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

“…Raman spectroscopy has proven to be a valuable tool in the study of geopolymers 8 , 9 . An advantageous feature is that their Raman spectra can be compared with those of SiO 4 glasses 7 . SiO 4 glass creation relies on the condensation of isolated SiO 4 tetrahedra (referred to as Q 0 ) by linking to each other via sharing one to four oxygen atoms (Q 1 –Q 4 ).…”

“…An advantageous feature is that their Raman spectra can be compared with those of SiO 4 glasses 7 . SiO 4 glass Table 1.…”

“…Geopolymer formation from solid materials is a complex, multistep process roughly comprising i) alkaline depolymerization of the poly(siloxo) framework and dissolution of aluminum ii) formation of monomers and oligomers from ortho-sialate (OH) 3 − Si–O–Al–(OH) 3 , and iii) polycondensation into higher oligomers and polymeric 3D networks 5 , 6 . Advantageously, it has been demonstrated that the degree of polymerization/depolymerization of glasses and geopolymers can be determined via Raman spectroscopy 7 . In essence, SiO 4 species in a silica network differ from each other spectroscopically according to the number of sharing oxygen atoms.…”

“…The reasoning extends then to Q 2 , Q 3 , and Q 4 meaning two, three, and four sharing oxygen atoms, respectively. Upon contacting glass or a silica-rich mineral with an alkaline environment it is expected that the amount of Q 0 -Q 3 species increase with time due to silica depolymerization, a phenomenon that can be tracked since each Q n species appear at distinct frequencies in the Raman spectrum [7][8][9] .Zinc (Zn 2+ ) species, as a retarder, have been under study where its mechanistic and kinetic aspects have been taken into consideration [10][11][12][13] . Zinc oxide (ZnO), for instance, is thought to dissolve into Zn 2+ which prolongs the setting time by sequestering calcium ions (Ca 2+ ) and forming calcium zincate [Ca(Zn(OH) 3 ) 2 .2H 2 O] 11 .…”

“…The reasoning extends then to Q 2 , Q 3 , and Q 4 meaning two, three, and four sharing oxygen atoms, respectively. Upon contacting glass or a silica-rich mineral with an alkaline environment it is expected that the amount of Q 0 -Q 3 species increase with time due to silica depolymerization, a phenomenon that can be tracked since each Q n species appear at distinct frequencies in the Raman spectrum [7][8][9] .…”