Ettringite Formation Research Articles

Development of novel mortar using fine river sand and pretreated waste flue gas desulfurization gypsum powder and pond fly ash

The objective of this study is to investigate the potential of combined pond fly ash (PFA) and flue gas desulfurization gypsum (FGDG) as a partial replacement for cement by mixing them with fine river sand for green mortar development. We studied the physical, mechanical, and microstructural properties of mortars by using techniques such as scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM-EDXS) and X-ray diffraction (XRD) for chemical component identification of the hydrated phase. These experimental results show that mortar containing pond fly ash, FGDG, and cement concentrations of 5 wt%, 5 wt%, and 90 wt% yields the maximum compressive strength (8.9 MPa) and flexural strength (3.4 MPa) after 28 days. This mortar has 12.6% higher compressive strength and 48% lower shrinkage in comparison with control specimens. A reduction in shrinkage is attributed to the denser structure of the mortar, as the calcium silicate hydrate (C–S–H) gel is found to exist in the SEM image and also identified through XRD studies. It is also concluded that excessive ettringite formation causes expansion in the volume of mortar, voids formation, and results in the reduction of strength. The application of this mortar can be in the internal plaster, bricks, and masonry.

Strength and Microstructural Characteristics of Activated Fly Ash–Cement Paste

Incorporating high volumes of fly ash (FA) in filling materials reduces costs and carbon emissions, but low early strength limits its use. This study investigates the effects of sodium sulfate decahydrate (Na2SO4·10H2O) and calcium hydroxide (Ca(OH)2) as activators at concentrations of 0.5%, 1.0%, 1.5%, and 2.0% on the mechanical properties and microstructure of tailings–cement–fly ash composites. Compressive strength testing, X-ray diffraction (XRD), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) were used to evaluate performance at different curing stages. Results indicate that all activators enhance early strength, with 2.0% Ca(OH)2 yielding the greatest improvement. Microstructural analysis showed that activators boost quartz reactivity and create denser structures. Na2SO4 promotes ettringite and gypsum formation, while Ca(OH)2 increases alkalinity, enhancing gel formation from FA. These findings clarify how activators improve the performance of activated fly ash–cement paste (AFCP).

Sulfate attack on portland-dolomite cement exposed to sodium sulfate solution at 5 ℃ and 20 ℃

The purpose of the study is to investigate the sulfate resistance of portland-dolomite cement (PDC) with varying dolomite contents, when exposed to sodium sulfate solution at 5 ℃ and 20 ℃. Visual appearance, expansion, mass change, and compressive strength of PDC mortars were tested, and XRD, FTIR, and SEM analyses were conducted to examine deterioration products and microstructure. Results indicate that at 20 ℃, the incorporation of 10 % dolomite in PDC had a negligible impact on sulfate resistance, whereas higher dolomite replacements of 20 % and 30 % notably diminished it. In contrast, exposure at 5 °C led to more severe deterioration in all PDC samples, regardless of dolomite content, and the higher dolomite content, the poorer resistance to sulfate attack. The increased formation of ettringite and gypsum resulted in reduced sulfate resistance of PDC at 20 ℃. However, at 5 ℃, the presence of dolomite significantly promoted the formation of thaumasite at later stages, which caused initial ettringite and subsequent late-stage thaumasite sulfate attack, ultimately leading to the failure of PDC. The findings suggest that PDC should be carefully considered when used in sulfate-rich environments, particularly at low temperatures.

Combined effect of self-stressing and confinement on GFRP-LC3 interface bond

The load capacity, failure mode and durability of reinforced concrete members are strongly affected by the bond strength between the concrete and the reinforcement. This work presents a physics-based approach to improve the bond strength based on self-stressing effect. Combination of limestone calcined clay cement (LC3) and calcium sulphoaluminate cement (CSA) with different ratios was employed to develop self-stressing function. The addition of CSA induces a remarkable volume expansion of matrix due to the formation of ettringite, it however leads to a decreased compressive strength because of the reduced Portlandite content and increased porosity. A push-out test was conducted to evaluate the interface properties between glass fiber reinforced polymer (GFRP) and LC3-based matrix. The results show that the interface bond strength is highly dependent on the matrix strength under unconfined condition. On the contrary, under confined condition, the interface bond strength increases linearly with the expansion value regardless of matrix strength, attributed to that the self-stressing effect can effectively refine the matrix pores and densify the interface transition zone. The findings from this work demonstrate that the self-stressing effect holds promise to be a plausible method to strengthen the overall properties of reinforced concrete without creating additional carbon emission.

