Showing papers on "Ettringite published in 2015"

Hydration of quaternary Portland cement blends containing blast-furnace slag, siliceous fly ash and limestone powder

Abstract: In this study the hydration of quaternary Portland cements containing blast-furnace slag, type V fly ash and limestone and the relationship between the types and contents of supplementary cementitious materials and the hydrate assemblage were investigated at ages of up to 182 days using X-ray diffraction and thermogravimetric analysis. In addition thermodynamic modeling was used to calculate the total volume of hydrates. Two blast-furnace slag contents of 20 and 30 wt.% were studied in blends containing fly ash and/or limestone at a cement replacement of 50 wt.%. In all cases the experiments showed the presence of C–S–H, portlandite and ettringite. In samples without limestone, monosulfate was formed; in the presence of limestone monocarbonate was present instead. The addition of 5 wt.% of limestone resulted in a higher compressive strength after 28 days than observed for cements with lower or higher limestone content. Overall the presence of fly ash exerts little influence on the hydrate assemblage. The strength development reveals that amounts of up to 30 wt.% fly ash can be used in quaternary cements without significant loss in compressive strength.

Effect of calcium sulfate source on the hydration of calcium sulfoaluminate eco-cement

Abstract: The availability of cements, including eco-cements, with tailored mechanical properties is very important for special applications in the building industry. Here we report a full study of the hydration of calcium sulfoaluminate eco-cements with different sulfate sources (gypsum, bassanite and anhydrite) and two water/cement ratios (0.50 and 0.65). These parameters have been chosen because they are known to strongly modify the mechanical properties of the resulting mortars and concretes. The applied multi-technique characterization includes: phase assemblage by Rietveld method, evolved heat, conductivity, rheology, compressive strength and expansion/retraction measurements. The dissolution rate of the sulfate sources is key to control the hydration reactions. Bassanite dissolves very fast and hence the initial setting time of the pastes and mortars is too short (20 min) to produce homogeneous samples. Anhydrite dissolves slowly so, at 1 hydration-day, the amount of ettringite formed (20 wt%) is lower than that in gypsum pastes (26 wt%) (w/c = 0.50), producing mortars with lower compressive strengths. After 3 hydration-days, anhydrite pastes showed slightly larger ettringite contents and hence, mortars with slightly higher compressive strengths. Ettringite content is the chief parameter to explain the strength development in these eco-cements.

Contribution of limestone to the hydration of calcium sulfoaluminate cement

Abstract: Calcium sulfoaluminate (CSA) cements can be blended with mineral additions such as limestone for properties and cost optimization. This study investigates the contribution of limestone to the hydration of a commercial CSA clinker regarding the hydration kinetics, hydrate assemblage and compressive strength. Nine formulations were defined at M-values of 0, 1.1 and 2.1 (M = molar ratio of anhydrite to ye’elimite) without and with medium and high limestone contents. Calorimetric results indicate that limestone accelerates the hydration reaction especially at M = 1.1, probably due to the filler effect. The phase assemblages were calculated by thermodynamic modeling using Gibbs Energy Minimization Software (GEMS). With increasing limestone content the formation of ettringite and calcium monocarboaluminate is predicted at the expense of calcium monosulfoaluminate. With increasing M-value more ettringite is predicted at the expense of the monocarbonate and less calcite takes part in the hydration reactions. The modeled results compare well with the experimental data after 90 d of hydration, except that calcium hemicarboaluminate was found instead of monocarbonate, which is assumed to be due to kinetics considerations. The lowest compressive strength occurs in ternary formulations, whereas in the absence of calcium sulfate, strength is significantly higher. The results presented here indicate that in CSA cements, limestone accelerates early hydration kinetics, takes part in the hydration reactions at M

Sulfate Resistance of Clay-Portland Cement and Clay High-Calcium Fly Ash Geopolymer

Abstract: This paper examines the short-term resistance ability of two different cementitious systems prepared using silty clay as a major component against 5 wt% sodium sulfate and 5 wt% magnesium sulfate solutions. The two cementitious systems are clay–portland cement and clay-high calcium fly ash (FA) geopolymer. The 28-day strength of the clay-FA geopolymer is up to 1.2 times higher than that of clay-cement. The results show that the physical performance of clay-FA geopolymer when exposed to sulfate solution is better than that of clay-cement. While the geopolymer phase is prominent in the clay-FA geopolymer system, gypsum, and ettringite phases are present in both systems especially in the clay-cement system. The strength reduction in clay-cement with duration of sulfate exposure is caused by the formation of ettringite and crystallization of gypsum (observed by scanning electron microscope and X-ray diffraction) as well as the decalcification of CSH phases. There is no major change in the microstructu...

