Microstructure,strength, and moisture stability of alkali activated glass powder-based binders

Thermally assisted alkali activation of silica-rich glass powder to produce sustainable binders is investigated. Glass powder activated using NaOH provides higher compressive strengths than NaOH activated fly ash binders at lower heat curing temperatures. Sodium silicate gel is the reaction product when glass powder alone is used as the source material, while a combination of sodium silicate and sodium aluminosilicate (N–A–S–H) gels form in activated glass powder–fly ash blends. The activated glass powder-containing binders are found to disintegrate and lose strength when exposed to moisture or an alkaline solution, with the pure glass powder binders suffering the highest strength loss. Structural changes to the reaction product on exposure to moisture are explained using microstructural and FTIR spectroscopic observations. Doping the systems with Al containing (metakaolin) and Ca containing (slag) source materials, while retaining glass powder as the major component (50% or more), result in the formation of moisture-stable reaction products thereby mitigating the strength loss to a large extent.     

Effects of nano-TiO2 on strength,shrinkage and microstructure of alkali activated slag pastes

For alkali-activated slag (AAS), high drying shrinkage is an obstacle which impedes its application as a construction material. In this investigation, nano-TiO₂ was added to AAS, and its mechanical properties and shrinkage were tested to examine its effect on hardened alkali-activated slag paste (AASP). To understand the impact of nano-TiO₂ on AASP at micro scale, FTIR, MIP and SEM were carried out. Experimental results indicate that the addition of nano-TiO₂ to AAS enhances the mechanical strength, and decreases the shrinkage of AASP. FTIR and SEM results demonstrated that the addition of nano-TiO₂ into the AASP accelerates its hydration process, resulting in more hydration products and denser structure. MIP results showed that the addition of nano-TiO₂ reduces the total porosity of AASP and changes the pore structure. The porosity of 1.25–25 nm mesopores, which is believed to be responsible for the high shrinkage of AASP, is remarkably reduced due to the addition of nano-TiO₂.     

Influence of the slag content on the chloride and sulfuric acid resistances of alkali-activated fly ash/slag paste

This study aims to investigate the influence of the slag content on the chloride and sulfuric acid resistances of alkali-activated fly ash/slag (AFS) paste. A series of tests were conducted to examine the effects of reaction products and their contents on the chloride and sulfuric acid resistance capabilities. It was shown from the tests that the deterioration of the AFS binder due to a sulfuric acid attack was caused by 1) the corrosion by means of SO₄²⁻ penetration through the surface of the AFS binder, which is associated with permeable voids and a rate of water absorption and 2) the corrosion of the reaction products resulting from the different degrees of resistance to sulfuric attack between the C-(A)-S-H and the N-A-S-H. Variation of the slag content led to differences in the reaction product content of the AFS binder, clearly affecting the chloride-binding capacity and the resistance to chloride penetration.     

Influence of crack width,cover depth and concrete quality on corrosion of steel in HPC containing corrosion inhibiting admixtures and fly ash

In this study the influence of crack width, cover depth and concrete quality on corrosion of steel in high performance concrete was investigated. Three mixtures, one control and two more containing corrosion inhibiting admixtures, Calcium Nitrite and Disodium Tetrapropenyl Succinate, in combination with 20% fly ash replacement with respect to the cement weight were prepared. Specimens were concrete cylinders measuring 100 mm in diameter and 65 and 105 mm in length, with a 16-mm steel bar centrally placed at two concrete covers of 25 and 45 mm. Before being exposed to a simulated marine environment, the specimens were pre-cracked under a controlled splitting test with crack widths ranging from 90 to 330 μm formed perpendicularly to the reinforcing bars. During a 16-month exposure, the corrosion risk of the reinforcing bars was evaluated by half-cell corrosion potentials and the corrosion rate by linear polarization method. Also, the total integrated corrosion current was estimated. Results show that, albeit to different degrees, cracking was found to be an influencing factor in promoting corrosion of the steel in concrete with either 25 mm or 45 mm concrete cover; nevertheless, the effectiveness of the concrete cover depended greatly on the crack thickness. Results also revealed that corrosion inhibitors and fly ash were effective in delaying corrosion even in cracked concrete.     

Influence of limestone content,fineness, and composition on the properties and microstructure of alkali-activated slag cement

The influence of the fineness, concentration, and chemico-mineralogical composition of limestone on the workability, reaction kinetics, compressive strength, microstructure, and binder gel characteristics of sodium carbonate–based waste-activated waste slag cement pastes was investigated in this work. Alkali-activated slag cements incorporated with limestone, containing 33–100% of calcite, at a content of up to 60% with a 28-day compressive strength of 26.2–48.8 MPa were proposed. The main reaction products of hardened alkali-activated cement pastes and those incorporated with limestone are CSH, CaCO₃, Na₂Ca(CO₃)₂·5H₂O, and Na₂CaSiO₄. “Physically active” limestone does not chemically react with the binder gel but it can improve the physical structure. The higher packing density of mixed cement, without an increase in the water demand, the satisfactory binding strength of limestone with the binder gel lead to the improvement in the physical structure and compressive strength of alkali-activated slag paste.     

Influence of the annealing in nitrogen atmosphere on the XRD, EDX, SEM and electrical properties of chemical bath deposited CdSe thin films

The influence of annealing in nitrogen atmosphere on the structure, optical and electrical properties of cadmium selenide (CdSe) thin films deposited by chemical bath deposition (CBD) onto glass substrates was studied. The samples were annealed in nitrogen atmosphere at various temperatures. A transition from metastable nanocrystalline cubic to stable polycrystalline hexagonal phase has been observed after annealing. The as-deposited CdSe thin films grow in the nanocrystalline cubic phase with optical band gap 1.93 eV. The electrical resistivity of the thin films has been measured in order of 10⁶ Ω cm. The activation energy of the samples has been found to be 0.26–0.19 eV at low temperature region, and 0.36–0.56 eV at high temperature region. It was also found that the activation energy and the resistivity of the films decrease with the increasing annealing temperature.