Ultra‐Fast Supercritical Hydrothermal Synthesis of Tobermorite under Thermodynamically Metastable Conditions

Abstract

Tobermorite is a fibrillar mineral of the family of calcium silicates. In spite of not being abundant in nature, its structure and properties are reasonably well known because of its interest in the construction industry. Currently, tobermorite is synthesized by hydrothermal methods at mild temperatures. The problem is that such processes are very slow (>5 h) and temperature cannot be increased to speed them up because tobermorite is metastable over 130 °C. Furthermore the product obtained is generally foil-like and not very crystalline. Herein we propose an alternative synthesis method based on the use of a continuous flow reactor and supercritical water. In spite of the high temperature, the transformation of tobermorite to more stable phases can be prevented by accurately controlling the reaction time. As a result, highly crystalline fibrillar tobermorite can be obtained in just a few seconds under thermodynamically metastable conditions.

Continuous Synthesis of Nanominerals in Supercritical Water

“…Nevertheless, pure crystalline tobermorite is arduous to achieve being usually associated with amorphous or poorly ordered phase. Several studies were carried out to improve the crystallinity with the study of different parameters such as reaction duration, temperature, Ca/Si ratio, pH, precursor and partial substitution of silicon by aluminum . Moreover, the effect of additives such as EDTA, sucrose, borosilicate or Ca‐formate was also the subject of many studies.…”

“…The hydrothermal process is usually performed at 200 °C during 5 hours to 56 days . The temperature is a critical parameter since tobermorite is unstable (Ca/Si ratio not in the range of 0.8 to 1) under hydrothermal conditions and xonotlite starts to be formed at 130 °C . Notwithstanding, many researches confirmed that partial substitution of silicon by aluminum increases the stability of tobermorite …”

“…[61,62] Nowadays, tobermorite has been proposed to be used as seedinga ddition for cement composites and as as tabilizing agent for the trap of impurities. [63][64][65][66][67] Three structuralv arieties are distinguished, depending on the amount of water in the interlayers pace:1 4 tobermorite, 11 tobermorite (the most common structure under ambient conditions) or 9 tobermorite. The 11 tobermorite, referenced here as synthetict obermorite (Figure 12), exists in two polymorphs:n ormala nd anomalous.…”

“…Several studies were carried out to improve the crystallinity with the study of different parameters such as reactiond uration,t emperature, Ca/Si ratio, pH, precursor and partial substitution of silicon by aluminum. [67] Moreover, the effect of additives such as EDTA, sucrose, borosilicate or Ca-formate was also the subject of many studies. The hydrothermalp rocess is usually performed at 200 8Cd uring 5hours to 56 days.…”

“…[51,[70][71][72] The temperature is a critical parameter sincet obermorite is unstable (Ca/Si ratio not in the range of 0.8 to 1) under hydrothermalc onditions and xonotlite starts to be formed at 130 8C. [67] Notwithstanding, many researches confirmed that partial substitution of silicon by aluminumi ncreases the stabilityo ft obermorite. [51,71,72] Diez-Garcia et al presents an innovative synthesis method which emphasizes again the use of supercritical water and a continuousf low reactor.…”