The effects of chemical environment on the nucleation, growth, and stability of ettringite [Ca3Al(OH)6]2(SO4)3·26H2O

Ettringite is responsible for both the initial set of Portland cement and for premature concrete deterioration. A new method of ettringite crystal growth by combining calcium hydroxide and aluminum sulfate solutions was devised to reliably produce crystals that could be seen with a light microscope (45×-320×). The nucleation, growth, morphology, and stability of ettringite in the presence of over 300 chemicals and admixtures, many of which are present in the concrete environment, was then investigated. The plasticizers sorbitol, citrate, and tartrate were found to inhibit ettringite nucleation and growth, as did certain lignosulfonate air-entraining admixtures. The Type B set retarder borax inhibited ettringite formation at <44 ppm. The consequences and implications of this are discussed.     

Stress from crystallization of salt

The thermodynamic and kinetic factors influencing crystallization pressure are reviewed for cases including capillary rise and evaporation, cyclic wetting and drying, and hydration of cement. Under equilibrium conditions, where the crystal is surrounded by a film of solution, high stresses are expected only in small pores, but when that film is discontinuous (as may occur during drying), high stresses can arise even in large pores. High crystallization pressure requires a substantial supersaturation of the pore liquid. In the case of sodium sulfate, supersaturation results from the difference in solubility between the anhydrate and decahydrate phases; for ettringite, supersaturation may develop following the cooling from elevated temperatures. During the hydration of Portland cement, crystallization pressure may result from the growth of ettringite and/or calcium hydroxide.     

Water chemical potential: A key parameter to determine the thermodynamic stability of some hydrated cement phases in concrete?

The CaO-Al₂O₃-SO₃-H₂O system at 25 °C under 1 bar of pressure has been investigated with phase diagrams software (Zen + k) based on chemical potentials (or activities). The reported invariant points are similar to those obtained previously using equilibrium calculations. However Zen + k enables us to calculate systems at different relative humidities, and in conditions where water is not in excess, calcium monosulfoaluminate could be a stable phase and thus, as observed experimentally, remain for long times in an ordinary Portland cement paste.     

Composite systems fluorgypsum-blastfurnance slag-metakaolin, strength and microstructures

The hydration and properties of composite cementitious pastes with 75% fluorgypsum were investigated; blastfurnace slag and metakaolin were the complementary cementitious materials. The pastes were cured under water at 20 °C for 360 days. All pastes developed and maintained strength under water, except those of commercial gypsum. The addition of metakaolin had a positive effect, after 360 days compressive strengths of 13.4, 13.8 and 14.6 MPa were registered for systems with 0%, 5% and 10% of metakaolin, respectively. The microstructure of the composite pastes was formed of a framework of gypsum crystals, which formed in the initial stages; the matrix was later densified by the formation of C-S-H and ettringite, as a result of the slag and metakaolin reactions. The fluorgypsum reacted rapidly in the first days, however it was still present after one year; the slag reacted in a slower fashion, and the metakaolin was very reactive and contributed with the ettringite since the early ages, which enhanced the strength.     

Removal of sulfate from high-strength wastewater by crystallisation

Sulfate causes considerable problems in anaerobic digesters, related to generation of sulfides, loss of electrons (and hence methane), and contamination of gas streams. Removal of sulfides is generally expensive, and still results in methane losses. In this paper, we evaluate the use of precipitation for low-cost sulfate removal, in highly contaminated streams (>1 gS L⁻¹). The main precipitate assessed is calcium sulfate (gypsum), though the formation of complex precipitates such as jarosite and ettringite to remove residual sulfate is also evaluated. The four main concerns in contaminated wastewater are:- high solubility, caused by high ion activity and ion pairing; slow kinetics; inhibition of nucleation; and poisoning of crystals by impurities, rendering product unsuitable for reuse as seed. These concerns were addressed through batch experiments on a landfill wastewater with a similar composition to other sulfate rich industrial wastewaters (high levels of organic and inorganic contaminants). Crystallisation rates were rapid and comparable to what is observed by others for pure solutions (2-5 h). The kinetics of crystallisation showed a 2nd order dependence on supersaturation, which have implications for crystalliser design, as discussed in the paper. No spontaneous nucleation was observed (seed was required). Seed poisoning did not occur, and product crystals were as effective as pure seed. Solubility was increased by an order of magnitude compared to a pure solution (2.6 × 10⁻³ M² vs. 0.22 × 10⁻³ M²). As evaluated using equilibrium modelling, this was caused equally by non-specific ion activity, and specific ion pairing. Jarosite and ettringite could not be formed at reasonable pH and temperature levels. Given the lack of complex precipitates, and relatively high solubility, gypsum crystallisation cannot practically be used to remove sulfate to very low levels, and gas-sulfide treatment will likely still be required. It can however, be used for low-cost bulk removal of sulfate.     

