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The hydration of siliceous fly ash (FA), slag (S) and metakaolin (MK) with calcium hydroxide (CH) was studied at 27 °C to isolate the roles of CH, alkali and sulphate. The influence on rate of hydration was studied along with phase assemblage and strength development. While the FA was the most slowly reacting SCM, it was also the least influenced by the addition of alkalis and sulphates. For MK and S, not only the rate of hydration is influenced, but the phases formed are also affected by the amount of alkalis and sulphates. The addition of sulphates delays the hydration of alumina in S and MK. In more reactive SCMs such as MK, the availability of CH can be a limiting factor in the reaction of the SCM. The results show that to achieve maximum possible hydration and contribution to strength by SCMs, an optimisation of the alkali and sulphate content is required. [ABSTRACT FROM AUTHOR]

This study assessed the feasibility of different methods for sulfate optimization of limestone calcined clay cements (LC3). LC3-based pastes were studied with kaolinite and montmorillonite calcined clays and different gypsum contents. Quartz powder was used in reference systems. The sulfate optimization was done using isothermal calorimetry, compressive strength, thermogravimetric analysis, and chemical shrinkage analysis at different ages. Overall, there is no single best technique to predict sulfate optimization in LC3, carefully combining multiple techniques can assist in more coherent decision-making, such as isothermal calorimetry with compressive strength within 72 h. We recommend the optimum LC3 sulfate content based on criteria established by the decision-maker (academia or industry). Either based on mechanisms and kinetics reactions, phase assemblage, compressive strength, or shrinkage of LC3 mixtures containing different SO 3 content. Also, the results of montmorillonite-LC3 systems were satisfactory without accelerated sulfate depletion. [ABSTRACT FROM AUTHOR]

Possibilities for enhancing the aluminate reaction were studied in a mix of OPC blended with 7.9 % hemihydrate (HH) or 7.4 % anhydrite (AH) in combination with different triethanolamine (TEA) dosages. Heat flow calorimetry and quantitative X-ray diffraction (XRD) were performed to study the hydration process. Using anhydrite as an additional sulfate carrier allowed a lower TEA dosage compared to hemihydrate systems to enhance the degree of hydration of C 3 A. High TEA dosages in hemihydrate systems were found to hinder C 3 S hydration. Nevertheless, it was possible to push the aluminate reaction ahead of the silicate reaction, thereby enhancing the degree of hydration of C 3 A. Additionally, the degree of hydration of C 3 S was not strongly affected when anhydrite and a well-balanced TEA dosage were used. The addition of tartaric acid to OPC-anhydrite-TEA systems retarded the subsequent aluminate reaction. [ABSTRACT FROM AUTHOR]

Degradation of cementitious materials by sulfate ions is commonly classified into chemical and physical sulfate attack. So-called "physical" attack dominates in many field situations, but laboratory testing focuses on "chemical" attack under full-immersion. This paper presents a new setup which looks at sulfate ion ingress under unidirectional capillary action, as a first approach to field conditions. Here, a high concentration of sulfate ions and w/c and Portland cement were used to accelerate degradation to see if the approach is feasible. The radial expansion and appearance of mortar and paste samples were tracked over time. Periodically, profiles of sulfate ingress and phase assemblage were studied in the SEM and by X-ray diffraction. The results show that physical and chemical sulfate attack occur in different areas of the same sample. The approach shows potential to uncover the mechanisms involved in sulfate attack including both "chemical" and "physical" aspects of the degradation process. [ABSTRACT FROM AUTHOR]

In County Donegal, northwest Ireland, thousands of homes built with concrete blocks show an increasing degree of severe structural defects attributed to high mica content in the aggregates. Consequently, the problem is popularly known as the "Mica Crisis". In this project the concrete blocks of four affected homes are investigated by microstructural and chemical analysis combined with thermodynamic modelling. Apart from mica, the aggregates contain iron sulphides mainly in the form of pyrrhotite. The sulfur content of the aggregates considerably exceeds the limit value defined by the European standard for concrete aggregates (EN 12620). The results of the microstructural analysis coupled with thermodynamic modelling demonstrate that the concrete suffers from internal sulfate attack triggered by pyrrhotite oxidation. The comparison of the results of this investigation with the data collected by chartered engineers on almost hundred damaged homes shows that the four investigated cases are representative of the situation in Donegal. [ABSTRACT FROM AUTHOR]

