Aluminate Hydrate Research Articles

STRUCTURE AND PROPERTIES OF THE ETRINGITE PHASE

Problem statement. Aluminate and sulfoaluminate cements are not produced in Ukraine, despite a rather significant need for binders of this class. The use of imported raw materials is limited by the high cost and certain disadvantages occurring during exploitation including rapid hardening, significant heat generation, which is associated with instability of some sulfoaluminates. At the same time, it is possible to highlight the following problems in the direction of expanding possibilities of using special cements of this type: stabilization over time and operating conditions of the hydrosulfate phase based on alumina cement, as well as modification of the compositions of mineral binders based on calcium sulfate dihydrate (CaSO4·2H2O) and development of binders of this class based on secondary production products. The main factor is that during hydration of sulfoaluminates and aluminates in the presence of gypsum (CaSO4·2H2O), a hydrosulfoaluminate phase is formed, which makes it possible to obtain a hardened cement paste structure with special properties. Then monocalcium hydrosulfoaluminate turns into hydrosulfoaluminate of the low-sulfate form С3А·СaSO4·12H2O with the release of gibbsite Al2O3·3H2O. Also, ettringite С3А·СaSO4·nH2O and 2(С2S)·nH2O is formed and hydrated calcium silicate CSH(B) can be formed. When gypsum is added to cement, ettringite is formed in this system. Etringite is one of the components. It is not formed initially, but through intermediate structures. During hydration reactions, ettringite is rearranged, neoplasms are formed, which can lead to gypsum corrosion. Etringite loses its stability. Herewith, the problem of primary and secondary ettringite arises. Primary ettringite creates conditions for strength. Secondary ettringite is formed already in the hardened system and leads to internal stresses. Formation of secondary ettringite can have both positive and negative consequences. The purpose of the atrticle is to investigate structure and properties of the ettringite phase. Conclusions. The hydration process depends on the Gibbs surface energy. Change in surface energy depends on the CaO/Al2O3 ratio. It was established that the surface energy increases with an increase in the CaO/Al2O3 ratio. The paper has studied influence caused by the ettringite phase on the main characteristics of alumina cement and gypsum in a modified gypsum binder. Studies have been conducted on formation of the maximum amount of ettringite phase. We have calculated the maximum amount of the ratio of alumina cement and gypsum to obtain the maximum amount of mineral – 70 % alumina cement and 30 % gypsum. The highest effect is achieved during the simultaneous use of С6АṤ3Н32 and АН3, which occurs during hydration of С4А3Ṥ. Taking into account the above, it is advisable to obtain clinker containing calcium sulfoaluminate and cements based on it.

Complex Effect of Portland Cement Modified with Natural Zeolite and Ground Glass Mixture on Durability Properties of Concrete

This article analyses the combined effect of a mixture of natural zeolite and ground soda lime silicate glass on the durability properties of concrete. The specimens were prepared by replacing 25% of the cement with a mixture of zeolite and glass at different proportions with zeolite particles of two different sizes and cured for 28 and 90 days. Chloride migration, the depth of water penetration under pressure, and freeze–thaw cycle tests were performed, and compressive strength was determined. The combined effect of the mixture on the composition and microstructure of the cement matrix was studied by analysing the results of XRD, TGA, SEM-BSE, and porosity tests. The test results showed that irrespective of the cement “dilution” effect, the mixture of additives had a beneficial combined effect: increased durability of concrete, a denser structure of the cement matrix, the formation of calcium aluminate hydrates, faster rate, and higher degree of hydration.

A comparative study of alkanolamines and inorganic additives on the microstructure development of blast furnace slag blended cement at low curing temperature

The initial reactivity and strength development of blast furnace slag cement (BFSC) are crucial factors that increase its utilization. Therefore, it is important to select the most suitable accelerator. In this study, the impact of four accelerators, namely sodium sulfate (Na2SO4), calcium nitrite (Ca(NO2)2), triethanolamine (TEA), and diethanolisopropanolamine (DEIPA), on the hydration and strength development of BFSC at curing temperatures of 5 and 20 °C was investigated. Isothermal calorimetry, X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were conducted. The results revealed that Na2SO4 effectively enhanced early age strength by accelerating the hydration of the clinker and slag. Ca(NO2)2 is sensitive to temperature changes. At 20 °C, Ca(NO2)2 promoted the reaction of the aluminates and slag in the initial stage. However, the presence of slag inhibited clinker hydration, and the addition of Ca(NO2)2 exacerbated this effect, resulting in reduced strength at 5 °C. Indeed, the use of TEA and DEIPA at 0.02 % at 5 °C is not recommended. Complex interactions between TEA and Ca2+ may inhibit the formation of calcium hydroxide crystals, further inhibiting aluminate hydration. This may lead to low strength development at low curing temperatures such as 5 °C.

