Some of the steel slag from ironworks and dredged soils from marine and waterfront engineering work are partially treated as waste. However, a mixture of these two kinds of waste has the potential to be used as construction materials when mixed, due to chemical reactions forming secondary phases. Utilizing waste of such kind as a resource will help to improve sustainability in society by reducing waste and replacing virgin resources such as cement. Recently, it was reported that mixtures of steel slag and dredged soil hardens under specific conditions. The phase compositions of dredged soils and steel slags vary depending on the quantity of each component, which results in unpredictable strength development of mixtures. The effect of the variations in the components of steel slags and dredged soils on strength development of the mixtures is not yet clear, limiting the utilization of both materials. Understanding the hardening mechanisms of the mixtures will enable the prediction of strength development. Focusing on the variations in the components in steel slags and, especially of dredged soils, this study aims to identify the components in both materials that affect the secondary phase formation that are responsible for strength development. We found support for suggestions that calcium silicate hydrate, C-S-H, is one of the secondary phases responsible for the strength development of the mixtures. From a comparison of two kinds of steel slags and various dredged soils, the amount of portlandite in the steel slags and the amount of amorphous silica in the dredged soils are suggested as a couple of the key components of starting materials involved in the C-S-H formation.

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The presence of portlandite (Ca(OH)2) in steel slag leads to potential hydration capacity, and this can be exploited to improve the engineering properties of high-water-content dredged clays. The strength and stiffness mobilisation characteristics of four dredged clays from Japan mixed with two steel slags were continuously investigated from early hours to 28 d of curing in this study, by using direct shear apparatus and bender elements, which allowed testing of mixed specimens from the initial, soft, uncemented states. The stiffness and strength increase rates in the second, main stage of evolution in the clay–slag mixtures were found to be loosely correlated to the amount of amorphous silica. The correlation was also influenced by the amount of portlandite in the slags. The results can be used to screen out clays that are hard to solidify in the medium term, based on quick on-site measurement of the amorphous silica content. For quality control in practice, the strength–stiffness relationship was critically examined as a means to assess the strength using non-destructive stiffness probes. Close observation revealed that to represent a wide range of curing time and mixing conditions by a single line, as proposed in existing studies, could be misleading. A new interpretation for the relationship is proposed.