Hemihydrate Gypsum Research Articles

Preparation of Medium–Low Strength Hemihydrate Gypsum from Phosphogypsum and Grey Correlation Evaluation of Autoclaving Energy Consumption

Preparation of Medium–Low Strength Hemihydrate Gypsum from Phosphogypsum and Grey Correlation Evaluation of Autoclaving Energy Consumption

Effect of Hemihydrate gypsum on the basic properties of oil-well cement

The purpose of the study was to determine the influence of gypsum in different phase states (dihydrate or hemihydrate), an active additive of granulated blast furnace slag, and the total SO3 content on free water separation, the spread of well cement slurry, and the flexural and compressive strength of cement mortar. Laboratory and production cement mills were used to compare the results. The conducted research cycle enabled the quantitative determination of these parameters. Gypsum was preheated in a furnace at 200 °C for 60 minutes. It was found that the presence of hemihydrate gypsum in the cement does not significantly affect free water separation but has some impact on the spread of the well cement slurry. Additionally, the study revealed the influence of hemihydrate gypsum on compressive strength at 28 days and flexural strength at 1 day. The research also discovered that the presence of hemihydrate gypsum in the cement reduces the thickening time by 60 minutes. The presence of granulated blast furnace slag in the cement reduced free water separation in the well cement slurry. The research conducted at the production cement mill confirmed that the presence of hemihydrate gypsum in the cement reduced the spread by 25 mm and free water separation by 1 ml. The results from the production cement mill and laboratory mill are comparable and representative.

DEVELOPMENT OF DRY BUILDING MIXTURES BASED ON REDISPERSIBLE POWDER FOR REGULATING THE PROPERTIES OF PLASTER MIXTURES

The relevance of the work. The results is theoretically substantiated and experimentally confirmed the possibility of obtaining a dry additive-retarder of gypsum setting based on lime and polyvinyl acetate dispersions due to the formation of polyvinyl alcohol and salts of calcium acetate in the result of alkaline hydrolysis of polyvinyl acetate dispersion (PVAD) with the slaked lime in hydrated lime. The results of the work implemented in the production of dry gypsum mixes. Purpose. The development of dry building mixes based on gypsum and time increase of time of their conformance through adding of complex additives on the basis of burnt lime and polyvinyl acetate dispersions [PVAD]. Methodology. The study used standard research techniques to determine physical and mechanical properties of gypsum binders and quality of complex additives according to ДСТУ Б В.2.7-82:2010, ДСТУ Б В.2.7-23-95, ДСТУ Б В.2.7-171:2008. Study of phase composition of materials, micro – and macrostructure was carried out by x-ray diffraction, electron and light microscopy. The results. The influence of the main components of the additive (polyvinyl acetate dispersion and lime) on the properties of hemihydrate gypsum is investigated. The chemistry of the interaction of components of the complex additive, according to which the result of alkaline hydrolysis of polyvinyl acetate is formed: polyvinyl alcohol, a feature of its molecules structure is the presence of hydrophilic OH−groups and the hydrophobic component − a hydrocarbon radical. The hydrophilic part of the molecule being ion adsorbed on the surface of the particles of the binder, forms a monomolecular layer that is oriented with the hydrophobic part of the gypsum particles. It is also possible the formation of calcium acetate Ca(СН3СОО)2, which increases the concentration of calcium ions in solution. The combined effect of hydrolysis products significantly reduce the solubility of hemihydrate, the rate of formation of crystallization centers, due to the slowing effect of these supplements.

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.

Effect of phosphorus impurities on sulfoaluminate cement-modified gypsum-based self-leveling mortar and improvement method