Fluorides immobilization through calcium aluminate cement-based backfill: Accessing the detailed leaching characterization under torrential rainfall

Urbanization and economic development have increased the demand for fertilizers to sustain food crop yields. Huge amounts of by-products, especially phosphogypsum (PG), are generated during the wet processing of rock phosphate to produce fertilizers. Chronic exposure to fluoride in phosphogypsum in groundwater as a result of the weathering of fluoride-containing waste poses a significant health risk to millions of people. We propose a method for using calcium aluminate cement (CAC) to remediate high fluoride contents in solid waste. Column leaching tests under harsh rainfall conditions confirmed the efficient fluoride immobilization capacity of a CAC binder. Although the fluoride concentrations in leachates during the first 1–2 days (1.25 mg/L) slightly exceeded the threshold of 1.00 mg/L, the concentrations over 3–28 days (ranging from 0.98 to 0.83 mg/L) consistently remained well within the acceptable range. Furthermore, our characterization and geochemical modeling revealed the fluoride retention mechanisms of CAC-stabilized PG under laboratory-simulated conditions of torrential rainfall. During leaching, physical encapsulation prevents fluoride from contacting leachate. However, an unfavorable pH value can cause the release of fluoride from the cement matrix, which is subsequently captured by aluminate hydrate through adsorption or co-precipitation. We quantified the carbon footprint of CAC for immobilizing 1 mg of fluoride in PG, obtaining a remarkably low value of 4.4 kg of CO2, in contrast to the emissions associated with the use of ordinary Portland cement (OPC). The findings suggest a unique opportunity for extensive PG remediation. This opportunity extends the horizons of achieving zero-waste emissions in the phosphorus industry and has practical significance in the context of reducing carbon emissions.

Effects of Fly Ash Composition to Mitigate Conversion of Calcium Aluminate Cement Composites

Calcium aluminate cement (CAC) is one of the alternative cements that is widely used for special applications. However, during the hydration process degradation of CAC microstructure, the so-called hydrate conversion process, hexagonal calcium aluminate hydrate (CAH10) transforms into a cubic (C3AH6) phase, resulting in increased porosity and reduced strengths. It is known that alternative means for stabilizing the CAC conversion are conducted by introducing fly ash (FA) in CAC, where its microstructure is attributed to aluminosilicates. However, no study has yet been conducted on different FA compositions influencing CAC performance. This study aims to evaluate the effects of different compositions of FA on CACs’ fresh and hardened characteristics. Results revealed that the microstructure was denser when CAC was mixed with FA. Regarding reactivity, CAC with calcium-rich FA systems is 13% faster than the silica-rich one. The higher the density and the lower the porosity of calcium-rich FA mixtures were found compared with silica-rich FA in both micro- and macro-structures. As seen in the microscopic structure, this is due to the calcium-rich phase formation.

Experimental study of mixtures soil-industrial waste using simplex design for application in paving

The large amount of solid waste generated by industry is a concern for society. A possible solution is to incorporate these wastes into soils to improve their properties and contribute to sustainable development. The objective of this research was to conduct an experimental study on the chemical stabilization of two samples of tropical soils using two industrial wastes, electric arc furnace slag (EAFS) and fly ash (FA), using a design of experiments (DoE) of mixtures in a simplex centroid network. Seven mixtures were determined from the DoE for each soil sample. Using specimens of these mixes prepared at optimum moisture content, after a curing period of 7 days in a humid chamber, the properties were evaluated: unconfined compressive strength (UCS), California Bearing Ratio (CBR), and resilient modulus (RM). An improvement in the evaluated mechanical properties was observed with the addition of waste, especially with the addition of EAFS. The statistical software Minitab 18 allowed to obtain response surfaces for each mechanical property studied as a function of the pseudo-components of the mixtures. To determine the proportion of components that optimized the soil-waste mixtures, the desirability function was used, which resulted in the proportions of 80% soil, 20% EAFS, and 0% FA for both soil samples. Based on SEM and XRD tests, it was found that calcium silicate hydrate (C–S–H), calcium aluminate silicate hydrate (C-A-S-H), and calcium aluminate hydrate (C-A-H) were formed, which were responsible for improving the mechanical strength. The use of industrial waste to stabilize tropical soils with low bearing capacity proved to be a technically and environmentally viable alternative.

