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Massive landfilling of red mud (alumina residue, RM) has posed a great challenge to the sustainable development of the alumina industry. To consume such vast industrial wastes, it is of great benefits to promote reuse of RM in land reclamation to replace partial traditional cementitious materials. This study investigates the potential use of the RM in stabilizing marine dredged clay (MDC) with a low dosage of cement, where sodium silicate is used as an alkali-activator. A series of laboratory tests are performed to characterize positive role of alkali-activated RM in improving mechanical and microstructural properties of cement-treated MDC. The variation of strength, permeability and micro characteristics are examined with different RM contents, sodium silicate contents and curing time. The results indicate an optimal content of sodium silicate regardless of curing time, which grows from 10% to 20% when RM content increases from 5% to 15%. Both hydration and polymerization reactions occur in the mixture of MDC, cement and alkali-activated RM, generating products including hydration gels (C–S–H and N–A–S–H) and geopolymer (analcime, faujasite and unnamed zelites). With optimal content of alkali, the stacked structure combining quadrate crystal clusters and gels can be observed, which contributes to significant strength improvement of stabilized MDC. [ABSTRACT FROM AUTHOR]

Cement-based treatment is a well-established technology for converting unwanted marine dredged clay (MDC) to qualified filling materials. However, a basic understanding of the strength development of cement-treated MDC is still lacking. In this study, we attempt to characterize the strength variation of cement-treated MDC under a wide range of water-to-cement ratios (w/c). The dominant influencing factors such as initial water content, cement content, curing time and pore salinity are examined through a series of laboratory tests. The results indicate that the strength of cement-treated MDC decreases exponentially with the initial water content. The strength approaches zero when the initial water content increases to 3.5–4.0 times the liquid limit, even with a high content of cement. The effect of pore salinity depends significantly on the cement content and curing time. Owing to the wide range of w/c, the extensively used Abrams' law fails to characterize the strength variation of cement-treated MDC precisely. To this end, a new strength prediction model is proposed based on the concept of minimum cement content. The volume ratio is found to be inferior to the mass ratio in defining the minimum cement content, which remains approximately constant even for a wide range of w/c. [ABSTRACT FROM AUTHOR]

Bayer Red mud (BRM) and phosphogypsum (PG) are known as two main industrial wastes around the world. Previous studies have demonstrated the contribution of BRM and PG in cement-based hydration reactions, but their collaborative role is still pending in cement-stabilized dredged soil. In this study, the collaborative role of BRM and PG was examined by comparing micro-macro properties of stabilized clay with different initial water contents, cement contents and BRM/PG proportions. The chemical and microscopic results are found highly complementary, indicating existence of alkali-activation effects, pore-filling effects and cementation damage effects in stabilized clay. The pH of 8% cement-stabilized clay increases by about 0.5 after addition of 10% alkaline BRM, which activates clay mineral in dredged soils and results in more sufficient pozzolanic reactions. In addition, the pore-filling effect of ettringite is much more contributory in soils with a higher water content of 140%. Such positive effects significantly weaken as the water content decreases to 80%, the cementation damage effects come into picture instead. The unconfined compressive strength (UCS) after 7 days curing nearly triples with an optimal proportion of BRM and PG, demonstrating effectiveness of the proposed approach. As the water and cement contents increase, the optimal proportion of BRM and PG evolves from R7.5P2.5 into R5P5. • This paper examines the role of RM and PG in cement-stabilized dredged soil with different water and cement contents. • The contribution of alkali-source from RM is significant at a low cement content and an early curing stage. • The pore-filling effect of ettringite is more important at a high water content. • The optimal proportion of RM and PG evolves from R7.5P2.5 to R5P5 as the water and cement contents increase. [ABSTRACT FROM AUTHOR]