The preparation of gypsum-based self-leveling mortar (GSLM) from solid waste-derived sulfoaluminate cement-modified hemihydrate gypsum is an effective path to enhance its mechanical properties. However, the phosphorus impurities in phosphogypsum (PG) still pose a challenge to the hemihydrate gypsum & sulfoaluminate cement composite system. To evaluate the impact of phosphorus impurities and develop targeted removal strategies, this study first assessed the effects of different types of phosphorus impurities on the mechanical properties and hydration characteristics of the GSLM. Then, a method of pretreating PG with carbide slag (CS) was proposed, and the removal efficiency and mechanism of phosphorus were examined by ICP-OES and XPS testing. The results indicated that the soluble phosphorus impurities severely inhibit the hydration of both CŜH0.5 and C4A3Ŝ, leading to a most notable reduced mechanical performance. The sequence of inhibitory action was as follows: H3PO4 >> Ca(H2PO4)2·H2O > CaHPO4·2H2O > Ca3(PO4)2. CS could effectively immobilize the soluble phosphorus in PG. The pretreatment by CS notably enhanced the hydration degree of the GSLM by converting soluble phosphorus into Ca3(PO4)2 beforehand. In addition, the alkaline environment provided by CS promoted the ettringite formation. When the CS content approaches 3%, a PG-based GSLM with good performances meeting the G20 standard requirements of GSLM can be prepared.

Effect of phosphate impurities on the properties of β-hemihydrate gypsum pastes plasticized by polycarboxylate superplasticizer

This study aims to investigate the effect of HPO42-/PO43- on the working performance and mechanical strength of β-hemihydrate gypsum pastes containing polycarboxylate superplasticizer (PCE) and reveal its underlying mechanism. The results showed that HPO42-/PO43- has a detrimental effect on the fluidity, compressive strength, and softening coefficient of β-hemihydrate gypsum pastes. However, in the presence of PCE, all properties were improved compared to the blank group when the HPO42-/PO43- content was less than 0.3 %. The fluidity and compressive strength of the pastes can be increased by up to 95 % and 32 %, respectively. It was found that HPO42-/PO43- reacted with Ca2+ to form insoluble calcium phosphate salt (ICPS), and the adsorption of PCE on the surface of ICPS increased the total adsorption amount, thereby enhancing the dispersion ability of PCE and increasing the fluidity of fresh slurry. In addition, the adsorption behavior of PCE on the surface of ICPS weakened its inhibitory effect on the hydration of hemihydrate gypsum, which was confirmed by TG-DSC. The increased hydration products and reduced water requirements significantly improved mechanical properties. These findings enhance the understanding of the interaction between phosphorus impurities and PCE, which can guide the improvement of the performance of β-HPG and promote the resource utilization of phosphogypsum.

Mechanical performance and water resistance of a carbonated hemihydrate gypsum and slaked lime composite

Mechanical performance and water resistance of a carbonated hemihydrate gypsum and slaked lime composite

Effect of Fluorine and Phosphorus Impurities in Phosphogypsum on Microstructure and Mechanism of α‐Type Hemihydrate Gypsum Crystals

AbstractIn this study, phosphogypsum (PG) is simulated by doping fluorine and phosphorus ions in an analytically pure reagent of gypsum dihydrate. The influence of fluorine and phosphorus impurity and content on the dehydration reaction process of phosphogypsum and its crystalline micromorphology is assessed during the preparation of α‐type gypsum hemihydrate in the reversed‐phase microemulsion system and its mechanism. The results show that when the fluorine content increases from 0 to 1.0 mol L−1 (ωNaF = 1.0 mol L−1), the dehydration process of dihydrate gypsum will be greatly slowed down. Scanning electron microscopy (SEM) analysis showed that even a small amount of F− (ωNaF = 0.2 mol L−1) can significantly inhibit the formation of α‐type hemi‐hydrated gypsum. When ωH3PO4 = 0.10 mol L−1, the water of crystallization content in the solid phase of the sample decreased to 5.24% after 90 min, which is significantly lower than during the same period of the benchmark group. However, there is a threshold value for the effect of phosphorus on the microscopic morphology of the α‐type gypsum hemihydrate crystals, when ωH3PO4 ≤ 0.04 mol L−1, the crystal morphology is basically unaffected. Moreover, when ωH3PO4 continued to increase, the defects on the crystal surface increased.

Development of Eco-Friendly Soil Improvement Agent for Enhanced Slope Stability and Erosion Control in Urban Areas

Due to the impact of climate change, extreme rainfall events are becoming more frequent, resulting in shallow slope collapse and erosion that trigger debris flows. While traditional reinforcement methods like anchoring and nailing are effective, they can be costly and environmentally unfriendly. To address this issue, researchers have investigated using in situ soil reinforcement with vegetation, which is a more sustainable and economical option. In this study, a soil improvement agent was developed using leaf mold and herbal medicine to promote vegetation growth. Adding microcement and gypsum hemihydrate increased the shear strength of the soil, preventing surface erosion. A laboratory test confirmed that the combination of these ingredients effectively increased the soil’s resistance to erosion caused by rainfall. The soil improvement agent proposed in this study was applied to the case of the slope failure in the Gwangju area, South Korea, to confirm the slope stability for 10 days of rainfall. The results of numerical analysis confirmed that the reinforced slope cured by the pozzolanic reaction using the developed material improved the slope stability by 36% compared to the original soil slope during the rainy season.