The in-situ formation of Mg-Al hydrotalcite and its effect on the properties of MgO-bonded alumina castables

MgO-bonded castables are one of the promising non-cement bonded refractories used for ladle lining. To make crack-free MgO bonded castables, the formation of layered double hydroxides (Mg-Al hydrotalcite) is proposed. In the present work, the influences of Al(OH)3 and hydratable alumina (ρ-Al2O3) and different curing temperatures (40 °C, 60 °C, 80 °C) on the formation of hydrotalcite are investigated. The hydration behavior and corresponding hydrates are examined using electrical conductivity, XRD, SEM, FTIR, and TG-DSC analyses. The results show that the increase in curing temperature is more conducive to the formation of hydrotalcite, where ρ-Al2O3 significantly promotes the formation due to the enhanced reactions between intermediates of Mg(OH)2 and Al(OH)3. After high-temperature treatment, the decomposition of a larger amount of hydrotalcite and Al(OH)3 and thereafter growth of microcrystalline spinel contribute to refined pore size distribution. Consequently, a higher hydration degree of ρ-Al2O3 and the formation of more hydrotalcite provide a higher bonding strength at room temperature, and the optimized microstructure enhances high-temperature strengths.

Study on the Corrosion Behavior of Cemented Organic Soil in Dianchi Lake, China.

To study the corrosion behavior of cement soil in peat soil, the experiment involves the preparation of peat soil by incorporating humic acid into cohesive soil with a lower organic matter content. Cement soil samples are then prepared by adding cement to the mixture. These samples are subjected to immersion in fulvic acid solution and deionized water to simulate different working environments of cement soil. The experiment considers immersion time as the variable factor. It conducts observations of apparent phenomena, ion leaching tests, and unconfined compression strength tests on the cement soil. The experiment results are as follows: (1) With increasing immersion time, the surface of the cement soil in the peat soil environment experiences the disappearance of Ca(OH)2 and calcium aluminate hydrate. Additionally, large amounts of bird dropping crystals precipitate on the surface and within the specimen. The cement soil undergoes localized disintegration due to extensive erosion caused by swelling forces. (2) In the peat soil environment, fulvic acid reacts with the hydration products of cement, resulting in partial leaching of ions such as Ca2+, Mg2+, Al3+, and Fe3+ into the immersion solution. The lower the pH of the fulvic acid immersion (indicating higher concentration), the more significant the ion leaching. Increasing the ratio of humic acid to cement can slow down the leaching of ions. The cement soil undergoes dissolutive erosion in the peat soil environment. (3) The peat soil environment exerts both strengthening and corrosive effects on the cement soil. Cement soil without humic acid exhibits noticeable corrosion in the peat soil environment, gradually decreasing strength as immersion time increases. The strength decreases by 83% from 28 to 365 days. In contrast, cement soil with humic acid experiences an initial period of strengthening, leading to a significant increase in strength in the short term (34% increase from 28 to 90 days). However, the corrosive effects gradually dominate, resulting in a decrease in strength over time. The strength decreases by 80% from 90 to 365 days. This study also explores the strengthening effects of peat soil on cement soil. It identifies phenomena such as extensive erosion and new substance precipitation in cement soil.

Shear strength and durability against wetting and drying cycles of lime-stabilised laterite soil as subgrade

Laterite soil is commonly used as a fill or pavement material. However, it tends to lose its natural bonding and experiences large deformation when subjected to cyclic rainy (wet) and hot (dry) seasons. This leads to cracking and deflection of road pavement. It has long been known that lime stabilisation can improve the engineering properties of laterite soils. Unconfined Compressive Strength (UCS) tests at different curing periods (0, 3, 7, 14, and 28 days) were conducted with various percentages of lime (3%, 5%, 7%, and 9%). The durability of stabilised soil against wet and dry cycles was also evaluated. In addition, the variation of shear strength parameters during the Consolidated Undrained (CU) triaxial test under different confining pressures has been presented. Microstructural analyses showed an increased soil strength due to the formation of calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH).

