Disruption of Lignin-Emissive Domains: Deciphering Fiber-Cement Interfacial Reactions by EEM Fluorescence and DUV Raman Spectroscopy.

Deciphering the fiber-cement interfacial reaction without chemical labeling and digestion remains a significant challenge in vibrational and fluorescence spectroscopy. Herein, we have elucidated the reaction between lignin and calcium hydroxide (Ca(OH)2) utilizing excitation emission matrix (EEM) fluorescence and deep ultraviolet (DUV) Raman spectroscopy. Tracking lignin photoluminescence via excitation-emission maxima (EX-EM max) reveals emission governed by both classical aromatic chromophores and supramolecular clusteroluminogen networks. In unbleached Kraft pulp (UBKP) fibers, the excitation-dependent shift in emission maxima reflects multiple coexisting emissive cluster populations. Cement matrix-induced chemical degradation disrupts lignin emissive domains and drives a characteristic hypsochromic shift in EX-EM max, which depends on fiber size fraction. We highlight the diagnostic advantage of selecting a single 330 nm excitation wavelength to concurrently probe phenolic (∼415 nm) and nonphenolic (∼540 nm) lignin domains. The observed fluorescence spectral evolution is consistent with the progressive lignin-Ca(OH)2 reaction. Spectral tracking over 2 years demonstrates that this reaction follows diffusion-controlled kinetics, culminating in progressive fiber mineralization. Incorporation of a rock dust blend (RDB) as a solid-state pH modifier preserves composite flexural strength across 5-20 wt % cement replacement, suppressing phenolic degradation while reducing the hypsochromic shift of nonphenolic emissions by ∼50%. Aqueous alkaline model systems (NaOH, pH 11-13) further support these findings, showing that phenolic reactivity (I415) increases with alkalinity. DUV Raman spectroscopy confirms that hydroxide ions from Ca(OH)2 preferentially attack ether (C-O) and carbonyl (C═O) linkages, causing structural breakdown of the lignin. In agreement with fluorescence tracking, the overall extent of the fiber-cement interfacial reaction strongly depends on both fiber size fraction and fiber content.