Effect of Na⁺ and K⁺ on aluminosilicate network polymerization across alkalinity and temperature regimes: A ReaxFF reactive molecular dynamics study

Abstract

Low-calcium geopolymers derive their performance from the connectivity of sodium- and potassium-aluminosilicate hydrate networks. Here, reactive molecular dynamics simulations were used to determine how alkali type, effective alkalinity, and temperature (293 and 600 K) govern the formation and structural evolution of N-A-S-H and K-A-S-H clusters in charge-neutral, Ca-free alkaline solutions. Structures were quantified by Si and Al Qn distributions, bridging-oxygen deprotonation, T-O-T angle statistics, and the radius of gyration of the largest cluster under bond-order filtering. Lower effective alkalinity promotes polymerization in both Na and K systems, shifting Si and especially Al from Q0–Q1 toward Q2–Q4, while elevated temperature further accelerates reorganization toward higher connectivity, most clearly under lower effective alkalinity. Bridging-oxygen deprotonation increases with hydroxide availability and temperature, yielding highly charged, weakly connected aggregates at high alkalinity. In contrast, lower effective alkalinity stabilizes larger and more persistent clusters whose compositions fall within reported ranges. Together, these results show that excessive residual alkalinity leads to a less connected aluminosilicate network and that alkalinity control is important in low-calcium binders.

The authors' abstract, as published at the source. Results in Engineering, 2026 · DOI ↗