Showing papers on "Ettringite published in 2016"

Effect of further water curing on compressive strength and microstructure of CO2-cured concrete

Abstract: This study aims to investigate the effects of further water curing on the compressive strength and microstructure of CO2-cured concrete. The results showed that concrete with a residual w/c ratio of 0.25 showed the most rapid strength development rate upon further water curing due to hydration of uncarbonated cement particles. Thermogravimetric, IR-spectrophotometric and scanning electron microscope examinations indicated that further hydration of the cement particles could form C-S-H gel and ettringite crystals. The results showed that the calcite formed during the initial CO2 curing was consumed during the further hydration of C3A, and produced calcium monocarbonaluminate hydrate. Also, Ca(OH)2 was not detected due to its reaction with the formed silica gel. Mercury intrusion porosimetry test results indicated that the porosity and pore size of the CO2 cured mortar decreased further after water curing.

Chemical activation of hybrid binders based on siliceous fly ash and Portland cement

Abstract: The hydration of Portland cement (PC) blended with a high amount of a siliceous fly ash (70% fly ash, 30% PC) has been examined. The fly ash contributes significantly to the long-term strength development, when compared to a reference sample with quartz powder. However the long setting time and the poor early strength prevent the use of such binders. Therefore the effect of different activators (sodium carbonate, potassium sodium silicate, potassium citrate and sodium oxalate) on the setting, the hydration kinetics and the strength development of the fly ash-PC blend has been investigated. The addition of the activators increases the pH and decreases thus the calcium concentrations in the pore solution, which leads to a faster reaction of alite and thus to early setting and increased early strength. On the long term, the high alkali concentrations lower the compressive strength and lead to a (partial) destabilization of ettringite. Sodium oxalate and potassium sodium silicate accelerate both the setting of the fly ash-PC blend and increase the early compressive strength. Furthermore, they show better compressive strengths at later ages compared to the other activators. Based on these findings, they can be considered as the most suitable accelerators among the investigated activators.

Influence of slag characteristics on strength development and reaction products in a CaO-activated slag system

Abstract: This study collected four ground granulated blast furnace slags from different manufacturing locations for CaO-activation to investigate the influence of the slag characteristics on strength development and the reaction products. Despite the seemingly similar characteristics of the slags, each slag developed significantly different strengths varying from 25 to 52 MPa at 28 days and different reaction products. The main reaction products were C-S-H and calcium hydroxide in all the samples; however, various forms of AFm phases appeared depending on the presence of calcite and calcium sulfates in the raw slags. Ettringite was also found in the case of raw slag with calcium sulfates. In this study, strength development was not governed by any single dominant material parameter of the raw slag, but rather by the combination of various favorable factors such as a higher content of calcium sulfates, a smaller overall particle size distribution and a higher basicity (or chemical indexes).

Strength enhancement and pore-size refinement in clinker-free CaO-activated GGBFS systems through substitution with gypsum

Abstract: This study presents an improvement in the compressive strength of the clinker-free CaO-activated GGBFS system with the addition of gypsum. The compressive strength was primarily governed by the amount of ettringite, but more importantly, the morphology of reaction products (e.g., size, slenderness) is proposed as being a more dominant factor. The gypsum content at the best strength produced fine ettringite crystals and removed large pores that were around 100 μm in diameter, resulting in significant pore-size refinement. However, excessive addition reduced the strength and generated a large volume of detrimental pores with sizes of 0.03–0.4 μm, which were probably due to expansive cracking or large voids between the large sizes of re-precipitated gypsum crystals. The SEM images demonstrated that the fine ettringite crystals at the gypsum dosage of the best strength filled the pores effectively, which led to dense matrices; while the coarse crystals of gypsum or ettringite at the excessive addition level produced porous matrices.

Hydration of calcium sulphoaluminate clinker with additions of different calcium sulphate sources

Abstract: In these last decades the complete comprehension of mechanisms that affect the hydration of calcium sulphoaluminate cement (CSA) has become crucial in the optic of an extensive use of CSA based systems as promising low-CO2 binders alternative to ordinary Portland cement. In this study the hydrated phases evolution of cementitious systems based on CSA clinker, mainly composed by ye’elimite (C4A3$) phase, blended with two calcium sulphate forms (i.e. gypsum and anhydrite) has been investigated. The adoption of 27Al magic-angle spinning NMR spectroscopy led to identify and follow the evolution of all the main CSA hydrated phases (i.e. ettringite, monosulfate, aluminium hydroxide) supporting the X-ray diffraction technique in the amorphous phase resolution. Thermogravimetric analysis has been adopted as complementary technique in the hydrated phases identification and the kinetic of the hydration process has been monitored by means of isothermal calorimetry. Experiments made on pastes hydrated at different ages (8, 16, 24 and 48 h) showed system reactivity strongly dependent on sulphate ion availability. In presence of gypsum, more soluble, a higher ettringite phase formation with a consequent greater C4A3$ consumption was detected in the first hours of hydration (8 h) slowing down the kinetic of phases formation only at later ages (48 h). Results showed how the different sulphate ion availability modifies the kinetics of CSA hydration process as well as the morphology of hydrated phases. SEM observations confirmed that the high solubility of gypsum led to formation of poorly-crystallized hydrated phases, with a consequent more dense matrix, while in the presence of anhydrite, less soluble, a well crystallized needle like ettringite has been detected as main hydrated phase.

