Delayed Ettringite Formation Research Articles

Influence of relative humidity on delayed ettringite formation

This experimental study aims to determine the effect of relative humidity on concrete expansion due to delayed ettringite formation (DEF), which still remains imperfectly known. Test methods were designed to trigger reaction in concrete specimens and determine how ambient RH affects expansion of the specimens. Saturated salts solutions were used to achieve distinct ambient RH conditions within containers. Also, chemical analysis was performed to determine and quantify the fractions of alkalis leaching out of the pore solutions of concretes. Results showed that the critical ambient RH value that corresponds to significant suppression of DEF could be greatly dependent on the alkalis fractions leached out. Finally, the consequences of swelling due to DEF on the concrete properties are assessed. After swelling, both of gas permeability and porosity was increased significantly and dynamic modulus was reduced by about 45%.

The effect of delayed ettringite formation on fine grained aerated concrete mechanical properties

Abstract Delayed ettringite formation (DEF) is a chemical reaction with proven damaging effects on the mechanical properties of hydrated cementitious composite (concrete). Ettringite crystals can cause cracks and the widening of cracks due to pressure on the crack walls caused by the positive volume difference in the reaction. In this paper, we investigated the potential to utilise the positive volume difference in DEF in order to improve the mechanical properties of hydrated fine grained aerated concrete. Fine dispersed crystallisation nuclei, achieved by adding air-entraining agent (AEA) and by short vibration of specimens, are presented as the main requirement for such improvements. Control tests of expansion and mechanical properties were performed on samples of concrete with and without AEA by inducing DEF. The microstructure of fine grained aerated concrete was examined with an optical microscope and scanning electron microscope. We found that the controlled DEF, which is guaranteed by adding AEA and with the formation of uniformly dispersed air bubbles, which are crystallisation sites for ettringite crystals, improves the mechanical properties. The specimens with induced DEF were measured and found to have a 6.8% increase of compressive strength.

Evaluation of the contribution of boundary and initial conditions in the chemo-thermal analysis of a massive concrete structure

The chemo-thermal response at early ages is an important question in the design of massive concrete structures since it affects directly the mechanical response and is related to durability issues such as Delayed Ettringite Formation (DEF). This response is largely dependent on both environmental and initial conditions of the structures. The goal of this paper is then to predict the contribution and the relative importance of the solar radiation, convection, reradiation and conduction on the chemo-thermal response of massive concrete structures at early-age. An approximate solution of the 1D heat equation, with boundary conditions accounting for the solar flux, convection and reradiation, in a semi-infinite domain, is proposed. This solution is used to define scenarios for 3D numerical simulations with more complex geometry and boundary conditions. A particular massive structure for nuclear waste disposal composed of walls and slab having thicknesses ranging from 0.7 to 0.85m, and for which the evolution of the temperatures was experimentally determined, is studied. The results highlight (i) the competition effects of convection and reradiation with the solar radiation; (ii) the determination of concreting temperatures in order to prevent DEF problems; and (iii) the influence of assembly dates of walls and slabs on the chemo-thermal response. The influence of model definitions such as the use of average or daily varying temperature and daily varying solar fluxes are also studied. It is shown that except in the case of ambient temperatures touching the maximum ambient temperature attained within a year (33°C for the case studied), the maximum temperature reached within the structure does not exceed the maximum admissible temperature of 70°C.

