Performance and Microstructure Evaluation of Lightweight Low-Heat Roof Tile Using Crushed Coconut Shell as Fine Aggregate

Abstract The growing demand for energy-efficient and sustainable construction materials has led to increasing interest in alternative aggregates derived from agricultural waste. This study presents a comprehensive investigation into the use of crushed coconut shell (CS), a low-cost and renewable by-product from the coconut milk industry, as a partial fine aggregate replacement in lightweight low-heat roofing tiles. The research focuses on optimizing both mechanical strength and thermal insulation performance of roofing tiles in accordance with ASTM C1492. Specimens were produced with two water-to-cement ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">w</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">c</mml:mi> </mml:mrow> </mml:math> ) ratios (0.50 and 0.55) and six CS-to-total aggregate ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi>CS</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">A</mml:mi> </mml:mrow> </mml:math> ) ratios (0.18, 0.22, 0.26, 0.30, and 0.34 by total aggregate weight), with a constant cement-to-aggregate ratio of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>∶</mml:mo> <mml:mn>2.5</mml:mn> </mml:mrow> </mml:math> . A key strength of this study lies in its integrated experimental design, which evaluates not only conventional engineering properties—including compressive strength, flexural strength, water absorption, density, water permeability, and thermal conductivity—but also microstructural characteristics through scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and mercury intrusion porosimetry (MIP). The multiscale approach enables a detailed understanding of how CS content affects the internal structure, interfacial bonding, and functional performance of mortar. Results demonstrate that increasing the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi>CS</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">A</mml:mi> </mml:mrow> </mml:math> ratio significantly enhances the thermal insulation of mortar (up to 52% reduction in thermal conductivity) while reducing unit weight (up to 20%), aligning with lightweight design principles. At moderate CS replacement ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi>CS</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">A</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.18</mml:mn> <mml:mo>–</mml:mo> <mml:mn>0.26</mml:mn> </mml:mrow> </mml:math> ), specimens maintain sufficient compressive strength ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mo form="prefix">≥</mml:mo> <mml:mn>21</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>MPa</mml:mi> </mml:mrow> </mml:math> ) and flexural strength ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mo form="prefix">≥</mml:mo> <mml:mn>4.88</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>MPa</mml:mi> </mml:mrow> </mml:math> ), exceeding minimum requirements for structural lightweight concrete. Additionally, the specimens meet water absorption and impermeability standards when <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi>CS</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">A</mml:mi> <mml:mo>≤</mml:mo> <mml:mn>0.30</mml:mn> </mml:mrow> </mml:math> . SEM and MIP analyses confirm that CS promotes internal curing through its porous structure but may compromise matrix cohesion when used excessively ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi>CS</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">A</mml:mi> <mml:mo>≥</mml:mo> <mml:mn>0.34</mml:mn> </mml:mrow> </mml:math> ), leading to larger porosity and lower mechanical performance. This study highlights the dual functionality of CS as both a lightweight aggregate and thermal insulator, offering substantial environmental and structural benefits. The findings not only provide practical guidance for optimizing CS content in lightweight low-heat roof tile but also contribute to circular economy goals by promoting the upcycling of agricultural waste into high-performance building materials.