Showing papers on "Ettringite published in 2010"

Calorimetric and thermogravimetric study on the influence of calcium sulfate on the hydration of ye’elimite

Abstract: Calcium sulfoaluminate (CSA) cements, which represent a CO2-friendly alternative to conventional Portland cements, are produced by blending CSA clinker with gypsum and/or anhydrite. The hydration kinetics and the hydrated phase assemblages of the main hydraulic phase ye’elimite (calcium sulfoaluminate) with calcium sulfate were studied by isothermal conduction calorimetry, thermogravimetric analysis, X-ray diffraction analysis and thermodynamic modelling. Two calcium sulfates with different reactivities (gypsum and anhydrite) were applied. It was found that the pure phase without any calcium sulfate addition exhibits very slow hydration kinetics during the first 10 h. The hydration can be accelerated by the addition of calcium sulfate or (less effective) by increasing the pH of the aqueous phase. The amount of the calcium sulfate determines the ratio between the hydration products ettringite, monosulfate and amorphous aluminium hydroxide. The reactivity of the added calcium sulfate determines the early hydration kinetics. It was found that the more reactive gypsum was better suited to control the hydration behaviour of ye’elimite.

The ternary system Portland cement–calcium sulphoaluminate clinker–anhydrite: Hydration mechanism and mortar properties

Abstract: Binders composed of ordinary Portland cement (OPC), calcium sulphoaluminate clinker (CSA) and anhydrite ( C S ¯ ) were examined in order to study the impact of variations of the OPC:CSA: C S ¯ ratio on the hydration process and related mortar properties. A first sample series had various anhydrite contents and fixed OPC to CSA ratio, and a second various OPC contents and fixed CSA to C S ¯ ratio. Experiments made on pastes and thermodynamic modelling showed that the phase assemblage formed during the hydration of the binders was not very sensitive to changes in modal composition, while the ettringite to monosulphoaluminate volume ratio was influenced. All mixes started to hydrate with the formation of ettringite during a reaction involving C 4 A 3 S ¯ and calcium sulphate. This generated high early strength. Until about 7 d, mainly the CSA clinker reacted, and 15–20% of the dry binder was converted to ettringite. From about 7 d on, the OPC clinker phase alite reacted significantly, stratlingite, C–S–H and monosulphoaluminate formed, while the ettringite content decreased. According to the laboratory experiments, the CSA clinker was mainly responsible for the early mechanical properties, while OPC played an important role at later ages.

Compressive strength, microstructure and thermal analysis of autoclaved and air cured structural lightweight concrete made with coal bottom ash and silica fume

Abstract: This research investigated the compressive strength, microstructure and thermal analysis of autoclaved and air cured structural lightweight concrete made with coal bottom ash and silica fume. The results show that bottom ash lightweight concrete autoclaved for 6 h gives compressive strength similar to the bottom ash lightweight concrete air cured for 28 days and found that the compressive strength of both bottom ash lightweight concrete increased when silica fume was added to the mix. The highest compressive strength obtained for all mixes was found when coal bottom ash was used at 20% with the addition of silica fume at 5% and that this strength value is significantly higher than that of Portland cement control. The thermal conductivity of all bottom ash lightweight concrete at 28 days and those autoclaved for 6 h were found to be slightly higher than that of Portland cement control concrete. Air cued hydration products such as ettringite, calcium silicate hydrate and gehlenite hydrate were detected using thermogravimetric analysis. The tobermorite phase detected in autoclaved bottom ash concrete with silica fume was found to give denser microstructure than the fibrous-like C–S–H phases detected in Portland cement control concrete.