FACTORS AFFECTING DEF AND ASR IN THE CONCRETE DAM AT VRANÉ NAD VLTAVOU

Two degradation mechanisms, delayed ettringite formation (DEF) as well as the alkali-silica reaction (ASR), were investigated in the concrete dam at Vrane nad Vltavou, CZE with an aim to: (1) find those aggregates causing the degradation of the concrete; (2) define those parts of the concrete most affected by the degradation; and (3) correlate the water attack with the extent of degradation. The following methods were used: measurement of residual expansion of the concrete (following the CSN 72 1179 standard), optical microscopy, petrographic image analysis, and scanning electron microscopy combined with energy dispersive spectrometer (SEM/EDS). Eight different sampling points were selected according to the cracks observed on the concrete's surface. DEF was found to be the most important degradation mechanism in the concrete samples; ASR and calcite-rich rims are regarded as minor. Influence of freezing/thawing degradation is also possible. The highest residual expansion was measured on one concrete sample periodically washed by water (this sample taken from an area one meter above the water level). The same sample shows the highest content of ettringite and microcracks, as well as minor contents of calcite-rich rims and alkali-silica gels. All samples that were taken below the water level as well as samples without any contact with water, showed lower contents of ettringite and microcracks, and very low (or no) contents of alkali-silica gels. The highest degree of degradation by DEF, freezing/thawing and ASR is explained by the following factors: (1) position of sampling points one meter above the water level; and (2) coarse, as well as fine aggregates that are mainly composed of acid volcanic rocks (alkaline rhyolite, tuff and tuffite).

SOME PROBLEMS WITH PRESTRESSED CONCRETE SLEEPERS DURABILITY

In the paper, the authors presented the hazards of concrete sleepers linked with delayed ettringite formation (DEF) as well as the need to maintained of low alkalis content in cement. The authors also discussed the advantages of mechanical anchorage in sleepers, applied in the BBRV technology. This method is based on the system of smooth bars anchorage, on the ends of the strings with higher dimensions and is realized with the anchorage plates. It prevents of microcracks formation after tension release.

Performance of RC Columns Affected by ASR. I: Accelerated Exposure and Damage

AbstractIn Texas, a number of RC bridges have developed early cracking. Most of this deterioration has been identified or suspected to be from alkali-silica reaction (ASR), and in some cases from delayed ettringite formation (DEF). An area of concern for the Texas DOT is the performance of columns, and specifically their lap-splice regions, which have varying levels of cracking primarily from ASR but also possibly from DEF. Therefore, a research program was carried out (1) to evaluate the experimental behavior of a typical column using large-scale specimens under varying levels of premature concrete deterioration and (2) to develop an analytical model that can evaluate the behavior of the column based on calibration with the experimental behavior. This paper provides a brief overview of ASR and DEF; documents the design, construction, curing conditions, and instrumentation of 16 large-scale specimens; and presents the findings on the specimen dimensional changes and cracking that occurred during the deter...

Properties of gypsum-free Portland cement

Calcium sulfate is the common set regulator in Portland cement. It is shown that ground clinker can be set regulated by other soluble calcium salts than gypsum. Rheology of paste is improved when gypsum is replaced by the other salts, due to less hydration and less crystals of acicular shape. For in particular calcium nitrate the cumulative hydration energy is reduced under isothermal conditions, which opens up for low-energy cement without changing clinker chemistry. The effect is confirmed by temperature profiles of mortar curing under semi-adiabatic conditions. Substituting calcium sulfate may also allow heat curing, or allow temperature in massive structures to exceed 70 °C, without risking delayed ettringite formation. Mortar based on clinker with 3.5% gypsum and 2.0% calcium nitrate, respectively, was heat cured at 80 °C for 3 days. Linear expansion vs. time is steadily increasing for mortar with gypsum, while mortar with calcium nitrate is dimensionally stable.

Review of Properties of Concrete , 5th Ed., by A. M. NevillePrentice Hall, Pearson, San Francisco, CA; 2012; ISBN 0273755803 and 978-0273755807; 846 pp.; $160 (paperback).

Review of <i>Properties of Concrete</i> , 5th Ed., by A. M. NevillePrentice Hall, Pearson, San Francisco, CA; 2012; ISBN 0273755803 and 978-0273755807; 846 pp.; $160 (paperback).