Revealing the hydration mechanisms in iron-rich calcium sulfoaluminate cements with elevated bauxite residue content

The goal of the study was to investigate the reactivity at different Blaine, hydration behavior, and performance of calcium sulfoaluminate-ferrite cement (CSA-F). The raw meal incorporated bauxite residue (38 wt%) fired at 1260 °C and 1300 °C for 30 and 60 minutes. Based on multi-objective optimization, an optimum dosage of citric acid ranges between 0.4<x<0.6 wt% with a w/b ratio of 0.45 was chosen for the mortar evaluation and hydration studies. At an early age, monosulfates-AFm only started appearing for 1300_30. In the later age, an increase in ettringite formation was only reported at 1300°C for 60 min due to the dissolution of the Ternesite phase after 7d of hydration. Later-age studies after 28d reported ettringite, monosulfates, hydrogarnets (except for 1300_60), and strätlingite had formed. The 28d highest compressive strength was reported for 1300_60 corresponding to the highest bound water and amorphous content from the pore filling ettringite and amorphous stratlingite.

Utilizing raw phosphogypsum to prepare one-part geopolymer: Mechanical properties, optimization, and hydration mechanisms

The accumulation of phosphogypsum (PG) poses environmental risks. This study investigates the feasibility and performance of one-part geopolymers derived from a ternary system, where raw phosphogypsum (PG) partially replaces ground blast furnace slag (GBFS) in combination with fly ash (FA). The effects of raw PG on geopolymerization kinetics, mechanical properties, and efflorescence were examined. PG addition affected compressive strength, gel formation, and efflorescence. An optimal PG replacement rate of 10–20 % for GBFS was identified to balance the positive effect on ettringite formation and the negative impact on gel phase formation. Using a composite alkali activator, combining NaOH and Na2SiO3, promotes strength development and enhances the anti-efflorescence properties of one-part geopolymers synthesized with PG. One-part PG-incorporated geopolymers demonstrate excellent performance in the fixation of heavy metal contaminants in raw PG, offering a more convenient and safer alternative to conventional two-part synthesis methods.

Performances enhancing of supersulfated cement (SSC) using waste alkaline activators: Red mud and carbide slag

Supersulfated cement (SSC), since its invention, showed lower early-age strength gain, which has limited its practical applications. The urgent issue is how to regulate and enhance its early-age performance. In this context, solid waste materials such as bauxite residue-red mud (RM) and carbide slag residue (CS) from acetylene production by calcium carbide hydrolysis were used to improve the hydration and performance of SSC. By utilizing a range of analytical techniques such as isothermal calorimetry (IC), X-ray diffraction (XRD), backscatter scanning electron images (BSE), and thermogravimetric analysis (TGA), the hydration process, phase assemblages and microstructure of CS/RM-modified SSC were systematically investigated. It’s found that CS, as a calcareous alkaline activator, act as a regulator for adjusting the two main hydration reactions of C-A-S-H deposition and ettringite formation in SSC. The alkaline activator induces the hydration of slag and promotes the formation of hydration products. However, an excessively high dosage of carbide slag delays the primary ettringite formation reaction because it inhibits the dissolution of slag and promotes the formation of outer products. It leads to a looser and more porous paste matrix, resulting in higher expansion strain and poor strength gain at full curing ages. In contrast, red mud improves the hydration rate and enhances the hydration degree of SSC within the studied content (up to 30 wt%). With addition of red mud, SSC hydration is significantly advanced and deepened, resulting in more hydrates, particularly inner products. Red mud facilitates the depolymerization of the slag glass network and promotes the forming of nuclei and growth of hydration products. However, an excessively high red mud content would lead to a looser and more porous paste matrix due to the characteristics of red mud particle itself. An optimal red mud content has been identified as 20 wt%.

