Recycling industrial alkaline solutions for soil stabilization by low-concentrated fly ash-based alkali cements

• Published in: Construction & building materials Vol. 393; p. 132083
• Main Authors: Lal Mohammadi, Esmaeil, Khaksar Najafi, Elmira, Zanganeh Ranjbar, Payam, Payan, Meghdad, Jamshidi Chenari, Reza, Fatahi, Behzad
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 22.08.2023
• Subjects: High-calcium fly ash; Low-calcium fly ash; Low-range alkaline cement; Recycling alkaline solution; Soil improvement; High-calcium fly ash; Low-calcium fly ash; Soil improvement; Low-range alkaline cement; Recycling alkaline solution
• ISSN: 0950-0618; 1879-0526
• DOI: 10.1016/j.conbuildmat.2023.132083

•The use of a residual alkaline solution to enhance the clay properties is examined.•UCS, ITS, UPV and microstructural (SEM, XRD) tests are conducted.•High- and low- calcium fly ash are introduced into the alkaline activation process.•A microstructural analysis showed the formation of binding gels.•An environmental assessment is performed to compare the CO2 emission. This study aims to evaluate the usability of a recycled alkaline solution, a residue from cleaning iron plates in the car manufacturing industry, to enhance the strength and stiffness of clayey soil by carrying out unconfined compressive strength (UCS), indirect tensile strength (ITS), ultrasonic pulse velocity (UPV) and microstructural (SEM, XRD) tests. High- and low- calcium fly ashes with percentages of 10% to 40% and 10% to 30%, respectively, were independently introduced into the alkaline activation process to determine the proper type and content of precursors at room temperature. Two mixtures of the residual cleaning solution blended with 2 M and 4 M NaOH solutions at a weight ratio of one were also utilized to determine how efficiently binary solutions could improve the performance of the parent clay soil. These experiments revealed that all mixtures of the residual alkaline solution work better to activate high-calcium fly ash as compared to the low-calcium fly ash in terms of both mechanical strength and stiffness. The sample of high-calcium fly ash with fly ash/solid and Na/ash ratios equal to 30% and 0.037, respectively, experienced the highest improvement. A microstructural analysis showed the formation of C-(A)-S-H and N–(C)–A–S–H gels in the corresponding sample, thus providing a considerably dense structure, which in turn significantly contributed to the enhancement of the maximum strength and stiffness of the treated soil. In fact, the high-calcium precursor proved to be the best candidate for a low-range alkaline activator because it contributes more calcium to a low-alkaline environment. An environmental impact assessment was also carried out to compare the CO2 emissions of clays treated with the 4 M NaOH binary solutions tested in this study with those conventionally stabilized by cement and lime and also those treated traditionally with commercial 2, 4, and 8 M NaOH. The samples in this study treated with environmentally friendly binary solutions (residual cleaning solution blended with 2 M and 4 M NaOH solutions at a weight ratio of one) performed better in terms of both CO2 emissions and strength gain compared to the samples stabilized with conventional materials.