Gypsum formation mechanisms during external sulfate attack were studied on Portland cements of various C 3 A content exposed to low sulfate concentrations. Though ettringite is acknowledged as the main expansive phase during external sulfate attack, the simultaneous formation of gypsum can cause expansion of laboratory samples prepared with SR Portland cement. The comparison of free expansion, expansion pressure, sulfur distribution, microstructure and thermodynamic modelling was used to identify three different gypsum formation mechanisms: (i) gypsum formation in mesopores of the hardened cement paste matrix (> 1500 mg l−1 SO 4 2−), (ii) gypsum formation in new macroscopic cavities caused by matrix expansion (> 1500 mg l−1 SO 4 2−) and (iii) replacement pseudomorphs of gypsum after portlandite (metastable, ≤ 1500 mg l−1 SO 4 2−). Only the first mechanism is likely to contribute to expansion of non-constrained samples. The second and third mechanism are not able to exert significant expansion pressure and are therefore unlikely to cause damage under field conditions. [ABSTRACT FROM AUTHOR]

The pore crystallization of ettringite (AFt) and the material expansion are investigated for cement paste subject to external sulfate attack with and without chlorides. Specimens of 2 mm thickness were exposed to solutions of 10 g/L Na 2 SO 4 , 10 g/L Na 2 SO 4 + 10 g/L NaCl and 10 g/L Na 2 SO 4 + 19 g/L NaCl. The evolutions of AFt formation, pore structure, transport property and specimen expansion were characterized through 27Al NMR, MIP, DVS and micrometer measurements. The analysis of results shows that: (1) chlorides decrease the AFt formation rate and the expansion capacity through depressing the supersaturation of AFt crystals; (2) AFt crystallization alters the pore structure in the whole pore size range, especially through the blockage of "ink-bottle" pores in micropore range; (3) AFt crystals change the apparent vapor diffusivity by pore-filling and microcracking, and the linear stage of expansion is well captured by poromechanics with pore crystallization pressure incorporated. [ABSTRACT FROM AUTHOR]

The addition of calcium sulfate in the preparation of autoclaved aerated concrete (AAC) leads to improved mechanical properties but the precise mechanism is not well constrained. To study this, the hydrothermal curing process was monitored quantitatively with and without sulfate addition using ex situ and in situ X-ray diffraction. Higher concentrations of sulfate ions delayed the formation of the intermediate C-(A)-S-H phase and the dissolution of quartz. Once the additional sulfate ions were depleted by hydroxylellestadite formation, C-(A)-S-H formation and quartz dissolution started simultaneously at higher rates. This delayed reaction is suggested to enhance the mechanical strength by i) improving the diffusion of silicate ions in hydrothermal solution, ii) improving the distribution of C-(A)-S-H and tobermorite crystallites, and iii) closing larger capillary pores in the range of 1–5 μm. This could explain some of the ambiguous results from previous studies and provide an opportunity to optimize the AAC production process. [ABSTRACT FROM AUTHOR]

A spalling rate model is established to estimate the damage of concretes subject to the combined action of leaching and external sulfate attack (ESA). Two sub-models are developed to consider the different damage patterns. The profiles of sulfate and hydroxide ions are obtained from the solution of a dual moving boundary problem (MBP). The supersaturation of ettringite crystal is evaluated from these profiles, the effective stress is calculated through poromechanics for pore crystallization, and the spalling depth is determined using the fracture criterion. The experimental validation confirms the proportionality between the spalling depth and the square root of exposure time, and the models give conservative results but on the correct magnitude order for spalling depth. The parametric analysis shows the foremost impact of aluminate content and sulfate diffusivity on damage rate. The main assumptions in models lead to conservative predictions, but yet the model results are meaningful for engineering use. [ABSTRACT FROM AUTHOR]