Addressing Sulfate-Induced Heave in Lime Treated Soils

Abstract: Civil engineers are at times required to stabilize sulfate-bearing clay soils with calcium-based stabilizers Deleterious heaving in these stabilized soils may result over time This paper addresses critical questions regarding the consequences of treating sulfate laden soils with calcium-based stabilizers The authors describe the nature (chemistry and structure) of the minerals (ettringite/thaumasite) blamed for deleterious reactions and explain why these structures may lead to damage The writers also describe the mechanisms of the mineral growth, and the extent of mineral growth based on the amount of sulfate minerals present in the soil The writers explain why the rate of ettringite growth in treated soils should not be expected to follow a controlled rate of ettringite development such as that which normally occurs in portland cement concrete The writers compare the rate and degree of ettringite development in soils to the classical model of nucleation and growth typical of most crystal structures Finally, the writers evaluate the role of soil mineralogy in controlling soil behavior at varying sulfate contents and verify the existence of a threshold level of soluble sulfates in soils that can trigger substantial ettringite growth

Thermal analysis and microstructure of Portland cement-fly ash-silica fume pastes

Abstract: Thermal analysis (thermogravimetry and differential thermal analysis) was used with scanning electron microscopy technique to investigate the hydration mechanisms and the microstructure of Portland cement-Fly ash-silica fume mixes. Calcium silicate hydrate (C–S–H), ettringite, gehlenite hydrate (C2ASH8), calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3) phases were detected in all mixes. In the mixes with the use of silica fume addition, there is a decrease in Ca(OH)2 with increasing silica fume content at 5 and 10% compared to that of the reference Portland-fly ash cement paste and a corresponding increase in calcium silicate hydrate (C–S–H).

Effect of ettringite morphology on DEF-related expansion

Abstract: In this study, time dependent ettringite formation in heat-cured mortars has been investigated. In order to clarify the effect of formation place and morphology of ettringite on expansion, secondary electron images of cracked surfaces of mortars at three ages were analysed by SEM–EDS. Also, the X-ray microtomography analysis has been performed to observe the crack formation. The expansive role of delayed formed ettringite was related with its time dependent morphology as a function of formation place. From these observations, mechanism of ettringite reformation after heat curing has been proposed. Alumina rich species were the primary sources of ettringite formation as the starting nuclei. At later ages, if S and Al sources are readily available, the mentioned alumina rich nuclei will grow up and build ball ettringite. At long term, ball type ettringites (non-expansive) converted to massive type (expansive). These conversions can only take places if the form of available space is narrow (preformed micro-cracks). Massive ettringites exert pressure in these narrow spaces and cause expansion of mortar. If the form of the available space is spherical (entrapped air voids) ball ettringites preserve their initial form and do not cause any expansion.

Mechanical properties, drying and autogenous shrinkage of blast furnace slag activated with hydrated lime and gypsum

Abstract: This article reports the characteristics of blast furnace slag (BFS) pastes activated with hydrated lime (5%) and hydrated lime (2%) plus gypsum (6%) in relation to compressive strength, shrinkage (autogenous and drying) and microstructure (porosity, hydrated products). The paste mixtures were characterized using powder X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and thermogravimetric analysis (TG/DTG). BSF activated with lime and gypsum (LG) results in larger amounts of ettringite when compared with BFS activated with lime (L). Although the porosities of the L and LG mixtures were about the same, there was a greater pore refinement for the BFS activated with lime, with an increase in mesopores volume with age. The presence of ettringite and the higher volumes of macropores cause the compressive strength of BSF activated with hydrated lime plus gypsum to be smaller than that of BFS activated with lime. For both chemical activators, compressive strength developed slowly at early ages. Autogenous and drying shrinkage were greater for the BFS activated with lime, believed to result from the more refined porous structure in comparison with the mixture activated with gypsum plus lime.

