Red　clay　exhibits　characteristics　such　as　softening　owing　to　water　absorption　and　cracking　because　of　water　loss,　which　can　lead　to　slope　instability,　road　cracking,　and　compromised　structural　integrity　when　used　directly　in　roadbed　filling.　Although　the　addition　of　industrial　materials　such　as　cement　is　a　common　engineering　treatment,　it　severely　impairs　soil　renewability.　Lignosulfonate　(LS)　extracted　from　paper　plant　waste　fluids　is　a　natural　bio-based　polymer　with　promising　applications　as　a　soil　improver.　In　this　study,　the　boundary　moisture　content　and　mechanical　properties　of　LS-treated　red　clay　were　investigated　using　Atterberg,　unconfined　compressive　strength,　and　direct　shear　strength　tests.　Additionally,　the　LS-treated　red　clay　modification　mechanism　was　explored　at　multiple　scales　using　zeta　potential　analysis,　X-ray　diffraction,　scanning　electron　microscopy　coupled　with　energy　dispersive　spectroscopy,　and　Fourier　transform　infrared　spectroscopy.　The　results　indicated　that　the　LS　dosage　significantly　affected　both　the　water　content　and　mechanical　strength　of　the　red　clay　boundaries.　The　optimal　dosage　of　LS　for　red　clay　was　3　wt.　%,　at　which　the　liquid　limit　was　reduced　by　32.97%,　the　plastic　limit　by　19.33%,　and　the　plasticity　index　by　48.37%.　The　28-day　compressive　strength　of　LS-treated　red　clay　was　increased　by　378.4%,　and　the　direct　shear　strength　was　increased　by　136%.　Analysis　of　the　microstructure　and　mineral　composition　revealed　that　the　LS-treated　red　clay　did　not　form　new　minerals,　but　primarily　filled　pores　and　connected　soil　particles.　Through　the　combined　effects　of　hydrogen　bonds,　electrostatic　interactions,　and　cation　exchange,　the　LS-treated　red　clay　reduced　the　size　of　the　mineral　particles　and　the　thickness　of　the　mineral　double　electric　layer,　resulting　in　increased　structural　densification.　These　results　are　of　great　scientific　significance　for　the　ecological　modification　of　soils.　Graphical　Abstract:　(Figure　presented.)　©　The　Author(s),　under　exclusive　licence　to　the　Iran　University　of　Science　and　Technology　2024.