Effect of a Modified Pozzolan on the Sulphate Attack Resistant of Mortar

DOI: 10.7763/IJET.2014.V6.665  Abstract—Durability of mortar/concrete is a vital property that affects its serviceability. One of the factors that affect concrete durability is sulphate attack, which can result to expansion, cracking, deterioration, and deformation of concrete structures. The effect of a modified pozzolan (PWC) on the sulphate attack resistant of mortar was investigated in this study. The additive, a blend of selected alkaloids and zeolite, is commercially available and effectively used in soil stabilization for road construction. The additive pozzolanic behaviour had been observed in previous study in terms of strength and permeability. However, the present study focuses on its resistant to sulphate attack as a pozzolan. PWC additive was used in the proportions of 0 %, 0.4 %, 0.6 % and 2.5 % by weight of cement. Samples were subjected to both internal and external sulphate attack tests, according to ASTM C1038 and ASTM C1012 respectively, after being cured for 14, 28, 60, 90, 120, 180, and 295 days. The results showed that PWC additive when used at low dosages of 0.4 % and 0.6 % PWC causes reduction in expansion due to external sulphate attack. At higher dosages of PWC additive, greater external expansion occurs when compared to control samples. return have contact with concrete. The reaction will change the composition and microstructure of the concrete to which it has contact. The effect of the change might result to external cracking, expansion or loss of bond between the cement paste and aggregate. The general effect of the changes will be loss of concrete strength. Internal sulphate attack occurs when source of sulphate, for example, sulphate-rich aggregate or excess gypsum content is incorporated during concrete mix. The sulphate phases transformation of over-sulphated cement is from anhydrite to gypsum and ettringite, and then to thaumasite. This will finally result to concrete mush (6). Delayed ettringite formation (DEF) is a form of ettringite that occurs in hardened concrete that has been cured at elevated temperature and contains excess sulphate, which reacts with calcium- and aluminium- containing phases of the cement paste and leads to increase in volume and expansion. Concrete composition, curing conditions and exposure conditions affect the potential/degree of DEF. Thaumasite form of sulphate attack requires adequate supply of sulphate and carbonate and it continues to form until the calcium silicate hydrate is completely decomposed. It is relatively unusual form of sulphate attack; it is associated with low temperatures and very wet environments (6). The oxide compositions of ettringite and thaumasite as reported by Hooton (6) are shown below:

Development of an accelerated test for Internal Sulfate Attack study

Internal Sulfate Attack (ISA) is a pathology that occurs under certain conditions in concrete having undergone heating above 70 °C at early age (through heating in pre-casting industry or due to hydration in large concrete parts). This reaction deemed very slow, numerous methods to speed up reactions leading to delayed ettringite formation have been developed. These methods are all based on the material damage. Another type of test is currently under development. It is based on rehabilitation techniques such as electrochemical chloride extraction (ECE) in order to accelerate the leaching of alkalis that could be one of the triggers of the pathology. The study presented in this paper focused on concrete specimens prepared from cement (CEM I 52.5 N) enriched with Na2SO4. These concretes have undergone a heat treatment typical of those used in precast plants (up to 24 hours with a maximum temperature of 80 °C). Various paths were explored for the development of the accelerated test. The first results showed that it was necessary to use a removable titanium anode ruthenium anode instead of stainless steel embedded in the concrete. Then tests with de-ionized water as the solute to the cathode did not accelerate the onset of expansions. The experiment has been modified and potassium carbonate was added to the solution. This modification didn’t show any significant improvement, and other experiments are being carried out to explain this result.

Coupling between mechanical and transfer properties and expansion due to DEF in a concrete of a nuclear power plant

This paper focuses on studying the consequences of expansion due to delayed ettringite formation (DEF) on transfer and mechanical properties of concrete in the case of nuclear structures. It concerns a concrete representative of a containment vessel of a nuclear power plant where temperature variations at early age are very large. An experimental heat treatment, representative of the temperature history in the raft foundation of the containment vessel was reproduced after a modeling of its temperature rise. After this treatment, concrete exhibits swelling due to the development of DEF. The gas permeability is increased significantly after swelling, and the safety requirements expected by these structures are thus affected.

