Qingjun Ding

Effect of curing regime on degree of Al3+ substituting for Si4+ in C-S-H gels of hardened Portland cement pastes

The effect of curing regime on degree of Al3+ substituting for Si4+ (Al/Si ratio) in C-S-H gels of hardened Portland cement pastes was investigated by 29Si magic angel spinning (MAS) nuclear magnetic resonance (NMR) with deconvolution technique. The curing regimes included the constant temperature (20, 40, 60 and 80 °C) and variable temperature (simulated internal temperature of mass concrete with 60 °C peak). The results indicate that constant temperature of 20 °C is beneficial to substitution of Al3+ for Si4+, and Al/Si ratio changes to be steady after 180 d. The increase of Al/Si ratio at 40 °C is less than that at 20 °C for 28 d. The other three regimes of high temperature increase Al/Si ratio only before 3 d, on the contrary to that from 3 to 28 d. However, the 20 °C curing stage from 28 to 180 d at variable temperature regime, is beneficial to the increase of Al/Si ratio which is still lower than that at constant temperature regime of 20 °C for the same age. A nonlinear relation exists between the Al/Si ratio and temperature variation or mean chain length (MCL) of C-S-H gels, furthermore, the amount of Al3+ which can occupy the bridging tetrahedra sites in C-S-H structure is insufficient in hardened Portland cement pastes.

Effects of molecular structure of polycarboxylate-type superplasticizer on the hydration properties of C3S

Effects of polycarboxylate-type superplasticizer (PC) molecular structure on the hydration heat of tricalcium silicate (C3S) paste and polymerization degree of hydration products (C-S-H gel) were researched by using TAM AIR isothermal microcalorimetry (TA) and 29Si nuclear magnetic resonance (NMR). Methoxy polyethylene glycol-methacrylates-based polycarboxylate superplasticizers with different side chain lengths and main chain lengths were employed. PC molecules with shorter main chain or longer side chains caused stronger retardation of C3S early hydration and lesser increase of C3S 3 d hydration degree. NMR measurement indicated that the incorporation of PC increased the hydration degree of C3S paste and the polymerization degree of silicon-oxygen tetrahedron of C-S-H gel. The tendency for C3S 7 d hydration degree to improve was more pronounced while PC molecules with longer main chain or shorter side chain were added. Whereas, PC molecules with longer main chains or longer side chains increased the 7 d polymerization degree of C-S-H gel.

Effect of corrosive solutions on C-S-H microstructure in portland cement paste with fly ash

By means of 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) combined with deconvolution technique, X-ray diffraction (XRD), scanning electron microscopy (SEM) as well as energy dispersive X-ray system(EDX), the effect of 5 wt% corrosive solutions (viz. 5 wt% Na2SO4, MgSO4, Na2SO4+NaCl and Na2SO4+NaCl+Na2CO3) on C-S-H microstructure in Portland cement containing 30 wt% fly ash was investigated.The results show that, in MgSO4 solution, Mg2+ promotes the decalcification of C-S-H by SO42-,increasing silicate tetrahedra polymerization and mean chain length (MCL) of C-S-H. However, the substituting degree of Al3+ for Si4+ (Al[4]/Si) in the paste does not change evidently. Effect of Na2SO4 solution on C-S-H is not significantly influenced by NaCl solution, while the MCL and Al[4]/Si of C-S-H in fly ashcement paste slightly change. However, the decalcification of C-S-H by SO42- and CO32- attack, as well as the activation of fly ash by SO42- attack will increase the MCL and Al[4]/Si, which are both higher than that under Na2SO4 corrosion, MgSO4 or Na2SO4 +NaCl coordination corrosion.

Hydration Process of Cement-Based Materials by AC Impedance Method

AC impedance is a new method to study the changes of pore structure and the hydration degree during the hydration and hardening process of cement paste by the change of the electrochemical parameters. Employing AC impedance method, we studied the hydration and hardening process of cement paste with fly ash and slag, and analyzed the influence of different hydration age, water-binder ratio and mineral admixture on the impedance parameters. Moreover, we compared the results with those by the conventional porosity testing method and X-ray diffraction method. The results showed that AC impedance could be taken as a new technology in cement and concrete research.