Review of the Properties of Sustainable Cementitious Systems Incorporating Ceramic Waste

Global carbon dioxide emissions can be attributed to Portland cement production; thus, an alternative cementitious system is essential to reduce cement demand. Ceramic waste powder (CWP), which contains high proportions of silica and alumina, has emerged as a promising alternative because of its chemical composition. This review discusses the potential of CWP as an alternative cementitious system and its effects on the physical, mechanical, and durability properties of cementitious systems. The findings revealed that the utilization of CWP in cementitious systems has positive effects on their physical, mechanical, and durability properties owing to the chemical composition of CWP, which can act as a filler material or contribute to the pozzolanic reaction. A pozzolanic reaction occurs between the silica and alumina in the CWP and calcium hydroxide in the cement, resulting in the production of additional cementitious materials such as calcium silicate hydrates and calcium aluminate hydrates. These additional materials can improve the strength and durability of cementitious systems. Various studies have demonstrated that CWP can be effectively used as a partial replacement for cement in cementitious systems. This can reduce the carbon footprint of construction activities by reducing the demand for Portland cement. However, the optimal amount and particle size of CWP have not been fully determined, and further research is required to optimize its use in cementitious systems. In addition, the technical and economic challenges associated with the use of CWP in construction must be further investigated to ensure its effective implementation.

Fabrication and characterization of calcium aluminates cement via microwave-hydrothermal route: Mayenite, katoite, and hydrocalumite

In response to the growing interest in functional building materials, the microwave-hydrothermal (M-H) technique was employed to prepare calcium aluminate-based cement. In this study, aqueous solutions containing active lime and alumina were treated using M-H method for one hour at temperatures of 100°, 120°, and 140 °C. Various starting compositions with calcia to alumina molar ratios (C/A) of 1, 1.3, 1.5, 2, 3, and 4 were investigated. Subsequently, the hydrothermally synthesized powders at 140 °C were subjected to calcination for 2hr at 1150°, and 1350 °C to obtain anhydrous phases. The formed powders were characterized using X-ray diffraction (XRD), differential thermogravimetric analysis (TG-DTG), Raman spectroscopy, and scanning- or transition- electron microscopy (SEM & TEM). The results revealed that for starting C/A molar ratio of 1.00–2.00, M-H treated samples at 100°–140 °C produced stable phases such as katoite and hydrocalumite. As the lime content increased in the C/A mixtures up to a molar ratio of 3.00, the tri-calcium aluminate hydrate was an abundant phase, with its crystallinity increasing with higher hydrothermal temperatures. The M-H treated samples at 140 °C, calcined for 2hr at 1150°, and 1350 °C, yielded mayenite powder (C12A7) with varying degrees of crystallinity. These findings provide valuable insights into the preparation of calcium aluminate-based cement using the microwave hydrothermal method.

Shock Wave–Induced Dynamic Mechanical Behavior of Calcium Silicate Aluminate Hydrate at the Molecular Scale

Calcium silicate aluminate hydrate (C-A-S-H) is the main hydration product of cement mixed with industrial wastes. The purpose of this study is to understand the dynamic mechanical behavior and structural transformations of molecular-scale C-A-S-H induced by shock waves. Three C-A-S-H models with Al/Si ratios of 0.0, 0.1, and 0.2 are constructed and reactive molecular dynamics simulations are used to perform shock compressions with different shock velocities from 0.1 km/s to 3.6 km/s. The distributions of particle velocity, pressure, and density along the shock direction are calculated using the binning analysis method, allowing Hugoniot pressure-specific volume curves to be derived. The results reveal that shock waves may induce elastic, elastic-plastic, or shock Hugoniot responses in molecular-scale C-A-S-H, depending on the Al/Si ratio and the shock velocity. Below the Hugoniot elastic limit (HEL), higher Al/Si ratios cause the elastic wave to propagate farther due to the cross-linking effect of aluminate units. Above the HEL, higher Al/Si ratios give rise to a distinct two-wave structure characteristic comprising a plastic front and an elastic precursor. This characteristic becomes less pronounced as the shock velocity increases. Analysis of the molecular structural transformations of C-A-S-H revealed that the main atomic deformation behavior below the HEL involves a reduction of interatomic distances; above the HEL the main response is a densification of water molecules followed by a general collapse of the layered structure as the shock velocity increases.

