Auxiliary Activators Research Articles

Biomass ash (BA) waste as an activator to produce carbon-negative cement

The use of biomass, as a renewable energy source, to run heating and power plants is propelled by sustainable European policy. Olive is an important resource in Mediterranean countries. The residues from the extraction of olive oil are used as biomass, either to produce the oil or to generate heat or electricity. The disposal of ash residue poses an important burden. This study uses olive pit bottom ash waste (OBA) to produce carbon-negative cement. The OBA is mixed with waste GGBS (GGBS), and neither calcination nor thermal curing are used to lower environmental impact.The cements produced contain up to 60 %OBA and have a carbon sequestration capacity up to -97.45 kg CO2e/m3. An optimum mix with 40 %OBA is developed (using auxiliary activator), with compressive strength of 36–44 MPa and a carbon sequestration capacity of 40–45 kg CO2/m3. A modified loss on ignition test is proposed to evaluate the embodied carbon of biomass ash.The OBA's main chemical constituents: K2O and CaO, afford outstanding activation and alkalinity to release Ca2+ Si4+ and Al3+ from GGBS to form calcite, hydrotalcite, C-(A)-S-H and amorphous cements. Using sodium carbonate (NC) and lime as supplementary activators enhanced the mechanical properties of the cements and slightly changed their composition and microstructure. NC is the most efficient activator, it increased dissolution, and produced a denser and stronger cement with higher Si and K concentration that includes gaylussite, N-A-S-H and C(K)-A-S-H. Pre-dissolving the NC prior to mixing increases the activator's efficiency, producing less calcite cement for the same amount of NC. By adding 4 % pre-dissolved NC, the compressive strength increased by 138.76 % (compared to OBA-GGBS mortar without auxiliary activators) and 113.94 % compared to the material with NC in powder form.

Properties and Microstructure of Na2CO3-Activated Binders Modified with Ca(OH)2 and Mg(OH)2.

Delayed strength development and long setting times are the main disadvantageous properties of Na2CO3-activated slag cements. In this work, combined auxiliary activators of Ca(OH)2 and Mg(OH)2 were incorporated in one-part Na2CO3-activated slag binders to accelerate the kinetics of alkali activation. The properties and microstructure evolution were investigated to clarify the reaction mechanism. The results showed that the additions of auxiliary activators promoted the hardening of the pastes within 2 h. The 28 days compressive strengths were in the range of 39.5–45.5 MPa, rendering the binders practical cementitious materials in general construction applications. Ca(OH)2 was more effective than Mg(OH)2 in accelerating the kinetics of alkali activation. The dissolution of Ca(OH)2 released more OH− and Ca2+ ions in the aqueous phase to increase alkalinity in the aqueous phase and promote the formation of the main binding gel phase of calcium-aluminosilicate hydrate (C-A-S-H). An increase in the Ca(OH)2/Mg(OH)2 ratios increased autogenous shrinkage and decreased drying shrinkage of the binders. The formation of a compact pore structure restricted the water evaporation from the binders during the drying procedure.

Trap Levels in Eu and other Rare‐Earth Ions Codoped CaAl2O4 Phosphors

Photoluminescence spectrum, trap depths, and densities of trapped carriers of CaAl2O4:Eu phosphor crystals doped with rare‐earth elements were studied using the thermally stimulated luminescence technique. Trap depths and densities of the specimens vary with rare‐earth elements doped as the auxiliary activators. Tm and Nd are found to be effective for the strong afterglow phosphorescence peaking at λ = 442 nm for several hours after the excitation. CaAl2O4:Eu phosphor crystals doped with Nd and Tm include high density of carriers trapped at E = 0.59 and 0.52 eV, respectively.

Unrestrained Short-Term Shrinkage of Calcium-Hydroxide- Based Alkali-Activated Slag Concrete

A total of 34 concrete mixtures were tested to examine the unrestrained shrinkage behavior of calcium-hydroxide (Ca(OH)2)-based alkali-activated (AA) ground-granulated blast-furnace slag (GGBS) concrete. The main parameters investigated were water-binder ratio (w/b), unit water content (Wc), and fine-aggregate-to-total-aggregate ratio (S/A). From the regression analysis of test results, the comprehensive basic design equations were formulated to predict the 28-day shrinkage strain and time function, which can be expanded to long-term shrinkage strain or ultimate results of Ca(OH)2-based AA-GGBS concrete. The test results showed that the increasing rate of shrinkage strain against age and the amount of 28-day shrinkage strain of Ca(OH)2-based AA-GGBS concrete were significantly affected by the w/b, Wc, and the type of auxiliary activators, while they were minimally influenced by the S/A. When the w/b was higher than 0.35, a higher 28-day shrinkage strain was observed in Ca(OH)2- and Na2CO3 activated GGBS concrete than in Ca(OH)2- and Na2SiO3-activated GGBS concrete. The proposed equations showed a significantly better accuracy in the test results than the existing empirical models of which agreement was dependent on the mixing proportions of the concrete tested.