The effect of sea water on the phase assemblage of hydrated cement paste

Abstract: When concrete is exposed to sea water, it has been observed that the composition of the outer most millimeters of the concrete is considerably altered compared to the composition of the bulk concrete. The limited size of this zone complicates the investigation of the phases formed. This paper presents a new experimental set-up in which hydrated cement paste is exposed to sea water allowing a detailed investigation of the phase changes observed in that zone on a bulk material. The paste was characterized before and after sea water exposure by XRD, DTA/TG, ICP-MS and SEM-EDS. In the exposed sample, calcium carbonate and calcium sulfate had formed and calcium hydroxide was depleted. Two types of agglomerates of hydrated cement paste were observed. One type consisted of decalcified C–S–H and a combination of ettringite and Cl-AFm phases. The other type consisted mainly of M–S–H. These findings are in line with the observations on long term marine exposed samples in which the formation of a magnesium rich phase at the concrete surface is detected, followed by a sulfate and chloride enriched zone. The knowledge of these phase changes are important to obtain a better understanding of concrete deterioration in marine environment.

SEM Investigation of Microstructures in Hydration Products of Portland Cement

Abstract: Portland cement is the one of the main ingredients in the manufacture of many building materials include concrete composites. The phase composition of hydration products is controlled by means of XRD and DTA/TG analysis. Observations of phase changes and microstructure of maturing cement pastes can also be observed using scanning electron microscopy (SEM) techniques combined with chemical analyzes in microareas (EDS analysis). Hydrating in time cement paste is composed mainly of hydrated silicates of calcium so called C–S–H-phase accompanied with a calcium hydroxide (portlandite) and hydration products of calcium aluminate i.e. ettringite. SEM analysis of changes in the morphology and microstructure of cement pastes allow to track the hydration progress observed mainly by changes in the C–S–H phase. The initial stages of hydration of this phase is characterized by radial concentration of fibers or needles, often narrowed at the ends. This fibers grow from the surface of the cement grains. The increase in the degree of C–S–H structure orientation is shown by formation of fibers lattice, sometimes three-dimensional plates so called “honeycomb”, which is transformed into the form of a closely-packed, isometric grains. In addition, besides C–S–H cement phase investigation, scanning electron microscopy can also be applied to observation of the crystals formation of tobermorite, ettringite and relicts of portlandite, that often can’t be detected by XRD and DTA/TG due to their small amount in mineral composition of concrete.

Effects of Different Reactive MgOs on the Hydration of MgO-Activated GGBS Paste

Abstract: Reactive MgO has recently emerged as a potential activator for ground-granulated blast-furnace slag (GGBS), which is one of the most widely used by-products in the cement industry. However, it is known that the characteristics of reactive MgO vary significantly, which may affect the activation process and hence the performance of MgO-GGBS blends. In this study, seven commercially available reactive MgOs, whose characteristics vary widely, were chosen to activate GGBS. The unconfined compressive strength (UCS) of MgO-GGBS pastes up to 90 days was measured, and the hydration products were studied by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The primary hydration products were identified as calcium silicate hydrate (C-S-H) and hydrotalcitelike phases (Ht). Minor hydration products included magnesium silicate hydrate (M-S-H) and ettringite. The reactivity and calcium oxide content of the MgO samples were found to be the two major factors affecting the hydration process of the MgO-GGBS blend. Higher reactivity did not change the hydration products, but resulted in more hydration products in the same time. Sufficient CaO content increased the pH of the system, which enhanced the slag dissolution degree and was beneficial to the strength development.

Testing and Modeling the Short-Term Behavior of Lime and Fly Ash Treated Sulfate Contaminated CL Soil

Abstract: In this study, the effects of calcium sulfate contaminated soil treatment methods on the index properties, compacted soil properties, free swelling and compressive stress strain relationship of a CL soil obtained from the field was investigated. Calcium sulfate concentration in the soil was varied up to 4 % (40,000 ppm) and the soil samples were cured for seven days at 25 °C and 100 % humidity before testing. With 4 % sulfate contamination the liquid limit and plasticity index of the soil increased by 44 and 81 % respectively. The free swelling increased from 7 to 18 % and the compressive strength decreased by 65 % with 4 % of calcium sulfate, Also the study investigated the effects of fly ash (class C) and hydrated lime treatment on the behavior of treated sulfate soils. X-ray diffraction (XRD) was used to characterize the soil and the reaction products of lime and fly ash treated soils. Based on XRD analyses, major constituents of the CL soil were calcium silicate (CaSiO3), aluminum silicate (Al2SiO5), magnesium silicate (MgSiO3) and quartz (SiO2). Addition of calcium sulfate resulted in the formation of calcium silicate sulfate [Ternesite Ca5(SiO4)2SO4] and aluminum silicate sulfate [Al5(SiO4)2SO4]. Treatment with lime resulted in the formation of ettringite [Ca6Al2(SO4)3(OH)12·26H2O]. Treating with fly ash resulted in the formation of calcium silicate hydrate (CaSiO3H2O) and magnesium silicate hydrate (Mg3SiO3H2O), cementing by products. Contaminated soil treatment with lime and fly ash reduced the index properties and free swelling and increased the short-term compressive strength of the soil. Behavior of the compacted sulfate soils, with and without treatment, has been quantified using a unique model that was used to represent both linear and nonlinear responses. Also the model predications were compared with other published data in the literature. Compressive stress–strain relationships of the sulfate soil, with and without lime and fly ash, have been quantified using a nonlinear constitutive model. The constitutive model parameters were sensitive to the calcium sulfate content and the type of treatment. Compared to the fly ash treatment, the lime treatment reduced the strain at peak stress making the lime treated soil more brittle.