Understanding the adsorption of calcium sulfate on tetracalcium aluminoferrite and tricalcium aluminate surfaces

AbstractCalcium sulfate (CaSO4), an essential retarder in cement, retards the hydration of tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) phases. However, its retarding mechanism remains unclear. This paper focused on the adsorption of CaSO4f on C4AF and C3A surfaces based on isothermal calorimetry, the measurement of the ionic concentrations in a diluted system, and density functional theory to enhance the understanding of the retardation mechanism. The results showed that the retarding effect of CaSO4 on C4AF was stronger than that on C3A due to the slower CaSO4 consumption rate, lower driving force for CaSO4 adsorption, and surface coverage of Fe(OH)3 gel. The adsorption of CaSO4 hindered Ca dissolution more markedly on C4AF than C3A, which was pronounced on Fe‐free C4AF surfaces. The adsorption of CaSO4 weakened the affinity of water on C4AF and C3A surfaces, lowering the driving force for H2O adsorption. The adsorption of H2O and CaSO4 promoted the dissolution of Al on the [AlO6] octahedral surface of C4AF, which may be responsible for the maintenance of a higher Al concentration in the solution. Based on the above results, the adsorption of CaSO4 on initial C4AF and C3A hydration was explained.

Sustainable incorporation of Plaster of Paris kiln dust for stabilization of dispersive soil: A potential solution for construction industry

The presence of dispersive soils in the region of Khushab, Pakistan, has been recognized as a significant element causative to numerous challenges in construction. This research intended to examine the efficacy of Plaster of Paris kiln dust (PKD) as a cost-effective and eco-friendly material in stabilizing the strength properties and dispersion potential (DP) of dispersive soil at optimal curing period. Different contents of PKD (from 0.5% to 3%) were added to the soil. A series of tests have been carried out over various curing times (0, 7, 14, 28, and 60 days) to evaluate the effectiveness of PKD as a stabilizing agent. The tests included a double hydrometer, Atterberg limits, specific gravity, compaction, unconfined compression strength (UCS), consolidation, physiochemical properties (pH, Electrical conductivity), and chemical analysis (percentage sodium, sodium adsorption ratio). The results illustrated that DP decreased from 74.12% to 8.57% while the increase in UCS value was observed from 124.3 kPa to 1212.5 kPa (an increase of 875%) by increasing PKD up to 3% after curing for 60 days. The X-ray diffraction and scanning electron microscopy exhibited the development of cementitious products (Calcium silicate hydrate, Calcium aluminate hydrate) due to the pozzolanic reaction with 3% PKD after a curing time of 28 days, which is responsible for the increment in strength. PKD exhibits both favorable economic and environmentally friendly characteristics, as it has the lowest cost among several additives used previously and provides a significant portion of the treated area (92.6%) free from pollution and landfill. PKD improved the stability and strength of dispersive soil and provided a sustainable waste management solution, making it a highly viable option for soil improvement applications in the construction industry.

Structure and Electric Properties of Anodized Aluminum with PEDOT:PSS Conductive Polymer Cathode

Aluminum solid electrolytic capacitors using a conductive polymer as a cathode material has the advantages of low equivalent series resistance (ESR) and high thermal stability compared to aluminum electrolytic capacitors using liquid electrolytes. A disadvantage of aluminum solid electrolytic capacitors with a conductive polymer is the low breakdown voltages, limiting the operating voltage of the capacitors to ∼100 V or less. In this study, for a possible future increment of the withstand voltages of solid electrolytic capacitors, PEDOT:PSS-coated aluminum with an amorphous or crystalline alumina layer formed at 500 V has been characterized using SEM, STEM/EDS, and conductive AFM to understand the distribution of PEDOT:PSS conductive polymer and its influence on the electric properties. The crystalline alumina layer, developed beneath a porous hydrated alumina layer, contains many voids, which increase the leakage current compared to the void-less amorphous alumina layer. However, the crystalline alumina layer with PEDOT:PSS exhibits a higher breakdown voltage than the amorphous one. The limited incorporation of PEDOT:PSS into the inner part of the porous hydrated alumina layer may introduce a resistive surface layer, possibly contributing to the increased dielectric breakdown.

