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 ℃) and variable temperature(simulated internal temperature of mass concrete with 60 ℃ peak). The results indicate that constant temperature of 20 ℃ is benefi cial 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 ℃ is less than that at 20 ℃ 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 ℃ curing stage from 28 to 180 d at variable temperature regime, is benefi cial to the increase of Al/Si ratio which is still lower than that at constant temperature regime of 20 ℃ 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 insuffi cient in hardened Portland cement pastes.     

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.     

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 SO₄^2- 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 SO₄^2- 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.     

Hydration products of cement-silica fume-quartz powder mixture under different curing regimes

Composition, morphology, and structure of hydration products in hardened pastes of three kinds of blended cement(cement-silica fume, cement-quartz powder and cement-silica fume-quartz powder) hydrated under different curing regimes(standard curing, 90 ℃ steam curing, 200 ℃ and 250 ℃ autoclave curing) were investigated by X-ray diffraction and field emission scanning electron microscope equipped with EDAX system. Results showed that the main hydration products in three kinds of hardened pastes under standard curing condition are all C-S-H gels, CH, and AFt. Under 90 ℃ steam curing condition, the main hydration products of cement-silica fume and cement-silica fume-quartz powder are C-S-H gels, whereas those of cement-quartz powder are C-S-H and CH. Under 200 or 250 ℃ autoclave curing condition, no obvious crystallized CH phase is found in hardened pastes of three kinds of blended cement, and C-S-H gels are transformed into one or more crystalline phases such as tobermorite, jennite, and xonotlite. The chemical composition and morphology of these crystalline phases depend on the composition of mixture and autoclave 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(NO_3)_2 solutions utilizing ~(29)Si, ~(27)Al 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(NO_3)_2 solutions as compared to the pastes cured in water. Moreover, Mg~(2+) 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, Mg~(2+) ions exert an impact on the intra-transition between Al~[6] species, from AFm and hydrogarnet to hydrotalcite-like phase. NO_3~-ions are interstratified in the layered Mg-Al structure and formed nitrated hydrotalcite-like phase(Mg_(1-x)Al_x(OH)_2(NO_3)_x·nH_2O). Results from both ~(27)Al NMR and XRD data show that ettringite seems not to react with Mg~(2+) ions.     

Effects of curing systems on properties of high volume fine mineral powder RPC and appearance of hydrates

The effects of different curing systems on the properties of high volume fine mineral powder RPC (reactive powder concrete) and the appearances of hydrates were studied. The experimental results show that dry-heating curing promotes the development of pozzolanic reactivity of fine mineral powder; due to low cement content, 0.20 water-bind ratio and high reactive fine mineral powder content, the strength of RPC increases by around 200% after steam curing and subsequent dry-heating curing. Scanning electron microscopy and energy spectrum diagram showed that: after the high volume fine mineral powder RPC with 0.16 water-bind ratio underwent steam curing and dry-heating curing, there was no significant change in the appearance of hydrates; after the RPC with 0.20 water-bind ratio, the cement content of 150 kg/m³ and more steel slag powder underwent dry-heating curing, there was a certain change in the appearance of C-S-H, the structure of gel was more compact and was uniformly distributed, and the Ca/Si of C-S-H gel decreased from 1.41 to around 1.20.     

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 SO ₄ ^2- ,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 SO ₄ ^2- and CO ₃ ^2- attack, as well as the activation of fly ash by SO ₄ ^2- attack will increase the MCL and Al[4]/Si, which are both higher than that under Na₂SO₄ corrosion, MgSO₄ or Na₂SO₄ +NaCl coordination corrosion.     

Effects of clinker contents of different C3A and C4AF on the performance of PSC cement

Phosphogypsum-slag-clink (PSC) cement were prepared by original phosphogypsum(PG), which was grinded after dried at 60 °C combined with different contents of phosphogypsum (15%, 20%, 30%), and a small amount of different C₃A, C₄AF contents of finely ground cement clinker, and ground granulated blast furnace slag (GGBFS). Physical mechanical and sugaring properties were studied. The results show that compressive strength of PSC cement with 20% phosphogypsum at 3 d ages would be higher than 17 MPa and even 50 MPa at 28 days. Compressive strength at longer curing stage and sugaring properties of PSC with lower content of C₃A higher C₄AF clinker were improved. pH value of PSC cement system at early stage was relatively low, and pH gradually increased with the addition of clinker. pH increased firstly and then decreased with the hydration stage. SEM analyses showed that the amount of ettringite, which was influenced by pH and content of Al, must be controlled in PSC cement system, which may cause damage to microstructure or even expansive cracks if large amount of ettringite formed in hardened paste.     

