Retarding mechanism of Zn²⁺ species in geopolymer material using Raman spectroscopy and DFT calculations

Geopolymers are the most promising alternative to Ordinary Portland Cement for oil-well cementing and well abandonment. To that end, the slurry needs a required pumping time ensured by the addition of retarders. Although zinc has been widely known to prolong the setting time of geopolymers, its mechanism of action has yet to be fully elucidated. It is herein hypothesized that zinc ions impede the first stages of silicate oligomerization (Si-O-Al), culminating in longer setting times. Pumping time measurements showed that Zn(NO₃)₂ delayed the setting time by 5 h in comparison to the zinc-less sample. DFT calculations revealed Si(OH)₄ to react with [Zn(OH)₄]^2- via a barrierless transition state, evidencing a kinetic ground for the retardation effect. Additionally, Raman spectroscopy corroborated the DFT results by showing that Q³ species in the proposed mechanism are formed more rapidly in the presence of zinc ions than in its absence.