TiO2- geopolymer based novel corrosion protective micro-coatings to emaciate mild steel oxidation in severe environments

Now-a-days, cementitious geopolymer coatings are one of the most studied and adaptable coating systems to combat corrosion on low-carbon steel/structural steel. Due to its green and sustainable nature, it has enormous potential to partially replace traditional organic coatings in terms of corrosion resistant properties and economic aspects. Herein, corrosion protection and antibacterial efficiency of titanium oxide (micro-sized, 5/10/15%) containing sodium aluminosilicate (N-A-S-H) geopolymer coating for mild steel is reported to enhance the lifetime of structural steel. Developed coatings were characterized using XRD, FTIR and FESEM. Characterization results indicated the presence of titanium containing new inorganic phase (Sodium titanium oxide hydrate) along with sodium aluminium hydrate phases in XRD, supported by Ti-O stretching vibrations in FTIR along with Si-O-Al stretching vibrations. Microstructural studies using FESEM indicated the uniform distribution of TiO2 in NA-S-H gel matrix. Results revealed that developed coatings depicted corrosion protective characteristics and exhibited adhesion strength between 16.14 and 17.3 MPa. The scratch resistance test revealed that the coating could bear a point load of greater than 5 kg. Minimum percentage weight change and adhesion loss were observed for coating after immersion in NaCl/H2O for 119 days. An accelerated corrosion test (ASTM B117) was performed in the salt environment for 500 h and the best corrosion protective performance was observed for coating formulation containing 10 % TiO2 with maximum rust grading 5. Electrochemical studies showed better corrosion resistance performance for coating formulation containing 10 % TiO2 as compared to the other two formulations containing 5 and 15% TiO2. The antibacterial efficiency of coatings was evaluated by colony count assay (viability method) against the most commonly found gram-positive bacteria Staphylococcus aureus and one gram-negative bacteria Escherichia coli. All three TiO2-incorporated geopolymer coating formulations revealed a good bacterial reduction in percentage with the best performance observed for coating formulation containing 15% TiO2. The accelerated weathering test (ASTM G154-500 h) was performed to ensure the durability and stability of the developed formation. This study developed a novel, environment-friendly, and lucrative cementitious coating material which is potentially suitable for preventing early oxidation and bacterial fouling of mild steel structures.

Investigation of the milling route for the development of colloidal suspensions to be used as binder in refractory castables

Refractories castables bonded with CAC (calcium aluminate cement) have a limitation when they are used at high temperature in micro-silica containing system and/or in acidic corrosive environments. The CAC binder reacts with the micro-silica and/or acidic components to form liquid/viscous compounds deteriorating the refractory material.Refractory castables bonded with HA (hydratable alumina) perform better in these environments. However, the drying step of such materials needs time and energy for the water evacuation to avoid the lining cracking.The use of CS (colloidal silica) as binder seems to hinder the drawback of CAC and HA binders. However, the presence of free amorphous silica in the castable composition limits their uses in basic environments by forming liquid/viscous phases at high temperature with a reduction of castable refractoriness.In this context, new colloidal bonding systems that could improve the high performances of refractory castables such as boehmite and spinel colloidal suspensions, were developed.

Investigating the retarding effect of CAC in alkali-activated cements

This paper discusses the design of hybrid alkali-activated binders at ambient temperature, specifically the development of mixes based on ground granulated blast-furnace slag (GGBFS) with calcium aluminate cement (CAC) as an additive, aiming to obtain high early strength binders with optimised Al incorporation, increased crosslinking and degree of polymerisation. The effects of 10 wt% CAC replacement of GGBFS, activated with sodium silicate with varying modulus (SiO2/Na2O molar ratio) or with sodium hydroxide, on fresh and hardened properties are investigated. The inclusion of CAC in GGBFS mixtures activated using sodium silicate results in an unexpected retardation of the polycondensation reactions required to form the main calcium aluminosilicate hydrate gel phase. This is due to incomplete dissolution of the GGBFS precursor, inhibited by the rapid formation of additional reaction products (especially cubic aluminate hydrates, C3AH6) resulting in lowered reaction kinetics, and thus delayed setting and hardening times. For sodium silicate activators, this retarding effect appears to be only slightly dependent on the activator solution modulus. When activating with 4 M NaOH, the retarding effect is reduced, along with the incorporation of CAC in the C-A-S-H gel, increasing the amount of reactive aluminium present in the binder to form a more compact gel product. Increasing the molarity of NaOH solutions results in a similar delay in reaction kinetics. The results suggest the existence of only a limited pH range in which the addition of CAC can promote the formation of a binding gel with enhanced mechanical properties, whilst other factors including the effect of sodium silicate inclusion in CAC systems are still unclear. The typical hydration products of CAC were not detected in this study.

