Contact-hardening Behaviour And Mechanism Of Amorphous Calcium Silicate Hydrate And Its Potential Applications

According to the literature review, it is found that apart from setting-hardening cementitious properties, amorphous calcium silicate hydrate(C-S-H) shows contacthardening properties. A powder which contains amorphous C-S-H can be used to prepare hardened specimen by direct compression. One advantage of this method is that a solid block can be produced in only several minutes. In this case, the development of this binding system provides a new approach to manufacture cementitious materials. However, there is lack of literature to express the compaction and hardening process that how the powder becomes a hardened block by compression. As a result, it is of significance to investigate the contact-hardening behaviour and mechanism of amorphous C-S-H. The result can be used as a guideline to manufacture man-made blocks from industrial wastes with excellent properties such as low apparent density, high strength, and low thermal conductivity.In the present thesis, calcium silicate hydrates with different crystallinity and compositions were synthesized via dynamic hydrothermal treatment. Nano silica and grounded quartz powder were used as siliceous materials and lime was used as calcareous materials. These powders were compressed at the pressure of between 20 and 80 MPa to produce compacts with varied bulk density and mechanical strength.The compaction process of the powder containing amorphous C-S-H was evaluated by using Heckel equation and Kawakita equation. Furthermore, the bonding principles were analysed with respect to the mechanical properties and water resistant property of the compacts, as well as the microstructure variation before and after compaction of the powders. Quantitative analysis was also done on the compaction properties and microstructure of the powders. The results showed that light-weight and high-strength compacts were produced. The bulk density was between 500 and 1500 kg/m3, the compressive strength ranged from 4.5 to 33 MPa, and flexural strength went up to 1.0-6.5 MPa. The compacts also presented certain water resistant property with softening coefficient ranging from 0.4 to 0.8. From the Heckel plots, it was known that when the compression pressure was from 20 to 80 MPa, the compaction of powders could be expressed by that the rate of density increasing with applied pressure was proportional to the volume fraction of pores at different pressures. In other word, the compaction of the powders was described as a first-order chemical reaction kinetic between the change in density with pressure and pore fraction.For further understanding the bonding mechanism of calcium silicate hydrate particles by compression, a compression calorimeter was designed to determine the temperature evolution during compaction of the powder. With respect to the temperature evolution, a key parameter was discovered to be available for predicting the compressive strength of a compact.Finally, the effect of drying method and recompression of the powders on the contact-hardening properties were discussed, and the application was evaluated as well. The results showed that the interlayer structure of C-S-H could be destroyed at a high oven-drying temperature, and consequently affecting the hardening properties of the materials. The bulk density of the sample would be too high if the drying temperature was relatively low. Besides, it was found that the contact-hardening cementitious properties were gradually lost attributed to the recompression of the powders. With the increasingly repeated compression times, the bulk density of compact was increased but the strength was reduced.With regard to the above investigation, “Munich model” was modified to express the compaction of calcium silicate hydrate. The main conclusions were listed as following:(1)The contact-hardening of calcium silicate hydrate includes the increase of contact area and the formation of interparticulate bonding. The hardening of the powder is a result of particle-particle contact. During the compaction process, low density C-S-H becomes much denser and more ordered. The contact and bonding between particles are not only resistant to the water attack but also partially reversible.(2)There are two stages for the compaction of calcium silicate hydrate powders, which are rearrangement of powder particles and the plastic deformation together with particle fragmentation. The former stage usually occurred at the compression pressure of 0-20 MPa. Point contact is dominant leading to the formation of cohesion force, such as van der Waals force and hydrogen bond. At a higher compression pressure(e.g. 40 MPa), the fragmentation and plastic deformation of powder particle occurred. Consequently, point contact gradually transforms into surface contact, bringing the development of condensed bonding, like solid bridges and mechanical interlocking, which are much stronger than the cohesion force.(3)The temperature evolution results show that the formation of interparticulate bonding was related to the energetic liberation. In general, four stages are included in contact-hardening process which are initial period, acceleratory period, decelerating period and declining period;(4)Initially, point contact(cohesiveness) is dominant and the main bonding types are van der Waals force and hydrogen bond. Plastic deformation and particle fragmentation occurs at higher compression pressure, leading to the formation of solid bridge or mechanical interlocking with higher bonding strength. Large amount of heat is generated. The decelerating period appears when the compression pressure is constant. Plastic deformation of powder particles is continuous, and a higher amount of bonding is developed. At the declining period, the heat diffusion rate rather than interparticulate bonding rate is dominant.(4)The contact-hardening properties of calcium silicate hydrate is also influenced by crystallinity and specific surface area of the powder. It is reduced by increasing the crystallinity or decreased specific surface area. Besides, it is found that moisture content in the powder is another factor to affect the contact-hardening property. Relatively low moisture content is beneficial to the formation of interparticulate bonding. However, if the powder is too moist, for example at a moisture content of higher than 32%, the bonding will be weakened due to the disjoining pressure.