Development of a new thermally insulating and structural green concrete for lean constructions

S. Ng, B.P. Jelle
SINTEF,
Norway

Keywords: thermal insulation, mechanical strength, aerogel, freeform, calcined clay, fibers

Summary:

Thermal insulation has always been a demanding requirement in energy efficient constructions, particularly at locations with harsher and more extreme climates. To optimize thermal insulation resistance, new insulation materials and solutions with low thermal conductivity values are being developed. The current and most common practice is to utilize insulation materials in ever increasing thicknesses in the building envelopes. While this may achieve the immediate energy efficiency targets, very thick and multilayered building envelopes are less desirable due to many reasons, including but not limited to space issues with respect to economy and useable floor area, transport volumes, refurbishment issues, architectural restrictions, material demands, existing and new building techniques, etc. In response to this, a new class of thermally insulating concrete with structural properties was designed and developed in-house. This was carried out by the fusion of current concrete technology with thermal insulation materials in an effort to produce a slim wall, while maintaining both structural strength and thermally insulating property. With the current drive towards sustainability, there is a strong motivation in employing green materials in this development. This study thus aims to give a review of the work performed in the development of such thermally insulating structural composites and the way forward. As thermal insulation material, aerogel granules which work based on the Knudsen effect, were employed. When aerogel was utilized by partial replacement in conventional concrete, the thermal conductivity of the composite decreased at the drastic expense of structural properties due to the brittleness of the aerogel and also due to the development of a gap space arising from the hydrophobic-hydrophilic interface between aerogel and concrete. To minimize the decrease in mechanical strengths when thermal conductivity was decreased, optimization of the mix designs, curing conditions, type of binders and additive employments, was performed. A stand-alone improvisation in the packing matrix was insufficient to significantly improve the strength and thermal conductivity combination of the composite material. However, when coupled with curing and storage effects, up to 50% improvement in strength was achieved; e.g. for concrete samples possessing a similar thermal conductivity of ~0.2 W/(mK), the strengths of conventional versus enhanced samples were 8.3 MPa versus 12.6 MPa, respectively. Further improvement in properties was observed when ordinary Portland cement binder was replaced with calcined clay, where a decrease in thermal conductivity by up to 2.5 times at similar strength level was registered. Other additives include fibers and mineral additions such as silica fume. While the development of the final structural insulating concrete is still an on-going investigation, the research thus far signifies the possibility of realizing the target of producing thermally insulating concrete with structural property. Additionally, the research development also demonstrates the potential of producing a single composite material with both thermal insulation and structural properties which for on-site production can be employed in a freeform casting and aesthetic lean construction through advanced techniques such as e.g. 3D printing.