Effect of ettringite on thaumasite formation

Deterioration of cementitious building materials is often caused by sulphate attack where ettringite and gypsum play the most destructive role at moderate ambient temperatures. In contrast, thaumasite [Ca₃Si(OH)₆·12H₂O](SO₄)(CO₃) is mostly observed at comparatively low temperatures of less then 15 °C. This mineral forms from calcium, sulphate, carbonate and silicon. The latter originates from the decomposition of C-S-H which results in deterioration of the hardened cement paste structure. To investigate the effect of ettringite on thaumasite formation, pastes were mixed using synthetic clinker phases, fly ash and nanosilica. Aqueous suspensions were prepared with the ground-hydrated pastes mixed with calcite and either gypsum or sodium sulphate. Following different storage periods, the solid phase was separated by filtration, dried and analysed by XRD using the Rietveld method as well as ESEM and TEM. The liquid phase was analysed by ICP-OES. The results indicate that thaumasite formation occurs through the heterogeneous nucleation of thaumasite on the surface of ettringite, due to the structural similarities of these minerals. This reaction is followed by further epitaxial growth of thaumasite from its components present in solution.     

Contribution to the hydration of expansive cement on the basis of metakaolinite

The study of hydration of expansive cement prepared from 64% portland cement clinker, 23% metakaolinite and 13% CaSO₄.2H₂O is described. It was found that in the course of a 10-day hydration period, all the gypsum entered the reaction with the formation of ettringite. In 7–10 days, after the termination of the expansion processes, typical stalk-like crystals were transformed into leaf-shaped or other formations. Ettringite was identified even after 4 months of hydration. Monosulphate (3CaO.Al₂O₃.CaSO₄.12H₂O) was found in none of the investigated high-expansion cement paste samples.     

Effect of cement C3A content, temperature and storage medium on thaumasite formation in carbonated mortars

The purpose of this study is to determine the effect of cement C₃A content, temperature and composition of the immersion medium (water, gypsum and magnesium sulphate solution) on the rate of thaumasite formation in cement mortars. It also aims to ascertain how the C₃A content influences the composition of the salt formed.The mortar prisms for this study were made with two different cements, one with low and the other with high Al₂O₃ content, with or without gypsum and/or calcium carbonate. After hydration, curing and carbonation, the prisms were partially immersed in distilled water and stored at temperatures ranging from 0 to 5 °C for up to 5 years. Some of the prisms were immersed in a 2% (w/w) gypsum solution or in 1.4% (w/w) magnesium sulphate solution at ambient temperature. Samples were taken at different ages and mineralogical and micro-structurally characterised.Some of the specimens tested were observed to expand, in a process concurring with the formation of thaumasite or a solid solution of thaumasite and ettringite, at both ambient and cooler temperatures. A correlation was found between cement C₃A content and the composition of the deterioration product involved in the expansive process: thaumasite forms in mortars made with low C₃A cement, whereas mixed crystals or solid solutions of thaumasite and ettringite form in mortars made with high C₃A content cement.     

Mineralogical and engineering characteristics of dry flue gas desulfurization products

Fifty-nine coal combustion products were collected from coal-fired power plants using various dry flue gas desulfurization (FGD) processes to remove SO₂. X-ray diffraction analyses revealed duct injection and spray dryer processes created products that primarily contained Ca(OH)₂ (portlandite) and CaSO₃·0.5H₂O (hannebachite). Most samples from the lime injection multistage burners process contained significant amounts of CaO (lime), CaSO₄ (anhydrite), and CaCO₃ (calcite). Bed ashes from the fluidized bed process were often dominated by CaSO₄ but also contained CaCO₃, CaO (lime), and MgO (periclase). Cyclone ashes were similar in composition to the bed ashes but contained more unspent sorbent and CaSO₄ and less MgO. Fly ash in all samples ranged from 10 to 79 wt%. Samples usually exhibited two distinct swelling episodes. One occurred immediately after water was applied due to hydration reactions, especially the conversion of CaO to Ca(OH)₂ and CaSO₄ to CaSO₄·2H₂O (gypsum). The second began between 10 and 50 d later and involved formation of the mineral ettringite (Ca₆[Al (OH)₆](SO₄)₃·26H₂O). The final pH after 112 d ranged from 10.0 to 12.1. If samples are incubated under ‘closed’ (i.e. incomplete recarbonation with atmospheric CO₂) and alkaline weathering conditions, gypsum and portlandite are initially formed followed by the conversion of the gypsum to ettringite. Closed, alkaline conditions typically can occur when FGD products are placed in confined settings such as a road embankment or buried as a discrete layer as occurs in some surface mine reclamation projects.     

