Lubomír Kopecký

Material and structural characterization of alkali activated low-calcium brown coal fly ash

The waste low-calcium Czech brown coal fly ash represents a considerable environmental burden due to the quantities produced and the potentially high content of leachable heavy metals. The heterogeneous microstucture of the geopolymer M(n) [-(Si-O)(z)-Al-O](n).wH(2)O, that forms during the alkaline activation, was examined by means of microcalorimetry, XRD, TGA, DSC, MIP, FTIR, NMR MAS ((29)Si, (27)Al, (23)Na), ESEM, EDS, and EBSD. The leaching of heavy metals and the evolution of compressive strength were also monitored. The analysis of raw fly ash identified a number of different morphologies, unequal distribution of elements, Fe-rich rim, high internal porosity, and minor crystalline phases of mullite and quartz. Microcalorimetry revealed exothermic reactions with dependence on the activator alkalinity. The activation energy of the geopolymerization process was determined as 86.2kJ/mol. The X-ray diffraction analysis revealed no additional crystalline phases associated with geopolymer formation. Over several weeks, the (29)Si NMR spectrum testified a high degree of polymerization and Al penetration into the SiO(4) tetrahedra. The (23)Na NMR MAS spectrum hypothesized that sodium is bound in the form of Na(H(2)O)(n) rather than Na(+), thus causing efflorescence in a moisture-gradient environment. As and Cr(6+) are weakly bonded in the geopolymer matrix, while excellent immobilization of Zn(2+), Cu(2+), Cd(2+), and Cr(3+) are reported.

Nanoindentation of Alkali-Activated Fly Ash

Low-calcium ground fly ash was activated with a sodium silicate solution and cured under ambient and heat conditions. Nanoindentation was employed for the characterization of reaction products, mainly alumino-silicate (N-A-S-H) gel. Samples were indented by a series of grids consisting of hundreds of indents each. The intrinsic Youngs modulus of the N-A-S-H gel was found to be around a mean value of 17.7 GPa regardless of the curing procedure. Other characteristic phases were also identified, namely partly activated slag, nonactivated slag, and fly ash particles. Results of pore size distributions measured by mercury intrusion porosimetry were systematically observed depending on the curing procedure. Additional measurements using the same methodology were performed on cement paste samples. The synthesized N-A-S-H gel was found to be mechanically close to calcium silicate hydrate gels in matured cement pastes.

Characterization of Alkali-Activated Fly-Ash by Nanoindentation

Nanoindentation was employed for the characterization of reaction products, mainly N-A-S-H gel, within alkali-activated fly ash samples. Heat and ambient-cured samples from ground fly ash were indented in a grid of hundreds of indents. The intrinsic Young’s modulus of N-A-S-H gel was found around the mean value 17.70 GPa, regardless on the curing procedure. Such finding elucidates intrinsic stiffness of mature N-A-S-H gel with different origin. Partly-activated slag, slag and fly-ash particles were further distinguished by histogram deconvolution.

Effect of Pva Modification on Properties of Cement Composites

Polymers are used for modification of the cement-based composites and others building materials since the thirties of 20th century. Based on the conclusions of recent studies, it is assumed that even water soluble polymers could be used as an admixture for such modification. Currently, there exist and are exploited several possibilities for polymer modification of mortars, wood-based products or bituminous asphalts. Various options differ in the way of modification, which can be basically applied to the entire volume or just a surface, but also in the form of the polymer used – either in the form of solution or fibers. The aim of our study was to investigate the influence of volume modification by the water soluble polymers, such as polyvinyl alcohol (PVA), on the properties of cement paste and find an optimum additive. It turned out that the addition of PVA solution into fresh cement paste results in an increase of porosity and therefore a stiffness and compressive strength reduction. On the other hand, the bending strength of PVA-rich specimens was significantly higher and their water absorption decreased, which may consequently result in enhanced frost resistance.

