The effects of physical and chemical properties of fly ash on the manufacture of geopolymer foam concretes

The development of sustainable construction and building materials with reduced environmental footprint in both manufacturing and operational phases of the material lifecycle is attracting increased interest in the housing and construction industry worldwide. Recent innovations have led to the development of geopolymer foam concretes (GFCs), which combine the performance benefits and operational energy savings achievable through the use of lightweight foam concrete, with the cradle-to-gate emissions reductions obtained through the use of a geopolymer binder derived from fly ash. Fly ash is a by–product of coal fired power stations, and has become a highly promising source material for geopolymer manufacture. Compared to clays, another type of usually used materials, fly ash is probably more technologically suitable as it requires less alkaline activator while providing good workability. However, fly ash particles are substantially heterogeneous in physical and chemical properties. The composition and mineralogy of fly ash have marked effects on the properties of geopolymers, such as setting behaviour. This will affect the pore structure of GFC. Unfortunately, there is very limited specification regarding feedstock utilisation in geopolymer manufacture at present. Understanding the effect of fly ash physics and chemistry on the manufacture of GFC is not only necessary for the development of commercially mature GFC technology but also important for the geopolymer technology as a whole section. Five fly ash samples sourced from different power plants around Australia were used to manufacture geopolymer binders, enabling investigation of the relationship between the physical and chemical properties of fly ash and the mechanical properties of geopolymer products. The results showed that fly ashes from different sources exhibit substantially different physical properties. One important property is the inter-particle volume of fly ash, which largely determines the liquid requirement. The liquid requirement furthermore affects the porosity of hardened binders and their production costs. Another factor is the reaction extent of fly ash, which determines the quantity and composition of gel phases. A general trend obtained is that fly ash with higher network-modifying cations seems to possess higher reactivity. Research by Rietveld quantitative XRD and XRF analysis found that the composition and chemistry of glassy phases play an equally important role as the quantity of these phases in affecting the reactivity of fly ash. In glassy phases, both FeO4 and AlO4 tend to randomly distribute and connect with SiO4 tetrahedra by sharing corners and this is due to the alkali/alkali earth cations, which act as charge compensators. A reactivity index (RI) was proposed in this thesis to quantify the reactivity of fly ash under geopolymerization conditions. If pentacoordinated Fe cations are regarded as network modifiers, in addition to alkali and alkali earth cations, and by considering the contribution of specific surface area, it was found that the RI order of the studied five ashes matched well with their reactivity order. Alkaline dissolution analysis under different liquid/solid ratios supported the RI results. Additionally, dissolution analysis also showed that the crystals such as mullite and quartz were also partially dissolved, particularly in the ‘impure’ fly ashes, which had relatively higher concentration of network modifying cations. The above stages of the works were very useful to understand and to obtain a strong geopolymer binder by selecting a reactive fly ash. However, GFC manufacture in the laboratory conditions showed that a fly ash suitable for making high strength solid geopolymers was not necessarily suitable for GFC manufacture. It appeared that fly ash physical properties played a more important role than fly ash chemistry in affecting the engineering performances of GFCs. Those fly ashes with lower particle density and irregular particle shape appeared best suited for the manufacture of foam geopolymers. For a foamed paste derived from a specific fly ash, quick setting was a key property to achieve fine pore size and a homogeneous microstructure. The orthogonal array study conducted showed that slag addition was an effective method to control, and shorten the setting time of the foamed paste. The pore structure and porosity were also changed significantly and contributed to an increase in compressive strength. Research of the characteristics of pore structure of a series of GFCs showed that the pore size distribution in GFC affected the compressive strength to a large extent, particularly for the large pores. Based on the statistical fitting and modelling, a new model was developed, called the ‘large void model’, which treated the porosity of critical size pores (>100 m) and total porosity separately. Two mathematical models relating the measured thermal conductivity with porosity and dry density were successfully developed. The mathematical models were proven to be able to predict the mechanical and thermal insulation properties precisely

Amyrin exerts potent anxiolytic and antidepressant effects via mechanisms involving monoamine oxidase and γ-aminobutyric acid in mouse hippocampus

