Gypsum Crystals Research Articles

Solidified lake sediment with industrial waste and construction waste used as barrier cover material: mechanical strength, water resistance performance, and microscopic analysis

ABSTRACT This study introduces a novel landfill cover material, employing lake sediment as a substrate, stabilised with fly ash, slag, desulfurisation gypsum and construction waste. The mechanical properties, including shear strength parameters, unconfined compressive strength, hydraulic conductivity, volumetric shrinkage, and water content, of the solidified sludge were evaluated. The microscopic mechanism of the solidified sludge were investigated through XRD, FTIR, and SEM-EDS techniques. A novel three-layer composite capping cover system for landfills is proposed, comprising an upper capillary barrier layer, a middle drainage layer and a bottom impermeable layer . Indoor rainfall simulation tests were conducted to assess the water retention performance of this capping cover system under repeated moderate, heavy, and torrential rainfall events. The early strength of the solidified sludge exhibited rapid development, with cohesion and internal friction angle reaching 382.56 kPa and 57.67°, respectively, after 3 days. After 28d, the unconfined compressive strength ranged from 6.93 to 14.29 MPa, with hydraulic conductivity between 3.98–23.1 × 10−8cm/s. The hydration reactions of the industrial waste residues resulted in the formation of Ettringite, Gypsum, and hydrous calcium (aluminum) silicates. The Ettringite and Gypsum crystals formed an internal support framework, while the generation of gel-like substances such as C-S-H and C-A-S-H facilitated product aggregation. The RSM was employed to optimise the material ratio of the solidified sludge, while the Pearson coefficient facilitated correlation analysis. This study provides valuable data for designing landfill solidified sludge cover systems and offers a new approach for the co-disposal of sludge and industrial waste.

Gypsum heterogenous nucleation pathways regulated by surface functional groups and hydrophobicity

Gypsum (CaSO4·2H2O) plays a critical role in numerous natural and industrial processes. Nevertheless, the underlying mechanisms governing the formation of gypsum crystals on surfaces with diverse chemical properties remain poorly understood due to a lack of sufficient temporal-spatial resolution. Herein, we use in situ microscopy to investigate the real-time gypsum nucleation on self-assembled monolayers (SAMs) terminated with −CH3, −hybrid (a combination of NH2 and COOH), −COOH, −SO3, −NH3, and −OH functional groups. We report that the rate of gypsum formation is regulated by the surface functional groups and hydrophobicity, in the order of −CH3 > −hybrid > −COOH > −SO3 ≈ − NH3 > − OH. Results based on classical nucleation theory and molecular dynamics simulations reveal that nucleation pathways for hydrophilic surfaces involve surface-induced nucleation, with ion adsorption sites (i.e., functional groups) serving as anchors to facilitate the growth of vertically oriented clusters. Conversely, hydrophobic surfaces involve bulk nucleation with ions near the surface that coalesce into larger horizontal clusters. These findings provide new insights into the spatial and temporal characteristics of gypsum formation on various surfaces and highlight the significance of surface functional groups and hydrophobicity in governing gypsum formation mechanisms, while also acknowledging the possibility of alternative nucleation pathways due to the limitations of experimental techniques.

The corrosion resistance of white ultra-high performance concrete under the coupling environment of acid rain and carbonization.

Acid rain and carbonization are two primary types of environmental corrosion that threaten the health of urban concrete structures over time. However, the coupling effects of acid rain and carbonization on concrete deterioration have been rarely reported. In this paper, four coupling regimes were designed using accelerated simulation experiments to investigate the deterioration properties of white ultra-high performance concrete (WUHPC). The results showed that under acid rain corrosion, the WUHPC surface was covered with white crystals before peeling off after 7 days, resulting first in an increase, followed by a rapid decrease in weight and strength, and the erosion depth linearly increased at a rate of 33.0 μm per day. Meanwhile, negligible changes occurred with only carbonization. However, under coupling corrosion, the deterioration worsened after environmental alternation. The strength of WUHPC with acid rain after carbonization decreased by 27.7%, reaching a minimum of 72.0 MPa. The erosion depth growth under acid rain followed by carbonization was 20.0 μm per day, which was much faster than that (3.9 μm per day) of single carbonization. The major corrosion products under acid rain were gypsum crystal, and the crystal shrank with time, leaving more voids and a weakened bonding strength. The calcium carbonate sediment generated during carbonization blocked the pores on the surface, hindered the diffusion of acid solution, and partly consumed acid ions via dissolution, resulting in facilitated acid rain corrosion. Once carbonate was consumed in a short time, more capillary pores were unblocked to promote further acid rain corrosion.