Effect of water–binder ratio on the mechanical properties of calcium hydroxide-based alkali-activated slag concrete

This paper presents mechanical properties of calcium hydroxide (Ca(OH)2)-activated ground granulated blast-furnace slag (GGBS) concrete with different auxiliary activators in order to provide basic data for developing the mixing procedure and design models for structural concrete. Twelve concrete mixes were prepared with the main parameters of water–binder (W/B) ratio. The measured mechanical properties were compared, wherever possible, with design equations specified in ordinary Portland cement (OPC) concrete code provisions. The investigated Ca(OH)2-based AA GGBS concrete showed an enhanced workability, delayed slump loss and similar increasing rate of compressive strength against W/B ratio to that observed in OPC concrete. Similarly to OPC concrete, mechanical properties of Ca(OH)2-based AA GGBS concrete could be expressed as a power function of compressive strength (fc′). The values of the power in the determined equations could be approximately set to be 0.5 for modulus of elasticity, 0.65 for modulus of rupture, tensile strength and direct shear strength, and 0.75 for bond strength. Overall, the present test results revealed that the tested Ca(OH)2-based AA GGBS concrete has a considerable potential for practical application to structural concrete when the W/B ratio is lower than 30%.

Hydration products and strength development of calcium hydroxide-based alkali-activated slag mortars

To strengthen the practical application of the Ca(OH)2-based alkali-activated (AA) ground granulated blast-furnace slag (GGBS) system, the hydration products, microstructural characteristics and compressive strength development of the AA GGBS mortars were examined for 3 pastes and 21 mortar mixes according to the different auxiliary activators and curing conditions. 7.5% Ca(OH)2 was used for the main activator and either 1% Na2SiO3 or 2% Na2CO3 was added for an auxiliary activator. Test results revealed that the diffraction peaks and molar Si/Ca ratio of the calcium silicate hydrate (CSH) gels were higher for water curing than for air-dried curing and with the addition of auxiliary activators than with the use of the single Ca(OH)2. Furthermore, the addition of auxiliary activators contributed to the stabilization of CSH gels at a long-term age, showing that the molar Si/Ca ratio in CSH gels increased with the hydration time. These growth and intensity of CSH gels were consistent with higher strength development. The strength gain ratio of water-cured mortars was lower at the early ages but higher at the long-term ages than that of the companion air-cured mortars. This trend was independent of the mixing proportions of the mortars and the type of auxiliary activators. Simple equations were also proposed to predict the strength development of Ca(OH)2-based AA GGBS mortars considering the influencing parameters including the curing condition, the type of auxiliary activators and the mixing proportions. The predicted compressive strength of Ca(OH)2-based AA GGBS mortars at different ages was in good agreement with the test results, though the 3-day compressive strength was underestimated.

Influence of co-doping different rare earth ions on CaGa 2S 4: Eu 2+, RE 3+(RE=Ln) phosphors

Photoluminescent phosphors CaGa 2S 4: Eu 2+, RE 3+ (RE 3+ including all rare earth ions except for Sc 3+, Pm 3+, Eu 3+ and Lu 3+) were prepared by sintering at high temperature in a reductive atmosphere, and their luminescent properties were studied intensively. The influences of co-doping rare earth ions on their luminescent properties were also investigated. No remarkable differences were found from excitation spectra of co-doped phosphors CaGa 2S 4: Eu 2+, RE 3+ in contrast with that of phosphor CaGa 2S 4: Eu 2+, but there were a few differences in emission spectra of Ce 3+, Pr 3+ or Ho 3+ co-doped phosphors. Phosphors CaGa 2S 4: Eu 2+, RE 3+ (RE=Ce, Pr, Gd, Tb, Ho and Y) had persistent afterglow, and very short afterglow was shown for Nd 3+ or Er 3+ co-doped phosphors, but no long afterglow appeared when auxiliary activator was La 3+, Sm 3+, Dy 3+, Tm 3+ or Yb 3+. Among the phosphors with long-lasting phosphorescence, in our experiments, CaGa 2S 4: Eu 2+, Ho 3+ had the longest and the highest brightness long yellow afterglow. Thermo-luminescence of all co-doped phosphors was measured to find the answer of different influences from different rare earth auxiliary activators.