Effect of curing regime on polymerization of C-S-H in hardened cement pastes

The effect of two different curing regimes on the polymerization degree of C-S-H in hardened cement pastes within 28 d were investigated by measuring the chemical environments of 29Si with magic angle spinning (MAS) nuclear magnetic resonance (NMR) and by analyzing the 29Si NMR spectra with deconvolution technique. The experimental results indicate that, at curing regime of constant temperature of 20 °C, the polymerization of C-S-H increases and then decreases with curing age, and the Al/Si ratio increases gradually with curing age, furthermore, the two non-bridging oxygen bonds of bridging silicate tetrahedra in C-S-H structure mainly bond to H+. At curing regime of variable temperature, the polymerization of C-S-H firstly increases then changes slightly and subsequently decreases with the temperature from low to high and then to low, and the Al/Si ratio firstly increases then keeps invariant and subsequently slightly decreases. Moreover, the temperature decreasing is advantageous for the Ca2+ to be bonded to the bridging silicate tetrahedra and entering into the interlayer of C-S-H structure. The polymerization of C-S-H at curing regime of variable temperature is higher than that cured at constant temperature, but the curing regime of constant temperature is more beneficial to the substitution of Al3 for Si4+ than that of variable temperature.

Effect of magnesium on the C-S-H nanostructure evolution and aluminate phases transition in cement-slag blend

The microstructural study was conducted on cement and cement-slag pastes immersed in different concentrations of Mg(NO3)2 solutions utilizing 29Si, 27Al NMR spectroscopy and XRD techniques. The results show that the hydration of both the cement and cement-slag pastes is delayed when the pastes are cured in Mg(NO3)2 solutions as compared to the pastes cured in water. Moreover, Mg2+ ions also exhibit an decalcifying and dealuminizing effect on the C-A-S-H in cement and cement-slag pastes, and thereby decrease Ca/Si and Al[4]/Si ratios of the C-A-S-H. The dealuminization of C-A-S-H is mitigated for cement-slag paste as compared to pure cement paste. The depolymerized calcium and aluminum ions from C-A-S-H gel mainly enter the pore solution to maintain the pH value and form Al[6] in TAH, respectively. On the other hand, Mg2+ ions exert an impact on the intra-transition between Al[6] species, from AFm and hydrogarnet to hydrotalcite-like phase. NO3- ions are interstratified in the layered Mg-Al structure and formed nitrated hydrotalcite-like phase (Mg1-xAlx(OH)2(NO3)x•nH2O). Results from both 27Al NMR and XRD data show that ettringite seems not to react with Mg2+ ions.

Microstructural Evolution Mechanism of C-(A)-S-H Gel in Portland Cement Pastes Affected by Sulfate Ions

The microstructural evolution of C-(A)-S-H gel in Portland cement pastes immersed in pure water and 5.0 wt% Na2SO4 solution for different ages was comparatively investigated, by means of 29Si NMR spectroscopy, and SEM-EDS analysis. Additionally, molecular dynamics simulation was performed to study the aluminum coordination status and interaction of sulfate ions in C-(A)-S-H gel. The results showed significant changes in the microstructural evolution of C-(A)-S-H gel in Portland cement paste. Sulfate attack has decalcifying and dealuminizing effect on C-(A)-S-H gel which is evident from increase in mean chain length (MCL) and decrease in Ca/Si & Al[4]/Si ratios of C-(A)-S-H gel. Additionally, Molecular dynamics simulation proves that Al[4] substituted in silicate chains of C-(A)-S-H gel is thermodynamically metastable, which may explain its migration from the silicate chains and transformation to Al[6], thus lowering the Al[4]/Si ratio of C-(A)-S-H gel. SO42- ions can carry the interfacial Ca2+ ions into the pore solution by the diffusion-absorption-desorption process, which unravels the mechanism of sulfate attack on C-(A)-S-H gel.

