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Characteristics of microhydrates formed under steam conditions
Abstract
This paper presents the physical properties and chemical composition of hydrates formed under steam conditions. The test specimens were made with cement as well as individual compounds. A comprehensive study on the characteristics of hardened hydrates in a microscopic scale was carried out. An improved polymerization of silicate anions resulting in an increased CaO/SiO2 ratio at an elevated temperature was verified by the nuclear magnetic resonance spectroscopy (NMR) analysis. Dense microstructures of hydrates and low content of Ca(OH)2 crystals were observed by the scanning electron microscope (SEM), energy dispersive spectromer (EDS), environmental scanning electron microscope (ESEM), thermogravimetric analyzer (TGA), and x-ray dtffractrometer (XRD) tests. Mercury intrusion porosimetry (MIP) had been used to examine the pore structures of hydrates. It revealed that the hardened paste made with the dry-mixture/steaminjection (DA/57) method developed microstructures with low porosity, small pore radius, and larger surface areas. These observations agree with the NMR/TGA test results. This paper also discusses the effects of sample fineness and compaction on the hydrate's microstructures formed under steam environment. The test results verify the validity of the DMSI concept.
... Furthermore, autoclave curing (160-200°C, 0.6-1.6 MPa saturated vapor pressure) products have higher strength than general standard curing (20 ± 3°C, RH > 95%) curing products mainly attribute to the more generating of C-S-H gel which resulted in a denser concrete structure [7,8]. ...
... In the conventional autoclaving technology, calcium hydroxide and PI 52.5 were both frequently-used calcareous materials in autoclaved products. However, former studies of lunar simulant regolith autoclaved concrete were adopting cement as calcareous material which was not accordance with the optimization reaction mechanism of autoclaved materials [7][8][9][10][11][12]. As the LRS already has 66.52% SiO 2 and Al 2 O 3 in content, additional siliceous material was not required, on the contrary, introducing pure calcareous material can improve the alkalinity of the material system so that it is closer to the optimum alkalinity coefficient range (K alk = 0.8-1.2) [16]. ...
... Meanwhile, DM/SI method prevented the material from being exposed to the vacuum and thus avoids the rapid evaporation of water [3]. The DM/SI method products can achieve higher strength than conventional cured products mainly owe to the more generating of C-S-H gel which resulted in a denser concrete structure [4,5]. Steaming technology can be separated into unsaturated and saturated steam curing. ...
The article provides an overview of findings and production processes of various mineral materials which have been developed and tested worldwide in the past with regard to the establishment of a lunar base. At the beginning, the aim and procedure of constructing a lunar base will be outlined briefly. Subsequently, the lunar environment factors and their influence on a possible construction will be described. Then the advantages and disadvantages of examined materials such as sulfur concrete, cast basalt, lunar concrete or polymer concrete, on the one hand, as well as previously investigated production processes like sintering, geothermite reaction and 3D printing, on the other hand, are presented. One promising method is the Dry-Mix/Steam-Injection method for producing a lunar concrete as a possible material which is based on cement made from lunar resources developed by T. D. Lin.
The coordination of A1 in several synthetic A1-substituted tobermorites was determined to be tetrahedral by solid-state 27Al MASNMR. Earlier results of tetrahedral Al coordination in Al-substituted tobermorites by x-ray emission were confirmed unequivocally by the new high resolution MASNMR technique. However, two 27Al chemical shifts at about 68 and 57 ppm were observed in the Al-substituted tobermorites which probably indicates two different environments for the Al atom in the structure. 27Al MASNMR technique clearly distinguishes the tetrahedral and octahedral coordination of Al in solid samples.
The grain boundary healing behavior of crushed rock salt was mainly studied by employing the environmental scanning electron microscope (ESEM) to study the consolidation mechanism of rock salt backfill. Dedicated miniature round rock salt specimens were prepared for observation of the water trapping effect by using a cold stage in the ESEM to reach saturation conditions. Comparable high pressure pellets were prepared for measuring the crystal growth. Consolidation tests using materials made at different pressures and containing different moisture levels were conducted in order to construct the proposed mechanism. Direct observation of specimens in the ESEM resulted in viewing water trapped on the surface and the formation of a water meniscus between two particles. The concentration of brine at the grain boundary was observed as contributing to the amount of recrystallization. From aforementioned observations, a schematic drawing of the dissolution and recrystallization process may be redrawn. The amount of water therefore has a great effect on the consolidation of rock salt and is possibly due to the sliding, rotation, or crushing of the contact zone of the granular material. From such a study, tentative healing and consolidation mechanisms can be deduced.