Site-Specific Microstructural, Mineralogical, and Particle-Size Baseline of Rammed Earth Wall Surfaces at Lianxi Academy Ruins: Implications for Lingnan Earthen Heritage Conservation

Lianxi Academy in Zhuhai, Guangdong, is a rare surviving rammed-earth academy ruin representative of Lingnan architectural tradition, yet its rammed-earth walls suffer persistent weathering under the subtropical marine monsoon climate, with no site-specific material baseline available. This study aims to characterize the microstructural, mineralogical and particle-size features of wall surfaces, clarify internal compositional differentiation, and provide quantitative baseline data for earthen heritage conservation. Four surface crust specimens from wall facades were analyzed via multiscale scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDS), X-ray diffraction (XRD), and laser diffraction granulometry. Results show all samples share an O–Si-rich background with notable local contrasts. Local EDS analyses record 12.93 wt% Ca in Sample 2 (accompanied by a unique intersecting rod-shaped network), 6.92 wt% Al and 4.97 wt% K in Sample 3, and 11.33 wt% Fe in Sample 4. Quartz dominates all XRD patterns with strongest reflections at 26.70–26.78° 2θ, accompanied by features consistent with subordinate carbonate-, aluminosilicate-, and Fe-bearing components. Particle-size distributions are highly heterogeneous: Sample 1 is the coarsest (D50 = 34.97 μm; 41.69% ≥ 63 μm), while Samples 3 and 4 are fine-fraction-dominated (D50 = 8.93 and 8.52 μm). These findings define three distinct surface material subgroups within a universal quartz-rich matrix, and are consistent with a heterogeneous wetting and selective particulate detachment weathering model. This work fills the data gap for Lingnan rammed-earth academy heritage, and the quantitative parameters serve as preliminary benchmarks for wall condition mapping, rainwater erosion regulation, and compatible repair material trial formulation.