Purpose: To investigate the anxiolytic and antidepressant effects of α- and β-amyrins in a mouse model of mild traumatic brain injury (mTBI), and the underlying mechanism(s) of action. Methods: Male Swiss mice (n = 165), weighing 25 - 40 g (mean weight = 32.5 ± 7.5 g), were used in this study. Existing mTBI model was modified and optimized for mild injury to brain capable of producing neurobehavioral changes. Forced swim test (FST) and tail suspension test (TST) were used to measure the antidepressant effects of α- and β-amyrins, while elevated plus maze (EPM) and light-dark model (LDM) were used for anxiety assessment. The probable mechanism of action of amyrin was also investigated through kinetics of serotonin uptake and activities of monoamine oxidases A (MAO A) and B (MAO B) in mouse hippocampus and cortex. Results: Induction of mTBI produced anxiety- and depression-like behaviors in mice. The 5th hitting righting time was 259 ± 25.11 s while duration of apnea was 27.33 ± 3.84 s. Apnea was significantly reduced on 5th and 7th hits, when compared with 4th and 6th hits (p < 0.05). The immobility time of mice was significantly reduced in FST and TST. A combination of the two forms of amyrin was more effective in reducing duration of immobility, relative to when each was used alone (p < 0.05). Amyrin significantly and dose-dependently increased entries, and time spent in open arms and light zone (p < 0.05). Mice in mTBI group exhibited a high degree of hopelessness, when compared with control group (p < 0.05). However, amyrin at a dose of 50 mg/kg significantly reduced the degree of hopelessness in the mice (p < 0.05). The specific activity of MAO A in hippocampal tissue (265.00 ± 12.07 µmol/min/g protein) was significantly higher than that of cortex (61.85 ± 5.14 µmol/min/g protein). In both tissues, there were no significant differences in the activity of MAO B among the groups (p > 0.05). Amyrin significantly reversed the effects of mTBI on the levels of amino acids in mice hippocampus and cortex (p < 0.05). The results of synaptosomal uptake of serotonin show that fluoxetin exhibited competitive inhibition of serotonin uptake, while amyrin exhibited mixed type inhibition. Conclusion: The results obtained show that α- and β-amyrins exert potent anxiolytic and antidepressant effects via mechanisms involving MAO and GABA in the hippocampus

Critical thinking on efflorescence in alkali activated cement (AAC)

Alkali-activated cement (AAC), also known as “geopolymer”, has been extensively investigated over the past 40 years and has been developed from laboratory mock ups to real structural usage in construction in the last decade. While numerous life cycle analyses and carbon accounting studies show the “green potential” of this material compared to Portland cement, some authors state that the high alkali concentration in AAC is a potentially unstable factor which may lead to, for example, efflorescence. This paper presents a critical thinking on the literature and some new experimental work regarding the possibility of efflorescence in AAC products. Subjects of the discussion include: (1) the role of alkalis in AACs, (2) the effect of alkali concentration on efflorescence, (3) the effect of solid precursor selection on efflorescence, (4) the effect of curing scheme and chemical additives on efflorescence, and (5) the impacts of efflorescence on the microstructural properties of AACs. Particular attention is given to the relationship between pore structure and efflorescence behaviour, and consequently the mechanical properties of AACs suffering from either efflorescence or alkali loss (by leaching). The changes in sodium aluminosilicate hydrate (N-A-S-H) gels due to efflorescence or alkali loss are critical to the durability of AACs. This paper emphasizes that the nature of the solid precursor and the pore structure of the resulting AAC are the two most important factors that control efflorescence rate. However, considering its alkaline nature, it seems difficult or impossible to avoid this issue in AAC products, although kinetically controlled diffusion of alkalis using phase transformation techniques may help to mitigate efflorescence. Efflorescence in AAC is a “skin issue” that needs to be carefully treated. It is recognized to be different from the visually similar, but chemically distinct, efflorescence that occurs in Portland cement based materials

