## 1 Introduction  
Lacustrine and marine diatomaceous soil (DS) is found extensively in many countries, such as Mexico [1], Japan [2], Korea [3], Columbia [4], and China [5]. DS contains numerous fossilized diatoms with sizes in a range of several to dozens of micrometers [6] as a result of the accumulation of unicellular diatom cell walls (the diatom cell wall is referred to as frustules) [7]. A diatom frustule comprises mainly amorphous SiO\({}_{2}\) (Opal A) and some feldspar, quartz, pyrite, etc. [8; 9; 10]. The geotechnical characteristics of DS are significantly different from those of fine-grained soil without diatom; for example, (i) the porosity of natural DS can be as high as 74% [11], (ii) DS is characterized by a low specific gravity (G\({}_{\text{S}}\)) of 2.26-2.38, which is lower than that of common clayey soil such as kaolin (G\({}_{\text{S}}\) = 2.775) and Singapore clay (G\({}_{\text{S}}\) = 2.770) [12], and (iii) natural DS has a high water content which can exceed 400% [13]. However, although DS has these relatively poor physical properties, it also has a high yield stress of ca. 540 kPa [14] and a high friction angle of 32-43\({}^{\circ}\)[15]. The Atterberg limits are among the most important physical properties of fine-grained soil and the plasticity index (\(I_{\text{P}}\)) is critical in soil classification. Many studies have investigated the plasticity properties of DS and provided a preliminary understanding of its Atterberg limits [6; 16; 17; 18; 12], showing that the Atterberg limits of DS are higher than the Atterberg limits of fine-grained soil without diatoms. For example, the liquid limit (\(w_{\text{L}}\)) and plastic limit (\(w_{\text{P}}\)) of artificial DS with a high content of pure diatoms can be as high as 289.7% and 205.5%, respectively [19]; additionally, the \(w_{\rm L}\) value was found to be over 400% for natural DSs from Mexico and Columbia, and their \(w_{\rm P}\) value was ca. 80% [1; 4]. Some studies showed that the Afterberg limits of DS are influenced by the mineralogy and fines fraction [12; 20]. More diatoms may lead to a steady increase in Afterberg limits and the horizontal rightward proceeding across the A-Line in the plasticity chart [6]. Different diatom species also affect the plasticity properties of the DS; for example, DS contains disk-shaped diatoms that cause higher plastic and liquid limits than that contain tube-shaped diatoms [16]. Despite the current studies, more work is required to better understand the Afterberg limits of DS, i.e., (i) how the NaCl concentration of pore fluid and crushing degree of diatoms influence the Afterberg limits of DS (these two factors should be considered for DS because it conventionally forms in marine environments and contains either completely or partially crushed diatoms under the effect of geological burial), (ii) the microstructural difference between DS and common natural fine-grained deposits without diatom (also the association between the diatom microstructure and the Afterberg limits of DS and the corresponding underlying mechanism), and (iii) the applicability of the Unified Soil Classification System (USCS) for fine-grained DS, because this classification method involves the Atterberg limits and the particle size characteristics. The study reported herein aimed to investigate the Atterberg limits of DS as affected by diatom content, diatom crushing, and pore fluid ionic concentration. For this purpose, the studied DS was obtained by mixing kaolin with pure diatoms of differing content and crushing degree, and NaCl solution replaced deionized water as the pore fluid for some of the diatomaceous samples. The plastic and liquid limits of this soil were measured by thread-rolling and fall-cone tests, the diatom and DS structures were studied microscopically via scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the pore characteristics of the DS were revealed via mercury intrusion porosimetry (MIP).