Showing papers on "Ettringite published in 2009"

Chemically Assisted Damage of Concrete: A Model of Expansion Under External Sulfate Attack:

Abstract: A micromechanical model is proposed to simulate the deformation of cementitious composites exposed to external sulfate attack. The model involves coupled physico-chemical processes of nonsteady diffusion with reaction, topochemical reaction of ettringite formation, expansion of ettringite inclusions, microcracking of hardened cement paste and percolation of sulfates through heavily deteriorated mortar. The Fick's second law with reaction term is assumed to govern the transport of the sulfate ions. The Eshelby solution and the equivalent inclusion method are used to determine the eigenstrain of expanding ettringite crystals in microcracked hardened cement paste. The degradation of transport properties is studied in the effective medium and the percolation regime. An initial boundary value problem (2D) of expansion of a mortar specimen immersed in a sodium sulfate solution is solved and compared with available test data.

Ettringite and calcium sulfoaluminate cement: investigation of water content by near-infrared spectroscopy

Abstract: Calcium sulfoaluminate (CSA) cement is a sulfate-based binder whose high-performance hydraulic behavior depends on the rapid formation of ettringite, when grinded clinker is hydrated in presence of gypsum. Ettringite is a calcium aluminum sulfate mineral characterized by high water content, estimated as 32 water molecules per formula unit. Three examples of utilization of near-infrared (NIR) spectroscopy are here shown. First of all, information on water distribution in pure ettringite was deduced and compared with infrared analyses. Then its thermal behavior has been followed up to 400 °C, allowing to improve the knowledge about water loss and thermal decomposition of this hydrated phase. Finally, the obtained results have been employed in order to follow hydration of CSA cement sample, demonstrating thus that NIR spectroscopy, being highly sensitive to water amount and distribution, can be an extremely useful tool for hydration studies.

Analysis of cubic and orthorhombic C3A hydration in presence of gypsum and lime

Abstract: Field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) have been used to study the microstructural changes and phase development that take place during the hydration of cubic (pure) and orthorhombic (Na-doped) tricalcium aluminate (C3A) and gypsum in the absence and presence of lime. The results demonstrate that important differences occur in the hydration of each C3A polymorph and gypsum when no lime is added; orthorhombic C3A reacts faster with gypsum than the cubic phase, forming longer ettringite needles; however, the presence of lime slows down the formation of ettringite in the orthorhombic sample. Additional rheometric tests showed the possible effects on the setting time in these cementitious mixes.

Durability and micro-structure of reactive powder concrete

Abstract: Durability of traditional reactive powder concrete (RPC) with rich cement and high volume of fly-ash reactive powder concrete (FRPC) were studied. The X-diffraction and scanning electron microscope (SEM) measurement was imployed to analyze the microstructure. The results show that both types of RPC have higher compressive strength, less volume shrinkage ratio and better carbonation-, chloride-, freezing-resistances than the conventional concrete. The results of X-diffraction indicate that they basically have C-S-H as the main composition without Ca(OH)2 crystal and ettringite. SEM results show that hydration products of FRPC is mainly III-C-S-H which is piled up closely like densely arranged stone body and it has very compacted structure, in addition, Ca/Si ratio of C-S-H gel is lower than 1.5.

Microscopic observations of samples affected by delayed ettringite formation (DEF)

Abstract: This article, which deals with the study of the microscopic modifications of DEF-affected materials, has two main objectives The first one is to study the influence of sample preparation on the microcracks of specimens observed in Scanning Electron Microscopy (SEM) From the results of this study, it can be concluded that direct observations of the samples are inappropriate because they produce cracks that are not ascribable to the pathology It is therefore preferable to use an indirect technique of observation such as the replica technique This technique was developed in the 1980s and used to study the evolution of microcracking due to mechanical damage to the concrete The results presented in this article show that it is possible to use this technique to study the microcracking associated with delayed ettringite formation The second objective is to study ettringite formation during the swelling of DEF-affected specimens The main results show that secondary ettringite is initially formed in the paste–aggregate interface even at low levels of expansion For these levels of expansion, there is no trace of secondary ettringite or microcracks observable in the paste However, for higher expansions, cracks filled with ettringite appear in the paste

