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Chemical reactions can lead to the expansion of the geomaterial because of the strong precipitation of minerals in the pores of the medium. The quantity and the variability of these reactions led to the development of several chemo-mechanical models. A generic chemo-poromechanical model is proposed to predict both the material degradation induced by various concrete pathologies and the environmental multi-factor impact on the macroscopic swelling. The model is based on the coupling between reactive transport (species diffusion and chemical reactions) and poromechanics. The mineralogy and the poromechanical properties are estimated by a hydration-micromechanics model which allows to consider the material effects. The main applications are dedicated to External Sulfate Attack and Delayed Ettringite Formation of cementitious material at mesoscale. The studies highlight the ability of the model to deal with complex and various phenomena such as coupling chemical reactions with calcium leaching, sulfate sorption and ettringite precipitation, and to consider their impact on the macroscopic expansion. Results also bring out the accuracy of the chemo-poromechanical model to simulate different swelling pathologies. The characteristic crack paths induced by differential strain, material impact, and swelling at the macroscopic scale, fit with experimental observations. [ABSTRACT FROM AUTHOR]

Calcium–silicate–hydrate (C–S–H), the most important phase of hydrated portland cement, is responsible for the cementitious system's mechanical properties performance and durability. In a severe environment, C–S–H often suffers from sulphate attack, leading to severe concrete deterioration. The present work describes a detailed mechanism of sulphate attack on C–S–H from 28 to 270 days of sulphate curing (10% Na2SO4 solution) using XRD, TGA, FTIR, 29Si-NMR and FESEM/EDX techniques. The results show that gypsum is the only sulphate-bearing product formed by decalcification of calcium hydroxide first and then of C–S–H. Decalcification enhances polymerization in the C–S–H structure as an additional Q3 peak was observed in FTIR and 29Si-NMR analysis. The microstructure of C–S–H becomes more crystalline as the sulphate curing period increases, as observed in FESEM micrographs. In addition, sulphate adsorption on C–S–H was identified through FESEM/EDX, which could play an important role in the deterioration of cementitious materials. [ABSTRACT FROM AUTHOR]

Cement–bentonite (CB) slurry cutoff walls are extensively utilized to restrict the horizontal flow of fluids. In this study, cement was replaced by four various contents of 0%, 10%, 20%, and 30% by natural zeolite to evaluate its effects on the unconfined compressive strength, flexural strength, tensile strength, and the coefficient of permeability of cement–bentonite–zeolite (CBZ) specimens. Since underground cutoff walls are always in danger of sulfate attack, the CBZ specimens were immersed in three different containers with three different pH values of 2, 5, and 8 to study the sulfate resistance of this material. Results have indicated that the strength of specimens decreases with an increase in zeolite content at early ages; however, increasing the amount of zeolite up to 20% improves the mechanical properties and permeability of cutoff wall materials at late ages. The bentonite/cement (B/C) ratio equal to 0.2 shows a relatively better response than B/C ratios equal to 0.14 and 0.3. Curing specimens in the sulfate environment increased the mechanical strength of specimens and decreased the coefficient of permeability at first ages. At the late ages, curing in sulfate solutions manifested disadvantageous effects on specimens; however, specimens containing zeolite indicated higher sulfate resistance than specimens without zeolite. [ABSTRACT FROM AUTHOR]

Fe-600 grade steels are recommended for high rises and other structures in India that require high-load carrying capacity (particularly in seismic zones) due to its excellent mechanical properties. But structures in food processing, agricultural waste handling, etc., industries and high rises are subjected to attack of sulphates as well as acid rain. The present work investigates the efficacy of electroless nickel coatings in corrosion prevention of Fe-600 grade rebars exposed to aggressive sulphate environment. Ni-P, Ni-P-W and Ni-P-Cu coatings were deposited on the rebars. Electrochemical tests such as potentiodynamic polarization and electrochemical impedance spectroscopy were employed to investigate the corrosion behaviour in 0.5 M sulphuric acid. It was seen that corrosion potential of rebars was nobler when they were coated with electroless nickel ternary variant coatings. Electrochemical impedance spectra also validated the same. Scanning electron microscope images of the surface of the bare rebars post electrochemical tests revealed a completely cracked morphology due to severe attack of sulphates. Whereas the surface of Ni-P-W coated rebar revealed the least damage and highest corrosion resistance. [ABSTRACT FROM AUTHOR]

Processed spent bleaching earth (PSBE) is the end product derived from the processing of de-oiled spent bleaching earth (SBE) after the oil is recovered. Bleaching earth is very fine powder clay, and its main component is silicon dioxide used for refining process of palm oil and its by product is known as SBE is commonly disposed to landfill. The bleaching earth that is used for decolouring during the palm oils refining process form a solid sludge called the SBE. The SBE is generated in large quantities and it is becoming increasingly difficult to ignore the disposal of SBE when production of palm oil increases rapidly. Hence, the problem can be overcome by reusing solid waste as construction materials. In this study, the effect of using PSBE as a partial cement replacement in foamed concrete on durability resistance sulphate attack is investigated. Sulphate resistance in terms of expansion, loss in mass and loss in compressive strength after 52 weeks immersed in 5% sodium sulphate was evaluated. On the other hand, the microstructure changes in the specimens were analysed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetry analysis (TGA). Comparisons are made between two mixtures which main ly control foamed concrete (FC) and foamed concrete containing 30% PSBE (PFC). Major findings show that when 30% PSBE added in a concrete mixture, after 52 weeks samples being immersed in 5% sodium sulphate, it will be more durable compared to control specimen (FC). It was recorded that change in mass, in expansion and loss in strength was 1.32%, 0.98% and 7.5% respectively. The microstructure study has confirmed that inclusion of PSBE in foamed concrete is known to improve the internal structure of PFC to be denser than control FC specimen with the visible image of crowded tiny of cotton shaped (CSH) and few hexagonal plates (CH) contributing to enhanced strength and durability. Conclusively, the foamed concrete containing 30% PSBE as partial cement replacement which exhibits enhanced strength and durability than plain foamed concrete has the potential to be used in construction application. [ABSTRACT FROM AUTHOR]

Concrete is widely used for lining structure in underground facility and unfortunately it is often attacked by coupling effects of external sulfate and triaxial compressive stress transferred from surrounding soil or rock formations. In this text, a model is developed to depict the coupling effects of mechanical loading and external sulfate attack on concrete lining. The total strain is composed of elastic strain and plastic strain characterised by modified Drucker–Prager criterion, and the deterioration of mechanical properties is related to both mechanical damage and chemical damage. The former considers microcracks generation through micromechanical operations, and the latter is defined by the molar concentration of total sulfates in concrete. A simple diffusion equation is deduced from the sulfate ions diffusion-reaction process to depict the distribution of the total sulfates in both solution and solid phase. A relationship between chemical damage and sulfates concentration is established based on experimental results. The proposed model is then implemented into COMSOL multiphysics to analyse the long-term stability of concrete lining subjected to external sulfate attack in a tunnel. The numerical results show that the simulation results match the experimental data. [ABSTRACT FROM AUTHOR]