Effects Of Inorganic Salt Admixtures On Microstructure Of Hydrated C₃S And C₃A In Cement

In china, chemical admixtures have been an essential composite in cementitious materials, and inorganic salts have been severed as primary accelerating admixture and anti-freezing admixture, which can increase early strength and decrease freezing point in cement and concrete. Therefore, inorganic salts are applied in cementitious materials for quite some time. The major objectives of this thesis were to study comprehensively the effect of four inorganic salts(Ca Cl2, Ca(NO3)2, Ca(NO2)2 and Na2SO4) on microstructure of the hydration products of C3 S and C3 A, and reveal the effect of inorganic salts on hydration mechanism by ways of scanning transmission X-ray microscopy(STXM) and high resolution X-ray diffraction. In addition, this thesis is also aimed at clarifying the effect of Ca Cl2 on the microstructure of synthesized calcium silicate hydrate(Syn-CSH) and the binding mechanism between Cl and AFm phase. The studies can provide essential theoretical basis and basic support for rational application of inorganic salts.Microstructural analysis of hydrated C3 S in presence of inorganic salts is conducted using STXM, near edge X-ray absorption fine structure(NEXAFS), 29 Si CP/MAS NMR, X-ray diffraction and differential scanning calorimetry(DSC), and the influence of inorganic salts on hydration mechanism of C3 S is revealed. The results indicated that Ca in C-S-H gel is not octahedral coordination, but irregular symmetric coordination; Ca Cl2 doesn’t not only chemically bind with C-S-H, but also not change the irregular 7-coordinated asymmetry in amorphous C-S-H structure. However, Ca Cl2 significantly shorten pre-induction period and induction period and speed the ongoing of acceleration period up. In addition, Ca Cl2 can also prompt the formation of the bridge Si O4 units and improve the degree of polymerization in the dreierketten silicate chains of C-S-H gel, therefore C-S-H in presence of Ca Cl2 grows in the two dimensional direction forming the thin sheet.Ca(NO3)2 does not also change the irregular 7-coordinated asymmetry in amorphous C-S-H structure, but increase slightly the crystal field of Ca Ox polyhedron in C-S-H gel and improve the symmetry of Ca coordination. Ca(NO3)2 and Ca(NO2)2 have similar and marginal accelerating effect: Ca(NO3)2 can extend induction period to some extent and slightly increase the rate of hydration of C3 S during acceleration period, while Ca(NO2)2 have no accelerating effect in initial several hours and after 1d. Ca(NO3)2 and Ca(NO2)2 have vitually no effect on the degree of polymerization in the dreierketten silicate chains of C-S-H gel expect that Ca(NO3)2 just increase the degree of polymerization in 3-day hydrated C3 S.Na2SO4 significantly shorten pre-induction period and induction period and speed the ongoing of acceleration period up; Na2SO4 does not change the asymmetric seven coordinated environment of Ca atoms in amorphous C-S-H structure; Na2SO4 has vitually no effect on the degree of polymerization in the dreierketten silicate chains of C-S-H gel but increase the dispersity of hydration products during C3 S hydration reaction.C-S-H(I)s with different concentration of Cl were studied by high-energy X-ray diffraction combining with PDF analysis. It has been observed that increasing concentration of Ca Cl2 results in a systematical increase in interlayer space from ~12 ? to ~14.5 ?. The unchanged tetrahedral coordination of the Si-O(1.64 ?) regardless of concentration of Ca Cl2. However, the coordination number of the Ca-O decreases gradually from 7 to 6.0 with an increasing concentration of Ca Cl2. This suggests that the C-S-H structure evolves from a tobermorite-like structure into a jennite-like structure.The bindin meachanism between Ca Cl2 and hydration products of C3 A is conducted using STXM, NEXAFS and XRD. The results indicate that the hydration products of C3 A in presence of Ca Cl2 are C3AH6 and Friedel’s salt. Kuzel’s salt forms in C3A+50%gypsum(by weight percentage of C3A) with 36 m M Ca Cl2 solution. Kuzel’s salt is unstable and then transform into Friedel’s salt in 1 M Ca Cl2 solution. In 1-day hydrated C3A+50%gypsum paste with 36 m M Ca Cl2, Friedel’s salt is forming at surrounding Kuzel’s salt and the interface between Kuzel’s salt and ettringite. The hydration products of C3 A cannot be stable in the presence of Ca Cl2, such as C3AH6, ettringite(AFt) and monosulfoaluminate(AFm). Since SO4- in AFt and AFm can be replaced by Cl-, Kuzel’ salt and Friedel’s salt is forming. The AFm phase, monosulfoaluminate, Kuzel’s salt and Friedel’s salt, have the similar Ca L3,2-edge NEXAFS spectra with three similar pre-peak of L3 edge, while AFt just have two prepeaks; monosulfoaluminate and Kuzel’s salt have well resolved leading(1&2) peaks, while the leading peak 2 of Friedel’s salt is indistinct. The three crystals are ranked by the resolution of leading peak 2: monosulfoaluminate > Kuzel’s salt > Friedel’s salt.