•Pure pollucite barely exists in nature due to coexistence of Cs and Na.•Pollucite, analcime and their solid solutions could be hydrothermally synthesized.•Most formed solid solutions were found to have similar properties to pollucite.•Even coexistence in nature, pollucite favors to form due to site preference for Cs over Na.Pollucite, as a perfect long-term potential host for radioactive Cs immobilization, barely exists in pure form naturally but in an isomorphism form between pollucite and analcime due to coexistence of Cs and Na. Pollucite could be hydrothermally synthesized with Cs-polluted soil or clay minerals which contain Cs and Na, and it is necessary to study the properties of the synthesis if Cs and Na contained. Pure pollucite, analcime and their solid solutions were hydrothermally synthesized with chemicals, and it was found that the most formed pollucite analcime solid solutions with Cs/(Cs + Na) ratios of 2/6–5/6 had very similar properties in mineral composition, morphology and size, structural water (Cs cations) and coordination environment to pollucite. This also suggests that even coexistence of Cs and Na in nature, pollucite favors to form due to site preference for Cs over Na, which leads to the property and the structure of the most solid solutions similar to that of pollucite.

•Silica-based materials with pore diameters from 3.5 nm to 10.9 nm were synthesized.•Pore sizes of 3.5 nm–7.1 nm conditioned the material humidity regulating capability.•Above 7.1 nm, the materials almost lost the humidity regulating capability.A humidity self-regulating material has been developed so as to improve the residential comfort level and save energy of buildings, and pore dimension of materials was found to condition the humidity regulating capability mainly. In order to confirm the effect of pore dimension of materials on humidity regulating, the silica-based materials with different pore sizes from 3.5 nm to 10.9 nm were synthesized and further used to measure the physical adsorption/desorption capabilities of water vapor. Experimental results showed that an effective humidity self-regulating performance of materials did depend closely on their pore dimensions (3.5 nm–7.1 nm in this study). Finer pores had a better capability of self-regulating humidity, but the too small pores (<3.7 nm) resulted in a minus influence on the humidity regulating property. The adsorption/desorption water vapor capability reduced quickly with increasing pore size from 4.2 nm to 7.1 nm, and above 7.1 nm the materials almost lost the humidity self-regulating capability.

A facile hydrothermal method has been developed to synthesize natural diatomite into hardened diatomite-based adsorbents with zeolite (analcime) formation for methylene blue (MB) adsorption. The results showed that the initial and final strengths were provided with the formed C–S–H gel and zeolite (analcime), respectively. Due to the low temperature synthesis, the formed analcime and retained diatomite were also found to exert a synergistic effect on MB adsorption. The NaOH concentration had a significant effect on the C–S–H and analcime formations, and a lower NaOH concentration (≤9 M) was favorable for C–S–H gel formation, while analcime formed readily at a higher NaOH concentration (≥12 M). The curing temperature and time also influenced the formation of analcime, a long curing time (≥12 h) or a high temperature (≥473 K) was favorable for analcime formation, while an over-long time (≥24 h) or over-high temperature (≥493 K) had a negative effect on the strength of the specimens. The adsorption of MB in this study followed pseudo-second-order kinetics, with a maximum adsorption capacity of 129.87 mg g−1 at 308 K, according to the Langmuir model. Thermodynamic studies also showed that the adsorption process was spontaneous and endothermic. As such, tough diatomite-based adsorbents with analcime formation could be synthesized hydrothermally, and could be used to capture MB in wastewater efficiently.

•Reduction of CO2 was carried out under subcritical and supercritical hydrothermal conditions.•Temperature influenced the generation of H2, and H2 produced largely under supercritical condition especially at 425 °C.•Formic acid yield was produced from CO2 reduction, and the temperature exerted a great effect on the formic acid yield.•Decomposition of formic acid became quick from 350 °C, which was decomposed into methane.Hydrothermal conversion of CO2 into organic chemicals in the presence of Fe and Ni powder under subcritical and supercritical hydrothermal conditions has been carried out, and the results showed that H2 could be obtained from water splitting under hydrothermal condition. According to Fe3O4 formation, the reaction temperature seemed to influence the generation of H2, and H2 could be formed largely at supercritical condition especially at 425 °C. The formic acid could be produced from CO2 reduction, and the reaction temperature had a great effect on the formic acid yield. The increase in temperature could improved the formic acid yield, however higher temperature (≥275 °C) seemed to exert a minus influence on the formic acid yield due to decomposition of formic acid. The decomposition of formic acid became quick from 350 °C, and the results showed that the formic acid was decomposed into methane at higher temperature (350 °C).

Low temperature (100 °C) solidification of clay brick waste has been achieved by use of NaOH solution after addition of calcium hydroxide. Flexural strength was approximately 26 MPa for specimens containing 30 % (m/m) calcium hydroxide which had been treated with 3 M NaOH then solidified for 48 h. The development of strength of the specimens was found to be mainly because of C–S–H gel or Al-substituted C–S–H gel formation. This C–S–H (Al-substituted C–S–H) gel formation within the specimen resulted in a denser matrix, which in turn enhanced the strength. Low-temperature (100 °C) solidification technology could have high potential for recycling and reuse of clay brick waste on a large scale by use of a continuous solidification process.

