Thermal Conductivity Coefficient Characterization of Lightweight Reactive Powder Concrete

Abstract

Lightweight reactive powder concrete (RPC) with enhanced thermal insulation is an interesting material due to its durability and mechanical properties. This paper presents the results of tests analyzing the thermal conductivity coefficient λ of several lightweight RPCs, obtained using expanded clay, Pollytag, expanded perlite, and expanded polystyrene beads in quantity from 30% to 60% by volume. Correlations between the λ coefficient and the density of the lightweight RPCs were defined. They were compared with correlations determined for different types of ordinary lightweight concrete based on data from previous studies in the literature. It was proven that, due to the significant influence of the thermal conductivity of the RPC cement matrix, with a relatively high λ coefficient, the thermal conductivity coefficients, λ, of lightweight RPCs reach higher values. Based on the microstructure analysis, it was found that the type and number of aggregate pores have also affected the density and, in consequence, the thermal conductivity of the lightweight reactive powder concrete. The expanded clay aggregate is the most suitable material to produce structural lightweight RPC, due to its low λ coefficient and high strength. A thermal conductivity coefficient λ value of 0.93 W/(m·K) and a density of 1,630 kg/m3 were measured for an RPC with 60 vol.% of expanded clay. This RPC reached a compressive strength of 64.5 MPa after 28 days and 77.9 MPa after one year and hence can be classified as structural lightweight. The use of expanded clay in RPC allows the subsequent material to achieve a significantly higher strength compared with conventional lightweight concrete with expanded clay, with a comparable density and slightly higher thermal conductivity coefficient λ. Reactive powder concrete with mineral lightweight aggregate could be an alternative to ordinary lightweight concrete with enhanced strength. Above all, the advantage of these new lightweight composites is a lack of strict dependence between the lightweight aggregate’s strength and the composite’s strength. Second, the water absorption of lightweight aggregate has minimal impact on RPC properties. In this case, the RPC matrix mainly influences the material’s water absorption and strength. Densely packed particles in a cement matrix and close adhesion of homogeneous RPC mix, with enhanced viscosity, to lightweight aggregate as well as placement of the concrete mix in large pores on the aggregate surface cause the low water absorption of lightweight RPC, similar to the water absorption of the RPC matrix. Further, lightweight RPC achieves relatively high strength. This fact makes the design of a lightweight RPC mix easier in comparison with ordinary lightweight concrete, where the water absorption of lightweight aggregate has to be taken into account when designing the concrete mix. In the case of ordinary lightweight concrete, the shaping of its properties is more complex, the designing of its composition is more complicated, and the production technology is more time-consuming, subjected to a greater risk of error.