混凝土硫酸盐侵蚀过程及主要产物研究进展

概述了在硫酸盐环境中混凝土的侵蚀破坏过程，以及硫酸盐类主要侵蚀产物包括石膏、钙矾石（AFt）、硅灰石膏（C3SC^-S^-H15）等生长特性的研究进展。硫酸盐侵蚀是一个比较复杂的过程，不同的环境介质、热湿条件等引起的侵蚀过程和产物也相应不同，尤其是钙矾石的生长特性和物理力学性能仍需要进一步的研究。针对硫酸盐侵蚀机理，实际工程中采用的一些抗硫酸盐侵蚀措施具有良好的效果。     

The effect of pozzolans and slag on the expansion of mortars cured at elevated temperature: Part I: Expansive behaviour

The expansive behaviour of heat-cured mortars containing pozzolans and slag was investigated. In most cases, the addition of any amount of these materials to the mixture typically reduced the long-term expansion, slowed the rate of expansion, and delayed the onset of expansion. However, the efficacy of a particular pozzolan or slag in controlling expansion may depend on its Al₂O₃ content. Metakaolin, which contains a high amount of reactive Al₂O₃, was the most effective at controlling expansion at relatively low cement replacement levels. Slag and fly ash, which are also sources of Al₂O₃, were also effective at suppressing expansion at higher replacement levels. Silica fume was less effective at controlling expansion at conventional replacement levels, and even at higher replacement levels expansion may only be delayed.     

Effects of setting regulators on the efficiency of an inorganic acid based alkali-free accelerator reacting with a Portland cement

Today, in the field of underground constructions, alkali-free accelerators are commonly employed, during tunnel excavation, to allow flash concrete setting. In this way, the cementitious sprayed material can firmly bond to the tunnel walls, controlling the convergence (the tendency of the section to squeeze). Their efficiency may be related to many parameters like: cement type, setting regulator, concrete composition, working temperature. Nevertheless, the influence of such factors on the accelerator performance has not been clarified yet. The accelerator efficacy is evaluated by real spraying test in job site or, when only laboratory equipment are available, by measuring the final setting times of cement systems admixed with the accelerator. Several alkali-free flash setting admixtures are available on the market. They can be divided into two main categories both containing aluminium sulphate complexes stabilized either by inorganic acids or by organic acids. In this paper, the influence of different setting regulators on the performances of an inorganic acid based alkali-free accelerator was analysed. Portland cement samples were obtained by mixing clinker with gypsum, α-hemihydrate, β-hemihydrate or anhydrite. The setting regulator instantaneous dissolution rates were evaluated through conductivity measurements. The setting time of cement pastes with and without the accelerator was measured. It was found that the shorter the final setting time (therefore the more efficient is the accelerator) the lower the setting regulator instantaneous dissolution rate. In order to understand this phenomenon, a comparison was performed between accelerated cement paste samples containing the setting regulator with the highest (β-hemihydrate) and the lowest instantaneous dissolution rate (anhydrite). The analytical work included morphological (Environmental Scanning Electron Microscopy-Field Emission Gun — ESEM-FEG), crystal-chemical (X-Ray Powder Diffraction — XRD), physical-chemical (hydration temperature profile) and chemical (Induced Coupled Plasma-Atomic Emission Spectroscopy — ICP/AES) evaluations. The results revealed significant morphological differences among the investigated samples.