Synthesis and Characterisation of Hemihydrate Gypsum-Polyacrylamide Composite: A Novel Inorganic/Organic Cementitious Material.

This study combined inorganic α-hemihydrate gypsum (α-HHG) with organic polyacrylamide (PAM) hydrogel to create a novel α-HHG/PAM composite material. Through this facile composite strategy, this fabricated material exhibited a significantly longer initial setting time and higher mechanical strength compared to α-HHG. The effects of the addition amount and the concentration of PAM precursor solution on the flowability of the α-HHG/PAM composite material slurry, initial setting time, and mechanical properties of the hardened specimens were investigated. The structural characteristics of the composite material were examined using XRD, FE-SEM, and TGA. The results showed that the initial setting time of the α-HHG/PAM composite material was 25.7 min, which is an extension of 127.43% compared to that of α-HHG. The flexural strength and compressive strength of the oven-dried specimens were 23.4 MPa and 58.6 MPa, respectively, representing increases of 34.73% and 84.86% over values for α-HHG. The XRD, FE-SEM, and TGA results all indicated that the hydration of α-HHG in the composite material was incomplete. The incompleteness is caused by the competition between the hydration process of inorganic α-HHG and the gelation process of the acrylamide molecules for water, which hinders some α-HHG from entirely reacting with water. The enhanced mechanical strength of the α-HHG/PAM composite material results from the tight interweaving and integrating of organic and inorganic networks. This study provides a concise and efficient approach to the modification research of hemihydrate gypsum.

Experimental and theoretical investigation on the mechanism of cationic surfactants-assisted phosphogypsum crystal transformation

The massive discharge and stockpiling of phosphogypsum became an increasingly severe issue on environment. The transformation of phosphogypsum into fibrous alpha-type hemihydrate gypsum (α-HH) with high L/D(ratio of length-to-diameter) is one of the effective routes to achieve the high-valued utilization. In this work, six types of cationic surfactants, including CTAB, STAC, CHEC, CGG, CPAM and PDDA were selected to investigate their different effects on shape of α-HH crystals through hydrothermal method under the condition of 160 °C(0.6 MPa saturated vapour pressure) for 3 h. The results showed that the introduction of all selected surfactants can increase the length-to-diameter (L/D) ratio of α-HH crystals. As the concentration of surfactants increases, the L/D ratio first increases and then decreases, and the highest L/D ratio of 171.3 was obtained by using CTAB at 0.6 %. The mechanism of regulated growth of α-HH crystals into fibrous structure following c-axis in the presence of six cationic surfactants was also demonstrated. Moreover, the adsorption mechanism of cationic surfactants on the α-HH surface was investigated by molecular dynamics simulation and theoretical analysis. It is found that the order of binding energy (Eb) between the addition and α-HH crystal faces is Eb(11¯0) > Eb(110) > Eb(200) > Eb(002). This work provides insightful guidance from both experimental and theoretical perspectives for controlling the crystal form of α-HH and preparing high-strength gypsum.

Insights into water absorption characteristics of various waste-based inorganic additives and their application for soil stabilization