Hydrothermal solidification/synthesis of diatomaceous earth based material has been carried out so as to investigate the relationship between porous and mechanical properties. Compaction pressure could make contact of particles closer, which accelerated the hydrothermal reaction and thus improved the strength of the solidified specimen; however, it could shrink the pore volume yet seemed to exert a small influence on the surface area. Curing time and temperature affected the porous and mechanical properties of synthesized specimens significantly, and the toughest specimen seemed to possess the largest surface area and optimum pore size distribution. As such, a porous and tough humidity regulating material seemed to be manufactured easily by the hydrothermal solidification/synthesis technology.

To simulate the thermal properties of cave dwellings, which are warm in winter and cool in summer, a mesoporous material with a good humidity-regulating performance was synthesized hydrothermally from loess. Through the calcination of loess, which both provides active calcium through the decomposition of calcite within loess and improves the reactivity by dehydroxylation, a tough mesoporous material could be synthesized without any additives, and tobermorite formation was found to exert a positive effect on its strength and porosity. Mesopores seemed to exert a positive influence on the humidity-regulating performance of the material from loess. Although calcination destroyed the original porosity of the loess, the porosity and thus the humidity-regulating performance could be recovered greatly through hydrothermal treatment. As such, using only calcined loess, a tough mesoporous material exhibiting good humidity-regulating performance was synthesized that could be used as a “cave-dwelling” building material in cities to save energy.

Municipal solid waste incineration (MSWI) bottom ash could be solidified with fly ash addition at 100 °C, and the flexural strength of solidified specimens reached almost 20 MPa. The strength development was found to be due mainly to C–S–H gel formation. Leaching tests were conducted to determine the amount of heavy metals dissolved from the solidified specimens, and the results showed that, under the hydrothermal conditions of this study, the leaching of heavy metals was very low. As such, it is possible to solidify MSWI ash by 100% (80% bottom ash + 20% fly ash) in a continuous production process with a low cost.

In order not only to manufacture a tough porous material from sepiolite but also to sustain the inherent properties of the sepiolite, a hydrothermal experiment was carried out under saturated steam pressure at 473 K for up to 24 h by mixing slaked lime. The experimental results show that the addition of slaked lime exerts a positive effect on the strength development, and the strength enhancement is found to be due to tobermorite and calcium silicate hydrate (CSH) gel formation. Curing time seems to condition the strength development. A fine particle size of sepiolite improves its activity during the hydrothermal process, thus offering a higher strength and quicker hardening rate of the synthesized specimen. The acid activation also has a beneficial effect on the strength development. The hardening mechanism of the synthesized specimen is different between the specimens synthesized with or without acid activation. The strength enhancement for sepiolite with acid activating is due mainly to CSH gel formation, instead of tobermorite formation without acid activation.

Municipal solid waste incineration (MSWI) bottom ash could be solidified with and without slaked lime (calcium hydroxide) addition by a hydrothermal method under steam pressure of 1.56 MPa at 200 °C for up to 72 h. Experimental results showed that CSH gel or tobermorite exerted a main influence on strength development, and without any additives CSH gel was easy to form, while slaked lime addition favored to form tobermorite. Tobermorite seemed to exert a larger effect on the strength development than CSH gel. Leaching results showed that the concentrations of heavy metals dissolved from the solidified specimens were effectively reduced after hydrothermal processing. The immobilization was mainly due to the tobermorite or CSH gel formation, and Pb2+ and Zn2+ seemed to be fixed more readily than Cr6+, which might be the reason that the structural Ca2+ within tobermorite or CSH gel was exchanged by Pb2+ and Zn2+ more easily than Cr6+. In addition, there existed a close relationship between leaching concentration and strength enhancement, and a higher strength seemed to exert a larger effect on immobilization of heavy metals.Highlights► MSWI bottom ash could be solidified hydrothermally without any additives. ► Hardening mechanism without additives was due mainly to CSH formation. ► Amount of heavy metals was greatly reduced after hydrothermal treatment. ► Immobilization of heavy metals was mainly due to CSH gel formation. ► A higher strength of specimen exerted a larger immobilizing capability.

•A simple three-dimensional (3-D) thermoelastic model is developed.•This model is incorporated into a 3-D stochastic network model.•Thermoelasticity exerted a significant influence on production temperature, injection pressure and water loss.•For a multi-well geothermal system, thermoelasticity seemed to have a potential to cause thermal short circuits.In order to assess the thermoelastic influence on the long-term performance of hot dry rock (HDR) reservoirs, a simple three-dimensional (3-D) thermoelastic model has been developed based on an assumption of a spherically symmetric volume of cooled rock within reservoir. This model has been incorporated into a 3-D stochastic network model, FRACSIM-3D, which incorporates a fracture network designed to mimic natural fracture distributions as well as stimulation and circulation. The model has been used to evaluate the possible long-term performance of the deep HDR reservoir at Hijiori, Japan. Simulation results showed that thermoelasticity could exert a significant influence on production temperature, injection pressure and water loss. For a multi-well geothermal system, thermoelasticity seemed to have a potential to cause the development of high flow rate/rapidly cooling flow paths (thermal short circuits).