Some Fundamental Properties of Porous Bodies, Studied on Hydration Products of Plaster of Paris

Abstract

In order to study some of the fundamental properties of porous body hydration products of plaster of Paris whose porosities were ranging from 45 to 88%, were prepared, and their properties were measured.The results obtained were summarized as follows:(1) The modulus of rupture of porous bodies was decreased exponentially with increase of porosity, but in the range of porosity higher than 70%, this was decreased more markedly (Figs. 3 and 4).(2) Dielectric constants in the direction of c axis, (100), and (010) of crystal from Yamagata district were measured at 20 Mc and found to be 8.7, 5.1 and 4.9, respectively.(3) Dielectric constants of the porous bodies were decreased directly with increase of their porosities (Fig. 6), and these relationship was formulated as follows:K′=K0(1-x)+Kp(x)where, K′: apparent dielectric constant of the porous body, K0: mean dielectric constant of solid phases composed of the body, Kp; dielectric constant of gas, mostly air, included in the pores, and x: volume ratio of pore in the body.(4) Permeability of the body was decreased with increase of the porosity. When the square root of permeability and the cubic root of porosity are chosen as ordinate and abscissa, the relationship can be represented by a set of straight lines knicking at a point corresponding to the porosity of 70% (Fig. 9).(5) From the relationship between the size of deposited crystals and stirring time (Fig. 11), process of the crystal growth was inferred as follows: the crystals formed in the earlier stage of mixing the solution of plaster of Paris and water, grew markedley in the direction of c axis, but after the stirring of 4 or 5 minutes, growth in this direction was distinguishedly diminished, while growth in the direction of a and b axes, especially in a axis, was gradually increased. This change of directional predominance of crystal growth affected the structure of cast bodies, and consequently, on their physical properties, e.g., the modulus of rupture or the permeability, as described above.