Cement is the most important binding agent used in soil stabilization and significantly improves the strength and compressibility of soils. However, owing to the significant environmental and socio-economic impact of cement production, alternative, low-carbon binders are increasingly being sought. Among them, waste-based inorganic additives used in construction provide sustainable soil treatment materials and reduce the need for landfills. Effective mud stabilization depends on the type of stabilizers and their mineralogical setup, as well as an understanding of their water absorption capacity. Therefore, this study investigated the factors influencing the water absorption characteristics of waste-based additives, including paper sludge ash (PSA), palm kernel shell ash (PKSA), rice husk ash (RHA), coal fly ash, hemihydrate gypsum, limestone powder, and basalt rock powder (BRP). The water absorption capacity (Wab) of each additive represents the amount of water absorbed per gram evaluated via the suction filtration method. The long-term water absorption capacities of the waste additives were compared with a conventional additive, blast furnace cement class B (BFCB). Although PSA, PKSA, and RHA absorbed more water than BFCB under short-term curing, BFCB exhibited the highest Wab under long-term curing. PSA, PKSA, and RHA had a water absorption capacity at 4 h (Wab)t = 4h of 74.5%, 107.3%, and 62.9%, respectively, whereas that of BFCB was 37.4%. In contrast, after 168 h, the water absorption capacity of BFCB increased to 126.1%. These evaluated water absorption capacities were categorized into immediate and time-dependent stages. Mercury intrusion porosimetry, X-ray diffraction (XRD), and scanning electron microscopy investigations were conducted to investigate the influence of factors such as porosity and hydration of the different additives on water absorption capacity. The results showed that porosity characteristics affect the immediate water absorption capacity, and hydrate formation contributes to increased water absorption during curing. The waste additives PSA, PKSA, and RHA with irregular surfaces and pores were highly porous and exhibited a high immediate water absorption capacity (Wab)t = 4h, unlike the additives with flat surfaces and less irregularities with lower (Wab)t = 4h. The XRD investigation revealed that the presence of ettringite and calcite contributed to increased water absorption during curing for PSA, PKSA, and BFCB. The generation of needle-like ettringite could promote a denser microstructure in PSA, PKSA, and BFCB, thus further improving Wab during curing. Finally, using cone index strength, we explored the effective application of waste-based additives as soil stabilizers, attempting to predict the effect of water absorption capacity on the properties of treated soil. The stabilizer addition ratio (A) and Wab of the waste additives were found to be essential indicators for predicting the strength development of treated clays. A clear correlation was observed between the cone index (qc) and parameter β(t), which is derived from Wab and A, of the treated clays.

Effect of Al2(SO4)3 Additive on the Properties of Calcined Gypsum Prepared from Flue Gas Desulfurization

In order to upgrade the utilization of flue gas desulfurization (FGD) gypsum, crystal modifier aluminum sulfate (Al2(SO4)3) was added into FGD gypsum to prepare calcined gypsum by calcining at normal pressure, and the effect of Al2(SO4)3 addition on the performance of calcined gypsum was studied. The results show that the addition of Al2(SO4)3 to FGD gypsum is slightly beneficial to promote the crystallization of hemihydrate gypsum (HH) along the a-axis direction. The Al2(SO4)3 addition also has a quick-setting effect on gypsum plaster during the hydration process, meanwhile inhibiting the growth of the (020) crystal plane of dihydrate gypsum (DH), then promoting uniform growth and aggregation along the c-axis. The close stacking makes hardened gypsum body dense and improves the strength of calcined gypsum. The strength of calcined gypsum prepared by calcining FGD gypsum with Al2(SO4)3 of 0.6 wt.% at 170 ℃ for 2 h is the highest. 2 h and 7 d flexural strength are 3.80 MPa and 7.20 MPa, and 2 h and 7 d compressive strength are 9.05 MPa and 19.23 MPa, respectively. In addition, 2 h flexural and compressive strength of the calcined gypsum with 0.6 ~ 1.0 wt.% of Al2(SO4)3 prepared at 170 ~ 180 ℃ for 2 h increase by 16 ~ 22 % and 20 ~ 33 %, respectively, which is very advantageous for the high value-added reuse of FGD gypsum for the preparation of high-quality calcined gypsum.

Coordinated disposal of FGD gypsum and power plant concentrated brine via preparation of α-hemihydrate gypsum

