The main measure to reduce energy losses is the usage of insulating materials. When the temperature exceeds 500 °C, silicate and ceramic products are most commonly used. In this work, high-crystallinity 1.13 nm tobermorite and xonotlite were hydrothermally synthesized from lime and Ca–Si sedimentary rock, opoka. By XRD, DSC, TG and dilatometry methods, it has been shown that 1.13 nm tobermorite becomes the predominant compound in stirred suspensions at 200 °C after 4 h of synthesis in the mixture with a molar ratio CaO/SiO₂ = 0.83. It is suitable for the production of insulating products with good physical–mechanical properties (average density < 200 kg·m⁻¹, compressive strength ~0.9 MPa) but has a limited operating temperature (up to 700 °C). Sufficiently pure xonotlite should be used to obtain materials with a higher operating temperature. Even small amounts of semi-amorphous C–S–H(I) significantly increase its linear shrinkage during firing. It has also been observed that an increase in the strength values of the samples correlated well with the increase in the size of xonotlite crystallites. The optimal technological parameters are as follows: molar ratio of mixture CaO/SiO₂ = 1.2; water/solid ratio W/S = 20.0; duration of hydrothermal synthesis at 220 °C—8 h, duration of autoclaving at 220 °C—4 h. The average density of the samples was ~180 kg·m⁻¹, the operating temperature was at least 1000 °C, and the compressive strengths exceeded 1.5 MPa.

In this work, the suitability of natural raw materials with various modifications of SiO₂—granite sawing waste (quartz) and opoka (a mixture of cristobalite, tridymite, quartz, and an amorphous part)—for the 1.13 nm tobermorite and xonotlite synthesis is examined, and their specific surface area, pore diameter and volume, and the predominant pores are determined. Hydrothermal syntheses were carried out at 200 °C for 12 and 72 h from mixtures with a molar ratio of CaO/SiO₂ = 1.0. X-ray diffraction analysis, simultaneous thermal analysis, and scanning electronic microscopy were used, which showed that in the lime–calcined opoka mixture the formation of crystalline calcium silicate hydrates takes place much faster than in the lime–granite sawing waste mixture. The high reactivity of amorphous SiO₂ results in the rapid formation of 1.13 nm tobermorite and xonotlite (12 h). According to Brunauer, Emmet and Taller (BET) analysis data, this product features a specific surface area of ~68 m²/g, a total pore volume of 245 × 10⁻³ cm³/g, and has dominating 1–2.5 nm and 5–20 nm diameter pores. This porosity of the material should provide good thermal insulation properties of the products made from it as no air convection occurs in the fine pores.

The idea of this work is to reduce the negative effect of ordinary Portland cement (OPC) manufacture on the environment by decreasing clinker production temperature and developing an alternative rankinite binder that hardens in the CO₂ atmosphere. The common OPC raw materials, limestone and mica clay, if they contain a higher MgO content, have been found to be unsuitable for the synthesis of CO₂-curing low-lime binders. X-ray diffraction analysis (ex-situ and in-situ in the temperature range of 25–1150 °C) showed that akermanite Ca₂Mg(Si₂O₇) begins to form at a temperature of 900 °C. According to Rietveld refinement, the interlayer distances of the resulting curve are more accurately described by the compound, which contains intercalated Fe²⁺ and Al³⁺ ions and has the chemical formula Ca₂(MgO_0.495·FeO_0.202·AlO_0.303)·(FeO_0.248·AlO·Si_1.536·O₇). Stoichiometric calculations showed that FeO and Al₂O₃ have replaced about half of the MgO content in the akermanite structure. All this means that only ~4 wt% MgO content in the raw materials determines that ~60 wt% calcium magnesium silicates are formed in the synthesis product. Moreover, it was found that the formed akermanite practically does not react with CO₂. Within 24 h of interaction with 99.9 wt% of CO₂ gas (15 bar), the intensity of the akermanite peaks does not practically change at 25 °C; no changes are observed at 45 °C, either, which means that the chemical reaction does not take place. As a result, the compressive strength of the samples compressed from the synthesized product and CEN Standard sand EN 196-1 (1:3), and hardened at 15 bar CO₂, 45 °C for 24 h, was only 14.45 MPa, while the analogous samples made from OPC clinker obtained from the same raw materials yielded 67.5 MPa.

The production of hydraulic binders, representing the essential constituent part of concrete and mortar, can be associated with high energy consumption and huge CO₂ emissions (at least 2.4 billion tons in 2022). Without appropriate measures, the situation will only worsen. The global annual output of cement stood at 4.4 billion tons of cement, whereas the annual production has been increasing at a rate of ca 5%. In order to significantly reduce CO₂ emissions, the following solutions are most widely used in the world: clinker additives; unconventional fuels; decreased energy-related expenses; and technological innovations. However, these are not sufficient to cut down on greenhouse gas emissions and bring them close to zero. Therefore, the utilization and development of alternative binders denoted by a reduced CO₂ footprint in comparison to that of conventional cement are among the main objectives of building materials manufacturers as well as researchers. This paper reviews obstacles, solutions and alternatives for the fabrication of hydraulic cementitious materials, along with the general principles of the carbonization of binders, such as natural processes and intensified processes, the impact of various parameters on the chemical and physical transformations, as well as the mechanism of interaction of OPC, belite, and blended cement with CO₂. The production of low-lime binders, along with time-optimized carbonation, can significantly improve carbon footprint values. However, due to the huge variety of blended cements, their hardening process by mineral carbonation needs to be investigated extensively and systematically, as it is emphatically dependent on many numerical values and criteria. Environmentally and economically acceptable production can only be achieved on the grounds of the optimized parameters of the entire process.