Application of Micromechanics on Alkali-Activated Materials

Research of alkali-activated materials has been a traditional domain of chemists. This paper exploits contribution of micromechanics to the subject. A new model for volumetric evolution of chemical phases is formulated. The first homogenization level identifies elasticity on the scale of N-A-S-H gel. Nanoindentation sensing technique yielded the intrinsic Youngs modulus of N-A-S-H gel as ~18 GPa, which was further downscaled to the solid gel particles. Percolation theory had to be introduced to match an early-age elasticity. The second homogenization level takes into account an unreacted fly ash. Homogenization models match well the experimental elasticity and demonstrate stiffening of N-A-S-H gel, induced by increasing packing density of the solid gel particles. The percolation model explains a long setting time of alkali-activated materials.

Using 2D Digital Image Analysis to Locate Position of Micro Fibers in Cross-Sections of Fiber-Reinforced Concrete

This work deals with determination of location of micro fibers positions in fiber-reinforced concrete. The digital images of sectioned cement-paste samples with dimension equal to 40 × 40 mm were used as an information source about the monofilaments positions. Properly acquired digital image of high resolution allows to determinate the number of fibers in samples cross sections and relate theirs coordinates to any point. Optical microscope Carl Zeiss Axio Zoom.V16 with camera and software allowing individual shots composition of examined samples surface was used to obtain these parameters. Cement pastes reinforced with PET (polyethylene terephthalate) micro fibers having diameter equal to 0.4 mm were studied. The total number and the fibers distribution along the height and width of the sample cross section were examined.

Cement Composite Reinforced with Synthetic Fibers: Comparison of Three-Point and Four-Point Bending Test Results

Presented article deals with the influence of PET fiber production on the bending strength of cement-based composite when incorporated into the fresh mortar, and comparison of results of 3-point and 4-point bending test. Cement paste samples were reinforced with 2 wt. % of primary or recycled PET fibers. The bending test was performed on prismatic samples with dimension of 40 × 40 × 160 mm. It was found that samples with recycled PET fibers, compared to primary ones, exhibit a decrease in bending strength. In the case of 4-point bending tests, the samples with recycled PET fibers exhibited higher bending strength than reference samples without any fibers. However, in the case of 3-point bending tests, the samples with recycled PET fibers had lower bending strength than the reference ones. The results suggest that recycled PET fibers could be used as an alternative to reinforce cement-based composites.

Influence of the Oxygen Plasma Treatments on Surface Wettability of Glass Micro Fibers Used as Reinforcement in any Mortars

Presented work deals with the surface treatments and its effect on micro fibers using as randomly dispersed reinforcement in many types of composite materials. Cool oxygen plasma was used to surface wettability modification of chopped glass fibers having diameter equal to 14 μm. Plasma treatments were carried out at three different times of exposition equal to 4 min, 8 min and 16 min. The influence of executed treatments was observed by the horizontal direct optical method enabling static contact angle measurements on micro fibers which were submerged in a distilled water. The identified differences between the contact angles size of original fibers and the treated fibers were equal to several tens of percent.

The Behavior of Fly Ash in Concrete Mixture

It is desirable to use secondary raw materials with regard to the sustainable development. One such suitable material is fly ash. Still enough unused possibility of using fly ash is the use in cement and concrete. This use brings a positive ecological and economic effect. However, it is important to devote to the behavior of fly ash in concrete mixture at the macro and micro level. This paper deals with selected properties as a positive influence on the development of hydration heat, the increase in compressive strength or problems with water absorption fly ash.

Influence of Oxygen Plasma Surface Treatment of PET Micro Fibers on Flexural Strength of Reinforced Cement Pastes

Presented work deals with PET (polyethylene terephthalate) fiber-reinforced cement pastes and cool oxygen plasma fiber surface treatment used to attain the better adhesion between fibers surface and the cement matrix. Three sets of cement paste samples were made with the same matrix (CEM I 42.5R with water to cement ratio equal to 0.4). The two sample sets contained micro fiber reinforcement varying in surface properties. One set was reinforced with unmodified fibers, while in to the other set plasma treated fibers were used. As a comparative indicator to bending response of the composite materials, four-point destructive tests were carried out. The samples reinforced with unmodified fibers exhibited deflection-softening behavior during the post-cracking phase, while samples with plasma treated fibers exhibited deflection-hardening behavior.