silica. RH also has several unique characteristics including a low bulk density, high specific surface area, and porous structure [
2,
3,
4,
5].
Historically, in the early 20th century, RH was used as an additive to building materials in Taiwan when agriculture was the main economic activity. Traditional Taiwanese architecture often uses a composite of RH and clay to create adobes in house construction [
6]. With the development of building materials and engineering technology, the adobes have become historical monuments, and RH is no longer used in modern building materials. In recent years, many researchers have studied the use of rice husk ash (RHA) in concrete materials [
7,
8,
9,
10,
11,
12]. RHA is produced by burning RH with sufficient oxygen. The organic substances of RH are completely removed, and RHA containing high-purity silica is thus obtained. RHA can be used as a pozzolanic material in concrete, and many studies indicate that the use of RHA can increase the compressive strength and durability of concrete materials [
7,
8,
11]. However, the combustion of RH increases CO
2 emissions and causes air pollution [
13,
14]. In addition, rice husk biochar (RHB) and rice straw have been used in building materials [
15,
16,
17]. Muthukrishnan et al. [
17] converted RH to RHB and then used it in cement mortar to improve the mechanical strength and durability. The addition of RHB also significantly reduced the autogenous shrinkage of the cement mortar. Pachla et al. [
16] used rice straw in cellular concrete and reported that the rice straw increased the three-point bending strength but decreased the compressive strength. Moreover, the rice straw improved the sound and heat insulation of the resultant cellular concrete. The Taiwan government has been committed to developing a circular economy in recent years, and one of the related policies is to advance the treatment and recycling technologies for agricultural wastes [
18]. Previous studies indicate that wastes derived from rice cultivation should have the potential to be used in concrete materials.
Owing to its low bulk density, high silica content, and porous nature, RH is suitable for the development of lightweight concrete products, such as autoclaved lightweight concrete (ALC). ALC is a lightweight precast component created by evenly distributing air bubbles within concrete via the addition of lightweight materials, foaming agents, or gases. ALC typically has a thermal conductivity between 0.2 and 1.0 W/m·K and a bulk density ranging from 0.3 to 1.8 g/cm³, which is approximately 25% that of conventional concrete [
19,
20,
21]. Compared with conventional concrete, ALC has low thermal conductivity, excellent sound and heat insulation, and fire resistance, and it has a wide range of applications [
19,
22,
23]. Some studies have indicated that the use of RH as a substitute in ALC production can not only reduce the bulk density but also improve the mechanical strength and durability. During ALC production, the silica from RH can react with Ca(OH)
2 to form calcium silicate hydrates (CSHs), and the transformation of CSHs into tobermorite during autoclave curing could improve the mechanical strength and durability [
24,
25].
The formation of tobermorite is closely related to the Ca/Si molar ratio in the material. Optimal tobermorite formation occurs when the Ca/Si molar ratio is between 0.8 and 1.0. Moreover, different Ca/Si molar ratios affect the structural characteristics of tobermorite. Generally, tobermorite in plate form provides greater mechanical strength than tobermorite in needle form [
26,
27]. However, some studies have reported that the mechanical strength of the materials decreases as the proportion of RH increases [
28,
29], which could be attributed to several factors. The use of RH increased the porosity of the composite, compromising the overall structural integrity [
30]. Moreover, the composite specimens exhibited high water absorption and air content. This can potentially lead to insufficient hydration reactions and consequently reduce the mechanical strength [
31]. Doumongue et al. [
32] indicated that the distribution and bonding of RHs within a composite are other crucial factors for maintaining mechanical strength. Chabannes et al. [
33] also suggested that the relatively lower stiffness and greater deformability of RHs further contribute to the reduction in mechanical strength.
Given the aforementioned challenges, the aim of this research is to optimize the use of RH in ALC production to increase structural integrity and performance. In the experimental work, the characteristics of raw RH were examined first, and the raw RH was then separated into four particle sizes for further analyses and experiments. The RHs with different particle sizes were used for ALC production, and the effects of the RH amount and water-to-solid (W/S) ratio on the properties of the ALC specimens were studied.
