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.
Topotactic route to novel layered titanosilicates via mild solution treatment of AM-4 at ambient temperature Stanislav Ferdov, Zhi Lin, Rositsa Titiorenkova, Nadia Petrova, Boris Shivachev, Rositsa Nikolova Applied Clay Science, 2026 Layered hydrated titanosilicates capable of topotactic transformations are known in nature. However, their preparation in laboratory conditions is challenging. This work demonstrated the synthesis and characterization of two novel layered titanosilcates, MSA-1, Na 2 [Ti 2 Si 4 O 12 (OH) 2 ]∙1.6H 2 O ( C 2 /c , a = 27.6518(12), b = 8.68987(37), c = 5.26316(21) Å, β = 90.8916(33) o , V = 1264.501(91) Å 3 ) and MSA-2, HTi 2 Si 4 O 11 (OH)(OH) 2 ∙ n H 2 O ( n is close to 1) ( C 2 /c , a = 26.4573(21), b = 8.75932(72), c = 5.21790(39) Å, β = 92.2808(63) o , V = 1208.28(17) Å 3 ), structurally similar to the minerals eliseevite and punkaruaivite, respectively. These materials were synthesized at room temperature by a 2D-2D transformation of AM-4 [Na 3 (Na,H)Ti 2 O 2 [Si 2 O 6 ] 2 ∙2H 2 O] titanosilicate, a synthetic sodium counterpart of the mineral lintisite. The transformations involved a gradual lattice contraction due to the removal of Na + cations from the interlayer space. This loss of positive charge was compensated by the protonation of underbonded framework oxygen atoms, resulting in the formation of hydroxyl groups (OH − ). These OH groups were structurally integrated into the titanosilicate framework, which helped to preserve charge neutrality. Other transformations were also found by heating MSA-2 to 200 °C which led to a lattice contraction, decreased symmetry ( P 2 1 /c , a = 11.9484(6), b = 8.7067(4), c = 5.2227(2) Å, β = 101.043(4) o , V = 533.26(4) Å 3 ) and resulted in the formation of the layered titanosilicate L3 [Ti(Si 2 O 5 )(OH)(OH)]. Further heating of MSA-2 to 300 °C led to the formation of a phase similar to L3 but with a substantially smaller lattice ( P 2 1 /c , a = 11.6257(8), b = 8.6809(5), c = 5.2235 Å, β = 100.906(4) o , V = 517.65(5) Å 3 ). The MSA-2 showed high CO 2 adsorption capacity. • Room-temperature topotactic transformation reveals new layered titanosilicates. • Immersing AM-4 in diluted H 2 O 2 results in the titanosilicate MSA-1, analogue of the mineral eliseevite. • Immersing AM-4 in diluted H 2 SO 4 results in the titanosilicate MSA-2, analogue of the mineral punkaruaivite. • MSA-2 titanosilicate reveals CO 2 adsorption properties.
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, Rositsa Titorenkova 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 chemistry of epsomite from three Bulgarian localities Rositsa Nikolova, Nadia Petrova, Vladislav Kostov‐Kytin, Rositsa Titorenkova, Petko Petrov Geologica Balcanica, 2025 Epsomite - MgSO4•7H2O has been first described as early as the eighteenth century and has attracted the interest of scientists for four centuries. Epsomite can be a constituent of evaporite mineral deposits and occurs as crusts and efflorescences in mine workings, in caves, in oxidation zones of sulphide ore deposits, on the surface of soils, and on the surface of objects of archaeological value. The so far published information on epsomite from Bulgarian deposits is based solely on microscopic and X-ray phase analyzes. In the present study, data on the crystal structure, spectroscopic and thermal characteristics of epsomite from three Bulgarian localities are reported for the first time. Data on natural compounds of the type (Mg1-x,Znx)SO4•7H2O (0.03<x<0.36) are also presented for the first time. The impact of impurity elements on certain crystal structural characteristics and in particular on the metal octahedron are discussed and this polyhedron degree of deformation has been estimated.
Optimal conditions for synthesis of Cr4+ doped Li2CaGeO4 glass-ceramics for laser emission in the near IR region V. Nikolov, I. Koseva, P. Tzvetkov, R. Nikolova, N. Petrova, R. Kukeva, D. Karashanova Ceramics International, 2025 The main goal of this research was to find a suitable starting glass composition and suitable conditions for it thermal treatment for preparing of Cr 4+ doped Li 2 CaGeO 4 glass-ceramics as an active medium for laser radiation in the range 1.1–1.6 nm. The crystallization of the glasses after thermal treatment from the four-component system Li2O-CaO-GeO2-B2O3 was studied, and the crystallization area of Li2CaGeO4 was determined for three sections of the system with content of B2O3 5.0, 7.5 and 10.0 mol%. The crystallization kinetic in the glass and the formation of different phases were experimentally investigated not only depending on the composition of the starting glass, but also on its thermal treatment regime. It is shown that the thermal treatment regime is decisive not only for the amount of a given crystallizing phase as well as the size and morphology of the nanoparticles , but also for the type, number and quantitative ratio of the crystallizing phases themselves. The obtained results are discussed in terms of the crystallization rules below the solidus line in multicomponent systems . As a result of the research, an optimal composition of the glass was found, allowing homogenization at 1150 °C for only 2 h, solidifying in an amorphous mass. An optimal heat treatment regime of the glass was chosen and as a result a Cr4+ glass-ceramic with very good properties was obtained. The obtained glass-ceramic contains up to 60 mass% Cr4+ doped Li2CaGeO4 nanophase with an average particle size of about 20 nm preserving transparency of about 70 % in relative to the no treated glass. X-ray powder diffraction, DSC , EPR , TEM and UV–VIS analysis were used for characterization of the glass-ceramics.