Research on the Elastic Properties of Deep Rock Composites Under High Pressure

This paper presents a three-dimensional model of the definitive structure of gypsum, calcite and quartz crystals, which are the primary components of deep rock mass. The optimization of the primary components of deep rock mass material was performed under pressures ranging from 0 to 120 MPa using the first principles method. This led to the determination of the structural constants of the primary component crystals within the deep rock material. Furthermore, the variation rule of mechanical parameters, including elastic constants, for the primary components of deep-seated rock materials under different pressures was also investigated. The findings indicate a clear downward trend in the partial crystal structure parameters of deep-seated rock materials with increasing pressure. The primary components of each rock mass material display disparate compressibility properties in distinct directions when subjected to uniaxial compression. In conclusion, the anisotropic elastic properties of gypsum crystals, calcite crystals and quartz crystals under different pressures were investigated. The anisotropic cloud diagrams of bulk modulus, elastic modulus and shear modulus were presented for the three types of rock material at 0 MPa, 60 MPa and 120 MPa, respectively. In conclusion, we derived the theoretical equations for the elastic tensor matrix of composite materials using the principle of superposition. Furthermore, we studied and obtained the elastic tensor matrix for composite materials within the deep rock mass. These results provide essential theoretical guidance for the development of underground space and the analysis of deep rock dynamics.

Preservation of Microorganisms (Chroococcidiopsis sp. 029) in Salt Minerals under Low Atmospheric Pressure: Application to Life Detection on Mars

Salt minerals on Mars represent a promising target for investigating potential past surface and subsurface life. Terrestrial salt minerals have been shown to incorporate microorganisms within crystals. However, the effect of Mars’s low atmospheric pressure on the preservation of microorganisms in salt minerals during their formation remains unclear. Here we investigated the interactions between microorganisms (Chroococcidiopsis sp. 029) and crystals of halite (NaCl), epsomite (MgSO4·7H2O), and gypsum (CaSO4·2H2O) during the rapid evaporation of brines under simulated Martian atmospheric pressure. Parallel experiments were conducted under terrestrial pressure for comparison. Halite, epsomite, and gypsum formed under both terrestrial and low-pressure conditions, though crystal morphologies varied depending on the pressure. Microorganisms were identified within fluid inclusions or as solid inclusions in the crystal matrix. Halite crystals exhibited a greater propensity to incorporate cells under low pressure compared to terrestrial pressure, while the entrapment of cells in epsomite was similar under both conditions. In contrast, significantly fewer cells were trapped in gypsum crystals under low pressure. The results demonstrate the feasibility of cell entrapment in salt minerals formed by rapid evaporation under low-pressure conditions on Mars, and atmospheric pressure exerts distinct influences on different types of salts. The variation in fluid inclusion size in halite under different pressures even shows promise as a possible paleobarometer. Our findings suggest that halite is the most promising candidate for preserving potential Martian life and could be an excellent target for future Mars sample return missions.