Leaching behaviour of major and trace elements from concrete: Effect of fly ash and GGBS

Abstract: The leaching of major and trace elements from concrete made with Portland cement, fly ash and GGBS (ground granulated blast-furnace slag) was studied using pH static availability and tank leach tests. The release of substances during the tank leach test occurs by surface dissolution of phases at the concrete surface and diffusion inside the concrete, the amounts depending on the phases controlling solubility and concrete porosity. Alkali release is controlled by diffusion and is thus reduced by lower water/binder ratios and the replacement of Portland cement by fly ash. Ca, Al and S release occurs mainly by surface dissolution of portlandite and AFt/AFm, respectively. The release of V is determined by surface dissolution of V substituted ettringite and/or calcium vanadate. Although fly ash can increase the total V content of concrete, enhancing release, only 2% of the total V content in concrete was available for release.

Thermal Stability of Ettringite Exposed to Atmosphere: Implications for the Uptake of Harmful Ions by Cement

Abstract: The decomposition behavior of ettringite, Ca6Al2(SO4)3(OH)12·26H2O, at different temperatures was studied by means of isothermal XRD experiments, in which the evolution of the solid is monitored as a function of time. The experiments were performed at 40, 50, 55, and 60 °C for a natural ettringite specimen. The experimental data were used to construct a temperature-transformation-time (TTT) diagram. Such a diagram enables the prediction of the reaction pathways during the transformation process. The decomposition behavior was also studied under nonisothermal conditions using thermogravimetry and differential scanning calorimetry, and the obtained results were correlated with the results of the XRD study. Finally, the transformation kinetics and the activation energy (Ea = 246.1 kJ·mol(-1)) of the reaction were estimated using the so-called "time to a given fraction" method. The temperature at which the initial transformation stage occurs (lower than 50 °C) indicates that ettringite cannot be considered a suitable host phase for the immobilization of radionuclides and other harmful elements, as is frequently proposed in the literature.

Performance characteristics of concrete based on a ternary calcium sulfoaluminate–anhydrite–fly ash cement

Abstract: This paper reports an assessment of the performance of concrete based on a calcium sulfoaluminate–anhydrite–fly ash cement combination. Concretes were prepared at three different w/c ratios and the properties were compared to those of Portland cement and blast-furnace cement concretes. The assessment involved determination of mechanical and durability properties. The results suggest that an advantageous synergistic effect between and ettringite and fly ash (Ioannou et al., 2014) was reflected in the concrete’s low water absorption rates, high sulfate resistance, and low chloride diffusion coefficients. However, carbonation depths, considering the dense ettringite-rich microstructure developed, were higher than those observed in Portland cement concretes at a given w/c ratio. It was concluded that the amount of alkali hydroxides present in the pore solution is as important factor as the w/c ratio when performance of this type of concrete is addressed.

Sulfate resistance of cement-reduced eco-friendly concretes

Abstract: Two newly developed cement-reduced eco-friendly concretes with high limestone powder content and low water/powder ratio were tested for sulfate resistance. Mortar samples with a paste composition of eco- as well as conventional concretes were immersed in 30 g l−1 Na2SO4 and saturated Ca(OH)2 reference solutions for 200 days at 8 °C. To evaluate the reaction mechanisms of progressing sulfate attack a combined approach of mechanical, mineralogical, and microstructural methods was applied. Gypsum and bassanite neo-formations related linearly to the expansion during sulfate exposure, except for one sample where ettringite co-precipitated. Thaumasite formation was not observed in spite of potentially favorable conditions. This is considered to be related to the evolution of the experimental solutions, kinetic effects, and the competing formation of CaCO3 polymorphs triggered by the usage of superplasticizer. Both eco-friendly mixes exhibited a better sulfate resistance than their corresponding reference samples and are therefore suggested to be applicable in low sulfate-loaded environments according to DIN EN 206-1. Eco-friendly concrete based on CEM III/B performed superior against sulfate attack and is expected to withstand even severe sulfate exposure despite a much higher water/cement ratio than required by the standard.

Raman Spectroscopy Study on the Hydration Behaviors of Portland Cement Pastes during Setting

Abstract: In the research reported in this paper, an in-situ observation on the hydration of cement pastes with various water-to-cement (w/c) ratios was implemented by Raman spectroscopy (RS). The RS was applied to study the paste continuously after cement mixing with water until the final setting was complete. Hydration products, including calcium hydroxide (CH), calcium-silicate-hydrate (C-S-H) gel, and ettringite were detected. The CH and C-S-H gel were present consistently with slight intensity changes in the RS patterns after their first detections. In all the pastes studied, the intensity of ettringite remained constant first, followed by a rapid increasing period; thereafter it began to decrease. Within the measuring duration a new peak assigned to monosulfate was detected in the pastes with w/c ratios of 0.35 and 0.40. These Raman detections were also analyzed together with the setting time and heat evolution to elucidate the early hydration mechanism.