α-hemihydrate (α-HH), known for its exceptional mechanical properties, is a widely used building material. Despite its advantages, the traditional method of producing α-HH through regular pressure brine poses significant drawbacks. These include substantial industrial salt demands, high production costs, and water pollution. A more environmentally friendly α-HH preparation process has been developed to counteract these issues, combining FGD gypsum and concentrated brine. This study delves into the effects of the principal ions present in concentrated brine on the phase transition process and the microstructure of the final product. Our findings reveal that the reaction rate accelerates with increased concentrations of Ca2+, Mg2+, K+, and Na+ in concentrated brine. However, excessive Na+ concentration can alter the phase transition process, converting the product to β-HH. Additionally, an overly high concentration of KCl may prompt a part of the K+ to engage in the reaction, leading to the formation of isomeric potassium gypsum. This process can disrupt the original structure of hemihydrate gypsum. The addition of maleic acid to the salt solution was found to foster anhydrous gypsum formation. In contrast, succinic acid can decrease the crystals' aspect ratio, resulting in the growth of short columns, thereby enhancing the product's strength performance. Our research also identified that the reaction rate elevates as the pH value declines. However, an acidic environment diminishes the size of the crystal and counteracts the crystal modifier's effects. Lastly, incorporating CaCl2 and MgCl2 into concentrated brine with a low Na+ concentration can convert FGD gypsum into high-strength α-HH. This transformation results in a product boasting a 1-day compressive strength of 40.5 MPa.

Green and effective route to convert chromium-free leather waste into protein retarding material for building gypsum

Leather manufacturing generates a large amount of leather solid waste, among which the effective treatment of leather waste scraps rich in protein resources has always been a challenge for the tanning industry. In this paper, a green and convenient method was devised for the first time to prepare protein retarding material (PRM) for gypsum from chrome-free tanned leather waste. Aldehyde tanned leather scraps as the main chrome-free leather waste were directly hydrolyzed through high-pressure hydrothermal treatment without any chemicals to obtain PRM. Compared with chemical treatment (acid or alkaline hydrolysis), high-pressure hydrothermal method avoided the decrease of protein product purity due to without the help of acid and alkali. The results showed that PRM obtained from leather waste had a significant effect on prolonging the initial setting time and delaying the release of hydration heat of gypsum. With the dosage or molecular weight of PRM increased, this retarding effect on gypsum was more remarkable. SEM, XRD, and FTIR analysis indicated that the presence of PRMs inhibited the hydration process of gypsum hemihydrate to gypsum dihydrate. Through polypeptide-calcium chelation, PRMs could bind free Ca2+ ions dissolved from gypsum to delay them from participating in the hydration reaction. The larger molecular weight or the more dosage of PRM, the stronger ability to bind calcium ions in gypsum. Chemical structure characterization and molecular dynamics simulation of PRM-Ca chelate further proved that PRMs mainly through carboxyl groups on molecular chain combined with Ca2+ in gypsum system to inhibit hydration reaction, thereby fulfilling the retardation demand. This study indicated that PRM from leather scraps can be a promising candidate for gypsum retardation, which provides a path for the efficient utilization of leather waste.

Interaction of various parameters on the properties of semi-dry gypsum-based blocks produced by compression forming method

The utilization of hemihydrate gypsum to produce blocks can be a more sustainable alternative to conventional cement concrete blocks. This study explores the fabrication of semi-dry gypsum-based blocks using the compression forming method to address the drawbacks associated with traditional blocks (clay block and cement concrete block) manufacturing processes, including high energy consumption and lengthy production time. However, the low mechanical strength, low water resistance, and high water absorption of gypsum-based blocks limit their application in construction. Therefore, in this study, semi-dry gypsum-based blocks were prepared using the compression forming method by investigating the interactions between the ground granulated blast-furnace slag (GGBS) content (0%, 20%, 45%), water-to-binder (w/b) ratio (0.20, 0.25, 0.30), and compaction pressure (CP) (10 MPa and 20 MPa). The compressive strength, water resistance, mineralogical composition, and microstructure of the produced gypsum-based blocks were investigated and compared with gypsum-based blocks without GGBS. It was found that adding GGBS can lead to formation of AFt and C-S-H gel when the water content reaches the required level for the reaction. These hydration products are beneficial to improve the compressive strength and water resistance. In addition, the porosity of the specimens was examined using water immersion and the mercury intrusion porosimetry (MIP) method. It was observed that when the w/b ratio is 0.25 or 0.30, a lower CP (10 MPa) should be applied. This is because higher CP (20 MPa) caused paste overflow during the forming process, resulting in undesirable pores having an adverse impact on the compressive strength. The optimal performance was obtained with GGBS content, w/b ratio, and CP at 20%, 0.25, and 10 MPa, respectively. In summary, the compression forming method could produce semi-dry gypsum-based blocks with adequate compressive strength (35.3 MPa) and water resistance (softening coefficient of 0.68).