Features of the formation of crystalline hydrosulfoaluminates during hydration of stoichiometric mixtures of calcium aluminates with calcium sulfate

Calcium aluminates as mineral binders allow to produce cements and concretes with high early strength, low porosity, increased heat resistance, frost resistance and corrosion resistance compared to materials based on Portland cement. An important structure-forming process in aluminate cements is formation of hydrosulfoaluminate phases (ettringite, monosulfoaluminate). Finding out the parameters that allow to control this process is of exceptional importance for construction practice. Hydration of various calcium aluminates C3A, C12A7, CA, and CA2 in the presence of stoichiometric amounts of various forms of calcium sulfate (CaSO4⋅2H2O, α-CaSO4⋅0.5H2O, anhydrite) was studied in order to determine the comparative ratio and the peculiarities of nucleation and growth of the hydrosulfoaluminate phase crystals. It was found that the highest yield of ettringite (∼100 wt% by 28 days) is achieved during hydration hardening of the composition C3A + 3CS¯H2, the yield is reduced in the series C12A7–CA–CA2. Monosulfoaluminate is not formed as the predominant product, its yield reaches 25 wt% during hydration of low-base aluminates. Ettringite crystals can form as a highly dispersed mass (from C3A) or prismatic (fibrous) crystals (from other aluminates). It was clearly shown for the first time that the origin and growth of ettringite crystals occurs in the matrix of aluminum hydroxide gel formed during hydration of calcium aluminates. Varying the pH of aluminate-sulfate mixtures changes the morphology, size and number of primary ettringite crystals. Lamellar hexagonal crystals of monosulfoaluminate are formed from hydroaluminates of type C4AH13-19 at low availability of calcium sulfate. The explanation of the experimental data was carried out using thermodynamic calculations. The results obtained can be used in practice to control the content, ratio and morphology of hydrosulfoaluminate phases in cement stone.

Associated Effects of Sodium Chloride and Dihydrate Gypsum on the Mechanical Performance and Hydration Properties of Slag-Based Geopolymer

The associated effect of sodium chloride and dihydrate gypsum on the mechanical performance of a slag-based geopolymer activated by quicklime was investigated by compressive strength, shrinkage, and square circle anti-cracking tests of mortar with a 0.5 water–binder ratio and a 1:3 binder–sand ratio, as well as paste soundness, powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), and mercury intrusion porosimetry (MIP) of the paste. The results indicate that (1) when dihydrate gypsum is used alone, it combines with calcium aluminate hydrate (C-A-H) to form calcium sulfoaluminate hydrate (AFt), which encourages the hydration process of slag. A 7.5% addition can result in an increase of 97.33% and 36.92% in 3-day and 28-day compressive strengths, respectively. When NaCl is used by itself, it facilitates the condensation of the aluminum silicate tetrahedron unit and generates zeolite. A 2% dosage can lead to a 66.67% increase in the 3-day compressive strength, while causing a 15.89% reduction in the 28-day compressive strength. (2) The combined effect of 2% NaCl and 7.5% gypsum results in the formation of needle-like and rod-shaped AFt, Friedel’s salt, and plate-like Kuzel’s salt in the geopolymer. This leads to an increase in 3-day and 28-day compressive strengths by 148% and 37.85%, respectively. Furthermore, it reduces the porosity by 18.7%. (3) Both NaCl and gypsum enhance the paste soundness of the slag-based geopolymer, and they do no harm to the crack resistance of the geopolymer. The drying shrinkage of the geopolymer at 28 days is just 0.48 × 10−3, which is only 66.7% of OPC. This slag-based geopolymer has a simple preparation process, good volume stability, low raw material cost, low energy consumption, and low carbon emissions. It can be used instead of 32.5 slag Portland cement in plain concrete applications, and has high engineering, economic, and environmental values.