Water as the Key to Expansion of Ettringite in Cementitious Materials

Abstract: Damage in sulfate-bearing soils and aggregate systems stabilized with additives containing lime, including lime and portland cement, has drawn considerable attention over the past two decades. Researchers and practitioners have made considerable contributions to the understanding of the problem, including the mechanisms involved in the formation of the two minerals, ettringite and thaumasite, that are most often associated with this damage. This paper provides a case history analysis of the expansion history compared with the ettringite growth history of three controlled low-strength mixtures containing fly ash with relatively high sulfate contents. Samples were subjected to three curing conditions: a dry cure, in which only mixing water was available for curing; a moist cure, in which an external source of water was available for curing; and a sulfate cure, in which an external source of sulfate-bearing water was made available for the duration of cure. Ettringite was quantified by using both differential scanning calorimetry and X-ray diffraction; the resulting volume changes in the samples were measured. Results suggested that sorption of water by the ettringite molecule was at least part of the reason for expansion. The importance of sorption was based on the fact that although expansion increased in moist cure compared to dry cure systems, the quantities of ettringite formed under each regime were about the same and remained about the same throughout the experiments. As expected, samples exposed to sulfate cure responded with the greatest expansion, which was concomitant with continued ettringite crystal growth due to a supply of the limiting reagent, sulfate.

Carbonation resistance of sulphoaluminate cement-based high performance concrete

Abstract: The influences of water/cement ratio and admixtures on carbonation resistance of sulphoaluminate cement-based high performance concrete (HPC) were investigated. The experimental results show that with the decreasing water/cement ratio, the carbonation depth of sulphoaluminate cement-based HPC is decreased remarkably, and the carbonation resistance capability is also improved with the adding admixtures. The morphologies and structure characteristics of sulphoaluminate cement hydration products before and after carbonation were analyzed using SEM and XRD. The analysis results reveal that the main hydration product of sulphoaluminate cement, that is ettringite (AFt), decomposes after carbonation.

Sulfate attack of cement matrix containing inorganic alkali-free accelerator

Abstract: This paper presents an experimental study on the durability of mortar specimens incorporating inorganic alkali-free accelerator (AFA) exposed to external sulfate attack. Visual examination, compressive strength, expansion and density of mortar specimens with or without AFA were monitored up to 360 days of exposure. Additionally, in order to identify the products formed by sulfate attack, mineralogical analysis based on x-ray diffraction (XRD) was also performed on paste samples. Test results confirmed that the mortar specimens with AFA suffered a serious deterioration by sulfate attack, especially in 10% sodium sulfate solution. Furthermore, XRD traces indicated that the formation of ettringite and/or thaumasite is primarily responsible for the sulfate deterioration of mortar specimens. Conclusively, it must be taken a special care when the shotcrete with AFA is applied under the sulfate-bearing environments.

Thick self leveling mortar, useful for coating surface, comprises ettringite binder comprising calcium sulfates and calcium aluminate mineral compound containing calcium oxide and aluminum oxide and aggregates

Abstract: Thick self leveling mortar (I) comprises: 10-50 wt.% ettringite binder comprising calcium sulfates and calcium aluminate mineral compound containing calcium (C) oxide and aluminum (A) oxide, where the binder is soluble and combined into one or more amorphous and/or crystallized mineralogical phases, and the molar ratio of C/A is 1.2-2.7, preferably 1.6-2; and aggregates (50-90 wt.%) at least 30 wt.% of which is synthetic inorganic alumino-calcic aggregates comprising 30 wt.%, preferably at least 35 wt.% of alumina. Thick self leveling mortar (I) comprises: 10-50 wt.% ettringite binder comprising calcium sulfates and calcium aluminate mineral compound containing calcium (C) oxide and aluminum (A) oxide, where the binder is soluble and combined into one or more amorphous and/or crystallized mineralogical phases, the molar ratio of C/A is 1.2-2.7, preferably 1.6-2, and the sum of the weight of phases (C + A) is at least 30%, preferably at least 50% of the total weight of the calcium aluminate mineral compound; and aggregates (50-90 wt.%), at least 30 wt.% of which is synthetic inorganic alumino-calcic aggregates comprising 30 wt.%, preferably at least 35 wt.% of alumina, where the total wt.% of mortar is based on the total weight of dry mortar. Independent claims are included for: (1) wet mortar obtained by mixing the dry mortar with water in an amount such that the water/solids is less than 0.5 l; and (2) self-leveling surface coating obtained from the mortar.