Assessing Fluoride Impact in Phosphogypsum: Strength and Leaching Behavior of Cemented Paste Backfill

Phosphogypsum (PG)-based cemented paste backfill (PCPB) is a widely used method for the large-scale disposal of phosphoric acid waste. However, fluoride contaminants in PG pose a significant risk of groundwater pollution. Effective pretreatment methods have been developed to mitigate this issue, but there is limited research on the specific pretreatment standards needed to control fluoride leaching. This study investigates the impact of initial fluoride content (IFC) in PG samples on the strength and leaching behavior of PCPB to establish appropriate pretreatment standards. Pure CaSO4·2H2O and NaF were used to simulate PG samples with varying IFC levels. The findings indicate that lower IFC leads to stronger PCPBs after 28 days of curing. To ensure compliance with Chinese national standards, the initial fluoride concentration must be below 0.0093 wt% to prevent excessive leaching. Microlevel analyses reveal that fluoride influences the hydration reactions of C2S and C3S in cement, affecting the formation of C-S-H and Ca(OH)2. Fluoride is primarily immobilized in PCPB as CaF2, which can either form gypsum crystals within the complex structure or be adsorbed onto hydration products. This research provides crucial insights into the pretreatment standards required for controlling fluoride leaching, offering a foundation for the industrial application of PG.

Gypsum crystal morphologies in lake sediments for paleoclimate reconstructions: a case study in Fuente de Piedra playa-lake (Málaga)

Gypsum (CaSO4∙2H2O) is one of the most common evaporitic minerals on Earth. Its crystals exhibit diverse morphologies that can provide insights into their depositional environment. We analyzed the morphologies of gypsum deposits in a 14 m sedimentary sequence of Fuente de Piedra playa-lake (Málaga, Spain) to link its variations to sedimentary facies and changes in the past lake level. Precipitation of primary prismatic crystals prevailed during lake highstand periods. In contrast, lenticular gypsum crystals are consistently present throughout the entire core and do not seem to be related to specific lake levels. An exception are macro-lenticular crystals, which seem to be correlated with dry periods and high salinity water. Subrounded gypsum grains, eroded from former primary crystals, are also abundant along the sequence and usually concentrate in facies related to stages of lake agitation during highstand periods. Gypsum is generally absent during lowstand periods, which may indicate relatively high dissolve organic matter or, more likely, less interaction of the lake with the saline groundwater aquifer beneath. Our results suggest that the occurrence of some gypsum morphologies can be used to infer paleo-lake levels.

Gypsum: From the Equilibrium to the Growth Shapes—Theory and Experiments

The gypsum crystals (CaSO4·2H2O) crystallizes in a low symmetry system (monoclinic) and shows a marked layered structure along with a perfect cleavage parallel to the {010} faces. Owing to its widespread occurrence, as a single or twinned crystal, here the gypsum equilibrium (E.S.) and growth shapes (G.S.) have been re-visited. In making the distinction among E.S. and G.S., in the present work, the basic difference between epitaxy and homo-taxy is clearly evidenced. Gypsum has also been a fruitful occasion to recollect the general rules concerning either contact or penetration twins, for free growing and for twinned crystals nucleating onto pre-existing substrates. Both geometric and crystal growth aspects have been considered as well, by unifying theory and experiments of crystallography and crystal growth through the intervention of βadh, the physical quantity representing the specific adhesion energy between gypsum and other phases. Hence, the adhesion energy allowed us to systematically use the Dupré’s formula. In the final part of the paper, peculiar attention has been paid to sediments (or solution growth) where the crystal size is very small, in order to offer a new simple way to afford classical (CNT) and non-classical nucleation (NCNT) theories, both ruling two quantities commonly used in the industrial crystallization: the total induction times (tindtotal) and crystal size distribution (CSD).

Effect of organic acid additives on sulfite oxidation in a calcium-based slurry

The forced oxidation of sulfite ions was conducted to obtain high-quality gypsum in the flue gas desulfurization system. The effects of formic, lactic, acrylic, acetic, propionic, and hexanoic acids added to calcium-based slurry on sulfite oxidation were investigated in this study. Sulfite oxidation was inhibited when the pKa value of organic acids was small or the carbon chain was long. In the identical slurry pH 6, the SO4 2- fractions of slurries with formic and lactic acids, which have the smallest pKa values, were lower than half that of the additive-free slurry. The addition of hexanoic acid with the longest carbon chain showed a similar SO4 2- fraction to that of additive-free slurry. These results indicate that the inhibitory effect on sulfite oxidation is more expressed by the acidity of organic acids. The SO4 2- fraction in the slurry affects the growth and quality of gypsum crystals. The slurry with acetic acid presented the highest SO4 2- fraction and resulted in the formation of high-quality gypsum crystals. The findings of this study can contribute to the selection of organic acid additives with high desulfurization efficiency and the production of high-quality gypsum.