2. Materials and Methods
The raw RH was obtained from a rice mill in southern Taiwan. To examine the characteristics of raw RH, proximate analysis (moisture content, loss on ignition (LOI), and ash content), chemical composition analysis, X-ray diffraction (XRD) analysis, and particle size analysis were conducted. The moisture content was determined by drying the RH at 105 °C for 24 h to obtain the weight loss of water. LOI was measured by calcining the RH at 850 °C for 3 h. The residual weight after the LOI measurement was used to calculate the ash content. The chemical composition of the RH was determined via an X-ray fluorescence (XRF) spectrometer (XEPOS, Spectro, Kleve, Germany). The XRD analysis was conducted by using an X-ray diffractometer (D8 Advance, Bruker, Karlsruhe, Germany) with Cu-Kα radiation. The XRD patterns were recorded between 5°2θ and 65°2θ with a scanning step of 0.02°. Before the XRF and XRD analyses, the RH was dried and ground into a powder with a particle size less than 0.075 mm. The particle size analysis for the RH was conducted by using a test sieve shaker (AS 200 Basic, Retsch, Haan, Germany) with standard test sieves of 0.075 mm, 0.15 mm, 0.3 mm, 0.6 mm, 1.2 mm, and 2.4 mm.
To obtain RHs with different particle sizes for the production of ALC, the RH was shredded with a pulverizing machine (RT-02A, Rong Tsong Precision Technology Co., Taichung, Taiwan) and then separated into four particle sizes, namely, RH-L (>1.2 mm), RH-M (0.6–1.2 mm), RH-S (0.3–0.6 mm), and RH-F (<0.3 mm). Photos of these RHs are shown in
Figure 1. It was observed that most of the >1.2 mm particles (RH-L) were intact RH. In contrast, RH-F (<0.3 mm) was a fine powder, and few features of RH remained. For the preparation of ALC specimens, slaked lime (Ca(OH)
2, 97.8%, Schaefer Kalk, Diez, Germany), silica powder (SiO
2, 99.5%, Alfa Aesar, Ward Hill, MA, USA), and cement (Taiwan Cement Corporation, Taipei, Taiwan) were employed as the ALC raw materials in this study. The weight percentages of slaked lime, silica powder, and cement were 45.0 wt.%, 45.0 wt.%, and 10.0 wt.%, respectively.
Table 1 shows the chemical compositions of the ALC raw materials.
The RHs (RH-L, RH-M, RH-S, and RH-F) were used as lightweight additives, and the amounts of RH were between 0 wt.% and 20 wt.%. The raw materials and RHs were mixed together with a rotary agitator for 30 min and then put into a mixing bowl. Water was added to the raw mixture at a specific W/S ratio (0.7–0.8 L/kg) and then blended into slurry with an electric blender. The slurry was poured into 50 mm cubic molds where it was allowed to expand. The expansion of the slurry nearly finished after standing for 30 min, and the bump on the top of the molds was removed. The molds were then moved into a moist closet for precuring, in which the temperature was 23 ± 2 °C and the relative humidity was ≥95%. After precuring for 24 h, the hardened specimens were obtained and the molds were subsequently removed. The hardened specimens were cured with an autoclave for 12 h to promote hydration reactions. The autoclave curing temperature was set at 189 °C, and the pressure of the saturated steam reached approximately 12 atm. The ALC specimens were prepared and then dried at 105 °C for 24 h before further testing. The bulk density and compressive strength of the ALC specimens were measured according to ASTM C1693. The compressive strength was determined with an electric testing machine (HCH-239-20T, Jin Ching Her Co., Ltd., Yunlin, Taiwan) at a loading rate of 1.0 mm/min. The water absorption test for the ALC specimens was conducted according to CNS 619. The dried ALC specimens were weighed (W
d) and then immersed in boiling water for 3 h. After cooling to room temperature, the specimens were taken out of the water, and the water attached to the surfaces was removed. The specimens saturated with water were weighed (W
s), and the water absorption percentage was obtained via Equation (1).
The scheme of the full experimental setup is presented in
Figure 2.