Optimization of synergy between cement, slag, and phosphogypsum for marine soft clay solidification

•Different types of industrial solid wastes are synergistically used in soft clay treatment.•The synergy of GGBS, PG and OPC proves its advantage over OPC and binary cement.•Chemical additives improve the mechanical properties of ternary cement solidified clay.•The enhancement is due to active pozzolanic reactions and ettringite expansion.•Types of alumina-containing additives greatly affect the growth characteristics of ettringite.