The Impact of Early Diagenesis on Biosignature Preservation in Sulfate Evaporites: Insights From Messinian (Late Miocene) Gypsum.

Due to their fast precipitation rate, sulfate evaporites represent excellent repositories of past life on Earth and potentially on other solid planets. Nevertheless, the preservation potential of biogenic remains can be compromised by extremely fast early diagenetic processes. The upper Miocene, gypsum-bearing sedimentary successions of the Mediterranean region, that formed ca. 6 million years ago during the Messinian salinity crisis, represent an excellent case study for investigating these diagenetic processes at the expense of organic matter and associated biominerals. Several gypsum crystals from the Northern Mediterranean were studied by means of destructive and non-destructive techniques in order to characterize their solid inclusion content and preservation state. In the same crystal, excellently preserved microfossils coexist with strongly altered biogenic remains. Altered remains are associated with authigenic minerals, especially clays. The results demonstrate that a significant fraction of organic matter and associated biominerals (notably biogenic silica) underwent early diagenetic modification. The latter was likely triggered by bottom sulfidic conditions when the growth of gypsum was interrupted. These results have significant implications for the interpretation of the Messinian Salt Giant.

Gypsum Crystals Formed by the Anhydrite–Gypsum Transformation at Low Temperatures: Implications for the Formation of the Geode of Pulpí

Determining the mechanisms of the formation of giant crystals is a challenging subject. Gypsum, calcium sulfate dihydrate (CaSO4·2H2O), is known to form crystals larger than one meter in several locations worldwide. These selenite crystals grow at different temperatures, either in sedimentary or hydrothermal systems. The famous selenite crystals of the geode of Pulpí (Almería, Spain) are known to have grown at a temperature T = 20 ± 5 °C and have been proposed to form in a subaqueous environment by a self-feeding mechanism triggered by anhydrite dissolution and the ripening of microcrystalline gypsum, enhanced by oscillations in temperature. This paper reports the monitored crystallization of gypsum crystals, from anhydrite powder dissolution, inside airtight evaporation-free reactors under oscillating low temperatures (15 °C < T < 25 °C). These crystals are clearly smaller than the ones in the Pulpí mine but exhibit similar habits (i.e., single blocky crystals and twins following the 100 twinning law). The growth rate of gypsum single crystals has been measured to be between 3.8 and 35.3 µm/day. Noteworthy, we document the occurrence of the 100 contact twinning law of gypsum, which is the most widespread twinning law in natural environments but never univocally reported in laboratory experiments. The selection of the 100 contact twinning law has been correlated to the low supersaturation values obtained in the experiment, where the concentration in these long-duration experiments can be safely assumed to be the equilibrium concentration, i.e., 0.3 (at 25 °C) ≤ SI ≤ 0.4 (at 15 °C). We discuss the relevance of our experiment for forming the gypsum crystals of Pulpí in the framework of the geological history of Pulpí mineralization. These laboratory model experiments contribute to a deeper understanding of mineral nucleation and growth processes in natural environments.

Insights into mechanism of electric field regulating calcium sulfate crystal structure and crystallization: A study by DFT calculation and experiment

Exploring the mechanism of external electric field (EF) regulating the structure and crystallization of calcium sulfate crystal is of great significance for solving the problem of inorganic salt scale in many industrial processes such as desalination and heat exchange. Here, the dispersion-corrected Density Functional Theory (DFT-D) calculations and a series of experiments were carried out to investigate the influence of EF on crystal structure, mechanical property and crystallization of gypsum crystal. The results show that under the influence of EF, the conventional cell of the gypsum crystal shortens in the direction of lattice parameter b. The water layer and the CaO8 octahedron are deformed. The stiffness and hardness of gypsum crystals decreases. The attachment energy and surface energy of (020) plane increase significantly. Both DFT-D calculations and experiment characterization results indicate that EF promotes a regular growth of gypsum crystal mainly along in the direction of (020) plane. Our study results demonstrate the possibility of electrical regulation of calcium sulfate crystal growth to solve the industrial calcium sulfate scale problem.