Influence of Polyepoxysuccinic Acid on Solid Phase Products in Portland Cement Pastes

The influence of polyepoxysuccinic acid(PESA) on the solid phase products in hydrated Portland cement pastes was investigated by isothermal calorimetry, X-ray diffraction(XRD), 29Si and 27Al nuclear magnetic resonance(NMR). The results indicated that PESA bonds Ca2+ ions in pore solution to prevent portlandite formation, and also combines with Ca2+ ions on the surface of silicate minerals to prolong the control time of phase boundary reaction process, leading to the retardation of silicate mineral hydration. Meanwhile, the interlayer Ca2+ ions in Jennite-like structure bridging PESA and C-S-H gels prevent silicate tetrahedron and aluminum tetrahedron from occupying the sites of bridging silicate tetrahedron, which causes the main existence of dimer in C-S-H structure, deceases the degree of Al3+ substituting for Si4+ and promotes the transformation from 4-coordination aluminum to 6-coordination aluminum. Furthermore, the -Ca+ chelating group from reacting PESA with Ca2+ ions combines easily with SO42- ions, resulting in transformation from ettringite, AFm to TAH(Third aluminum hydrate). However, with the higher addition of PESA, it will bridge the excess PESA by Ca2+ ions to form a new chelate with ladder-shaped double chains structure, which not only reduces the amount of PESA bonding Ca2+ ions, but also decreases its solidifying capability for SO42- ions, leading to the transformation from TAH to AFm or ettringite. Meanwhile, at later hydration, the inhibition effect of PESA on cement hydration is weakened, and the transformation degree from TAH to AFm is higher than that to AFt with the addition of PESA.

Mechanical behavior and failure mechanism of recycled semi-flexible pavement material

The mechanical behavior and failure mechanism of recycled semi-flexible pavement material were investigated by different scales method. The macroscopic mechanical behavior of samples was studied by static and dynamic splitting tensile tests on mechanics testing system (MTS). The mechanical analysis in micro scale was carried out by material image analysis method and finite element analysis system. The strains of recycled semi-flexible pavement material on samples surface and in each phase materials were obtained. The test results reveal that the performance of recovered asphalt binder was the major determinant on the structural stability of recycled semi-flexible pavement material. The asphalt binder with high viscoelasticity could delay the initial cracking time and reduce the residual strain under cyclic loading conditions. The failure possibility order of each phase in recycled semi-flexible pavement material was asphalt binder, reclaimed aggregate, cement paste and virgin aggregate.

Effect of curing regime on the distribution of Al3+ coordination in hardened cement pastes

The effect of curing regime on the distribution of Al3+ coordination in hardened cement pastes within 28 d were investigated by 29Si and 27Al magic angle spinning (MAS) nuclear magnetic resonance(NMR) with deconvolution technique. The results indicate that the tetrahedral coordination Al3+ incorporated in C-S-H structure mainly originate from the Al3+ incorporated in the alite and belite phases in the Portland cement. The curing regime of constant temperature of 20 °C is beneficial to the octahedral coordination Al3+ transforming to tetrahedral coordination Al3+ incorporated in C-S-H structure. However, at curing regime of variable temperature, the temperature rising process is more advantageous to the transformation from ettringite to monosulphate, substitution of Al3+ for Si4+ in the C-S-H structure and the formation of the third aluminate hydrate (TAH) than that at constant temperature of 20 °C. The high temperature of 60 °C in the holding temperature process promotes the decomposition of ettringite, and enhances the consumption of the Al3+ incorporated in C-S-H phases and the Al3+ in TAH for the monosulphate forming. The temperature decreasing promotes the transformation from monosulphate to ettringite, and increases the consumption of the Al3+ incorporated in C-S-H phases, and then increases the quantity of the TAH.