Abstract:

Excess-sulfate phosphogypsum-slag cement （ESPSC） represents a low-carbon cementitious material that simultaneously valorizes phosphogypsum and granulated blast furnace slag （GGBS）， with carbon emissions only 20% to 30% of ordinary Portland cement. However， residual soluble phosphorus and fluorine impurities in phosphogypsum severely retard hydration， resulting in critically low early-age strength that limits practical application. Red mud， a strongly alkaline byproduct of Bayer-process alumina refining with global stockpiles exceeding 4 billion tonnes， theoretically provides reactive aluminum to promote hydration， yet its active components are predominantly locked within stable crystalline minerals including cancrinite， muscovite， and kaolinite. Traditional activation routes such as thermal calcination and mechanical grinding are energy-intensive and incompatible with low-temperature ESPSC system， necessitating a greener activation approach.

This study employed an electrochemical method to activate red mud at a constant potential of 1.15 V versus Hg/HgO in 4 mol/L NaOH solution at 60 ℃ for 3 h， exploiting cathodic reduction of iron oxides to disrupt the mineral lattice and liberate reactive aluminum species. The electrochemically activated red mud was incorporated into ESPSC at replacement levels of 10%， 15%， and 20% by mass， and its effects on mechanical properties， hydration products， microstructure， and hydration kinetics were systematically characterized using XRD， TG-DTG， SEM-EDS， ICP-MS， and isothermal calorimetry. Electrochemical treatment induces partial transformation of crystalline minerals to amorphous phases， evidenced by the weakening or disappearance of cancrinite and muscovite diffraction peaks. Ion dissolution analysis shows that aluminum concentration in the filtrate increases 18.6-fold to 259.521 mg/L， iron concentration decreases by 95.4% to 0.030 mg/L， and solution pH rises from 10.64 to 12.48， confirming substantially enhanced pozzolanic reactivity.

Incorporation of 20% electrochemically activated red mud achieves a 3 d compressive strength of 24.1 MPa， a 6.3-fold improvement over the reference group， and a 28 d strength of 45.8 MPa， a 40.5% increase. TG-DTG analysis shows that the mass loss ratio in 60 to 120 ℃ range rises from 3.32% to 7.61% at 3 d， confirming markedly greater ettringite and C-（A）-S-H gel formation. Isothermal calorimetry reveals that the hydration induction period is shortened from approximately 40 h to as little as 3 h. SEM-EDS observations confirm a denser microstructure with uniformly distributed ettringite and abundant C-（A）-S-H gel filling the pore space， while high Al and Si atomic fraction of 9.25% and 7.84% （paste with 20% electrochemically activated red mud） in partially reacted red mud particles， which verifies that reactive aluminum participates in hydration. The enhancement is attributed to two synergistic mechanisms： the greatly increased reactive aluminum supply accelerates early ettringite formation， and the retained alkalinity further activates slag by promoting Si—O and Al—O bond breakage to sustain C-（A）-S-H gel growth at later ages. This work provides an effective technical pathway for red mud valorization and addresses the inherent early-strength deficiency of ESPSC systems.

Key words: red mud, excess-sulfate phosphogypsum-slag cement, electrochemical activation, performance enhancement, hydration mechanism