Controlling ettringite formation in FBC fly ash geopolymer concrete

Fluidized bed coal combstion (FBC) is extensively used in small self-generation power plants. The fly ash obtained from this FBC process contains high quantity of calcium and sulfate compounds which hinders its use in the construction industry. In addition, its reactivity is low and additional source material or additive is, therefore, needed to increase the reaction. This research studied the use of Al(OH)₃ and high concentrations of NaOH to control ettringite formation in the FBC fly ash geopolymer. Two replacement levels of 2.5 wt.% and 5.0 wt.% of Al(OH)₃ and three NaOH concentrations of 10, 12 and 15 M were used in the study. Results indicated that the NaOH concentration affected the ettringite formation and strength of the FBC geopolymer. No ettringite was formed at high NaOH concentration of 15 M which helped the dissolution of calcium sulfate and formed the additional calcium hydroxide. The subsequent pozzolanic reaction led to strength gain of the geopolymer. For 15 M NaOH, the addition of 2.5 wt.% Al(OH)₃ promoted the reaction and formed a dense matrix of alumino silicate compound. Relatively high 7-day compressive strength of 30 MPa was obtained.     

含钙矾石和水镁石双膨胀源水泥硬化浆体及其混凝土的体积稳定性分析

本文建立了在水泥硬化浆体中来自钙矾石和水镁石结晶膨胀作功方式的模型，计算了结晶膨胀能全部被用作体外膨胀功时所产生的膨胀率，由此分析了现有含钙矾石膨胀源或水镁石膨胀源水泥硬化浆体体积的稳定性，并研究得到含钙矾石和水镁石双膨胀水泥或胶体硬化浆体及其混凝土的长龄期膨胀及其体积稳定性，适用于补偿水泥混凝土的早期收缩和后期冷缩与干缩。     

The influence of alkali-free and alkaline shotcrete accelerators within cement systems: Influence of the temperature on the sulfate attack mechanisms and damage

The resistance to sulfate attack of mixtures accelerated with alkali-free and alkaline accelerators was found to be mainly influenced by the Al³⁺ and SO₄²⁻ added via the admixtures. Microstructural observations showed decalcification and disintegration of the CSH gel, which acted as an additional Ca²⁺ supplier as compared to the CH for ettringite formation. The CSH decalcification was mainly observed with a homogeneous distribution of the alkali-free admixture. The disintegration of the CSH gel increased the porosity and allowed more sulfate solution to penetrate into the specimens. This process promoted the swelling of the specimens and directly contributed to the expansion, explaining the lack of a direct relationship between the ettringite formation and the expansion. Moreover, the CSH gel disintegration, typical for MgSO₄ attack, also occurred with Na₂SO₄ solutions and depending on the aluminate-sulfate distribution and the extent of the CSH gel disintegration, different damage types were detected. At higher temperatures (65 °C) the damage was mainly controlled by the growth, the rearrangement and the thermal stability of ettringite.     

Temperature dependence, 0 to 40 °C, of the mineralogy of Portland cement paste in the presence of calcium carbonate

Thermodynamic calculations disclose that significant changes of the AFm and AFt phases and amount of Ca(OH)₂ occur between 0 and 40 °C; the changes are affected by added calcite. Hydrogarnet, C₃AH₆, is destabilised at low carbonate contents and/or low temperatures < 8 °C and is unlikely to form in calcite-saturated Portland cement compositions cured at < 40 °C. The AFm phase actually consists of several structurally-related compositions which form incomplete solid solutions. The AFt phase is close to its ideal stoichiometry at 25 °C but at low temperatures, < 20 °C, extensive solid solutions occur with CO₃-ettringite. A nomenclature scheme is proposed and AFm-AFt phase relations are presented in isothermal sections at 5, 25 and 40 °C. The AFt and AFm phase relations are depicted in terms of competition between OH, CO₃ and SO₄ for anion sites. Diagrams are presented showing how changing temperatures affect the volume of the solid phases with implications for space filling by the paste. Specimen calculations are related to regimes likely to occur in commercial cements and suggestions are made for testing thermal impacts on cement properties by defining four regimes. It is concluded that calculation provides a rapid and effective tool for exploring the response of cement systems to changing composition and temperature and to optimise cement performance.     

