Hydrothermal processing of calcium silicate hydrate minerals using industrial raw materials: Effect of relatively short synthesis durations on properties

IntroductionMinerals such as xonotlite and related calcium silicate minerals have wide applications, including insulation boards, refractory boards, fireproof materials, ceiling boards, microporous materials, architectural... [ view full abstract ]

Introduction

Minerals such as xonotlite and related calcium silicate minerals have wide applications, including insulation boards, refractory boards, fireproof materials, ceiling boards, microporous materials, architectural boards and light weight boards. This paper examines the effect of hydrothermal synthesis time on mineralogy, size, and morphology of calcium silicate hydrates.

Methods

Calcium carbonate and quartz minerals directly obtained from rock exposures in the field were used in this work. A stoichiometric mixture (Ca/Si=1) of CaO, SiO2 and distilled water (w/s=1.5) was ground and mixed for 30 min at 250 rpm in the agate ball mill. Gel formed precursor was divided into portions and treated from 1 to 4 days at 200 C in PTFE inserts within 4 different steel autoclaves. Samples were labeled as S1, S2, S3 and S4 where numbers indicate synthesis times as days. The raw materials and the final products were characterised by XRD, SEM, EDS and particle size analyzer.

Results

In Figure 1, it can be seen that after hydrothermal treatment for 1 day, tobermorite occurs near xonotlite. and its peak appears at 2.98 Å. However, as the reaction proceeds, tobermorite peak decreases while xonotlite peaks continue to grow. The characteristic peak of xonotlite at 4.21 Å (400) grows and becomes sharper and well-defined with longer synthesis time. EDX spectra given for 4th day synthesis sample also confirms the formation of xonotlite (Fig. 2). In the first day of synthesis, lath-like minerals are in evidence. With increasing synthesis durations, needle-like minerals accompany lath-like minerals with increasing quantities (Fig.3). Median diameters of minerals increases with synthesis time (Fig.4). In the first 3 days, mineral sizes ranges between 3 and 4 µm. There is a sharp increase after 3th day. At 4th day, xonotlite minerals reach 6 µm size.

Discussion

In literature, many studies including hydrothermal synthesis of calcium silicate hydrates use higher w/s ratios (20-35 for periodic synthesis and 100-200 for continuous synthesis). Here, compositions including tobermorite and xonotlite and pure xonotlite have been synthesized in an autoclave under autogeneous steam pressure without mixing at temperature of 200 C for 1 to 4 days with relatively lower w/s ratios.