X-ray absorption fine structure spectroscopy and X-ray diffraction study of cementitious materials derived from coal combustion by-products

Cementitious materials derived from coal combustion by-products have been investigated by means of X-ray diffraction (XRD) and S and Ca K-edge X-ray absorption fine structure (XAFS) spectroscopy. The XRD analysis revealed that these materials are a complex mixture of a small amount of quartz [SiO₂] and three calcium-bearing compounds: hannebachite [CaSO₃·1/2H₂O], gypsum [CaSO₄·2H₂O] and ettringite [(Ca₆(Al(OH)₆)₂(SO₄)₃·26H₂O)]. Analysis of the S XAFS data focused on deconvolution of the X-ray absorption near-edge structure (XANES) regions of the spectra. This analysis established that sulfate and sulfite are the two major sulfur forms, with a minor thiophenic component contained in unburned carbon in the fly ash. Increasing sulfate and decreasing sulfite correlated well with increasing gypsum and ettringite and decreasing hannebachite content in the samples. Different calcium compounds were identified primarily through simple comparison of the Ca K-edge XANES and radial structure functions (RSFs) of the cementitious samples with those of reference compounds. Because of the complex coordination chemistry of calcium in these materials, it was difficult to obtain detailed local atomic environment information around calcium beyond the first CaO peak. Analysis of the extended X-ray absorption fine structure (EXAFS) and the RSF gave average CaO distances in the range 2.44-2.5 Å, with each calcium atom surrounded roughly by eight oxygen atoms. In certain samples, the average CaO distances were close to that in ettringite (2.51 Å), suggesting that these samples have higher ettringite content. The results of S and Ca K-edges XAFS and the XRD data were in reasonable agreement.     

Anion water in gypsum (CaSO4·2H2O) and hemihydrate (CaSO4·1/2H2O)

The bonding nature of water in gypsum, CaSO₄·2H₂O, and hemihydrate, CaSO₄·1/2H₂O, was suggested by characteristic absorption bands of water and sulphate ions. The infrared spectral data indicate the presence of anion water in gypsum and hemihydrate through hydrogen bonding. Negative shifting of bending vibration of SO₄ ion and lowering and broadening of the O-H stretching vibration at around 3600 cm⁻¹ indicate the presence of both anion water and hydrogen bonding in gypsum and hemihydrate     

Stability and reactivity of thaumasite at different pH levels

Thaumasite (CaCO₃·CaSO₄·CaSiO₃·15H₂O) has been reported to form at low temperatures (below 15 °C) during sulfate attack. Reactions between calcium silicate hydrate (C-S-H) and Ca⁺², CO₃⁻², SO₄⁻², CO₂ and water, or between ettringite and C-S-H, CO₃⁻² and/or CO₂ and water, result generally in the formation of thaumasite. In some instances, thaumasite may be affected by the presence of other chemicals in the surrounding environment (i.e., phosphates and ammonia in agricultural soil). There are insufficient data regarding the stability of thaumasite at different pH levels in the presence of other chemical ions. Understanding this issue might help in the detection of the thaumasite form of sulfate attack, and, therefore, in one's choice of the appropriate protection technique. This work reports the reactivity of thaumasite with phosphate, carbonate and bicarbonate ions at different pH levels ranging from 6.00 to 12.00, as well as the stability of thaumasite at high pH levels (greater than 12.00). Thaumasite was found to react with these ions at pH levels at and below 12.00; however, thaumasite was stable with minimal reactivity at pH levels greater than 12.00.     

Thaumasite formation in a tunnel of Bapanxia Dam in Western China

A site investigation and sampling was carried out on a sulfate-attacked concrete structure in Bapanxia Hydraulic Power Plant in Western China. The concrete had been exposed to ground water containing substantial concentrations of salts (SO₄²⁻, CO₃²⁻ and Cl⁻) for about 6 years and was analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), laser-Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. It is shown that a white mushy mixture consisting of thaumasite, ettringite, gypsum and calcite is present in the residual concrete. This paper reports the first instance of the thaumasite form of sulfate attack of concrete in China.     

Thausamine in failed cement mortars and renders from exposed brickwork

The mineral thaumasite occurs within certain cement-based building materials as a direct result of sulphate attack. It readily forms within certain types of brickwork and not only contributes to expansive cracking of the brickwork but is also accompanied by severe softening of the cement matrix. Samples were taken of both brickwork mortars and renders, some of which were still relatively sound whereas others had deteriorated into a paste. The mortar samples were investigated using a quantitative Xray diffraction technique in order to determine the amounts of thaumasite, ettringite and gypsum present. Results show that if conditions are favourable then thaumasite formation can proceed rapidly and can even result in the complete breakdown of very strong renders.     