Thermoluminescent properties of Eu 2+ and RE 3+ co-doped phosphors CaGa 2S 4:Eu 2+, RE 3+ (RE=Ln, excluding Pm, Eu and Lu)

The thermo-luminescence (TL) of rare earth ions RE 3+ (RE=Ln, excluding Pm, Eu and Lu) co-doped phosphors CaGa 2S 4:Eu 2+, RE 3+ was studied between room temperature and 300 °C, and 3D thermo-luminescence of the phosphors were measured from room temperature to 400 °C. The basic material CaGa 2S 4:Eu 2+, showed at least two bands in the TL glow curve. Changing the auxiliary activator RE 3+ (rare earth ion), intensities and the positions of the TL glow curve peaks were affected significantly. For the phosphors with long afterglow, auxiliary activator such as Ce 3+, Pr 3+, Gd 3+, Tb 3+, Ho 3+, or Y 3+ created some new defects in these compounds at lower trap levels and enhanced their TL intensities. The Nd 3+ or Er 3+ auxiliary activator only enhanced TL intensities to a low extent, so these two phosphors have short persistent luminescence at room temperature. TL intensities of La 3+, Sm 3+, Tm 3+ or Yb 3+ co-doped phosphors were suppressed greatly and no afterglow was shown. The relationship between auxiliary activators and corresponding thermo-luminescence curves of phosphors CaGa 2S 4:Eu 2+, RE 3+ are discussed in detail. According to our results, suitable activation energy and enough high corresponding trap density are necessary for the phosphor with long afterglow.

Trap Levels in Eu‐Doped SrAl2O4 Phosphor Crystals Co‐Doped with Rare‐Earth Elements

SrAl2O4:Eu2+ phosphor crystals co‐doped with auxiliary activators such as La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or Y have been grown by the floating zone (FZ) technique. Photoluminescence spectrum (PL), time‐resolved PL, and thermally stimulated luminescence (TSL) are evaluated to clarify the long‐duration phosphorescence mechanism of SrAl2O4, Eu, and Ln phosphors. TSL spectra were measured in the temperature range from RT to 600 K to evaluate the depth and densities of the traps generated by the doping of auxiliary activators that are responsible for the long‐duration phosphorescence. The peak wavelength of PL does not vary with auxiliary activator elements, while decay curves vary greatly with the auxiliary activators. The trap depth and the densities of the trapped carriers estimated based on the hole trap model also vary with the auxiliary activator elements. The traps generated at around E=0.5 eV by the auxiliary activators, Nd, Dy, and Tm, with sufficient densities are effective for the long‐duration phosphorescence.

Long afterglow phosphorescent characteristics of BaAl 2O 4:Eu,Dy films

BaAl 2O 4:Eu,Dy (BAO) films have been fabricated on Si substrate by laser ablation, and their fundamental optical property and afterglow characteristics are discussed in comparison with the SrAl 2O 4:Eu,Dy (SAO) films. The intense green emission near 500 nm that originates from 5d to 4f transition in Eu 2+ ions was clearly observed from the BAO films. This photoluminescence peak was at a shorter wavelength than that of the SAO films ( λ = 520 nm). The afterglow intensity from the BAO films disappeared within a few minutes whereas that of the SAO films lasts over 20 min. The hole-trap depth ( E t) created by Dy as the auxiliary activators, which strongly affects the afterglow characteristics, was estimated on the basis of the thermally stimulated luminescence (TSL) result. The TSL glow curve for BAO films showed two broad peaks at 320 K and 450 K. The calculated E t for each peak was 0.2 eV (for the 320 K peak) and 1.2 eV (for the 450 K peak). On the other hand, E t = 0.5 eV was obtained from the SAO films. The hole-trap depths of the BAO film are either too shallow or too deep to affect the afterglow characteristics at room temperature.

Fibre-optic thermometer using sensor materials with long fluorescence lifetime

A fibre-optic thermometer using newly developed phosphor sensor materials is described. A highly sensitive fibre-optic fluorescence thermometer is developed using chromium ion (Cr3+) doped phosphor crystals with a longer lifetime than that of conventional ruby sensors. Long fluorescent europium doped strontium aluminate phosphors co-doped with various auxiliary activators (SrAl2O4:Eu2+, Ln3+, Ln = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), were also grown and evaluated. In the fibre-optic thermometer using Cr3+ doped phosphor, PL lifetime decreased at temperatures ranging from 270 K to 750 K. The sensitivity of the fibre-optic thermometer varies with the host materials of Cr3+ ions. In the fibre-optic thermometer using SrAl2O4:Eu2+, Ln3+ doped phosphor, the value and lifetime of afterglow phosphorescence from the SrAl2O4:Eu2+, Ln3+ phosphors varied greatly in the narrow temperature region because the afterglow phosphorescence was dominated by thermal excitation of trapped carriers. Since the trap depth and the trap density depend on auxiliary activator elements, the sensitivity and temperature range of the fibre-optic thermometer using SrAl2O4:Eu2+, Ln3+ phosphors varied with auxiliary activator elements of SrAl2O4:Eu2+, Ln3+ phosphors.