Rapid CO2 catalytic activation of binary cementing system of CSA and Portland cement

This study presents a novel approach to activating a binary cementing system composed of calcium sulfoaluminate (CSA) cement and ordinary Portland cement (OPC) using an instant CO2 catalysis. The activation led to significant and rapid strength enhancement, with the binary cement achieving a strength of 15.42 MPa immediately after activation, all within 1 min. The rapid CO2 activation catalyzed a notable heat release, accelerating the hydration of ye'elimite to form ettringite, which is a crucial component capable of bridging gaps and imparting high initial strength. Simultaneously, the CO2 activation catalyzed the increase in sulfur concentration, which in turn, also facilitated the formation of ettringite at an early age. Subsequent strength development was attributed to belite hydration. Apart from the rapid strength gain, employing CO2 activation facilitated control over the pH value of the pore solution, thus enabling the manipulation of ettringite's crystal morphology to strategically enhance the microstructure. Samples with lower pH values exhibited needle-like ettringite formations, whereas samples with higher pH values yielded rod-like and column-like ettringite crystal structures. Comprehensive analytical investigations were analyzed using XRD, FTIR, TGA, 27Al NMR, MIP, SEM, and ICP-OES. The present study provides a new perspective on the potential application of CSA-based cement, such as precast concrete element, instant concrete product delivery, and urgent reconstruction.

Effects of kaolinite and montmorillonite calcined clays on the sulfate balance, early hydration, and artificial pore solution of limestone calcined clay cements (LC3)

This study investigated the physicochemical effects of kaolinite (CK) and montmorillonite (CM) calcined clays on the sulfate balance, early hydration, and artificial pore solution of limestone calcined clay cement (LC3). The effects of fineness, clay dissolution, and ion-adsorption capacity were evaluated by isothermal calorimetry, compressive strength, ICP-OES, and zeta potential within 72 h, respectively. Increasing the fineness of both calcined clays did not significantly affect the sulfate depletion kinetics or the compressive strength and the adsorption of Ca2+ ions onto the calcined clay’s surface is not the main factor responsible for differences in sulfate demand. The higher dissolution of ions Al in CK provided an intensified and accelerated formation of ettringite that competes for the available sulfate. We demonstrate that the chemical effects have a significant impact on the sulfate balance of LC3, revealing the lesser impact of alternative clays like montmorillonite compared to metakaolin (MK) which can minimize the problem of accelerated sulfate depletion of LC3 mixes with MK.

Simultaneous enhanced phosphorus removal and hydration reaction: Utilisation of polyaluminium chloride and polyaluminium ferric chloride to modify phosphogypsum-based excess-sulphate slag cement

This study employed polyaluminium chloride (PAC) and polyaluminium ferric chloride (PAFC) for the phosphorus removal released during the hydration of phosphogypsum-based excess-sulphate slag cement (PESSC), as well as microstructure modification. The results indicated that the PESSC samples presented shorter setting times and bleeding rates with further modifier addition due to the effective phosphorus removal and rapid establishment of the flocculated structure. Compressive and flexural strengths were also significantly enhanced in modified samples at each age, where they exceeded 58 MPa and 13 MPa at 180 d, respectively, when PAC and PAFC supplements were within 1.0%. Furthermore, the distinctions in the effect mechanism of two functional compositions (Keggin-Al13 and iron phase) in modifiers have also been studied. In contrast to affecting the microstructure of C-(A)-S-H gel, the released Al(OH)4‐ and Fe(OH)4‐ during hydrolysis often acted with sulphate and portlandite, leading to a higher level of ettringite formation, as well as hydration degree. Comparatively, the introduction of the iron phase adversely impacted pH value development of pore solution and retarded the activation process of slag at early age, while also promoting continuous hydration at later age. The optimisation in the microstructure of PESSC also contributed to impurities solidification, presenting the lower leaching concentration in an acid environment. Overall observations suggested that adding 0.5%–1.0% PAC or PAFC in the PESSC are capable of enhancing not only its engineering properties but also promoting its sustainability.

