Study On The Structural Characteristics Of Gel Products Of Alkali-activated Binders

According to the composition of raw materials,there are two alkali-activated binder systems,one is the case of“Si+Ca”system(i.e.alkali-activattion of slag)and the other is“Si+Al”system(i.e.alkali-activattion of metakaolin and low calcium fly ash).The main reaction products of“Si+Ca”system are aluminum-substituted calcium silicate hydrates(C-A-S-H)with lower calcium to silicon ratio than ordinary Portland cement,and the other is a highly disordered aluminosilicate gel(N-A-S-H).However,the factors that influence the precipitation mechanism for gels,their microstructural characteristics and the co-existance-are not yet well understood.This paper reports a research on the effect of Ca/Si on the formation and properties of synthetic aluminosilicate hydrate gels as models for the gels formed in alkali-activated binders.And the evolution of C-A-S-H and N-A-S-H gels with reaction time was also studied.The microstructural characteristics of reaction products of alkali-activated materials were studied based on the alkali-activated metakaolin/slag blend.The results show that in the case of Al/Si=0.5,N-A-S-H gels coexisted with C-A-S-H gel at Ca/Si<0.6.At Ca/Si?0.6,only C-A-S-H gel can be detected.At Ca/Si<1.4,the increase of calcium concentration improve the content of C-A-S-H gel phase with low degree of polymerization,and reduced the structural units with hi gh degree of polymerization.At Ca/Si?1.4,the system form silica-rich gel rather than more C-A-S-H gel.It is confirmed that the increase of Ca/Si favored in the formation of C-A-S-H instead of the highly disordered N-A-S-H gel,which is almost undetectable at Ca/Si>0.6.The structure of N-A-S-H gel maintained a homogeneous 100-200nm cotton-like structure with reaction time,and the porosity decreased.When Ca/Si=0.2 remained unchanged,C-(N)-A-S-H gel was formed in the gel phase of Al/Si=0.5 and 1.0.The binding of Na~+was not stable.It gradualy disappeared at the longer age.The system tended to change to pure C-A-S-H with reaction time,and the morphology of C-A-S-H tends to from granular shape to foil shape with the development of reaction time.The alkali-activated metakaolin/slag blends had no obvious induction period and second exothermic peak.The accumulated exothermic heat in the early stage(within24 hours)was smaller than that in the single raw materials,which corresponded to a lower compressive strength in 1d.The decrease of content of metakaolin(the amount of slag increases),made the reacted product changing from amorphous phase to semi-crystalline or poor crystalline phase,and the FTIR band value of main vibration decreased.By comparing with the alkali-activated material and the synthetic gels,the change of phase formation of alkali-activated material with increasing Ca/Si were similar to that of synthetic gels.The Si-O-T asymmetric stretching band of synthetic gel tended to shift the lower wavenumber with Ca/Si increaseing,and the decrease of the content of metakaolin(the increase of slag content)had similar effect on the Si-O-T asymmetric stretching band of the alkali-activated materials.