A comprehensive understanding of hydration kinetics, sulfate requirement and phase assemblage of Portland cement is crucial for further optimization of early-age performance, especially when part of cement is replaced by mineral additions in blended cements. Gypsum is added to PC to control the reaction of the aluminate phase. The addition of gypsum also enhances the reaction of C 3 S independently of the interaction with the aluminates. In this study, the influence of gypsum in C 3 S and C 3 S/C 3 A hydration was studied in terms of the morphology and composition evolution of the C-S-H formed. The addition of gypsum modifies morphology of the C-S-H (needle length) precipitated and increase the nucleation density. The extent of the enhancement is dependent on the specific surface area (reaction rate) of C 3 S. In C 3 S/C 3 A systems, no differences in C-S-H morphology were observed before and after the aluminate peak. [ABSTRACT FROM AUTHOR]

Understanding the mechanism controlling the sulfate requirement of blended cements is crucial for a successful deployment of these technologies towards a more sustainable industry. Particularly, clarification regarding the role of alumina content on the sulfate requirement is essential to establish suitable procedures to achieve proper sulfation in practice. In this study, the role of alumina content on the sulfate requirement of cementitious systems is investigated in PC, blends of PC and slag and LC3-type systems. It is observed that adsorption of sulfate on surfaces, mainly C-(A)-S-H, is the overwhelming factor controlling the sulfate demand of these systems. Dissolution of alumina from slag and metakaolin and adsorption of sulfate in other surfaces seem to have only a minor effect on the overall sulfate requirement. Furthermore, these effects are not correlated with the total amount of alumina present in the SCM used. [ABSTRACT FROM AUTHOR]

Synthesized cement systems made with variable C 3 A/C 4 AF ratios, containing C 3 S, gypsum and, optionally, calcite, were stored long-term at humid conditions at 5 or 20 °C, without any protection against atmospheric carbonation. Analytical techniques able to assess both the crystalline and amorphous phases were used. Experimental results were compared with thermodynamic simulations. The systems with C 3 A/C 4 AF < 1 better preserved the soundness of the C S H phase, which hosted iron, and prevented thaumasite formation. The addition of calcite in these systems inhibited carbonation. When occurred (mixtures without calcite), the carbonation was significantly more intense at ambient temperature. In the systems that underwent extensive deterioration, cross-linking of silicate structures, AFt decomposition, and iron release from the deteriorating C S H, occurred, while Al-incorporating amorphous silica, calcium carbonate polymorphs and hydrous iron oxide formed. The presence of unreacted C 3 A in the systems with C 3 A/C 4 AF = 1, suggested that C S H decomposition was contributed by available sulfates. [ABSTRACT FROM AUTHOR]

This research studied the hydration of C 3 S-C 3 A-calcium sulfate systems made of combinations of two C 3 S (pure triclinic and Al-doped monoclinic), two C 3 A (pure cubic C 3 A and Na-doped orthorhombic), and two calcium sulfates (gypsum and hemihydrate). For each system, the hydration of four different SO 3 contents (0.25–2.0 wt%) was assessed by calorimetry. The optimum SO 3 content was fixed from calorimetry results, and the mixtures were evaluated by in-situ XRD and TGA. The type of C 3 S was the factor that most affected the sulfate balance of the systems. The mixes with Al-C 3 S produced a higher amount of ettringite in the first hours, resulting in much earlier sulfate depletions when compared to the mixes with pure C 3 S. The mixes with ort-C 3 A also showed faster sulfate depletion due to its higher reactivity compared with cb-C 3 A. Finally, the replacement of gypsum by hemihydrate resulted in faster sulfate depletion caused by the faster hemihydrate dissolution. [ABSTRACT FROM AUTHOR]