Mineralogical and chemical assessment of concrete damaged by the oxidation of sulfide-bearing aggregates: Importance of thaumasite formation on reaction mechanisms

Damages in concrete containing sulfide-bearing aggregates were recently observed in the Trois-Rivières area (Quebec, Canada), characterized by rapid deterioration within 3 to 5 years after construction. A petrographic examination of concrete core samples was carried out using a combination of tools including: stereomicroscopic evaluation, polarized light microscopy, scanning electron microscopy, X-ray diffraction and electron microprobe analysis.The aggregate used to produce concrete was an intrusive igneous rock with different metamorphism degrees and various proportions of sulfide minerals. In the rock, sulfide minerals were often surrounded by a thin layer of carbonate minerals (siderite). Secondary reaction products observed in the damaged concrete include “rust” mineral forms (e.g. ferric oxyhydroxides such as goethite, limonite (FeO (OH) nH₂O) and ferrihydrite), gypsum, ettringite and thaumasite. In the presence of water and oxygen, pyrrhotite oxidizes to form iron oxyhydroxides and sulphuric acid. The acid then reacts with the phases of the cement paste/aggregate and provokes the formation of sulfate minerals. Understanding both mechanisms, oxidation and internal sulfate attack, is important to be able to duplicate the damaging reaction in laboratory conditions, thus allowing the development of a performance test for evaluating the potential for deleterious expansion in concrete associated with sulfide-bearing aggregates.     

Effects of gypsum formation on the performance of cement mortars during external sulfate attack

Sodium sulfate attack was studied on C₃S mortars, along with ASTM Type I Portland cement (PC) mortars, in an attempt to independently evaluate the effect of gypsum formation on the performance. The quantity of gypsum and ettringite, as measured by differential scanning calorimetry (DSC), increased with the time of immersion in the sulfate solution. An increase in length of the mortar specimens was also registered along with the increase in the quantity of gypsum. This result suggests that the formation of gypsum could be expansive. Indeed, considerable expansion, although delayed compared to PC mortars, was observed in the C₃S mortars. Thus, it can be concluded that the expansion of the PC mortars occurred due to the combined effect of gypsum and ettringite formation, while the expansion of C₃S mortars occurred as a result of gypsum formation.Thaumasite formation as small inclusions was also detected in both the C₃S and the PC mortars, especially in regions of high gypsum deposition. The formation of thaumasite, despite the absence of carbonate bearing minerals and low temperatures, could be because of the carbonation of the surface zones of the mortars. However, it would be speculative to attribute any expansion to the formation of thaumasite, since it was detected only in minute amounts in the microstructural investigation.     

Tricalcium aluminate hydration in the presence of lime,gypsum or sodium sulfate

The influence of Ca(OH)₂, CaSO₄·2H₂O and Na₂SO₄ on the C₃A hydration was examined in order to study the retardation mechanism of C₃A hydration caused by lime and/or gypsum additions. When C₃A hydrates in the presence of gypsum, the results do not confirm the retardation mechanism based on sulfate ions adsorption or C₄AH_x impervious coating. They substantially confirm the mechanism based on ettringite crystals coating C₃A grains. In the absence of gypsum C₃A hydration is retarded by C₄AH_x formation coating C₃A grains.     

Utilization of borogypsum as set retarder in Portland cement production

Boron ores are used in the production of various boron compounds such as boric acid, borax and boron oxide. Boric acid is produced by reacting colemanite(2CaO·3B₂O₃·5H₂O) with sulphuric acid and a large quantity of borogypsum is formed during this production. This waste causes various environmental problems when discharged directly to the environment. Portland cement is the most important material in the building industry. This material is produced by adding about 3-5% gypsum (CaSO₄·2H₂O) to clinker as a set retarder. The aim of this study was to stabilize borogypsum, and to produce cements by adding borogypsum instead of natural gypsum to clinker. Concrete using cement produced with borogypsum was tested to find the mechanical properties and the test values were compared with those of concrete from cement with natural gypsum. Compressive strength of concrete from cement produced with borogypsum was found to be higher than that of natural gypsum. Also, the setting time of cement with borogypsum was longer than that of the Portland cement.     

Factors influencing the reaction between phlogopite and CaSo4·2H2O plus CaO mixtures

According to earlier studies phlogopite reacts with CaSO₄.2H₂O and a mixture of CaSO₄.2H₂O and CaO exchanging 40–80% of its potassium with calcium. The same reactions occur only poorly in the case of muscovite. The structural difference between phlogopite and muscovite is small so that some unknown factors must effect the exchange reaction. In the present work the factors influencing the reaction between phlogopite and a CaSO₄.2H₂O/CaO mixture have been studied by means of X-ray diffraction and an electron microscope. A rod mill has been employed in the separation of the solid phases produced during the reaction. The results have been calculated and drawn by computer programs created for the study. The effects of gypsum, calcium oxide, iron, aluminium and magnesium have been treated in the exchange reaction. Also the poor reactivity of muscovite in the reaction conditions used has been discussed.     

