Synergistic sulfate-alkaline activation of one-part volcanic pumice–cement binders: Mechanisms and microstructural evolution

The synergistic activation of a novel one-part volcanic pumice-PC hybrid binder with a Na₂SO₄-Al₂(SO₄)₃-Ca(OH)₂ (NŜ-AŜ-CH) ternary system was elucidated using a suite of advanced characterization techniques. The reaction proceeds via two distinct pathways: a rapid sulfatic pathway, where Al₂(SO₄)₃ promotes ettringite (AFt) formation for early strength, and a primary alkaline pathway, where the Na₂SO₄-Ca(OH)₂ synergy generates in-situ NaOH, driving VP dissolution and C-(N)-A-S-H formation. This resulted in a nearly threefold increase in 1-day strength, with optimized binders reaching over 70 MPa at 90 days. Long-term analysis reveals the “dual role” of AS, as its persistent AFt provides microstructural reinforcement. This leads to a “composite strength mechanism,” a key finding where high strength is achieved even in systems with a less polymerized silicate network (low Mean Chain Length). The NS-CH synergy, in contrast, is the primary driver for high polymerization, informing a new model for designing sustainable binders. • A novel ternary NŜ-AŜ-CH system successfully activates one-part VP-PC hybrid binders. • Activation proceeds via synergistic alkaline and sulfatic reaction pathways. • Al 2 (SO 4 ) 3 has a dual role, promoting early ettringite and long-term reinforcement. • Strength increased threefold at 1 day and reached 70 MPa at 90 days. • Advanced analysis (ICC, NMR, XRD, FTIR) tracks gel formation and evolution, and provides a mechanistic model.