Consecutive pozzolanic layerings to depress internal-sulfate-attack corrosion of OPC by phosphogypsum-based cold bonded aggregates

Phosphogypsum-based cold bonded aggregates (PCBAs), as incorporates excessive soluble sulfate, potentially induce Internal Sulfate Attack (ISA) corrosion to OPC, which severely constrains its utilization and advancement. This work applies consecutive pelletization using pozzolanic layerings (fly ash (FA) and phosphorus slag (PS) basis) for surface modification. The basic, exterior hydration and microstructure properties of PCBAs, as well as the interfacial corrosions of PCBAs-OPC, are investigated via XRD, TG-DTG, X-CT, SEM-EDS, μ-XRD, and nanoindentation technologies. Results indicate the FA and PS layerings block sulfate leaching by 15–30%. When blended with OPC, these layerings capture releasable sulfate within PCBAs exterior via interfacial hydration that also depletes portlandite to generate tighter ITZ. Therefore, the reactive layerings effectively alleviate ISA corrosion via in-front blockage and back-end reaction, as supported by reduced sulfate ingress within 30–40 μm, less degradation products (ettringite and gypsum) formation, and improved hardness of vicinity OPC. The FA-based layering performs slightly poorer due to its lower density causing inefficient consolidation of PCBAs. Consequently in macroscopic, the ISA-inducing fissures of concrete surface are diminished, followed by improved 90-d compressive strength from 36.43 MPa to optimally 48.38 MPa. This study elucidates the rich-gypsum aggregates-type ISA corrosion mechanisms to OPC and proposes an instructive solution using pozzolanic layerings.

Effect of Inorganic Anions on the Structure of Alkali-Activated Blast Furnace Slag

Analyzing the effect of anions on the structure of geopolymers is crucial because anions can significantly influence the material’s chemical stability, mechanical properties, and long-term durability. Understanding these effects helps optimize geopolymer compositions for various applications, such as construction materials and waste encapsulation. This research report describes the effects of nitrate, sulfate, and phosphate anions on alkali-activated blast furnace slag’s structural integrity and properties. Advanced techniques like XRD, FT-IR, Raman spectroscopy, and XPS have been employed to analyze structural modifications caused by anions, providing insights into their interactions and effects. These anions generally decrease compressive strength by disrupting geopolymerization and altering microstructure. For example, sulfate ions lead to the formation of ettringite, while phosphate ions bind calcium into separate phases. We can also observe microstructural changes, such as increased porosity with phosphate, which significantly reduces strength. Nitrate’s effect is less detrimental but still influences the overall structural dynamics.

Phase evolution and performance of sodium sulfate-activated slag cement pastes

This study evaluates the reaction kinetics, phase assemblage, and microstructure evolution of Na2SO4-activated slag cements produced with three commercial slags. The main reaction products identified are ettringite and calcium aluminosilicate hydrates, alongside a poorly crystalline SO42- intercalated Mg-Al-layered double hydroxide (LDH) phase. Results revealed that the Al2O3 slag content alone does not correlate with the cement performance. While pastes made with a higher Al2O3 content slag exhibit faster reaction kinetics, those made with a slag with a higher Mg/Al ratio developed superior compressive strength and reduced porosity over extended curing periods. Thermodynamic modelling simulations indicate that sulfate consumption occurs via ettringite and LDH phase formation, influencing the slag reaction degree, pH value, and porosity reduction in these cements. This research highlights the critical role of slag composition in controlling microstructure and, consequently, performance of sodium sulfate activated slag cement pastes.

Corrosion shielding effect of polyaluminium sulphate on metallic aluminium during the solidification of radioactive incineration bottom ash by low alkalinity cement