Ettringite formation: A crucial step in cement superplasticizer compatibility

The rheology of cementitious system containing superplasticizer is the consequence of a physical process due to the electrostatic repulsion between particles, but also of a chemical process linked to the nature of the phases that are formed. Ettringite crystallization play as a key role in this matter and the nature of the sulfate phase added to control cement setting is as important as its dosage. Alkali sulfates, which provide only SO₄²⁻ ions, do not promote the formation of ettringite for which the presence of large amounts of Ca²⁺ is necessary. The adsorption of superplasticizer molecules on hydrated cement grains slows down the dissolution rates of the constituents and modifies the nature of the compounds formed. It could result in a modification of the ettringite morphology.     

Impact of zeta potential of early cement hydration phases on superplasticizer adsorption

The zeta potential of early hydration products of cement was found to be a key factor for superplasticizer adsorption. A highly positive zeta potential results in a strong superplasticizer adsorption whereas a negative zeta potential does not allow adsorption. Synthetic ettringite precipitated from solution shows a highly positive zeta potential, hence it adsorbs great amounts of negatively charged superplasticizer. Monosulfate (AF_m) has a less positive zeta potential. Therefore, it adsorbs smaller amounts of superplasticizers. For syngenite, portlandite and gypsum, the zeta potential is around zero or negative. These phases do not adsorb superplasticizers. Consequently, a hydrating cement grain is best represented by a mosaic structure, with superplasticizer molecules mainly adsorbed on ettringite and some on monosulfate and C-S-H nucleated at surface.     

Effect of Zn,Cu,Cr and Pb Chlorides on the Formation of Tricalcium Aluminate Trisulfate Hydrate

The effect of addition of Zn, Cu, Pb and Cr chlorides as admixtures on the hydration reaction of the system 3CaO·Al2O3-gypsum with molar ratio 1：3 was studied. Different ratios of each salt were used, namely 0.5%, 2% and 4% by weight of the solid mixture. Hydration reaction was carried out at 35℃ for various time intervals from 0.5 h to up to 7 d. Hydration rate of the system 3CaO·Al2O3-CaSO4·2H2O in absence and presence of different salts was studied via the determination of the combined water contents. X-ray diffraction analysis showed that the ettringite was the only hydration product formed in the different mixes. The hydration products were investigated by scanning electron microscopy （SEM） and thermal gravimetric analysis. The results indicated that the rate of formation of ettringite and its microstructure depend on the admixture and its dosage.     

Evaluation of the treatment of chromite ore processing residue by ferrous sulfate and asphalt

The effectiveness of the treatment of chromite ore processing residue (COPR) with ferrous sulfate and encapsulation into asphalt were explored separately and in combination. The asphalt treatment was conducted by mixing COPR or ferrous sulfate pretreated COPR with varying amounts of asphalt. To assess the efficacy of the treatment, the leachability of toxicity characteristic leaching procedure (TCLP) total chromium (Cr) from all treated samples was determined for curing periods up to 16 months. X-ray absorption near edge structure (XANES) analyses were also performed to evaluate the Cr(6+) concentration in the selected samples. The combination treatment of ferrous sulfate and the encapsulation of the treated COPR into asphalt reduced the TCLP total Cr concentration to lower than the regulatory limit of 5mg/L for Cr contaminated soils, after 16 months. However, the Cr concentrations were still higher than the universal treatment standards (UTS) of 0.6 mg/L for hazardous waste. On the other hand, treatment with ferrous sulfate alone or the encapsulation of the COPR in asphalt failed to meet the TCLP total Cr concentration of 5mg/L, after 16 months. XANES analyses results showed that more than 75% Cr(6+) reduction was achieved upon pretreatment with ferrous sulfate.     

Transformation of calcareous oil-shale circulating fluidized-bed combustion boiler ashes under wet conditions

The hydration and transformation of Ca-rich kerogenous oil-shale (kukersite) ashes, formed in circulating fluidized bed (CFB) combustion furnaces, were investigated during a one-year experiment. During the first stages the process was characterized by rapid hydration of free lime hydration into portlandite and dissolution of anhydrite and periclase, whereas the formation of (crystalline) ettringite was somewhat delayed. During the final process, ettringite and portlandite that are unstable under atmospheric conditions were replaced by stable calcite and Ca-sulphate.     

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.