Orbital Calculations of Kaolinite Surface： on Substitution of Al^3＋ for Si^4＋ in the Tetrahedral Sites

The surface properties of kaolinite were determined using density functional theory discrete variational method （DFT-DVM） and Gaussian 03 program. A SiO4 tetrahedral hexagonal ring with two A1 octahedra was chosen to model the kaolinite crystal. The total density of states of the kaolinite cluster are located near the Fermi level at both sides of the Fermi level. Both the highest occupied molecular orbit （HOMO） and the lowest unoccupied molecular orbit （LUMO） of kaolinite indicate that kaolinite system can not only readily interact with electron-acceptor species, but also readily interact with electron-donor species on the edge surface and the gibbsite layer surface, and thus, shows amphoteric behavior. Substitution of Al3^＋ for Si4＋ in the tetrahedral site linking the vacant Al3^＋ octahedra does not increase the surface chemical reactivity of kaolinite, while substitution of Al3^＋ for Si^4＋ in the tetrahedral site with the apex O linking Al3^＋ octahedra increase the surface chemical reactivity of the siloxane surface of kaolinite, especially acting as electron donors. Additionally, substitution of Al3^＋. for Si^4＋ in the tetrahedral site results in the re-balance of charges, leading to the increase of negative charge of the coordinated O atoms of the AlO4 tetrahedra, and therefore favoring the formation of ionic bonds between cations and the surface O atoms in the basal plane.     

Influence of elevated curing temperature on the properties of cement paste and concrete at the same hydration degree

Three different curing temperatures(20 ℃, 40 ℃, and 60 ℃) were set, so that the nonevaporable water(w_n) contents of plain cement pastes cured at these three temperatures were measured to determine the hydration degree of cement. Tests were carried out to compare the pore structure and strength of cement paste, as well as the strength and permeability of concrete under different temperature curing conditions when their cements were cured to the same hydration degree. The experimental results show that either at a relatively low hydration degree(w_n=15%) or high hydration degree(w_n=16.5%), elevated curing temperature has little influence on the hydration products of cement paste, while it has a negative influence on the pore structure and compressive strength of cement paste. However, this negative effect is weaker at high hydration degree. The large capillary pore(100 nm) volumes of cement pastes remain almost the same at high hydration degree, regardless of curing temperatures. As for the concrete, elevated curing temperature also has negative influence on its compressive strength development, at both low hydration degree and high hydration degree. And this negative effect is stronger than that on cement paste's compressive strength at the same hydration degree. On the whole, elevated curing temperature has little influence on the resistance of concrete to chloride ion penetration.     

Effect of matrix composition on physical properties of Al<Subscript>3</Subscript>CON in-situ reaction reinforced corundum composite

60% white corundum used for aggregate, 5% aluminium powder for fixed additions and 35% various additives for matrix were prepared for specimens 1#,2#,3#. They were mixed uniformly with the suitable resin as a binder and pressed under pressure of 315 ton forging press, then dried at 200℃ for 24 h. Effects of various additives on 1500 ℃×2 h creep properties of Al3CON reinforced corundum composite were researched. The experimenal results show that creep coefficients of specimens 1#,2#,3# at 1500 ℃×2 h are 1.4×10^- 4, -9.4×10^-4, -22.6×10^-4, respectively. Crushing strength of the slide plate added with suitable additive A after fired at 1500 ℃ ×3 h reaches to 225 MPa, the creep rate is positive all the time from 0% to 0.014% at 1500 ℃ for 2 h. The microstructure result analysis shows that reinforced phases of Al3CON fiber composite have been formed after fired with Al powder in coke at high temperatures for specimen 1#, and the strength of the composite is increased. The hot modulus of rupture is up to 59 MPa at 1400 ℃ and the RUL is obviously higher than that at 1700 ℃. Its service life is two times as that of Al2O3-C slide plate when used in the process of pouring steel. The mechanism of creep rate resistance of the composites can be discovered by means of SEM and EDAX analysis. It is concluded that the active Al3CON and Al2O3 multiphases that were formed by N2 in gas, C, Al and Al2O3 inside the matrix of the composites during in-situ reaction,which gives the composites outstanding creep rate resistance for the dense zone resuiting from Al3CON oxidation that inhibits contraction at the high temperature. Besides, the matrix will turn into the multiphase with high refractoriness, N content and its Al3CON reinforced fiber will further increase accordingly. In addition, Al3CON formed by Al2O3 and C, Al in the matrix with N2 in gas will inhibit the creep rate and also greatly improve the creep rate resistance of the composites.     