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.
Metastable microporous lanthanide silicates – Light emitters capable of 3D-2D-3D transformations Stanislav Ferdov, Boris Shivachev, Iovka Koseva, Petia Petrova, Nadia Petrova, Rositsa Titorenkova, Rositsa Nikolova Chemical Engineering Journal, 2024 The synthesis of zeolites and porous heteropolyhedral silicates is usually accompanied by crystal phase transformations. Typically, this process is irreversible and characterized by a conversion from a metastable phase to a stable one. Here we demonstrate a reversible transformation between porous (K3LnSi3O9·nH2O) and layered (K3LnSi3O8(OH)2) structures exemplified by a dual synthesis (hydrothermal and solid-state transformation). We discuss structural and photoluminescent properties of the obtained microporous lanthanide silicates MS-4 (Minho-Sofia, solid number 4; K3LnSi3O9·nH2O, Ln = Y, Dy; orthorhombic, Pmc21). These materials are metastable, transforming from 3D porous to a 2D layered structure after prolonged crystallization time at hydrothermal conditions and returning to the same but distorted (≈ 2 % smaller lattice volume) porous 3D framework by calcination of 2D structure. The mechanism of 3D-2D structure transformation is driven by interzeolite-like transformation, while the rearrangement of common structural fragments controls 2D-to-3D conversion. We also show how the avoidance of Dy and preference for Y ions during MS-4 hydrothermal synthesis may be overcome by a 2D-3D conversion, allowing the production of white light emitters with controllable photoluminescence.
Li4GeO4-Li2CaGeО4 phase equilibria and Li2+xCa1-xGeO4 solid solutions V. Nikolov, R. Nikolova, N. Petrova, P. Tzvetkov, I. Koseva Heliyon, 2024 Detailed studies of the Li 4 GeО 4 -Li 2 CaGeО 4 system by solid-phase syntheses of various compositions from pure Li 4 GeО 4 to pure Li 2 CaGeО 4 in the temperature range from 25 to 1125 °C is investigated for a first time. Solid state synthesis powders are characterized by X-ray and DSC/TG methods. Concentration and temperature two-phase regions of Li 4 GeО 4 and Li 2 CaGeО 4 as well as two-phase regions of Li 2 CaGeО 4 and Li 2+2x Ca 1-x GeO 4 are established. Region of pure Li 2+2x Ca 1-x GeO 4 solid solution are detected too and it structure is investigated. Being structural analog to Li 2+2x Zn 1-x GeO 4 and Li 2+2x Mg 1-x GeO 4, Li 2+2x Ca 1-x GeO 4 has own specific local environment of the metal ions. The obtained results are compared with those for Li 4 GeО 4 -Li 2 ZnGeО 4 system and for Li 2+2x Zn 1-x GeO 4 solid solution. The differences of the phase diagrams and structural features of the solid solutions are discussed.
New Data on Crystal Phases in the System MgSO4–OC(NH2)2–H2O Rositsa Nikolova, Vladislav Kostov-Kytin, Nadia Petrova, Krasimir Kossev, Rositsa Titorenkova, Gergana Velyanova 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.
Unusual large pore copper silicate for CO2 adsorption Stanislav Ferdov, Boris Shivachev, Nikola Drenchev, Konstantin Hadjiivanov, Svetlana Simova, Rositsa Titorenkova, Nadia Petrova, Mihail Tarassov, Rositsa Nikolova Microporous and Mesoporous Materials, 2024
Indomethacin loading and in vitro release properties from vinyl acetate homo- and co-polymer nanoparticles, coated with polyzwitterion and carbopol® shells International Journal of Pharmacy and Pharmaceutical Sciences, 2014
The high energy milling effect on positional redistribution of CO 3-ions in the structure of sedimentary apatite Bulgarian Chemical Communications, 2013
Characterization of the pozzoulanic properties of the fly ash evolved from tpp "republika", bulgaria Journal of the University of Chemical Technology and Metallurgy, 2011
Structural changes in the system natural apatite - NH4 clinoptilolite during triboactivation Bulgarian Chemical Communications, 2011
