Development of energy-efficient wall products based on lightweight concretes with improved thermal insulation properties

Abstract

The stepwise tightening of energy efficiency requirements confronts the designer with the incompatibility of two properties of lightweight concrete on porous aggregates: the gain in thermal conductivity is bought by lowering the mean density, while compressive strength falls faster, and a single-layer wall ceases to be load-bearing before it becomes warm enough. The aim was to find a combination of density, strength and thermal conductivity at which a wall product reaches the required heat transfer resistance at a structurally acceptable thickness. Seven model mixtures were designed on a combined porous aggregate with partial replacement of Portland cement by fly ash and silica fume and with air entrainment of the mortar matrix. Thermal conductivity was measured according to GOST 7076-99, the thermal verification was performed according to SP 50.13330.2024 at 4383 degree-days of the heating period. Within the density range from 612 to 842 kg/m³ thermal conductivity follows a power law with an exponent of 1.498 and compressive strength with an exponent of 3.047, so a single-layer wall would require from 525 to 850 mm. A two-layer product 400 mm thick, with a 150 mm load-bearing layer of class B3.5 and a 250 mm insulating layer of density 342 kg/m³, provides a reduced heat transfer resistance of 2.99 (m²·°C)/W at a mass of 211 kg/m² and a utilisation factor of the load-bearing layer of 0.48. Separating the two functions between the layers of one product removes a contradiction unsolvable in the single-layer scheme, and the thermal protection reserve depends on the thermal uniformity of the masonry about 1.7 times more strongly than on the operating moisture of the material.