Question

Thermal conductivity variation with moisture content

Answer 1

Answer:

Explanation:

Cementitious composites contain a significant amount of pores of different size. As the thermal conductivity of the air is 0.026 W/mK [1] and the thermal conductivity of cement stone is (depending on the amount and the type of aggregates) in the range of 1-3 W/mK [2], both the total pore volume and the distribution of pores can affect the thermal conductivity of a cementitious material in a very significant way. In usual service conditions, cementitious composites always contain certain amount of water. The thermal conductivity of water is 0.60 W/mK [1], which is more than 20 times higher than of the air. Therefore, if water is present in the pore space, its effect competes with the effect of air, and the thermal conductivity of a composite material can be considered as a result of this competition together with the effect of the cement matrix.   Thermal conductivity as the main heat transfer parameter was often subject of measurement for various types of cement-based composites. However, mostly just one single value was determined (see, e.g., the reviews in [2]-[4]). The dependence of thermal conductivity on moisture content was studied for instance in [5]-[8] where empirical relations were obtained but such measurements can still be considered as relatively rare.   Homogenization theories working with the concept of an effective medium were proven as very useful in a variety of applications in mechanics and in the theory of electricity and magnetism where they already belong to well established treatments (see, e.g. [9], [10]). Their utilization in heat transfer was much less frequent until now. Within the last couple of years, some references appeared on using the effective media theories for estimation of thermal conductivity of refractory materials [11], [12], syntactic foams [13], polymer-based composites [14] but not a single reference was found by the authors for cementitious composites, in common sources.