Microstructural, physical, and thermal analyses of Portland cement–fly ash–calcium hydroxide blended pastes

Abstract: The effect of calcium hydroxide (CH) on the properties of Portland–fly ash cement pastes, at up to high-volume fly ash mixes has been investigated using normal consistency, setting time, compressive strength, thermal analysis and scanning electron microscope. CH as an additive material (5 and 10 wt%), lignite fly ash (FA) up to 50 wt% was used to produce Portland cement (PC)–FA–CH pastes at w/PC + FA ratio of 0.5. Water requirement for normal consistency was found to increase with increasing CH content while a decrease in initial setting time was found. Furthermore, the compressive strengths of all FA mixes with CH were found to be higher than the mixes without CH. Thermal analysis and scanning electron microscope were used to study the hydration of PC–FA–CH system. The results showed that the first phase transition detected by thermal analyses was attributed to ettringite, calcium silicate hydrate, gehlenite hydrate and was found to be higher in PC–FA–CH mixes than in pure Portland–FA cement paste resulting in an increase in compressive strength. Moreover, the hydration phases were also found to increase with increasing curing time. Overall, the results show that the additional of 5 wt% CH in Portland–FA mixes especially at high-volume FA mixes was found to accelerate FA pozzolanic reaction at early ages (7 and 28 days), resulting to an increase in compressive strength.

Mechanisms of degradation of concrete by external sulfate ions under laboratory and field conditions

Abstract: The durability of concrete is a major challenge for the construction, which devotes one third to one half of its annual investment to building maintenance. The lack of field data regarding concrete durability, especially in the case of exposure to sulfate ions ("sulfate attack") makes it difficult to determine the appropriate test methods and performance criteria. Additionally, the increased use of blended concretes (cement with mineral admixtures) suffers from a lack of experience regarding their long-term performance. Most results for sulfate resistance are derived from accelerated laboratory tests where performance criteria are based only on macroscopic properties, especially expansion. To fill this gap and better understand the mechanisms of sulfate attack under real conditions, a parallel study of laboratory micro-concrete and field concrete samples under sulfate exposure was undertaken, focussing on microstructural changes in addition to the typical macroscopic characterisation. Four exposure regimes were designed in the laboratory: full immersion (traditional test in "ponding"), pH-control, semi-immersion and wet/dry cycles. Pure Portland blends and slag blends with high level of slag replacement (70 wt.-%) were investigated. The exposure regime has been found to play a main role in the damage process. In ponding conditions, the damage process takes place three stages characterised by a first period of induction, followed by surface damage that finally extends to the bulk of the material. Paradoxically, the w/c-ratio does not seem to have much impact on the ionic transport phenomena but might be more decisive in the microstructure mechanical strength against local stresses. The slag blends, considered as sulfate resistant in ponding exposure, revealed bad performance under wet/dry cycles. This behaviour was attributed to poor proper physical resistance of the slag hydrates to the applied drying. Field observations tend to confirm the laboratory results and validate the test settings. It has been underlined that a direct relationship between the damage (e.g.; cracking/expansion) and the phase assemblage was not evident. However, the study highlights that sulfate combination with the hydrates of cement (e.g.; C-S-H) and to those of slag was found to play a role in the initiation of expansion, which would be initiated either by a swelling of the hydrates or by the precipitation of fine ettringite when the saturation level in sulfate of the hydrates has been reached.