Studies on the Stability of Calcium Sulphoaluminate

High calcium sulpho-aluminate (3CaO. Al₂O₃. 3CaSO₄. 31H₂O—C.S.A.) is a deterioration product of Portland cement found in concrete. It is formed by the attack of sulphate solutions on two of the Portland cement components: hydrated calcium Aluminate and lime. These phenomena fostered a study of the synthesis and stability of calcium sulpho-aluminate. Results of the experiments on stability of calcium sulphoaluminate in different media are discussed. Measures for preventing the formation of C.S.A. in Portland cement are described.     

A thermodynamic and experimental study of the conditions of thaumasite formation

The formation of thaumasite was investigated with the progressive equilibrium approach (PEA). This approach experimentally simulates the conditions of various levels of sulfate addition in hardened cement pastes. The influence of limestone, time, C₃A content, temperature and leaching on thaumasite formation was investigated. The results show that thaumasite formation is favoured at lower temperatures (8 °C) independently of the type of cement clinker (high or low C₃A content) used. Thaumasite was found to form only in systems where limestone was present and where sufficient sulfate had been added. Thaumasite precipitated only in systems where the Al present has already been consumed to form ettringite and the molar SO₃/Al₂O₃ ratio exceeded 3. In leached samples (reduction of portlandite and alkalis) slightly less thaumasite was formed whereas gypsum and ettringite are favoured under these conditions. The PEA, used to investigate the chemical aspects of sulfate attack was found to be a good tool for simulating external sulfate attack. Generally, thaumasite was detected were it was modelled to be stable in significant amounts. However, in this study equilibrium conditions were not reached after 9 months.     

Characteristics and so2 capture capacities of sorbents prepared from products of spray-drying flue gas desulfurization

The characteristics and the SO₂ capture capacities of sorbents prepared from products of spray-drying flue gas desulfurization (FGD) have been studied. Sorbents were prepared by first slurrying Ca(OH)₂ and CaSO₃ and/or CaSO₄ with and without the addition of fly ash and then drying. Compared to the use of pure Ca(OH)₂, the SO₂ capture and Ca(OH)₂ utilization decreased for sorbents prepared without fly ash and increased for sorbents with fly ash. Flakelike ill-crystallized tobermorites were observed for all the sorbents containing fly ash. In addition, significant amounts of needle-shape Ca₄Al₂(OH)₁₂SO₄ . 6H₂O and Ca₆Al₂ (OH)₁₂(SO₄)₃ . 26H₂O (ettringite) were also observed for the sorbents containing CaSO₃ and/or CaSO₄. These newly formed compounds dissociated into CaSO₃ . 0.5 H₂O and inert precursors upon sulfation, and were responsible for the high SO₂ capture capacities and Ca(OH)₂ utilizations of the sorbents prepared with fly ash.     

Selection of threshold agents for calcium sulfate scale control on the basis of chemical structure

The effects of polyvinyl sulfonate (PVS) and polyglutamic acid (PGA) on the precipitation of 2CaSO₄· H₂O and CaSO₄ · 2H₂O were compared. PGA strongly retards CaSO₄ · 2H₂O precipitation from sea water, and also significantly changes its crystal habit. PVS has a similar effect on 2CaSO₄ · H₂O precipitated at ca. l00°C. The selectivity is related to structural compatibility between the additive and the crystal lattice. A possible mechanism of heterogeneous nucleation of the two crystallographic species upon the suitable polymers is proposed. The hardening of scale in the presence of an unsuitable additive is explained within the framework of the proposed mechanism.     

Effect of trace impurities on calcium sulphate precipitation

Induction periods for the precipitation of CaSO₄.2H₂O in the concentration range 0.05-lM have been determined by the stopped-flow method utilising changes in the solution conductivity after nucleation and growth. The measured induction periods were insensitive to calcium sulphate concentration, suggesting that the nucleation process was heterogeneous. At first additions of Al³⁺ increasingly prolonged the induction period, but above a certain Al³⁺ concentration further additions caused the induction period to reduce again. Tentative explanations based on the poisoning of growth sites by [Al(H₂O)₆]³⁺ complexes for the first effect and the deaquation of aluminium complexes followed by formation of polymeric species for the second are given. It is suggested that the small crystallites which appeared on the surfaces of the gypsum crystals precipitated in the presence of relatively high fluoride concentrations are CaF₂.