CO 2 mineralization using alkaline waste materials and by-products is a promising technology to reduce CO 2 emissions. This work investigates the impact of alkali sulfates on the carbonation process. Detailed investigations of the carbonation reaction show that the sulfates are important phases impacting the mineralization. Alkalis alone accelerate the kinetics and change the morphology of the precipitated phases. Sulfates additionally reduce the amount of CO 2 mineralized since a part of calcium precipitates as calcium sulfate. The mutual interaction between alkalis and sulfate ions provides a tool to control the mineralization kinetics and especially the extent of carbonation by preventing calcium sulfate precipitation at surplus of alkalis. Furthermore, sulfate-containing materials, which so far have not been considered as suitable for CO 2 mineralization (e.g. gypsum), can now be considered for CO 2 sequestration. A modification of the Steinour formula, used for calculations of the carbonation potential, is proposed to account for the observed effects. • CO 2 mineralization of recycled concrete paste allows significant reductions of the environmental impact of the cement and concrete industry. • Mechanisms of carbonation reactions in presence of sulfates are investigated. • The interaction between soluble sulfates and alkalis influences the carbonation kinetics and its products. • The carbonation potential of recycled paste can be significantly increased by preventing calcium sulfate precipitation. [ABSTRACT FROM AUTHOR]

BYF cements are a promising alternative to Portland cement due to their lower CO 2 emissions and energy demand. However, it is essential to assess how production parameters affect BYF clinker composition. In this study, clinkers with varying SO 3 content were synthesized at temperatures of 1100–1400 °C and retention times of 15–120 min. The mineralogy of the produced clinkers was analyzed using X-ray powder diffraction with Rietveld phase analysis. According to the results, firing temperatures of 1300–1350 °C, retention times of 30–60 min, and excess SO 3 are needed to obtain the desired clinker composition. It was proved that SO 3 contributed to the stabilization of β-C 2 S and the formation of ye'elimite. Additionally, increasing SO 3 resulted in higher contents of cubic ye'elimite relative to orthorhombic. It was demonstrated that the level of Fe 2 O 3 incorporated into C 4 A 3 $-c increased with the increase of SO 3, as indicated by the decrease in C 4 AF and unit cell volume. • BYF clinker composition was tested as a function of SO 3 , firing temperature, and time. • The influence of SO 3 on the polymorphism of belite and ye'elimite was assessed. • The formation of ye'elimite is controlled by firing temperature and SO 3 in raw meal. • Optimal firing temperature, SO 3 content and, retention time were determined. [ABSTRACT FROM AUTHOR]

Supplementary cementitious materials (SCMs) can alter significantly the early hydration behavior of blended cement. While the underlying mechanisms have been intensively discussed in literature, they need to be extended regarding SCMs with complex mineralogy and surface properties, such as calcined clays. In this study, a blended Portland limestone cement was monitored during the first 48 h of hydration using in situ X-ray diffraction and isothermal calorimetry. The influence of a fine limestone and four different calcined clays, which vary in mineralogical composition and particle properties, was investigated at a replacement level of 30 wt%. The sulfate content was gradually adjusted and its influence on heat evolution and phase development was examined. It was shown, that depending on their particle properties, the investigated SCMs influence the aluminate and silicate clinker reaction by different mechanisms and to varying extents. The results underline the importance of proper sulfation of blended cements. [ABSTRACT FROM AUTHOR]

External sulfate attack (ESA) and delayed ettringite formation (DEF) are well-known concrete degradation phenomena induced by ettringite crystallization. This paper proposes a poromechanical model based on the surface-controlled ettringite growth mechanism and the chemical composition of the solution, explaining both ESA and DEF in a similar model. The driving force for this expansion is the crystallization pressure from the supersaturation of the sulfate solution that leads to the formation of ettringite in pores. The growth of ettringite is separated into two processes: (1) the pore invasion process, where ettringite forms in the existing largest pores and then penetrates progressively into the ever-smaller capillary and gel pores, and (2) the deformation process during which damage and cracks occur. Two kinetic constants corresponding to these two processes are proposed in this paper, and the predicted expansions show good consistency with the experimental measurements found in the literature. [ABSTRACT FROM AUTHOR]