Fly ash -limestone ternary composite cements: synergetic effect at 28 days

Abstract: During cement production large amounts of CO2 are emitted, about 1 tonne CO2 per tonne clinker, if no measures are taken. About 40% originates from fuel combustion, grinding and other operations, and 60% from the de-carbonation of limestone to form the clinker phases. One way to reduce these emissions on the short term is by replacing part of the clinker with other materials such as slag, limestone powder, fly ash, silica fume and natural pozzolans. The type of replacement materials used depends on their availability (e.g. amount available, price and transportation) and is therefore dependent on the geographical location of the cement plant. The aim of this study is to contribute to the development of a novel all-round Portland composite cement for the Norwegian market. When this study was started, the cements produced at the Norwegian cement plants were: CEM I Portland cements containing up to 5% limestone powder and CEM II/A-V Portland fly ash cements containing up to 18% fly ash but no limestone powder. In this study, the effect of increasing the replacement levels of the ordinary Portland cement (OPC) (up to 35% replacement), and combining siliceous fly ash (FA) and limestone powder (L) to replace OPC are investigated. Using a combination of fly ash and limestone to replace OPC seems to be better than using only one of them. Limestone powder accelerates the early hydration more than fly ash, but fly ash contributes to strength development at later ages due to its pozzolanic reaction. Additionally a chemical interaction between fly ash and limestone has been observed, first in simplified cementitious system and later also in Portland composite cement. Limestone powder interacts with the AFm and AFt phases formed during the hydration of OPC. At first, ettringite forms during the hydration of OPC. When all gypsum is consumed, ettringite will react with the remaining aluminates and form monosulphate. In the presence of limestone, hemi- and monocarboaluminate are formed instead of monosulphate. The ettringite does, therefore, not decompose. This leads to higher volume of the hydrates, which on its turn might reduce the porosity and enhance the compressive strength. The effect of limestone powder on OPC is limited due to its low aluminate content. However, when part of the OPC is replaced by fly ash, the fly ash will introduce additional aluminates to the system as it reacts. This will lower the SO3/Al2O3 and increase the AFm/AFt ratio and thereby amplify the impact of limestone powder. These changes in the AFm and AFt phases have been experimentally observed by TGA, XRD and EDX, and predicted using thermodynamic modelling. Only a few percent of limestone powder are required to prevent ettringite from decomposing to monosulphate. The changes in hydration products resulting from these small limestone powder contents coincides with an increase in compressive strength. Replacement of 5% fly ash with 5% limestone powder in a 65%OPC+35%FA cement resulted in a compressive strength increase ranging between 8 and 13% after 28 days of curing. At higher limestone contents the compressive strength decreases again as the additional limestone mainly serves as an inert filler. Replacing 5% of OPC with limestone powder resulted, on the other hand, in a strength reduction or a slight increase up to 4% after 28 days of curing. The beneficial effect of limestone is maximal at 28 days, and reduces slightly upon further curing. It is furthermore valid at 5, 20 and 40°C. However, at 40°C the fly ash reaction is accelerated and over time the fly ash content is more important than the synergetic effect. The observed increase in compressive strength has to be partly due to the chemical interaction described above as an inert filler (crystalline quartz) with a similar psd does not have the same beneficial impact on strength as limestone. Additionally, the presence of limestone powder does not seem to affect the reactivity of OPC and fly ash significantly. The observed effect between fly ash and limestone enables higher replacement levels than when only one of them is used. The applicability of the study is demonstrated by the fact that cement with the optimal composition found in this study (65%OPC+30%FA+5%L) has recently been used in the construction of the Meteorological Centre in Oslo and the Science Centre in the county of Ostfold

Heat evolution in hydrating expansive cement systems

Abstract: Calorimetry was applied to follow the hydration of special cement mixtures exhibiting expansion or shrinkage compensation. The standard, common cements show generally less or more visible shrinkage on setting and hardening but mixed with and expansive agent, usually of aluminate and sulfate nature, they can exhibit the increase of volume. The calcium aluminate cement CAC 40 was ground together with special sulfate–lime sinter to produce an expansive additive to Portland cement (CEM I 42.5R). The expansive additive in the environment of hydrating cement transforms into ettringite at “right time” to give expansion before the final setting and hardening takes place. In the experiments the proportions of components of expansive mixture and basic cement were variable. The rate of hydration versus time for common cements is commonly known and reflects the moderate setting and early hardening during the first days after mixing with water (two peaks and the induction period between them). The aim of measurements presented in this study was to show the course of heat evolution curve and the heat evolved values, equivalent to the acceleration/retardation of hydration, in case of the paste with the expansive mixture, as well as the pastes produced from Portland cement and the components of expansive additives added in variable proportions. It was possible to see how the calorimetric curve and consequently the hydration process itself declines from the controlled setting/hardening. These measurements were supplied by the examples of phase composition studies by XRD.