Integrated above and below-ground responses of the gypsum specialist Helianthemum squamatum (L.). to drought

Gypsum endemics (i.e. gypsophiles) have adapted to live in gypsum-rich soils where nutrient unbalance and drought can be extreme. Despite their relevance as plants adapted to extreme conditions, a complete analysis of the physiological responses of gypsophiles to drought is still lacking. Helianthemum squamatum (L.) Dum. Cours. is a conspicuous Iberian gypsophile that has been reported to use gypsum crystallization water during the driest period, but the mechanisms behind this process are unknown. To characterize gypsophile responses to drought and unravel the mechanisms underlying gypsum crystalline water use, H. squamatum plants were grown in pots with natural gypsum soil and gypsum soil with deuterium-labelled crystalline water. After three years, a drought experiment was carried out and whole-plant responses were investigated. Unexpectedly, the labelling treatment affected soil physicochemical characteristics and reduced microbial biomass and organic matter content, decreasing plant aerial biomass. H. squamatum plants did not use gypsum crystallization water during simulated drought neither in the labelled soil, nor in the natural one. Drought reduced plant transpiration, stomatal conductance, water use, photosynthetic rate and the foliar concentration of most elements except P and N, which were higher in the drought stressed plants. We detected increased root exudation of choline, an osmoprotector, by drought stressed plants. The drought treatment also affected the structure of microbial communities but did not reduce the relative abundance of functional microbial groups, highly adapted to the natural drought pulses. Our results highlight an integrated water-saving strategy of H. squamatum plants in the short-term, where responses at the leaf level are combined with belowground processes, like altered root exudation. Our findings also underline the remarkable resistance to drought of microbial communities present in gypsum soils.

Insights into iron-accelerated gypsum scaling mitigation strategies in nanofiltration membrane process

In this study, the impact of ferric ions on gypsum crystallization and NF membrane performance was investigated to mitigate iron-accelerated gypsum scaling. Gypsum scaling in the presence of ferric ions led to a significant decline in the water flux, but a relatively higher reversible fouling ratio was observed compared to that under individual gypsum and iron-oxide scaling. Ferric ions influenced the crystallization of gypsum on NF membranes by serving as additional nucleation sites, leading to accelerated gypsum scaling. Various mitigation methods (e.g., feed spacers and cleaning processes) were evaluated to effectively control iron-accelerated gypsum scaling. The introduction of spacers exhibited a high mitigation efficiency by enhancing the mass transfer of scalant ions due to changes in the water channel hydrodynamics, but the overall efficiency decreased over time. To enhance the mitigation efficiency, different cleaning methods (e.g., CO2 oversaturated solution, HCl, and NaCl flushing) were conducted with feed spacers. A CO2 saturated solution demonstrated high cleaning efficiency owing to the high turbulence induced by CO2 bubbles, whereas HCl flushing showed negligible efficiency. NaCl flushing exhibited the highest cleaning efficiency through mono/divalent ion-exchange effects on iron-gypsum scaling. This study provides comprehensive insights into iron-accelerated gypsum scaling mitigation strategies for nanofiltration membranes, highlighting the impact of coexisting ferric compounds on gypsum scaling.