Dehydration Characteristics of C-S-H with Ca/Si Ratio 1.0 Prepared Via Precipitation

Calcium silicate hydrate(C-S-H) with Ca/Si ratio 1.0 was prepared via precipitation in solution and heated at various temperatures to investigate its dehydration behavior. The dehydration, structural collapse and recrystallization characteristics of C-S-H and its microstructural change during heating process were investigated by XRD, SEM, Raman and TG-DSC techniques. C-S-H gradually lost non-evaporable water upon heating, about 50% and 80% non-evaporable water was removed below 200 and 400 ℃, respectively, and the rest was removed up to about 1 000 ℃. At 400 ℃, dehydrated C-S-H exhibited the increasing disordering of calcium/silicon environment and the decreasing symmetrical bending vibration of Si-O-Si of Q~2 silicate chains. At 650 ℃ non-bridging oxygen atoms(O_(non)) attached to silicon were almost removed, and significant structural change occurred, and at 815 ℃ C-S-H dehydrated to wollastonite.     

Devitrification process in rapidly solidified Al-Ni-Cu-Nd metallic glass

In the present study, rapidly solidified ribbons of A187 Ni7 Cu3 Nd3 metallic glass was prepared by using melt spinning. Devitrification process of the totally amorphous ribbons was investigated by high temperature X-ray diffraction analysis, combining with differential scanning calorimetry, under continuous and isothermal heating regime. The X-ray diffraction intensity and full width at the half maximum (FWHM) were analyzed to investigate the increase of crystallized amount and growth of α-Al crystal particles. The results show that under continuous heating regime, the metallic glass devitrifies via two main stages: primary crystallization, resulting in two-phase mixture of α-Al plus residual amorphous phase, and secondary crystallization, corresponding to rapid precipitation of some inter-metallic phases in the form of dispersion or eutectic mixture. Under isothermal heating regime, only Al crystal precipitates from the Al-rich amorphous matrix at low temperature, and when heating at 280 ℃ only Al     

Influence of curing duration on thaumasite formation of Portland-limestone cement pastes

Extensive researches have been carried out on the conventional sulfate attack, while it has been found that the thaumasite form of sulfate attack(TSA), sulfate attack at low temperature, has just been discovered and its mechanism is not well understood so far. In this study, the sulfate attack of cement paste incorporating 30% mass of limestone powder was investigated. After 20 ℃ water cured for 7, 14, and 28 days,respectively, 20 mm cube specimens were exposed in a 5% magnesium sulfate solution at(6 ±1) ℃ for periods up to 240 days. Their appearance change, compressive strength development were examined at different storage time, and selected paste samples were examined by X-ray diffraction(XRD), Fourier transform infrared spectroscopy(FTIR), scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS). The results indicate that all Portland-limstone cement pastes suffer from appearance deterioration to some extent. The compressive strength of cement paste initially increases and after 120 days decreases with increasing exposed period. In addition, the cement paste with short curing time is more susceptible to sulfate attack, which directly leads to the formation of non-binder thaumasite crystal accompanied by the formation of ettringite, gypsum and brucite, and becomes a white, mushy, and incohesive matrix. Additionally, the extent of sulfate attack is greater and the formation of thaumasite is observed earlier for shorter curing time.     

Moessbauer spectra of MnFe2-2x Al2xO4 （0 ≤x≤ 0.4） ferrites

The effect of non-magnetic Al³⁺ ion doping on the magnetic properties of MnFe_2−2x Al_2x O₄ (0≼x≼0.4) spinel ferrites was studied using M?ssbauer spectroscopy measurements at room temperature. From the M?ssbauer study, it is observed that the resolved hyperfine sextets are due to the distribution of Fe ions on the two sublattices of the spinel ferrites. The value of the isomer shift obtained from the fitting of the M?ssbauer spectra indicates that Fe ions are in +3 state. A paramagnetic doublet is observed at degree of inversion x=0.4, superimposed on the hyperfine sextets, indicating that the super-exchange interaction A–B decreases due to the dilution of sublattice by Al³⁺ ions. The hyperfine magnetic field decreases at both interstitial sites of tetrahedral (A) and octahedral (B) with the increase in Al concentration.     

Properties of supersulphated phosphogysumslag cement (SSC) concrete

Supersulphated phosphogysum-slag cement （SSC） is a newly developed non-burned cementitious material mainly composed of phosphogysum （PG） and ground granulated blast furnace slag （GGBFS）, with small amount of steel slag （SS） and clinker （CL）. SSC is a kind of environmentally-friendly cementitious material due to its energy-saving, low-carbon emission, and waste-utilization. We prepared concretes with supersulphated phosphogysum-slag cement, and studied the mechanical properties, micro- properties and resistance to chloride penetration of concrete in comparison with those of portland slag cement （PSC） and ordinary portland cement （OPC） concrete. The test results show that the compressive strength of SSC concrete can reach 38.6 MPa after 28 d, close to PSC concrete and OPC concrete. Microanalyses indicate that large quantities of ettringite and C-S-H, and little amount of Ca（OH）2 are generated during the hydration of SSC. The dense cement paste structure of SSC is formed by ettringite and C-S-H, surrounded unreacted phosphogysum. The property of resistance to chloride penetration of SSC concrete is better than PSC and OPC concrete due to the fact that SSC can form much more ettringite to solidify more Cl^-.     

