Concrete has a tremendous influence on the environment since the majority of its composition is cement, which is a material that emits high levels of carbon dioxide. It is possible for concrete construction to have a lower impact on the environment if the usage of cement is reduced as much as possible by the addition of mineral admixtures such as fly ash, without sacrificing the durability standards at the same time. The disposal of fly ash, which is produced by power stations that burn coal for fuel, is recognised as one of the most pressing environmental issues. When there is a simultaneous increase in the amount of fly ash and a decline in the capacity of landfills, it is much more difficult to find a solution to this problem. The research on fly ash admixed concrete is analysed and discussed in this publication. There have been many studies conducted on the topic of fly ash concrete and its beneficial effects. In this study, the fresh and hardened properties of fly ash concrete, such as mechanical properties, durability parameters, and microstructural characteristics, are studied. Additionally, the useful application case studies of fly ash concrete published by the American Coal Ash Association are also summarised. 

Fly ash for sustainable construction: A review of fly ash concrete and its beneficial use case studies

This study reports the enhanced chemical resistance of a blended concrete mix (CFNI) made with 40 wt.% fly ash, 2 wt.% nanoparticles, and 2 wt.% sodium nitrite inhibitor as partial replacement of cement against calcium leaching, acid and sulfate attacks. The concrete test specimens of four different compositions were fabricated and immersed in natural seawater, 3% sulfuric acid solution, and 10% magnesium sulfate solution for 120 days. Long-term chemical deterioration of the concrete systems is evaluated by assessing visual changes of the specimens and solutions along with the changes in percentage mass loss, compressive strength of the concrete, pH of the solution, and dimensions. The results indicate that CFNI concrete exhibits a superior resistance against chemical attack under all the three aggressive environments. Detailed chemical characterization of the specimens, carried out using XRD, FTIR, and thermogravimetric analysis, reveal a reduced CaO content, absence of deterioration phases like ettringite, brucite, and enhanced C-S-H content in the CFNI concrete. The addition of nanoparticles and inhibitors into fly ash concrete has lowered w/c ratio, increased surface pH, enabled conversion of soluble calcium hydroxide into insoluble calcium silicate hydrate, filled pores/voids, and reduced shrinkage and cracking. The compact microstructure of the CFNI prevented leaching and reduced the ingress of aggressive chemical ions into the concrete. Our results demonstrate that incorporation of nanoparticles and inhibitor into the fly ash concrete composition is ideally suited for the design of high-quality, low-permeable concrete structures that is the key for enhanced chemical resistance in natural and industrial environments.

Binary blended fly ash concrete with improved chemical resistance in natural and industrial environments

This research focused on the modification effects on recycled concrete (RC) prepared with nano-SiO2 and CO2 cured recycled coarse aggregates (RCA) subjected to an aggressive ions environment. For this purpose, RCA was first simply crushed and modified by nano-SiO2 and CO2, respectively, and the compressive strength, ions permeability as well as the macro properties and features of the interface transition zone (ITZ) of RC were investigated after soaking in 3.5% NaCl solution and 5% Na2SO4 solution for 30 days, respectively. The results show that nano-SiO2 modified RC displays higher compressive strength and ions penetration resistance than that treated by carbonation. Besides, we find that ions attack has a significant influence on the microcracks width and micro-hardness of the ITZ between old aggregate and old mortar. The surface topography, elemental distribution and micro-hardness demonstrate that nano-SiO2 curing can significantly decrease the microcracks width as well as Cl- and SO42- penetration in ITZ, thus increasing the micro-hardness, compared with CO2 treatment.

A Comparative Study of the Properties of Recycled Concrete Prepared with Nano-SiO2 and CO2 Cured Recycled Coarse Aggregates Subjected to Aggressive Ions Environment

This study compared the effects of the sulfate dry–wet cycle on the properties of ordinary concrete and nano-TiO2-modified concrete, including the mass loss rate, ultrasonic wave velocity, compressive strength, and XRD characteristics. In addition, a series of compression simulations carried out using the PFC2D software are also presented for comparison. The results show the following: (1) with an increase in dry–wet cycles, the damage to the concrete gradually increased, and adding nano-TiO2 into ordinary concrete can improve the material’s sulfate resistance; (2) after 50 sulfate dry–wet cycles, the mass loss rate of ordinary concrete was –3.744%, while that of nano-TiO2-modified concrete was −1.363%; (3) the compressive strength of ordinary concrete was reduced from 41.53 to 25.12 MPa (a reduction of 39.51%), but the compressive strength of nano-TiO2-modified concrete was reduced from 49.91 to 32.12 MPa (a reduction of 35.64%); (4) after a sulfate dry–wet cycle, the nano-TiO2-modified concrete surface produced white crystalline products, considered to be ettringite based on the XRD analysis; (5) when considering the peak stress and strain of the concrete samples, the numerical results agreed well with the test results, indicating the reliability of the method.