Relationship between the evolution of organic impurities and properties of β-hemihydrate phosphogypsum

Transforming phosphogypsum (PG) into β-hemihydrate gypsum plasters is one of the most effective and high-value-added pathways to consume and utilize PG. However, the relationship between the evolution of organic impurities and the properties of gypsum plaster is still unclear. In this paper, the composition and evolution of organic impurities, including pure organic matter (PO) by flotation and polyacrylamide (PA), were characterized, and their influence on the dehydrate behavior of PG and water requirement for normal consistency, setting time, mechanical strength, microstructure, and appearance of gypsum plasters at the calcination temperature of 150 °C and 200 °C were explored. The results showed that at 150 °C, PA and PO could increase the water requirement for normal and setting time of gypsum plaster and reduce the mechanical strength due to the reduced wettability and bonding strength causing by their intact structure and covering on HPG surface. At 200 °C, decomposed products of PO containing P-O-P and OH- groups could interact with Ca2+ and inhibit the dissolution of hemihydrate gypsum, resulting in coarsened crystals, thereby reducing the compressive strength. PA could partially decompose into the micro-molecular polymer and cover on the hydration products, leading to an increased densification compared to 150 °C. Besides, organic impurities could cause blackening, spots, and mildew on the gypsum plaster surface.

Real-time gypsum quality estimation in an industrial calciner: A neural network-based approach

Total bound moisture (TBM) is a typical quality indicator of industrial-grade gypsum. This gypsum is comprised of three distinct phases, namely anhydrite, dihydrate, and hemihydrate, of which only the latter is of much industrial use. TBM analysis is a lengthy laboratory procedure, and an artificial neural network (ANN) TBM inference measurement is proposed as a fast and online alternative. An ANN inference model for gypsum TBM based on plant data was developed. The inputs to the network were primarily focused on the plant's calciner, and different network topologies, data divisions, and transfer functions were investigated. Furthermore, the applicability of the TBM value as a quality indicator was investigated based on a gypsum phase analysis. A strong correlation between TBM and the gypsum hemihydrate and anhydrite content was found, validating the plant target TBM of 5.8% as a quality indicator. A network topology consisting of one hidden layer with logarithmic-sigmoid (logsig) and pure linear (purelin) transfer functions showed the best performance (R > 90%).

Gypsum plastering mortars with Acacia dealbata biowaste additions: Effect of different fractions and contents on the relative humidity dependent properties

Hemihydrate gypsum is a very eco-efficient binder. Gypsum plasters were commonly used in the past and should be still chosen nowadays for being an eco-efficient choice. However, their hygroscopicity and contribution to act as moisture buffer are not very high. The present study analyses the hygrothermal behaviour of mortars based on gypsum and modified with the addition of residual biomass of A. dealbata, an invasive species in Portugal. Five different fractions of the plant were tested as additions for mortars, at incorporation levels of 5% and 10% by volume, with the purpose of enhancing the moisture buffering of the plasters without jeopardizing other properties. The study found that the addition of A. dealbata increases their hygroscopic behaviour. In some cases (bark fraction) the Moisture Buffering Value of the reference mortar is triplicated, and the behaviour is comparable with high hygroscopic plasters as clay-based ones. However, biological colonization must be controlled.

Study on the Effect of Three Types of Calcium Sulfate on the Early Hydration and Workability of Self-Compacting Repair Mortar.

Despite having a high early mechanical strength and using sulfoaluminate cement as the primary cementitious material, self-compacting repair mortar (SCRM) suffers from rapid hydration rates leading to construction time constraints. This study examined how several forms of calcium sulfate, including hemihydrate gypsum, anhydrite, and dihydrate gypsum, affected SCRM's workability, hydration process, and microstructure. The outcomes demonstrated that adding hemihydrate gypsum sped up SCRM's early hydration rate and boosted its expansion rate. For a cement with 8% hemihydrate gypsum, 6 h after adding the water, the flexural strength and compressive strength increased by 39.02% and 34.08%, respectively. The hydration rate of SCRM can be efficiently delayed by dihydrate gypsum, although the result is subpar. The material exhibited the lowest fluidity loss in 20 min, the setting time was extended, and the 28-day flexural and compressive strengths were raised by 26.56% and 28.08%, respectively, after adding 8 percent anhydrite.