Exploring the feasibility of using fluidized bed homogeneous crystallization technology to recover sulfate ions from briny water as calcium sulfate pellets

The increasing salinity of water sources poses significant operational and environmental challenges, highlighting the importance of developing efficient methods for recovering [SO42−] ions from briny wastewater. In this study, the potential of fluidized bed homogeneous crystallization (FBHC) technology for recovering [SO42−] ions from briny water is explored. FBHC is an innovative technique that enhances scalability and separation efficiency by promoting the growth of crystalline particles in a fluidized state. To achieve maximum [SO42−] recovery and maintain stable fluidized bed conditions, a pH of 8 is recommended. A removal ratio (RR) of 71 % was recorded at a [SO42−] concentration of 0.14 M. Various [SO42−] concentrations exhibited a molar ratio (MR) of [Ca2+]/[SO42−] = 1.5. Smaller particles, which are more soluble and exhibit less nucleation, are typically formed under conditions with lower pH and [Ca2+] levels. Intermediate-sized particles are favored at neutral to slightly alkaline pH levels and near-stoichiometric [Ca2+]/[SO42−] MR. Greater calcium content and higher pH values promote the development and agglomeration of larger particles, with 50 % of the particles exceeding an average size of 0.38 mm. In the XPS study of CaSO4 crystals (gypsum), the Ca 2p1/2 and Ca 2p3/2 peaks are observed at binding energies of 351.5 eV and 347.7 eV, respectively. Gypsum is identifiable due to the presence of clear and distinct peaks in XRD patterns. SEM analysis reveals the coexistence of amorphous patches and tiny crystalline domains on the surface. These gypsum crystals resemble stacked needle-like structures accompanied by small, powdery crystals. In an acidic environment, the primary phase is CaSO4·2H2O, which generates well-formed, elongated, or tabular crystals that are characteristic of gypsum. The FBHC process, characterized by a high crystallization ratio (CR = 68 %) and the recovery of crystal pellets (>0.3 mm), significantly reduces sludge production compared to traditional chemical precipitation methods. Its cost-effectiveness and recoverability make FBHC a promising technology for the recovery of sulfate ions from briny wastewater.

Prominent differences in the adsorption and substitution characteristics of As(III) and As(V) on gypsum during wet flue gas desulfurization

Arsenic-bearing gypsum (ABG) is a hazardous by-product of wet flue gas desulfurization (FGD) and must be strictly controlled. This study investigated the migration of arsenic species during wet FGD and proposed strategies to reduce the toxicity of FGD gypsum in operation. During gypsum crystallization, the adsorption and substitution characteristics of As(III) and As(V) exhibit significant differences. The amount of As(III) captured by CaSO4·2H2O is greater than that of As(V). The As(III) adsorbed on the growth surface of CaSO4·2H2O can be incorporated into the gypsum by substituting for SO42−, while As(V) tends to remain adsorbed on the surface. Moreover, the arsenic adsorption capacity of the incompletely oxidized product CaSO3·0.5H2O was almost three times that of CaSO4·2H2O, especially for As(III). Based on the differences in the migration characteristics of As(III) and As(V), a strategy was proposed to reduce the arsenic content and biotoxicity in gypsum during operation by enhancing the oxidation of sulfite and highly toxic arsenite, thus ensuring the safe utilization of FGD gypsum.

Bio-Inspired Fluorescent Calcium Sulfate for the Conservation of Gypsum Plasterwork.

In this work, the potential of bio-inspired strategies for the synthesis of calcium sulfate (CaSO4·nH2O) materials for heritage conservation is explored. For this, a nonclassical multi-step crystallization mechanism to understand the effect of calcein- a fluorescent chelating agent with a high affinity for divalent cations- on the nucleation and growth of calcium sulfate phases is proposed. Moving from the nano- to the macro-scale, this strategy sets the basis for the design and production of fluorescent nano-bassanite (NB-C; CaSO4·0.5H2O), with application as a fully compatible consolidant for the conservation of historic plasterwork. Once applied to gypsum (CaSO4·2H2O) plaster specimens, cementation upon hydration of nano-bassanite results in a significant increase in mechanical strength, while intracrystalline occlusion of calcein in newly-formed gypsum cement improves its weathering resistance. Furthermore, under UV irradiation, the luminescence produced by calcein molecules occluded in gypsum crystals formed upon nano-bassanite hydration allows the easy identification of the newly deposited consolidant within the treated gypsum plaster without altering the substrate's appearance.