Solidification/Stabilization of Fly Ash from a Municipal Solid Waste Incineration Facility Using Portland Cement

Abstract: This study investigated the solidification/stabilization of fly ash containing heavy metals using the Portland cement as a binder. It is found that both the cement/fly ash ratio and curing time have significant effects on the mechanical (i.e., compressive strength) and leaching behaviors of the stabilized fly ash mixtures. When the cement/fly ash ratio increases from 4 : 6 to 8 : 2, the increase of compressive strength ratio raises from 42.24% to 80.36%; meanwhile, the leaching amount of heavy metals decreases by 2.33% to 85.23%. When the curing time increases from 3 days to 56 days, the compressive strength ratio of mixtures raises from 240.00% to 414.29%; meanwhile, the leaching amount of heavy metals decreases by 16.49% to 88.70%. The decrease of compressive strength with the lower cement/fly ash ratios and less curing time can be attributed to the increase of fly ash loading, which hinders the formation of ettringite and destroys the structure of hydration products, thereby resulting in the pozzolanic reaction and fixation of water molecules. Furthermore, the presence of cement causes the decrease of leaching, which results from the formation of ettringite and the restriction of heavy metal ion migration in many forms, such as C-S-H gel and adsorption.

Characterisation of pre-industrial hybrid cement and effect of pre-curing temperature

Abstract: This study aimed to determine the physical-mechanical, mineralogical and microstructural properties of a pre-industrially manufactured hybrid cement (HYC) containing 5% alkaline activator and less than 30% clinker. The effect of the initial curing temperature (25 ± 1 or 85 °C for 20 h) on hydration kinetics and the development of compressive strength were also explored. The hydration products formed were characterised using XRD, SEM/EDX and 27 Al and 29 Si MAS-NMR. The findings showed that pre-industrial hybrid cement sets when hydrated with water and hardens to a 28-day mechanical strength of 35 MPa. The main reaction product formed was a mix of cementitious gels: C-(A)-S-H and C-A-S-H. Curing at 85 °C for 20 h, shows a behaviour similar to OPC, inhibited ettringite formation and generated more polymerised gels, enhancing 3-day but not 28- or 90-day mechanical strength.

XRD and TGA, Swelling and Compacted Properties of Polymer Treated Sulfate Contaminated CL Soil

Abstract: In this study, the effect of up to 4 % calcium sulfate contamination on the soil properties of a natural clay with low liquid limit (CL) soil with and without polymer treatment was investigated and compared to 6 % lime treated soil. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) methods were used to identify and quantify the changes in the contaminated CL clay soil. XRD analyses showed the major constituents of the soil were calcium silicate (CaSiO3), aluminum silicate (Al2SiO5), magnesium silicate (MgSiO3), and quartz (SiO2). With 4 % calcium sulfate contamination, the liquid limit (LL) and plasticity index (PI) of the CL soil increased by 30 and 45 %, respectively. The addition of calcium sulfate resulted in the formation of calcium silicate sulfate (Ternesite Ca5(SiO4)2SO4) and aluminum silicate sulfate (Al5(SiO4)2SO4). TGA analyses showed a notable reduction in the weight of calcium sulfate contaminated soil between 600 and 800°C, possibly due to changes in soil mineralogy. In addition, the total weight loss at 800°C for 1.5 % polymer treated soil was about 40 % less than the 4 % calcium sulfate contaminated soil, and it was similar to the weight loss observed in the uncontaminated CL soil. The maximum dry density of compacted soil decreased and the optimum moisture content increased with 4 % of calcium sulfate. The addition of 4 % calcium sulfate increased the free swelling of compacted soil by 67 %. The addition of 6 % lime resulted in the formation of ettringite (Ca6Al2 (SO4)3(OH)12·26H2O). Polymer treatment decreased the LL, PI, swelling index, and optimum moisture content of the soil and increased the compacted maximum dry density. Behavior of sulfate contaminated CL soil with and without treatment was quantified using a unique model that represented both linear and nonlinear responses. Also the model predictions were compared with published data in the literature.

Stability of ettringite in CSA cement at elevated temperatures

Abstract: Calcium sulfoaluminate cements are promising low carbon dioxide alternatives to Portland cement. Their main hydration products are ettringite and aluminium hydroxide. Ettringite has recently been identified as a potential heat storage material. The reversible dehydration of ettringite to metaettringite at elevated temperatures and under dry conditions involves a relatively high enthalpy, which can be used for (seasonal) heat storage. In this context, the effect of elevated temperatures (up to 110°C) and humidity conditions (steam curing and dry curing) on the stability of ettringite in calcium sulfoaluminate was studied experimentally using thermogravimetric analysis, X-ray diffraction and thermodynamic modelling. The experimental results show that ettringite decomposes under steam curing conditions at temperatures far below 100°C to monosulfate. This may lead to delayed ettringite formation when the temperature is lowered again under humid or wet conditions. Under dry conditions at low water vapour satur...