The higher reactivity of orthorhombic C 3 A (ort-C 3 A) in sulfate-containing solutions, compared with cubic C 3 A (cb-C 3 A), was previously related to the differences in crystal structure or the sodium in the ort-C 3 A pore solution. We analyzed the hydration of cb-C 3 A (in water and NaOH solution) and Na-doped ort-C 3 A in the presence of gypsum and hemihydrate. Calorimetry, in-situ XRD, TGA, and rheological tests were conducted. NaOH accelerated the hydration of cb-C 3 A, but ort-C 3 A still presented higher ettringite formation rate and earlier sulfate depletion. Ort-C 3 A pastes showed 10-20 times higher viscosities and yield stresses. Replacing gypsum by hemihydrate increased the ettringite precipitation rate and anticipated the sulfate depletion of ort-C 3 A but did not significantly influence cb-C 3 A hydration. The crystallization of hemihydrate into gypsum resulted in early (<10 min) stiffing of all C 3 A-hemihydrate pastes. Overall, the higher reactivity of ort-C 3 A is related to differences in crystal structure rather than the sodium in the pore solution. [ABSTRACT FROM AUTHOR]

The use of pozzolans to partially replace Portland cement in concrete has generally demonstrated beneficial impacts on the durability characteristics of concrete for decades. In this paper a diverse range of pozzolans including natural pozzolans, ground glasses and industrial by-products such as coal ash (fly ash and bottom ash) and silica fume were investigated for their synergistic potential in binary or ternary blends with Portland cement in improving resistance to chemical sulfate attack and alkali-silica reaction (ASR). It is generally considered that pozzolans improve most of the durability issues encountered in concrete, including reducing the risk of sulfate attack or ASR. But this is not always the case. For example, it was found that ground glasses were very efficient in improving sulfate resistance, but their ability to mitigate expansion due to ASR was dictated by the equivalent alkalis content (Na 2 O e) of the glass and high-alkali soda glass was generally not effective in this role. On the other hand, metakaolin, a highly reactive pozzolan, was highly effective in reducing ASR expansion, but may actually increase the damage due to sulfate attack when used at moderate replacement levels. Most pozzolans, such as low-CaO coal fly ash and ground coal bottom ash, silica fume, and pumice, were effective in controlling expansion due to both ASR and sulfate attack. The results demonstrated that the extent of the positive impact of using natural pozzolans on both properties was variable. The pozzolanic reactivity of materials alone was an unreliable indicator to assess the ability of the pozzolan to suppress expansion due to sulfate attack or ASR. [ABSTRACT FROM AUTHOR]

This paper reports the strength-promoting mechanism of alkanolamines on limestone-calcined clay cement (LC3). The hydration kinetics, phase formation and microstructure of LC3 with triethanolamine (TEA) or triisopropanolamine (TIPA) were investigated. TEA enhanced the strength of LC3, while TIPA was effective at 28 days, and a correct sulfate addition promoted their effects. Alkanolamines accelerated the aluminate reaction of LC3 during early hydration, increased sulfate slowed this reaction by producing more ettringite. TIPA favoured the hydration of ferrite in clinker, not only accelerated the silicate reaction of metakaolin, but also resulted in a rapid limestone reaction. Alkanolamines altered the morphology and chemical composition of the aluminate hydrates and C-A-S-H. Alkanolamines enabled the reduction of porosity of hardened pastes and the increased sulfate further refined them. It was the large pores being induced by TIPA at early ages that were considered to have an adverse impact on the early strength of LC3. [ABSTRACT FROM AUTHOR]

This paper evaluates how early aggressive exposure affects the sulfate resistance of blended cements containing limestone filler and/or calcined clay. Mortar and cement paste specimens were elaborated with different combinations of limestone filler and two different illitic calcined clays and exposed to a sodium sulfate solution shortly after casting. Assessment of sulfate resistance was based on expansion, mass variation, and compressive strength. Microscale evolution and distribution were examined by mercury intrusion porosimetry, X-ray diffraction, and scanning electron microscopy with energy-dispersive spectrometry measurements. Results prove superior sulfate resistance of compositions with high calcined clay content over limestone filler addition. Furthermore, the results reported suggest that the pozzolanic reaction progresses enough to reduce the sulfate ingress even at early exposure conditions. Therefore, calcined clay replacement can still provide effective pore refinement to limit sulfate penetration, increase strength, and reduce available CH to limit expansive phase formation. [ABSTRACT FROM AUTHOR]