## 1 Introduction  
An artificial algal reef (AAR), a type of artificial reef, is an underwater structure created to improve the environment for seaweed growth and increase the protection of fishery resources [1; 2; 3]. The many types of artificial reef material have different adhesive properties and biological effects [4; 5]. In general, concrete is the most common material for AARs due to its stable structure, availability, and ease of modeling. Artificial reefs made of concrete can provide a substrate for seaweed attachment, which provides a habitat for the accumulation and growth of fish and food and shelter for invertebrates and crustaceans [6; 7; 8; 9]. With the expanding application of artificial reefs around the world, a lot of recycled waste or low-value material is gradually being used to develop artificial reefs, with the aim of environmental protection and for economic reasons, including fly ash [10; 11; 12; 13], blast furnace and steel slag [14; 15; 16], and shells [17; 18; 19; 20]. AARs mixed with tidal sludge can continuously dissolve nutrients containing nitrogen and phosphorus into the surrounding water, an effect similar to slow-release fertilizer [3]. Concretes with bio-activators admixtures can improve marine colonization, and the pre-carbonation of cementitious materials accelerates the growth of microorganisms forming a biofilm by lowering the pH of the surface [21; 22]. Therefore, the production of artificial reefs based on AAR concepts and technologies can save a lot of raw material and effectively consume waste, thus protecting land resources and the environment while avoiding the pollution caused by man-made waste disposal.  
In recent decades, China's coastal zone has experienced intense development and utilization. In particular, large numbers of aquaculture facilities such as fish cages and kelp or shellfish rafts have been set up in coastal waters to develop marine aquaculture. On the one hand, large quantities of bait and excrement from farmed objects are deposited on the seabed; on the other hand, large-scale aquaculture facilities hinder tidal movement and reduce the exchange efficiency of seawater, greatly changing the hydrodynamic patterns and sedimentary environment in the sea area [23; 24]. The resulting sediments, combined with large amounts of silt brought by land soil erosion and continental runoff, have formed extensive silty seabed, which have semi-permanently changed the benthic environment of these seas. For example, in the marine aquaculture areas of Shuangdao Bay, Shandong Province, the silt layer is particularly thick. An anoxic layer can therefore easily form on the seabed, causing the large-scale death of benthic organisms, a major cause of environmental disasters in nearby seas [25]. If the seabed silt could be used as artificial reef material in the same way as tidal sludge, this would not only make full use of seabed waste to achieve green circulation but also improve and restore the coastal seabed environment. However, few studies relevant to this concept have been conducted.  
In this study, seabed silt was used as a raw material and mixed with cement in four proportions to form concrete mixtures for the preparation of silt artificial reef (SAR) specimens. The compressive strength development of the SARs was then evaluated, and the hydration products and microstructure of the SAR paste were investigated by XRD, SEM, and differential scanning calorimetry (DSC) techniques. The aim of this study was to explore the feasibility of using seabed silt in the preparation of AARs and provide technical support and a scientific reference for the ecological restoration of the coastal seabed environment.