Effect of Rice Husk Addition on the Hygrothermal, Mechanical, and Acoustic Properties of Lightened Adobe Bricks

Abstract

In the context of efforts to reduce greenhouse gas emissions in the building sector, the reintegration of traditional earthen construction into modern architectural and renovation practices offers a sustainable alternative. To address the mechanical and water-resistance limitations of adobe bricks, the use of agricultural waste-such as rice husk-is increasingly being explored. This experimental study evaluates the effects of rice husk addition on the mechanical, hygrothermal, and acoustic properties of adobe bricks. Two soil types-one siliceous and one calcareous-were combined with 1, 2, and 3 wt% rice husk to produce bio-based earthen bricks. The influence of rice husk was found to depend strongly on the soils' mineralogical and granulometric characteristics. The most significant improvements were in hygrothermal performance: at 3 wt%, thermal conductivity was reduced by up to 35% for calcareous soil and 20% for siliceous soil, indicating enhanced insulation. Specific heat capacity also increased with husk content, suggesting better thermal inertia. The moisture buffering capacity, already high in raw soils, is further improved due to increased surface porosity. Mechanically, rice husk incorporation had mixed effects: a modest increase in compressive strength was observed in siliceous soil at 1 wt%, while calcareous soil showed slight improvement at 3 wt%. Acoustic performance remained low across all samples, with minimal gains attributed to limited macro-porosity. These findings highlight the importance of soil composition in optimizing rice husk dosage and suggest promising potential for rice husk-stabilized adobe bricks, especially in thermally demanding environments.

Keywords: acoustics; adobe; compressive strength; lightened earth; moisture buffer value; rice husk; thermal performance.

Figure 1

(a) Geographic origin of the soil samples—from Polidori et al., 2025 [25]; (b) 51-AT adobes collected from a traditional barn; (c) excavation site of 51-CH soil.

Figure 2

Bulk rice husk (a); rice husk in detail (b).

Figure 3

Overview of the soil and rice husk volumes for a 3 wt% content (a); drying of cubic earth-rice husk samples (b).

Figure 4

Compression test on a 3 wt% rice husk sample. (a) Specimen in place between compression platens; (b) Crushed specimen with corresponding failure mode.

Figure 5

PSD of soil samples with wet sieving (top) and laser granulometry (bottom).

Figure 6

Size distribution of rice husk.

Figure 7

SEM image of 51-AT soil (×5000 magnification).

Figure 8

SEM image of 51-CH soil (×3000 magnification).

Figure 9

(a) Rice husk (×40 magnification); (b) cross-section (×1000 magnification).

Figure 10

EDX spectra of 51-AT, 51-CH, and rice husk.

Figure 11

Dry density vs. rice husk content.

Figure 12

Normalized stress–strain curves for 51-AT (left) and 51-CH (right).

Figure 13

Compressive strength vs. rice husk content.

Figure 14

Compressive strength vs. dry density.

Figure 15

Thermal characteristics vs. rice husk content for 51-CH (left) and 51-AT (right).

Figure 16

Classification of moisture buffer values (from Rode [33]).

Figure 17

Moisture buffer value of biocomposites vs. rice husk content.

Figure 18

Weighted acoustic absorption coefficient ${\alpha }_w$ vs. rice husk content.