Decoupling between water absorption and radial expansion in bio-SAP cores of Typha domingensis for natural geocomposites

Natural composites for slope protection must act as water-retentive interfaces while preserving geometry under prolonged wetting, yet most superabsorbent systems used in soil applications are granular or substrate-mixed and do not resolve the coupling between uptake and swelling in a continuous core. This study quantifies the decoupling between water absorption and radial expansion in continuous superabsorbent biopolymer (Bio-SAP) cores of Typha domingensis over 480 h of saturation, comparing an integral fiber-reinforced composite (N1) with a concentrated polymer-matrix variant (N2) and a consolidated particulate-matrix variant (N3). An unconsolidated plant aggregate (N4) was included as a basal thermal control. Scanning electron microscopy, thermogravimetry, transmission FTIR spectroscopy, prolonged immersion, gamma generalized linear modeling, exploratory OLS regression and static puncture testing were integrated to relate interfacial consolidation, sorption, deformation, thermal stability and handling resistance. N1 retained up to 474.2% moisture at mean radial expansions between 2.0% and 5.0%, sustaining a hydrofunctional efficiency of 174 to 292 percentage points of absorption per percentage point of diametral swelling, against 80 to 268 in N2 and 68 to 274 in N3. Across the saturation plateau, N1 and N2 showed similar mean water retention, with 401.1% and 401.4%, whereas N3 retained 349.5%; mean radial expansion was 3.4% in N1 against 6.8% in N2 and 6.6% in N3 ( $$p < 0.001$$ ). OLS slopes between absorption and radial expansion were non-significant in all formulations ( $$R^2 \leq 0.012$$ ), indicating that retained water did not propagate linearly into diametral deformation after equilibrium uptake. Thermogravimetry showed that polymer consolidation suppresses the low-temperature degradation step of the raw biomass, raising the temperature of 5% mass loss from 195.8 $${}^\circ\textrm{C}$$ in N4 to between 252.4 $${}^\circ\textrm{C}$$ and 261.9 $${}^\circ\textrm{C}$$ in N1–N3, while the main pyrolysis maximum remained near 324 $${}^\circ\textrm{C}$$ in every consolidated formulation. Transmission FTIR spectroscopy showed a dose-dependent rise in the aliphatic C–H to cellulose C–O–C band ratio, from 0.35 to 0.89, at an invariant lignin aromatic ratio, indicating polymer deposition without degradation of the cell-wall constituents. The puncture resistance of N1 reached 1633 N (Weibull $$\beta = 4.46$$ ), with a conservative $$P_{10}$$ of 1078 N. The results indicate that fibrillar architecture governs deformation anisotropy and converts water uptake into functional storage rather than radial swelling, supporting continuous Bio-SAP cores as hydroretentive components for natural geocomposites within the laboratory saturation window tested.