Radioactive incineration bottom ash containing metallic Al is difficult to be directly solidified in the cement matrix due to the generation of hydrogen gas during the reaction of aluminium with alkali pore solution of cement paste. In this study, the simulated incineration bottom ash (SIBA) containing metallic Al was successfully and directly solidified by polyaluminium sulphate (PAS)-modified low alkalinity cement (LAC), and the mechanism of the corrosion shielding effect of PAS on metallic Al was characterized by a range of analytical techniques. The results indicate that the cement waste form containing 29.56 wt% SIBA achieved a compressive strength of 16.6 MPa at 28 days of hydration, which increased by 13.3 % and 36.1 % after freeze-thawing test and immersion test, respectively. The 42d cumulative fraction leached of Nd3+ and Ce3+ were 1.74×10−7 cm and 3.40×10−7 cm, respectively. The corrosion degree and corrosion rate of Al sheets in LAC with PAS addition was much lower than that without PAS at early and late age hydration, by transforming thick porous corrosion layer to thin dense one. The mineral phase of corrosion product remained as bayerite (Al(OH)3), regardless of whether PAS was added. The addition of PAS reduced the hydrogen gas generation and prevented the dissolution of Al(OH)4- from metallic Al sheet. The hydrolysis of PAS produced a large amount of Al(OH)4- in pore solution, which extended the pH range for Al(OH)3 passivation by preventing the transformation of Al(OH)3 protection layer on the Al sheet surface to Al(OH)4- in the pore solution, and accelerated the formation of ettringite to decrease the pH of pore solution. This study cast the light on the direct solidification of metallic-aluminium-containing incineration bottom ash or Al bulks by cement-based materials without the need for complex pretreatment processes.

Effect of ground granulated blast-furnace slag (GGBS) and sulphoaluminate cement (SAC) on the strength and microstructure of ordinary Portland cement paste at elevated temperatures

This study explored the potential of GGBS and SAC to enhance the thermal resistance of cement. The research explores changes in compressive strength and pore structure at various temperatures, employing microscopic analysis to elucidate underlying mechanisms. The results indicated that an optimal GGBS content enhanced strength, whereas SAC significantly reduced strength. The strength variation was not correlated with porosity or pore size distribution but rather with the hydration products of the blended cement. GGBS promoted the generation of C-A-S-H gels by introducing Al to replace Si in C-S-H gels, thereby increasing both the quality and quantity of gels. This enhancement suggested that GGBS improved thermal resistance by augmenting hydrate formation. SAC promoted ettringite formation even at high temperatures but reduced the quantity of hydrate gels, illustrating that SAC did not provide Al for the generation of C-A-S-H gels, consequently diminishing strength.

Experimental study on flexural behaviour of DEF–ASR-affected prestressed concrete beams

When concrete is exposed to high temperatures during initial hydration, delayed ettringite formation (DEF) may occur and cause expansion, with consequences similar to that of the alkali-silica reaction (ASR). This study investigated the structural performance of prestressed concrete (PC) beams exhibiting expansion owing to DEF and the combined effects of DEF and ASR. In both cases, concrete cylinders were cast to determine the expansion, compressive strength, and dynamic elastic modulus at different expansion levels. The compressive strength and Young’s modulus of the plain concrete specimens were significantly reduced with large free expansion, which caused map cracking on the surface. Because of the restraining effects of the reinforcement and prestressing load, the expansion of the PC beams was anisotropic, with more longitudinal cracks. The Young’s modulus determined from stiffness of the cross-section of the DEF and ASR–DEF-affected beams at a low load level without bending cracking decreased moderately (22 % for DEF and 30 % for ASR–DEF beams) as compared to that of the non-reactive companion beam. The load capacities of DEF and ASR–DEF-damaged beams, even with many cracks, were reduced by 16 % and 23 %, respectively, compared to those of the non-reactive beam. Though the compressive strength of deteriorated plain concrete decreased significantly, less compressive strength reduction in concrete of DEF and ASR–DEF-damaged beams owing to confinement resulted in the lesser load capacity reduction in beams.

Effect of compound mineral capsules on the self-healing performance of cementitious materials under marine environment

Marine concrete has been subjected to the dual threats from cracking and the corrosion of ion. To achieve the self-healing of cracks in concrete and the binding of Cl− and SO42− in marine environment, a novel compound mineral capsule with MgO expansive agent, quicklime and metakaolin as healing agents was developed in this study. To evaluate the ion binding capacity of capsules, after the broken capsules were immersed in synthetic seawater for different times, the concentrations of Cl− and SO42− in seawater were measured, and the binding products on the capsules were analyzed. Then the capsules were incorporated into mortar in varying amounts, and the self-healing capacity at different healing ages was quantitatively characterized by the tests of crack observation, sorptivity and water permeability. Through microstructure analysis of healing products, the self-healing process and mechanism of the mortar containing capsules under marine environment were revealed. Results showed that the compound mineral capsules effectively bound Cl− and SO42− by forming Friedel's salt and ettringite. The addition of capsules significantly improved the crack closure and conspicuously reduced the transport performance of cracked mortar. With the dosage of capsules increased, the self-healing process was accelerated. The underlying reasons were that the capsules promoted the formation of Friedel's salt, ettringite, hydrotalcite and brucite, in addition to portlandite, calcite and C-S-H. Under the filling effect of these products, cracks achieved self-healing while Cl− and SO42− were simultaneously bound. Consequently, the impermeability of the cracked cementitious materials in marine environments was effectively increased.