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Corrosion Performance of Nano-TiO2-Modified Concrete under a Dry–Wet Sulfate Environment

Nowadays, utilizing large amount industrial by-product fly ash (FA) as the alternatives for cement in self-compacting concrete (SCC) had attracted more attention. In this study, FA was employed in SCC at five levels (0 %, 20 %, 30 %, 40 %, 50 %). The mechanical behaviors, the water porosities, the transport properties, and the sustainability of FA series SCC were investigated. At the initial
 curing stage (3 days), the use of FA in SCC reduces mechanical properties and increases water porosity, water absorption and water absorption coefficient (sorptivity) of SCC. FA series SCC have the lower resistance against carbon dioxide
 attack and chloride ion penetration than cement-based SCC. The prolonging curing time is beneficial to improve the long-term behaviors of FA- blended SCC. After SCC made by 20 %, 30 %, and 40 % FA water-curing for 90 days, there are the reduction of 0.44–2.09 % in the mechanical behaviors and the increase of 0.082–0.41 % in the water porosity, compared to pure-cement SCC. Beyond the content of FA (40 %), the difference
 s of the mechanical properties and the water porosity between SCC with 50 % FA and fully cement SCC are below the value of 2.5 %. With the progress in the curing time, the largest reduction rates of the water absorption and the sorptivity in all SCC mixtures were found in 50 % FA-blended SCC. Utilizing 50 % FA in SCC reduces the total charge passed values of SCC. The manufacture of 50 % FA-blended SCC has the lowest energy consumption
 and released amounts of CO2, NOx, and SOx in all series SCC mixtures. The application of high-level FA to SCC is the positive assistance to prepare sustainable SCC with satisfying long-term behaviors.

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Sustainable self-compacting concrete containing high-amount industrial by-product fly ash as supplementary cementitious materials.

Sulfate attack on concrete structures is a major durability concern wherein concrete interacts with marine water, swamp water, groundwater, sewage water, freshwater, etc. In this study, the supplementary cementitious materials such as fly ash (FA) and nanoparticles are together incorporated into conventional concrete aiming to enhance the resistance of concrete against the penetration of sulfates. The present work is focused to understand the degradation in FA concrete modified with nanoparticles by surface sulfate attack. Concrete mix such as FA and FA modified with 2 wt% nano-TiO2 (FAT), nano-CaCO3 (FAC), and 1:1 ratio of nano-TiO2 to nano-CaCO3 (FATC) was fabricated. The specimens were exposed in 3% of ammonium and sodium sulfate for 90 days. The deterioration effects and changes in microstructural properties in all the specimens were comparatively studied. Results showed FAT, FAC, and FATC concrete have been deteriorated in ammonium and sodium sulfate solution compared with FA concrete. Partial replacement of cement with fly ash decreases the quantity of freely available reactive aluminates. Consumption of free lime by the fly ash prevents to react with sulfate. The enhanced properties of fly ash concrete against sulfate attack could be achieved with less C3A content thus reducing the available Ca(OH)2 and reducing the possibility of development of deleterious ettringite and gypsum.

Investigation on surface sulfate attack of nanoparticle-modified fly ash concrete

This study is focused on polymeric coatings on fly ash concrete submerged under seawater. The specimens were casted and coated with acrylic resin and epoxy resin of three layers each. The mechanical, durability, and microstructural properties of coated and uncoated fly ash concrete specimen were studied as pre- and post-exposed in seawater. Fly ash concrete coated with epoxy and acrylic had attained more strength compared to uncoated specimens. An increased strength in coated specimens and a decrease in value were observed in uncoated fly ash concrete specimens during split tensile strength. Coated specimens showed less reduction in pH value as compared to uncoated specimens. Rapid chloride permeability test (RCPT) analysis confirmed that epoxy and acrylic-coated concrete specimens appear to be denser than uncoated specimens leading to more resistance against the penetration of aggressive chemicals. The X-ray diffraction (XRD) comparative analysis of 56 and 90 days acrylic resin and epoxy resin-coated and uncoated specimen showed higher intensity in 90 days coated specimens than the uncoated specimens. Field emission scanning electron microscope (FESEM) investigation of uncoated 56 and 90 days concrete specimens subjected to seawater demonstrated dense appearance of hydrated products, whereas epoxy and acrylic-coated specimens were verified with no visible micro-cracks or holes on the surface, even at higher magnification. The epoxy and acrylic-coated fly ash concrete showed high physical strength and good bonding with concrete and will be appropriate for construction.

Ramachandran Dasnamoorthy

Papers

Investigation on surface sulfate attack of nanoparticle-modified fly ash concrete

Study on polymeric coatings on fly ash concrete under seawater.