Cryptanalysis of publicly verifiable authenticated encryption

Ma and Chen proposed a new authenticated encryption scheme with public verifiability. This scheme requires less computational costs and communication overheads than the conventional signature-then-encryption approaches. In this letter, we show that the Ma-Chen scheme does not satisfy three security properties: unforgeability, confidentiality and non-repudiation

Corrosion Performance of Embedded Steel Bar in Cl--contaminated Limestone Calcined Clay Cement (LC3) at initial stage of hydration

Limestone Calcined Clay Cement (LC3) presents brilliant properties in binding Cl- so that the embedded steel bars are probably protected in Cl--contaminated condition, which meets the need of sea sand application. However, the corrosion performance of steel bars embedded in LC3 paste with Cl‑ is unclear, especially in early age hydration. Thus, a series of experiments were carried out to evaluate the corrosion performance of steel bars on initial and hardened stages of hydration, including concentration of OH- and Cl- in real pore solution, open circuit potential (OCP) and chemical elements of steel bars. In terms of early age hydration, the OCP of steel bars and ions concentration in pore solution indicated that both specimens embedded in PC and LC3 pastes were at a highly corrosion state, however, elemental results showed that no obvious corrosion happened at this stage. With respect to hardened age hydration, visual corrosion could be seen on PC-embedded steel bars, with more Fe3+ and O2-, in comparison with LC3-embedded one, which was related to the much lower absolute OCP and Cl- concentration in pore solution. Overall, LC3 cement demonstrates protective effect on steel bar in special contaminated-Cl- concentration

Shadow operator: Effective dynamic load change operation training in air separation processes based on industrial nonlinear MPC and Bloom's taxonomy

A novel human-machine interactive training method for dynamic load change operation in air separation processes (ASPs) is proposed. A shadow operator (SO) is developed in this method to train ASP operators through industrial model predictive control (IMPC) and Bloom's taxonomy. First, a nonlinear two-layer IMPC machine algorithm is developed for dynamic load change operation. The IMPC uses a linear parameter varying prediction model and an iterative multi-step linearization algorithm to compute accurate control decisions. Second, a hierarchical human-machine cooperation model is established to improve the effectiveness of operation training. The model is inspired by an educational psychology framework (Bloom's taxonomy) and assists ASP operators in enhancing their dynamic operational skills. Finally, five dynamic training modes of the SO are designed based on the IMPC algorithm and the human-machine cooperation model. The practical application results demonstrate that the SO improves the effectiveness of skill acquisition for novice operators and the safety of dynamic operations.Comment: 16 pages, 18 figure

Study on the activation of calcined kaolin

Calcined temperature is a key factor to the activity of metakaolin. Structure characteristics and alkali activation of kaolin and its calcined products at different temperatures were analyzed by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), infrared spectrometry (IR) and isothermal calorimetry. The results show that the activity of kaolin calcined at 900°C is best. The characteristic absorption peak of kaolin disappears, a large amount of Al atoms convert from 6-coordination to 5-coordination; some characteristic vibration peaks of kaolin disappear while characteristic absorption peaks of metakaolin appear; There is much heat evolution after mixing it with alkali and the compressive strength is the highest. The strength of samples cured at 80°C for 3 days and 7 days reaches 33.8 and 35.3 MPa respectively

The pore characteristics of geopolymer foam concrete and their impact on the compressive strength and modulus

The pore characteristics of geopolymer foam concretes (GFCs) manufactured in the laboratory with 0–16% foam additions were examined using image analysis (IA) and vacuum water saturation techniques. The pore size distribution, pore shape, and porosity were obtained. The IA method provides a suitable approach to obtain the information of large pores, which are more important in affecting the compressive strength of GFC. By examining the applicability of the existing models of predicting compressive strength of foam concrete, a modified Ryshkevitch’s model is proposed for GFC, in which only the porosity that is contributed by the pores over a critical diameter (>100 μm) is considered. This 'critical void model' is shown to have very satisfying prediction capability in the studied range of porosity. A compression-modulus model for Portland cement concrete is recommended for predicting the compression-modulus elasticity of GFC. This study confirms that GFC have similar pore structures and mechanical behavior as those Portland cement foam concrete and can be used alternatively in the industry for the construction and insulation purposes