Impact of calcium formate and anhydrous sodium sulfate on the flowability, mechanical properties, and hydration characteristics of UHPC

Ultra-high performance concrete (UHPC) is not an ideal material for rapid repairs of bridge expansion joints due to its slow early strength development. To mitigate this early strength limitation, this study investigated the use of calcium formate (CF) and anhydrous sodium sulfate (SS) as early strength agents to enhance the early strength of UHPC. The findings indicate that a composite addition of 0.25 % SS and 0.25 % CF significantly improves both strength and workability, resulting in a 28-day compressive strength of 141.3 MPa, a 28-day flexural strength of 12.6 MPa, and a flowability of 227 mm. SS facilitates the formation of ettringite (AFt) and calcium hydroxide (C-H), while CF promotes the generation of smaller gismondine particles. This substantial increase in hydration products reduces the porosity of UHPC, thereby densifying the matrix and enhancing its mechanical strength. The synergistic effect of both additives leads to a higher concentration of calcium ions (Ca2+) around the smaller gismondine particles, which generates additional hydration products, optimizes the pore structure of UHPC, and narrows the width of the interfacial transition zone (ITZ). Consequently, this combination significantly improves the early strength of UHPC, achieving a 45.6 % enhancement in the early hydration rate with the use of 0.25 % CF and 0.25 % SS.

Improving foamed mixture lightweight soil containing red mud properties by utilizing nano-SiO2

Red mud (RM) is an aluminum industrial solid waste with a global inventory exceeding 4 billion tons. To utilize RM efficiently, this study used RM as the primary material, with cement, steel slag powder, and desulfurization gypsum as supplements, to prepare foamed mixture lightweight soil containing RM (FMLSR) for subgrade filling. And the effect of nano-SiO2 on the fresh properties, unconfined compression strength, durability, and microscopic properties of FMLSR was investigated. The test results indicated nano-SiO2 significantly reduced the flowability of FMLSR. Therefore, to maintain suitable flowability, the nano-SiO2 content should not exceed 1.5%. When the content ranged from 1.0% to 1.5%, nano-SiO2 effectively enhanced the compression strength of FMLSR, reduced its volumetric water absorption, and improved its resistance to dry-wet cycles. Nano-SiO2 can also reduce the environmental risk of FMLSR by significantly lowering the pH of FMLSR leachate. Microanalysis revealed that nano-SiO2 enhanced the density of the skeletal structure of FMLSR by promoting the formation of ettringite, thereby improving its properties. The addition of nano-SiO2 increased the CO2 emissions of FMLSR, but as nano-SiO2 improves various aspects of FMLSR properties, this increase is acceptable. In conclusion, the suitable content of nano-SiO2 for FMLSR ranged from 1.0% to 1.5%.

Electromagnetical and ultrasonic characterizations of concretes subjected to internal swelling reactions

Among pathologies of reinforced concrete structures, internal swelling reactions (ISR), including alkali-aggregate reaction and delayed ettringite formation, are at the origin of cracks and major disorders due to rebar corrosion. Visual evaluation of crack density combined to non-destructive testing techniques can be used to characterize the global swelling and then give some structural diagnosis. For the last ones, an intermediate step (assimilated to a calibration step) can be performed at laboratory to evaluate the sensitivity of electromagnetic, electrical and ultrasonic properties of concretes subject to ISR.This present study focuses on such characterizations of concrete samples presenting different levels of ISR and for several water content. Numerous samples have been extracted from mock-ups representative of two massive concrete structures, affected one by alkali-aggregate reaction and the other by delayed ettringite formation, and conditioned in homogeneous and controlled conditions. After describing the experimental campaign, results are shown and commented.