Influence of Silica Fume Source on Crystallization of Xonotlite in a New Process Making Medium Density Ca-Silicate Based Products

Abstract

This paper investigates the influence of different types of silica fume on the crystallization process of medium density calcium silicate based products. The products are formed by a new technology that consists of two steps. In the first step, a mixture containing calcium silicate hydrates (C-S-H) is formed by reaction of lime with special silicas at temperatures below 100°C. This mixture is then molded into boards by a filter-pressing technique. In the second step, the boards are treated in hydrothermal conditions enabling the conversion of the C-S-H into important contents of xonotlite (Ca6Si6O17(OH)2); this is the most stable calcium silicate hydrate phase at high temperatures. In order to make C-S-H in pressure less conditions, the use of reactive forms of silica is required. In this work we used silica fume as reactive silica. To understand the influence of the silica fume on the formation of xonotlite, several properties were studied, such as particle size, purity and specific surface area (BET). It was found that the particle size distribution and degree of agglomeration for the silica fume were the most important properties. A proper dispersion technique must be applied in order to break the silica fume agglomerates, forming particles small enough to react with dissolved lime and to form C-S-H phases that are able to be converted into xonotlite under hydrothermal conditions. Finally, it was also found that the formation of xonotlite is favored by the use of high purity silica fume.

Development of Hydrated Calcium Silicates by Hydrothermal Treatments

“…( 8) and ( 9) are the compositionstructure relationships where charge compensation of the substitution of Al 3+ for Si 4+ ions is entirely by alkali cations or Ca 2+ ions, respectively. The close relationship between Richardson and Groves's formulation and the models due to Taylor [16], Cong and Kirkpatrick [21,22], Nonat and Lecoq [23] and Chen et al [1] has been demonstrated by Richardson [6]. Glasser et al's [24] and Stade's [15] models are closely related.…”

“…This C-S-H present in hardened pastes of C3S/ C2S or neat Portland cements generally has a mean Ca/Si ratio of about 1.75, with a range of values within a given paste from around 1.2 to 2.1 [5]. Higher values of Ca/Si in the literature could be explained by mixtures of C-S-H with portlandite, Ca(OH)2 [1]. Unlike the highly crystalline hydrated calcium silicate phases, some people associate the C-S-H gel term as a nearly amorphous or even amorphous material.…”

“…However, a large list of them have been synthesized applying hydrothermal conditions. Furthermore, there are calcium silicate hydrates prepared at room temperature or higher up to ~100ºC with structure ranging from nearly amorphous to semi-crystalline [1,2]. This group of hydrated calcium silicates is usually named as C-S-H gel.…”

“…In this work, we found that the particle size distribution of amorphous silica plays a crucial role in the chemical reaction with Ca(OH)2 for the conversion into C-S-H, as well as in the crystallization into xonotlite and/or tobermorite later on. Therefore, methods to disperse and break up agglomerates of amorphous silica were needed in order to have a sufficiently reactive silica fume [1]. Other factors such as impurities were found to influence the final crystallization of xonotlite and/or tobermorite.…”

“…In mixtures with quartz, the C-S-H gel forms around the quartz grains in the initial phases of the hydrothermal step. This C-S-H typically is Ca-rich due to the lower solubility of quartz compared to that of portlandite [1] and has a molar C/S ratio varying from 1.5 to 1.9, depending on the size of the quartz crystals . In advanced stages, when more crystalline silica is solubilized and diffuses in the formed C-S-H (II), the latter will convert into C-S-H (I) and transforms into tobermorite.…”