Protein-Decorated Reverse Osmosis Membranes with High Gypsum Scaling Resistance.

The global challenge of water scarcity has fueled significant interest in membrane desalination, particularly reverse osmosis (RO), for producing fresh water from various unconventional sources. However, mineral scaling remains a critical issue that compromises the membrane efficiency and lifespan. This study explores the use of naturally occurring proteins to develop scaling-resistant RO membranes through an eco-friendly modification method. We systematically evaluate three protein modification techniques, namely, polydopamine (PDA)-assisted coating, protein conditioning, and protein drying, for fabricating membranes resistant to gypsum scaling. Protein conditioning is found to be the most effective approach, resulting in protein-decorated membranes with an exceptional resistance to gypsum scaling. We also demonstrate that a hydrated protein layer is essential for optimal scaling resistance. To further understand the mechanism underlying the scaling resistance of protein-decorated membranes, five proteins (i.e., bovine serum albumin, casein, lactalbumin, lysozyme, and protamine) with distinct physicochemical properties are used to explore the key factors governing membrane scaling resistance. The results of dynamic RO experiments indicate that the molecular weight of proteins plays a crucial role, with higher molecular weights leading to higher membrane scaling resistance through steric effects. However, static experiments of bulk crystallization highlight the importance of electrostatic interactions, where proteins with more negative charge delay gypsum crystallization more effectively. These findings suggest the difference between gypsum scaling in the RO and gypsum crystallization in bulk solutions. Overall, this research offers a novel approach to developing resilient and sustainable RO membranes for the desalination of feedwater with high scaling potential while elucidating mechanistic insights on the mitigating effects of protein on gypsum scaling.

Experimental investigation into the effect of porosity on the strains developing during anhydrite to gypsum transformation

The swelling of anhydritic claystones often leads to severe tunnel damage. Even though this phenomenon has gained significant scientific interest, particularly in the last decades, there are still open questions which introduce uncertainties in tunnel design. One question concerns the strains developing during the anhydrite to gypsum transformation (AGT). These depend, among other factors, on whether the gypsum crystals grow within the available pore space or whether they tend to push the particles apart, leading to an expansion of the matrix and, in turn, larger macroscopic strains. The experimental investigations of this paper aim to assess the influence of the initial porosity on the strains developing during AGT. Specimens consisting of highly compacted anhydrite and kaolin powders are created with varying initial porosities between 0.22 and 0.35. It is concluded that, ceteris paribus, the strains developing during AGT decreases with increasing initial porosity. The results also indicate that in the case of high initial porosity the gypsum crystals grow in the available pore space, thus decreasing the porosity, while in the case of low initial porosity, gypsum growth leads to an increase of the pore space. The results are applicable to porous media where crystallisation may occur within the pores.

Early length changes and microscopic phase analysis of phosphorus hemihydrate gypsum fireproof mortar

Abstract Coating the surface of steel structures with phosphohemihydrate gypsum fireproof mortar can improve the fire resistance of steel structures. The hydration curve of phosphorus hemihydrate gypsum (PHG) showed a monotonic increasing trend, and the hydration time was about 1 day. In order to study the hydration process of phosphorus hemihydrate gypsum fireproof mortar, its length change in the early hardening stage was experimentally investigated. The results showed that compared with the hydration time of phosphohemihydrate gypsum, the duration of early length changes in phosphohemihydrate gypsum fireproof mortar was significantly prolonged to about 4 days, and four intervals were clearly observed during this process. Hydroxypropyl methyl cellulose (HPMC) as a water retaining agent, hydroxypropyl starch ether (HPS) as a thixotropic agent, and dispersed latex powder (LP) can all make the four intervals of length changes of fireproof mortar more significant. After adding HPMC, HPS, and LP, it can promote the formation of dihydrate gypsum mainly in fireproof mortar, but dihydrate gypsum crystals were more likely to grow along the [021] direction. The addition of additives can also effectively improve the pore structure of phosphogypsum fireproof mortar.