Experimental Studies on Xonotlite in the SiO2－CaO－H2O System: With Special Reference to the Spherical Secondary Particles

Synthetic xonotlite is one of the most important constituents of the industrial material for heat insulating and fire-resistant building materials. In this paper, formation mechanism of xonotlite as well as spherical secondary particle were experimentally examined in the SiO2—CaO—H2O system. Special attention was paid for the process of crystal growth and effects of the crystalline state of the starting materials. Effects of Al2O3 in the starting materials were also investigated using both pure and industrial materials. The products obtained were examined by X-ray diffraction and electron diffraction in addition to the detailed observations under the stereoscope and electron microscope. Amorphous to semi-crystalline state of C—S—H was characteristically formed at the initial stage of reaction and the morphology and the crystalline state of C—S—H varied complicatedly according to the experimental conditions.
The main results obtained are as follows: Morphology of C—S—H and its aggregate depend largely upon the crystalline state of the starting materials. In the experiments used Brazilian quartz as source of silica, fine and fibrous C—S—H is aggregated, forming angular surfaced massive agglomerate. Using silica gel (reagent), fine aggregate particle of crumpled foil of C—S—H is entangled with long and fibrous C—S—H, resulting in an irregular massive agglomerate. Most of C—S—H formed in the Brazilian quartz system transform to platy tobermorite and to strip and needle crystals of xonotlite through platy tobermorite as reaction proceeds, and simultaneously massive agglomerate of slightly rounded form changes to oolitic (A1) and spherical-shelled (A2) secondary particles. The formation process of the spherical secondary particle composed of xonotlite is basically the same as that when industrial silica powder (α-quartz) containing a very small quantity of Al2O3 is used. With increasing particle size of CaO, hollow spherical secondary particles change to those of dense aggregates composed of needle crystals of xonotlite (A3) through the spherical shell (A2) and oolitic (A1).
In the experiments used silica gel (reagent), C—S—H transforms directly to needle crystal of xonotlite which aggregates in the forms of bundle or irregular massive agglomerate, and no spherical agglomerate is formed. With rising temperatures, however, prisms of hillebrandite are partly formed and transform to xonotlite, forming the spherical secondary particle (B1) composed of extremely coarse aggregates with long needle xonotlite on the surface. By addition of very small amount of Al2O3, spherical secondary particles are formed in such a way that long fibrous C—S—H formed at the initial stage of the reaction becomes gradually short resulting the number of bundled aggregates decrease and then, as the reaction proceeds, the platy crystal of tobermorite is partly formed which later transforms into xonotlite forming spherical secondary particles. The secondary particle is composed of relatively coarse aggregates of needle crystals (B2). The spherical secondary particle is unevenly hollow and has long needle crystals on the surface. To be noted is that the texture and morphology of these spherical secondary particles are similar to those produced in the industrial processes when byproduct amorphous silica containing a very small quantity of Al2O3 is used.
The spherical secondary particle of types A1, A2, B1 and B2 play an important role producing light-weighted products suited for insulation and heat insulating materials and that of A3 is suited for fire-resistant building materials because of the high density property. Aggregates composed of the needle crystals of xonotlite do not form the secondary particle and the products have drawbacks such as poor mouldability and contraction and distortion during the drying. These aggregates could not be used as the industrial materials.