Forensic Investigations to Evaluate Sulfate-Induced Heave Attack on a Tunnel Shotcrete Liner

Abstract: This paper presents the results of a comprehensive research study to determine the potential causes for an inordinate distress developed on a shotcrete liner material of a tunnel located near Dallas, TX. This tunnel was originally founded on a limestone material. Distress locations were identified where possible delamination of shotcrete layer and moisture leaks were either suspected or noticed. As a part of the research, rock cores and white powderlike substance behind the liner were collected around the distressed locations, and these cores and powder material were subjected to chemical, mineralogical, and engineering tests to understand the potential causes of this distress. Mineralogical tests, in particular, X-ray powder diffraction analysis on a powder material and gel-like substances collected on the liner, showed the presence of anhydrite, gypsum, and ettringite traces. High amounts of sulfate measurements in chemical and energy dispersive X-ray spectroscopy studies also showed that both gypsum and ettringite formations were possible in and around the limestone material. Upon hydration, mineral expansion of ettringite and anhydrite led to heaving and subsequent cracking of the adjacent shotcrete layer. Engineering characterization tests including unconfined compression strength (UCS), indirect tensile strength (ITS), and triaxial tests on rock cores embedded with a powder type sulfate material revealed that low strength cores were obtained near high distress zones and high strength cores were collected at low distress zones. The UCS values ranged from 6.2 (high distress) to 13.8 MPa (low distress) whereas the ITS values of the cores varied from 0.5 to 1.1 MPa for the same distress locations. This indicates the potential loss of strength of these rock materials from the presence of gypsum material in them. Possible methods to mitigate this heaving problem behind the liner are also discussed.

Boron Uptake Behavior During Ettringite Synthesis in the Presence of H3BO3 and in a Suspension of Ettringite in H3BO3

Abstract: Exchange between the SO42− ions in ettringite and borate is believed to occur during borate removal from wastewater by the Ca(OH)2–Al2(SO4)3 addition method. To investigate borate uptake by ettringite, we synthesized ettringite in the presence of H3BO3 and prepared suspensions of ettringite in H3BO3 solution. At the theoretical Ca(OH)2 / Al2(SO4)3 ratio for ettringite, with an increase in the H3BO3 / Al2(SO4)3 ratio, the ettringite concentration decreased but the concentration of amorphous compounds increased; further, the amount of borate uptake by the precipitate formed also increased. Upon alkalization of the H3BO3–Al2(SO4)3 solution to pH = 10 with NaOH solution, borate-containing amorphous Al(OH)3 was precipitated. It was proposed that the borate coprecipitated along with amorphous Al(OH)3 (which showed boron uptake) and did not undergo exchange with SO42− ions in ettringite; this was strongly supported by the results of IR and TG analyses performed on the boron-containing precipitates. At a high H3BO3 / Al2(SO4)3 ratio, ettringite decomposed to afford amorphous Al(OH)3, and borate coprecipitation was promoted. Upon suspension in H3BO3 solution, the ettringite precipitate showed low boron uptake, which was presumably due to the substitution of SO42− in ettringite with B(OH)4−.

Deterioration mechanisms of sulfate attack on concrete under alternate action

Abstract: By micro- and macro-observations, the deterioration mechanisms of concrete under alternate action between repeated sub-high temperature/cooling by water and sodium sulfate solution attack (TW-SA) were studied; meanwhile, the single sodium sulfate solution attack (SA) was also done as comparison. Micro-observations included the analysis of attack products by thermal analysis method and the determination of sulfate-ion content from surface to interior by chemical titrating method (modified barium sulfate gravimetric method). Macro-observations mainly included the mechanical behaviors such as compressive strength, splitting strength. The experimental results indicate, in both cases, the main attack product is ettringite, only in the first layer of case SA some gypsum is checked; in case SA, the sulfate ions mainly concentrate in the surface layer, so the attack is relatively mild; but in case TW-SA, the repeated sub-high temperature/cooling by water promotes the sulfate ions diffusing inwards, which leads to obvious strength degradation.