Uptake and loss of water in a cenosphere-concrete composite material

Cenospheres are hollow, aluminum silicate spheres, between 10 and 300 mum in diameter. Their low specific gravity (0.67) makes them ideal replacements for fine sand for producing low-density concrete. In an effort to understand the potential for practical use of the cenospheres as a fine aggregate in concrete, the moisture uptake and loss by cenospheres and water uptake and loss in cenosphere-concrete composites have been studied in this paper. The equilibrium moisture content of cenospheres is about 18 times higher than that of sand, reflecting the porous nature of cenospheres. The temporal evolution of water penetration into the cenosphere-concrete is modeled using Washburn kinetics. The effective pore size using this model is of the order of several nanometers. These results imply a lack of connectivity within the pores, leading to a low permeability. SEM images of the concrete reveal pore sizes of the order of 2-5 mum. The drying flux for cenospheres shows a classical behavior-a constant rate followed by a linear failing rate period. Thus, experiments done at these conditions can be used to predict drying times for wet cenospheres exposed to other environments. The flux of water vapor away from both the cenosphere-concrete as well as the normal concrete shows a nonlinear change with moisture content throughout the drying cycle, implying that the pore structure within the concrete strongly influences the drying behavior. (C) 2003 Published by Elsevier Ltd.