Toward Carbon-Negative Construction Materials: CO2-Storing Alkali-Activated Waste-Based Binder Aleksandar Nikolov, Nadia Petrova, Miryana Raykovska, Ivan Georgiev, Alexander Karamanov Buildings, 2026 This study examines the carbonation behavior and CO2 storage potential of a Ca-rich alkali-activated binder produced entirely from industrial residues-ladle furnace slag (LFS), coal ash (CA), and cement kiln dust (CKD). The system was designed as a one-part alkali-activated material (AAM), with CKD acting as an internal activator, and subjected to ambient curing, water curing, and accelerated CO2 curing at ambient pressure. Phase evolution, microstructural development, and pore-structure characteristics were investigated using X-ray diffraction, FTIR spectroscopy, DSC–TG analysis, scanning electron microscopy, and X-ray micro-computed tomography, together with measurements of density, water absorption, and compressive strength. Loss-on-ignition measurements combined with chemical analysis were further used to quantify CO2 uptake and evaluate the degree of carbonation of the binder system. CO2 curing fundamentally altered the reaction pathway of the binder, shifting it from hydration-dominated to carbonation-controlled phase evolution, leading to the decomposition of calcium-bearing hydrates and complete carbonation of non-hydraulic γ-belite with the formation of vaterite, aragonite, and calcite. These transformations induced pronounced microstructural densification, reflected in a near-doubling of compressive strength (>48 MPa), increased apparent density, reduced water absorption, and simplified pore-network topology. A preliminary carbon footprint assessment indicates that the production of 1 m3 of the developed LFS–CA–CKD concrete generates about 14.36 kg CO2-eq, while the carbonation process enables significant CO2 sequestration, resulting in a net negative carbon balance. The results demonstrate that controlled carbonation is an effective post-treatment strategy for waste-derived alkali-activated binders, enabling simultaneous performance enhancement and permanent CO2 sequestration.
Comparative Multi-Stage TG-DSC Study of K+, Na+, Ca2+ and Mg2+-Exchanged Clinoptilolite Forms Tsveta Stanimirova, Nadia Petrova, Georgi Kirov Molecules, 2025 A multi-stage TG-DSC approach consisting of five heating/holding and five cooling/holding stages within one experiment in the temperature range 20–320 °C was applied to investigate the dehydration/hydration processes in K+, Na+, Ca2+, and Mg2+ clinoptilolite forms. The influence of extra-framework cations on the parameters characterizing these processes (such as mass changes, dehydration and hydration heats calculated per gram zeolite, amounts of water molecules leaving and entering the structure, and enthalpy values calculated per mol water) was established. The values of molar enthalpy of dehydration for different cationic clinoptilolite forms increase in different ways with temperature increasing (within the framework of 50–120 kJ mol−1). The data on the molar enthalpy are in good agreement with the distributions of the two types of water molecules—weakly bound to cations and water molecules coordinating cations in the applied crystal chemical models of the cationic exchange samples. The data obtained for water molecules and their molar enthalpies of dehydration for the various cationic forms are useful in studying the sorption of water vapor and other sorbates, in choosing a desiccant and an object to dry at room conditions, etc. The first data on the hydration energy of sequentially added water molecules in a dynamic cooling mode in the temperature range 320–20 °C were obtained.
Sunflower Shells Biomass Fly Ash as Alternative Alkali Activator for One-Part Cement Based on Ladle Slag Aleksandar Nikolov, Vladislav Kostov, Nadia Petrova, Liliya Tsvetanova, Stanislav V. Vassilev, et al. Ceramics, 2025 This study explores the synergistic potential of ladle slag (LS) and sunflower shell fly ash (SSFA) in alkali-activated binder systems, focusing on their chemical and mineralogical characteristics and the influence of SSFA addition on the mechanical performance of LS-based pastes. X-ray fluorescence and XRD analysis revealed that LS is rich in CaO and latent hydraulic phases such as γ-belite and mayenite, while SSFA is dominated by K2O, SO3, and KCl/K2SO4 phases, reflecting its biomass origin. Infrared spectroscopy and thermal analysis confirmed the presence of carbonate, hydroxide, and hydrate phases, with SSFA exhibiting more complex thermal behavior due to volatile-rich composition. When used alone, LS produced weak binders; however, a 10 wt% SSFA addition tripled compressive strength to nearly 30 MPa, indicating a significant activation effect. Further increases in SSFA content led to strength reduction, likely due to increased porosity and excess salts. Microstructural analysis showed that SSFA promotes the formation of AFm phases such as Friedel’s salt and hydrocalumite, altering hydration pathways and enhancing early strength through chemical activation and carbonation processes. The findings highlight the potential of combining LS and SSFA as a sustainable binder system, offering a waste-derived alternative for low-carbon construction materials.
Crystal phases in the system MgCl2–OC(NH2)2–H2O: thermal stability and decomposition Nadia Petrova, Vladislav Kostov-Kytin, Krasimir Kossev, Rosica Nikolova Journal of Thermal Analysis and Calorimetry, 2025 Crystal phases in the system MgCl 2 –OC(NH 2 ) 2 –H 2 O have been studied and discussed. The conditions for obtaining MgCl 2 ·OC(NH 2 ) 2 ·4H 2 O, MgCl 2 ·4OC(NH 2 ) 2 ·2H 2 O, MgCl 2 ·6OC(NH 2 ) 2 and MgCl 2 ·10OC(NH 2 ) 2 have been specified. The crystal structures of MgCl 2 ·OC(NH 2 ) 2 ·4H 2 O and MgCl 2 ·6OC(NH 2 ) 2 have been solved for the first time. The thermal behavior of the four studied phases has been investigated in the temperature range from room temperature to 600 °C applying simultaneous thermal and mass-spectroscopy analyses. In-situ time-resolved powder diffraction analysis in the temperature range from 25 to 230 °C has been used in order to clarify the structural transformations upon heating. Research has revealed that the most stable compound upon heating is MgCl 2 ·6OC(NH 2 ) 2 . For all the studied samples, the decomposition of the urea component has been observed to start closely after the melting. The decomposition processes are more complex for the H 2 O-containing phases, where the water molecules releasing occurs in stages. Magnesium analog of bis-biuret-zinc-chloride have been detected among the intermediate phases obtained during the decomposition of the studied compounds.
New Data on Crystal Phases in the System MgSO4–OC(NH2)2–H2O Rositsa Nikolova, Vladislav Kostov-Kytin, Nadia Petrova, Krasimir Kossev, Rositsa Titorenkova, et al. Crystals, 2024 Urea complexes of magnesium sulfate have been intensively studied due to their application in many areas of life, including agricultural chemistry, pharmacy, medicine, etc. The aim of this study is to add new knowledge about the trends and consistencies in the preparation procedures of MgSO4·nOC(NH2)2·mH2O phases. A set of analytical methods was used to characterize their structure, thermal and spectroscopic properties. The conditions for obtaining the three complexes in pure form were specified and the crystal structures of MgSO4·OC(NH2)2·2H2O and MgSO4·OC(NH2)2·3H2O were determined. The spectroscopic data of the considered compounds were analysed with respect to their structural and chemical properties. Thermal analyses showed that both the melting point and the urea decomposition temperature depend on the OC(NH2)2: H2O ratio in the octahedral environment of the magnesium ion in the studied structures.