Calorimetric study of calcium aluminate cement blended with flue gas desulfurization gypsum

Abstract: The use of by-product gypsum is an important alternative in concrete design. In present experiment, conduction calorimetry was applied to investigate the early hydration of calcium aluminate cement (CAC)/flue gas desulfurization (FGD) gypsum paste, supplemented with the determination of setting times and analysis of hydrates by X-ray diffraction (XRD). It was found that different profiles of heat evolution rate were presented depending on the CAC/FGD gypsum ratio. Two distinct exothermic peaks, associating with CAC hydration and ettringite formation respectively, appeared when the FGD gypsum content was less than 20%. Hydrate barrier mechanism was introduced to explain the difference in induction periods of the pastes with or without FGD gypsum. It is concluded that the blending of FGD gypsum accelerates the hydration of CAC for the quick formation of ettringite and generates greater hydration heat from per gram of pure CAC for the high exothermic effect of ettringite formation. The dissolution and diffusion of gypsum plays an important role of reacting controller during the hydrations of the pastes with FGD gypsum. The modified hydration process and mechanism in this case is well visualized by means of calorimetry.

Full process analysis of damage and failure of concrete subjected to external sulfate attack

Abstract: Based on Fick’s second law, a nonsteady diffusion equation of sulfate ions, in which chemical reactions depleting sulfate ions in concrete are considered during diffusion, is presented, and the finite difference is applied for solution of the equation to obtain the sulfate ion concentration distribution in concrete. Based on the amount of ettringite produced from the chemical reactions between sulfate and aluminates, the expansion strain due to the growth of the reaction product in concrete is calculated, and the constitutive relation of concrete was used to determine the expansion stress for evaluating the cracking failure of concrete. Finally, numerical simulation is utilized to analyze the process of the sulfate ion diffusion, the increment of the expansion strain and stress and the cracking of a concrete plate immersed into 2% sodium sulfate solution. The results show that the suggested analytical methods can quantify the cracking process of concrete under sulfate attack.

Preferred orientation of ettringite in concrete fractures

Abstract: Sulfate attack and the accompanying crystallization of fibrous ettringite [Ca{sub 6}Al{sub 2}(OH){sub 12}(SO{sub 4}){sub 3} {center_dot} 26H{sub 2}O] cause cracking and loss of strength in concrete structures. Hard synchrotron X-ray microdiffraction is used to quantify the orientation distribution of ettringite crystals. Diffraction images are analyzed using the Rietveld method to obtain information on textures. The analysis reveals that the c axes of the trigonal crystallites are preferentially oriented perpendicular to the fracture surfaces. By averaging single-crystal elastic properties over the orientation distribution, it is possible to estimate the elastic anisotropy of ettringite aggregates.

Acidic Rocks as Aggregates in Concrete: Engineering Properties, Microstructures and Petrologic Characteristics

Abstract: In this research, the possibility of replacing different portions of the normally used aggregate by acidic rocks was investigated. These rock types outcrop at the northern part of Eastern Desert, southwest Ras Gharib area where large quantities of good quality acidic rocks are available their. Portland cement concrete is a composite material made up of the hydrated cement matrix, fine aggregate and coarse aggregate. The scanning electron microscope equipped with an energy dispersive X-ray analysis system (EDX) has been applied to investigate several aspects of Portland cement concrete microstructure. The topics investigated so far include the influence of aggregate composition on the development of the cement paste-aggregate interface and the formation of calcium silicate hydrate CSH/calcium silicate aluminate CSA formation. The silicate gel coated the aggregates in the concrete paste and crystallized into well defined needle like shape, cotton shape as well as euhedral to subhedral crystals of silicate/alumiante and ettringite minerals with free lime librated from the hydrolysis process. The free lime can react again with the aggregates leading to the formation of cementing materials which increase strength and durability of the concrete paste by increasing the interfacial bonds between the used aggregates.