Long afterglow phosphorescent sensor materials for fiber-optic thermometer

Long duration phosphorescent SrAl2O4 phosphors doped with various auxiliary activators (Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) have been evaluated for the fiber-optic thermometer application. In this thermometer, temperature was measured based on the temperature dependence of the lifetime and/or intensity of the afterglow phosphorescence which was dominated by the thermal excitation of trapped carriers. Characteristics of the long phosphorescent fiber-optic thermometer are quite different from those of the conventional thermometer using fluorescent lifetime of the phosphors. Lifetime and intensity of the afterglow phosphorescence from the long afterglow phosphorescent sensor heads show large positive temperature coefficients in the narrow temperature region around room temperature. Sensitivity and temperature range of the long phosphorescent fiber-optic thermometer depend strongly on the trap depth and the trap density of phosphors. Since the trap depth and the trap density vary with auxiliary activator elements, a variety of fiber-optic thermometers with high sensitivity in various temperature ranges will be able to be developed using Eu2+ doped SrAl2O4 codoped with various auxiliary activator Ln3+ ions (Ln=Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu).

Characterization of trap levels in long-duration phosphor crystals

A mechanism for the long-duration phosphorescence from Eu 2+ ions in SrAl 2O 4 crystals has been studied by evaluating the energy level and the density of the traps generated by the auxiliary activators. SrAl 2O 4:Eu 2+ crystals doped with Nd, Sm, Gd, Dy and Y have been grown by the floating zone technique. Trap depth and trap density are evaluated by the thermally stimulated luminescence technique. A tentative energy diagram for the traps in the crystals has been drawn based on the hole-trap model. The trap levels vary from 0.0024 to 0.78 eV with the added elements. The long-duration phosphorescence is influenced strongly by the depth and the density of the traps. The traps at around Et=0.5 eV, which are seen in Dy- and Nd-doped specimens, are responsible for the long-duration phosphorescence.

Growth and characteristics of long persistent SrAl 2O 4- and CaAl 2O 4-based phosphor crystals by a floating zone technique

Long persistent phosphor crystals, SrAl 2O 4 doped with Eu and Dy, and CaAl 2O 4 doped with Eu and Nd have been grown by a floating zone technique. The intensities and the persistent times of the phosphorescences are found to depend on the growth atmosphere. SrAl 2O 4 crystals grown under H 2Ar atmosphere and CaAl 2O 4 crystals grown under Ar atmosphere exhibit strong and long persistent green and blue phosphorescences peaking at λ = 520 and 450 nm, respectively. The spectra of the phosphorescence from the crystals are quite similar to those obtained with sintered powders used for luminous pigments. The persistent time of the phosphorescence from CaAl 2O 4 crystals is similar to that obtained with sintered powders, while in SrAl 2O 4 crystals, it is shorter than in sintered powders. The difference in the persistent time is found to be due to Dy and/or Nd dopant concentration added as auxiliary activators. In SrAl 2O 4, the Dy concentration in the crystal is about 1/100 of the sintered powders. The Dy 3+ ion is responsible to the persistent time of the phosphorescence.

Effects of Composition on the Long Phosphorescent SrAl2 O 4 : Eu2 + , Dy3 + Phosphor Crystals

Effects of the composition on the afterglow characteristics of phosphors have been evaluated using crystals grown by a floating zone technique for the first time. Phosphorescence spectrum due to ions peaking at λ = 520 nm does not vary with the compositions. Persistent times vary greatly with different compositions. Dy concentrations added as auxiliary activators are changed dramatically with the Sr/Al molar ratio of the starting materials. The phosphorescence decay consists of three exponential components with different decay times. The faster decay components play an important role for the long persistent phosphorescence.

Vanadium activated zinc and cadmium sulphide and selenide phosphors

Vanadium activated ZnS, ZnSe, CdS and CdSe powders were prepared. All these powders show a fluorescence at about 2μ. Addition of copper and silver as auxiliary activators enhances the vanadium emission, while the well known silver and copper emissions do not appear.