A contribution to the study of lightweight geopolymer concrete with expanded clay: evaluation of its thermal performance

Abstract

Purpose: The objective of this work was to evaluate the thermal insulation performance of lightweight geopolymer concretes with expanded clay, considering their implications for energy consumption and construction efficiency.   Theoretical framework: The urgent need for sustainable construction practices amid global concerns about climate change and environmental degradation has been increasingly discussed. With the cement industry being a major contributor to CO2 emissions, alternative materials like geopolymers offer a promising solution, once the consumption of concrete tends to grow bigger. The production of geopolymer concrete, known for its strength and low environmental impact, involves combining a precursor rich in aluminosilicates with an alkaline activator, usually sodium hydroxide and sodium silicate. Notably, geopolymer cement can cut up to 64% of greenhouse gas emissions. Expanded clay, as a lightweight aggregate, garners attention for its porous structure and ability to provide thermal and acoustic insulation to concrete. Its application in constructing vertical enclosures, such as concrete walls, enhances thermoacoustic comfort and aids in assembly and transportation on construction sites. Effective thermal insulation, achieved through materials with low thermal conductivity, plays a pivotal role in creating thermally suitable environments, impacting user satisfaction, productivity, and energy conservation.   Method and materials: The materials used for concrete production, including metakaolin, sodium hydroxide, sodium silicate, expanded clay, crushed stone, and sand, were initially characterized. Subsequently, the dosage calculation for the production of test specimens was performed, involving mini concrete slabs with dimensions of 20x40x12 cm, compacted on a vibrating table. The concretes were produced with volume substitutions of crushed stone by expanded clay at 0% (GP) 30% (GP30%) and 70% (GP70%). For the thermal insulation test, the slabs were exposed to a heat source, and temperatures on the exposed and opposite faces were measured over 12 hours, at 30-minute intervals. After obtaining the experimental data, logarithmic equations with three parameters were fitted to achieve the stabilization temperature of the surfaces.   Results and conclusion: Despite the expectation of stabilization in temperature after exposure to a heat source, it was not observed for the faces of the tested panels. A mathematical curve was derived through a curve-fitting process using logarithmic equations with three parameters. All curve fittings yielded R² values equal to or higher than 0.95, indicating satisfactory representativity and suggesting viability in analyzing panel thermal insulation through the adjusted equations. Considering the adjusted data, thermal insulation values for GP, GP30%, and GP70% were 31.82 °C, 35.30 °C, and 40.24 °C, respectively. Expanded clay's effect on increasing thermal insulation was evident, aligning with references indicating its insulating characteristics. Moreover, the substitution of crushed stone with expanded clay led to a noticeable reduction in specific mass, highlighting the lightweight nature of the compositions. Both 30% and 70% mixes fall under the lightweight category.   Research implications: To produce a more environmentally friendly concrete using geopolymer cement, combined with expanded clay, aiming for a weight reduction in precast concrete wall structures and an improvement in the thermal insulation of these systems. Originality/value: To assess the feasibility of using lightweight geopolymer concretes with locally sourced materials through tests conducted in Brazilian studies, aiming to contribute to the existing gap in knowledge regarding the behavior of this alternative binder.