The Use of Numerical Extrapolation to Accelerate Delayed Ettringite Formation Tests

Abstract Current test methods for evaluating the potential for delayed ettringite formation (DEF) are quite time consuming. It is common for these tests to take over one year to complete due to the fact that there are no minimum expansion criteria at a prescribed number of days. A novel numerical extrapolation technique is presented that can predict the final expansion of the specimen after expansion starts. The accuracy of the method is shown for 30 tests from laboratory and field concrete with expansion from DEF. The technique was able to accurately predict the ultimate expansion within 0.18 % and shorten the test by over 80 %. This time savings would be especially useful for owners who need to evaluate the potential for DEF expansion for an existing structure.

Early carbonation for hollow-core concrete slab curing and carbon dioxide recycling

Early carbonation curing was developed for hollow-core concrete slab production to partially replace heat curing and recycle carbon dioxide recovered from the cement kiln. The cavities in hollow core slab were utilized as a CO2 gas chamber to facilitate a carbonation curing at a low gas pressure. Concrete performance was evaluated by carbon dioxide uptake, compressive strength, pH values and resistance to air permeation. The microstructure was examined by X-ray diffraction patterns, thermogravimetric analysis and scanning electron microscope. It was found that early carbonation curing could reduce the conventional heat curing duration and produce concrete with higher strength and lower permeability due to the precipitation of calcium carbonates on surface. The pH value of the carbonated concrete at the reinforcement could be kept above the corrosion threshold through a subsequent hydration and the delayed ettringite formation owing to heat curing was totally eliminated by early carbonation. The process offers economical, technical and environmental benefits.

Importance of considering the coupling between transfer properties, alkali leaching and expansion in the modelling of concrete beams affected by internal swelling reactions

Alkali Aggregate Reaction (AAR) and Delayed Ettringite Formation (DEF) cause expansion of the affected concrete that generally leads to cracking and decrease of its mechanical properties. Consequently, these pathologies raise severe problems regarding serviceability, sustainable operation and structural integrity. Thus it is necessary to provide predictive models able to re-assess the mechanical state of the affected structures. Since cracking may alter diffusion properties of concrete, and since water and alkali content affects AAR and DEF progress, the necessity of taking full coupling of damage and water transfer is questioned for proper structural reassessment. Documented structural tests help evidence this necessity.

A nonlinear meso–macro approach to modelling delayed ettringite formation and concrete degradation

We show how a two-scales FE model—within a sequential approach—leads to consistently simulate the DEF macroscopic consequences: cracks pattern and opening, residual modulus decrease. The mesoscopic part of this model is adapted to heterogeneous materials and is based on two kinematics enrichments. The first one leads to a very fast meshing process along the randomly chosen set of aggregates and, the second leads to directly compute the cracks openings. Assuming DEF is driving to a progressive and homogeneous mortar expansion, we show how the macroscopic—concrete—expansion as well the cracks pattern are consistent with the experiments. For the latter we are interested in the characterization of the aggregates effect on the amplitude of DEF swelling, so that we focus on parameters such as the size and the volume fraction of granular inclusions.

The effect of natural pozzolan on delayed ettringite formation of the heat-cured mortars

Delayed Ettringite Formation (DEF) is an internal sulfate attack caused by early age heating to a temperature of over 70°C. In this paper, the effect of natural Algerian pozzolan on the expansion of cement mortars caused by DEF was investigated. For this purpose, a portion of cement was replaced by natural pozzolan, with three different dosages (10%, 20% and 30%) and finenesses (fine, medium, and coarse). The results obtained highlighted the significant effects of natural pozzolan on DEF and the correlation existing between the expansion of the heat-cured mortars and the evolution of their mechanical properties.