Early hydration of composite cement with thermal activated coal gangue

Composite cement samples were prepared by mixing clinker, gypsum with burnt coal gangues which was calcined at various temperatures. The mechanical strength and Ca(OH)₂ content in the cement paste were tested, and the paste composition and microstructure were analyzed by thermogravimetry-differential thermal analysis (TG-DSC), X-ray diffraction(XRD), scanning electronic microscopy (SEM) and pore structure analysis. Results demonstrate that the thermal activated coal gangue could accelerate the early hydration of cement clinker obviously, which promotes the gangue hydration itself. The early hydrated products of the cement are C-S-H gel, Ca(OH)₂ and AFt. The cement with 30% (in mass) the gangue exhibits higher mechanical strength, and among all the cement samples the one with the gangue burnt at 700 °C displays the highest hydration rate, mechanical strength, the most gel pores and the lowest total porosity.     

The mechanism of ground granulated blastfurnace slag preventing alkali aggregate reaction

Three different methods were applied to study the alkali content of gelpores in cement. In the closed system, the concentration of K⁺, Na⁺ and OH⁻ have not reduced with the increase of age. In the open system, the diffusion and transferring of K⁺ and Na⁺ towards free space leads to the decrease of total alkali content. In the micro-analysis system, the contents of K⁺ and Na⁺ in the first hydrated layer of ground granulated blastfurnace slag (GBFS) are very low, while the contents of calcium and magnesium are relatively high. This phenomenon shows that the mechanism of GBFS preventing alkali aggregate reaction (AAR) is: when GBFS is dissolved by alkali medium, SiO₂ and Al₂O₃ are dissolved into the cement matrix, then around GBFS particles form reaction rings rich in Ca²⁺ and Mg²⁺, and the C-S-H gel of positive charges formed in the area repulses K⁺ and Na⁺, which are forced to transfer to the mortar’s matrix, pore or mortar sample surface. The transferred K⁺ and Na⁺ form alkali gel products with other dissolved ions, then become evenly distributed in the mortar sample and react with Ca(OH)₂ in pore solutions to form (Na,K) _x−2z ·zCa·(SiO₂)_y·(OH)_x gel products; and thus changes the AAR gel products’ structure. The gel products will not expand, and so they can delay expansion destruction.     

Characterization of pozzolanic reaction and its effect on the C-S-H Gel in fly Ash-cement paste

High resolution solid-state ²⁹Si MAS NMR, combined with XRD, SEM and FTIR were used to characterize the pozzolanic activity of FA, type of main pozzolanic reaction products, and the effect of pozzolanic reaction on the C-S-H microstructure in fly ash-cement (FC) paste. The experimental results indicate that in the hydrated FC paste with 30% dosage of FA at 3 d, FA partially participated in the pozzolanic reaction, while, at 120 d, FA largely reacts. During the hydration of FC paste at laboratory temperature, the pozzolanic reaction products are C-S-H gel rather than zeolitic gel. Moreover, after the covalent bonds of Si-O-Si, Si-O-Al and Al-O-Al in the structure of FA are broken, monosilicates Si-OH and Al-OH groups form, these chemical species can connect C-S-H dimers, thus producing more Al-free C-S-H and aluminous C-S-H than in the plain cement paste. The increased content of Al for Si substitution in the bridging tetrahedra of C-S-H may decrease the stability of C-S-H, which results in a rather obvious loss in the mechanical strength of hardened FC paste.     

Shrinkage Behavior of High Performance Concrete at Different Elevated Temperatures under Different Sealing Conditions

The shrinkage behavior of high performance cement concrete made from Portland cement, ultra fine granulated blast furnace slag and pulverized fly ash with addition of superplasticizer at different temperatures from ambient temperature to 120 ℃ under different seuliug conditions was investigated by means of length change measurement on cylindrical concrete specimens along with curing age. Results show that drying shrinkage deformations of titled concrete specimens increased rapidly as the curing temperature rose. The development of dryiing shrinkage deformatian can be efficiently controlled with the aid of aluminum tape sealing as compared with the unsealed specimens, especially when the curing temperature is below 60℃ , although it will increase dramatically when the curing temperature is elevated to above 90%＂ . Polymer coating on concrete specimens showed a similar effect on the control of drying shrinkage as the sealing operation with aluminum tape.