The paper describes a field trial using deep in situ dry soil mixing to stabilize soft peat deposits beneath the route for the Channel Tunnel Rail Link in the SE of the UK. The aim of this trial was to evaluate whether a ground granulated blast furnace slag (GGBFS)–red gypsum mixture could be used as a replacement for a cement-based binder. Red gypsum is produced as a co-product to the production of the white pigment titanium dioxide. Trial columns were installed adjacent to the main stabilization works. A combination of in situ and laboratory testing has shown that the blended binder was as effective as ordinary Portland cement in increasing the strength of the peat. The blended binder columns also displayed similar durability characteristics. Using the alternative binder required no alteration to existing equipment and caused no reduction in production rates. The minerals formed in the samples from the GGBFS–red gypsum columns were of the type formed by pozzolanic reactions and the strength of the GGBFS–red gypsum columns was dependent on achieving a pH of 10.5. The paper recommends that peat samples must be stored in airtight containers prior to initial design mix testing. If required, the pH of the soil–binder mixes should be adjusted (using a small addition of lime) during the treatment process.