Potential reduction of concrete deterioration through controlled DEF in hydrated concrete

Delayed ettringite formation (DEF) is a chemical reaction with proven damaging effects on hydrated concrete. Ettringite crystals can cause cracks and their widening due to pressure on cracked walls caused by the positive volume difference in the reaction. Concrete may show improvements in strength at early ages but further growth of cracks causes widening and spreading through the concrete structure. In this study, finely dispersed crystallization nuclei achieved by adding air-entraining agent (AEA) and short vibration of specimens is presented as the main prerequisite for reducing DEF-induced deterioration of hydrated concrete. The study presents the method and mechanism for obtaining the required nucleation. Controlling long-term DEF by providing AEA-induced crystallisation nuclei, prevented excessive and rapid initial strength improvements, and resulted in a slight increase of compressive strength of fine grained concrete with only marginally lower density.

Five year monitoring of curing solutions of heat-cured mortars affected by delayed ettringite formation

The work presented in this paper studies the leaching of heat-cured mortars affected by delayed ettringite formation (DEF). For this, the expansion and the leaching of various elements (OH−, Ca2+, Na+ and K+) were monitored for more than 5 years in three mortars: a reference mortar and two compositions in which 3·11% of Na2SO4 (sodium sulfate) and 1·75% of NaOH (sodium hydroxide) were added to the mixture. The four mortars expanded but with different kinetics. pH, conductivity and leaching of calcium had the same global variation: a fast decrease during the first few days of curing, a strong increase after the beginning of the expansions and, lastly, a plateau of several months followed, or not (depending on the mixtures and the progress of the reaction), by a strong decrease in leaching. Finally, the results suggest that the reasons for DEF are not only related to the leaching of alkalis.

Kinetic model to predict cement susceptibility to delayed ettringite formation. Part 1: Theoretical concept

A model is proposed for predicting the intrinsic potential of Portland cements to exhibit delayed ettringite formation, when used in cementitious systems. The model has been developed on the basis of kinetic reaction theory and consists of a set of two equations which define the primary conditional requirements on cement chemistry. The important conditions – sufficient alumina content exceeding a certain unknown threshold and sulfite/aluminate (SO3/Al2O3) ratio high enough to promulgate ettringite infilling pressures over and above counter-microstructural factors – are mathematically expressed. Both conditions have been reported in the literature by various authors based on experimental observations. The proposed model can be a useful screening tool for rapid evaluation of cements against their proneness to delayed ettringite formation and may be applied in cement manufacture, import depots, ready-mix plants and many others, or for the selection of cements for specific applications. Owing to the intrinsic nature of the model, it has the potential to reveal the underlying expansive behaviour that may be suppressed or not easily observed from experimental tests alone. Validation of the model against experimental data from the literature has been undertaken in Part 2 of this work.

Kinetic model to predict cement susceptibility to delayed ettringite formation. Part 2: Model validation and application

This work is a continuation of the development of a model for predicting the potential of Portland cements to exhibit expansion due to delayed ettringite formation (DEF). The model was derived based on kinetics theory applied to cement chemistry and evaluates two primary conditions – the presence of sufficient active alumina in cement and the requirements for sulfate-to-alumina ratio applied against the countering microstructural effects – to determine the underlying kinetic characteristics for DEF in the system. The primary purpose of the model is determination of the intrinsic presence of the potential for DEF in cements. In this second part of the work, the model is validated and tested for robustness by its application to a wide range of existing experimental data for DEF studies, taken from various global sources available in the literature. Over 169 data sets taken from various independent works were used in the validation and the model was found to be in strong agreement with experimental data for both expansive and non-expansive cements. However, a few outputs showed disagreement between model predictions and experimental observations. Most of the important disparities appear to occur with cements of high Fe2O3/Al2O3 ratios and may be related to the inadequacy of the Bogue formula used in the model. These aspects reveal where future refinement of the model is required. The model at this stage is applicable to plain Portland cements with a future possibility of improvements to examine more complex systems containing pozzolanic materials.