In Japan, volcanic fine soil as well as highly organic soil are categorized as 'problematic soil'. Since the engineering properties of these problematic soils greatly differ from those of textbook-type geomaterials, these soils are often improved in the engineering practice by mixing them with cement, for instance. It has been reported that some natural agents included in the problematic soils such as allophane and organic substance often impede cement solidification reaction. It has also been reported that the inhibition of the solidification reaction may be avoided by adding steelmaking slag comprising free lime. However, laboratory investigation into the effects of content of these agents on the cement solidification reaction is scarce. In this paper, the mechanical as well as chemical properties of various problematic soils are examined by using a single indicator, i.e., the content of alkali adsorption capacity. First, a fairly good relationship between the amount of allophane/organic substance and the alkali adsorption capacity was found for of several problematic soils tested. Second, it was manifested that the unconfined compression strength of cement/steel-slag mixed soil was much higher than that of comparative strength of cement mixed soil. The tendency was more pronounced as the alkali adsorption capacity increased.

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