Hydration mechanism of low quality fly ash in cement-based materials

The hydration mechanism of low quality fly ash in cement-based materials was investigated. The hydration heat of the composite cementitious materials was determined by isothermal calorimetry, and the hydration products, quantity, pore structure and morphology were measured by X-ray diffraction(XRD), thermalgravity-differential thermal analysis(TG-DTA), mercury intrusion porosimetry(MIP) and scanning electron microscopy(SEM), respectively. The results indicate that grinding could not only improve the physical properties of the low quality fly ash on particle effect, but also improve hydration properties of the cementitious system from various aspects compared with raw low quality fly ash(RLFA). At the early stage of hydration, the low quanlity fly ash acts as almost inert material; but then at the later stage, high chemical activity, especially for ground low quality fly ash(GLFA), could be observed. It can accelerate the formation of hydration products containing more chemical bonded water, resulting in higher degree of cement hydration, thus denser microstructure and more reasonable pore size distribution, but the hydration heat in total is reduced. It can also delay the induction period, but the accelerating period is shortened and there is little influence on the second exothermic peak.     

Model Analysis of Initial Hydration and Strueture Forming of Portland Cement

The auto efficiently hydration heat arrangement and the non-contacting electrical resistivity device were used to test the thermology effect and the resistivity variation of Portland cement hydration. The structure forming model of Portland cement initial hydration was established through the systematical experiments with different cements,the amount of mixing water and the chemical admixture.The experimental results show that,the structure forming model of cement could be divided into three stages,i e,solution-solution equilibrium period,structure forming period and structure stabilizing period.Along with the increase of mixing water,the time of inflexion appeared is in advance for thermal process of cement hydration and worsened for the structure forming process.Comparison with the control specimen,adding Na_2SO_4 makes the minimum critical point lower,the flattening period shorter and the growing slope after stage one steeper.So the hydration and structure forming process of Portland cement could be described more exactly by applying thc thermal model and the structure-forming model.     

Model Analysis of Initial Hydration and Structure Forming of Portland Cement

The auto efficiently hydration heat arrangement and the non-contacting electrical resistivity device were used to test the therrnology effect and the resistivity variation of Portland cement hydration. The structure forming model of Portland cement initial hydration was established through the systematical experiments with different cements, the amount of mixing water and the chemical admixture. The experimental results show that, the structure forming model of cement could be divided into three stages, i e, solution-solution equilibrium period, structure forming period and structure stabilizing period. Along with the increase of mixing water, the time of inflexion appeared is in advance for thermal process of cement hydration and worsened for the structure forming process. Comparison with the control specimen, adding Na2SO4 makes the minimum critical point lower, the flattening period shorter and the growing slope after stage one steeper. So the hydration and structure forming process of Portland cement could be described more exactly by applying the thermal model and the structure-forming model.     

Characteristics and Mechanism of Modified Triethanolamine as Cement Grinding Aids

Effects of modified triethanolamine as cement grinding aids on particles characteristics and mechanical property of cement were studied, and its reaction mechanism was analyzed by IR, Zeta potential, SEM, XRD and TG-DTA. The results show that the content of 3-32 μm particles for cement with 0.015% modified triethanolamine(M-TEA) is increased by 12.4%, and the compressive strengths of cement with 0.03% M-TEA are increased by 5.5 and 8.2 MPa at 3 and 28 days, respectively. And both the grinding and enhancement effects of M-TEA on cement are better than triethanolamine. The mechanism analysis shows that M-TEA not only has the amino and hydroxyl groups of TEA, but also has the ester, carbonyl, carboxyl groups which easily combine with metal ions of cement minerals, resulting in that M-TEA can promote surface adsorption and shield the unsaturated charges in the surface and crack section of particles, thus particles reunion is prevented and grinding efficiency is improved. Enhancement of M-TEA on cement mainly lies in that it can promote or induce hydration reaction of cement mineral with gypsum and water, which accelerates formation of hydration products, and then improves the structure and morphology of cement hydration products, thus the uniformity and compactness of product structure is increased.     

Hydration heat effect of cement pastes modified with hydroxypropyl methyl cellulose ether and expanded perlite

Hydration heat effect of cement pastes and mechanism of hydroxypropyl methyl cellulose ether (HPMC) and expanded perlite in cement pastes were studied by means of hydration exothermic rate, hydration heat amount, FTIR and TG-DTG. The results show that HPMC can significantly delay the hydration induction period and acceleration period of cement pastes. As mixing amount increased, hydration induction period of cement pastes enlarged and accelerated period gradually went back. At the same time, the amount of hydration heat gradually decreased. Expanded perlite had worse delay effects and less change of hydration heat amount of cement pastes than HPMC. HPMC changed the structure of C-S-H during cement hydration. The more amount of HPMC, the more obvious effect. However, EXP had little influence on the structure of C-S-H. At the same age, the content of Ca (OH)2 in cement pastes gradually decreased as the mixing amount increase of HPMC and expanded perlite, and had better delay effect than that single-doped with HPMC or expanded perlite when HPMC and expanded perlite were both doped in cement pastes.     

Effects of Coal Metakaolin on the Mechanical Properties and Microstructure of High-belite Sulphoaluminate Cement

The effects of coal metakaolin on the mechanical properties of high-belite sulphoaluminate cement under compressive loading were investigated. The composition and microstructure of hydration products at different hydration times were analyzed by X-ray diffraction and scanning electronic microscopy. The hydration process of blended cement was studied via electrochemical impedance spectroscopy. In particular,replacing a part of cement with CMK (10%, 20%, and 30%) was found to promote the hydration...     

Effect of Triisopropanolamine on the Compressive Strength and Early Hydration of Cement at Low Temperature

Triisopropanolamine (TIPA) was used as an early strength component to study its effects on mortar strength,cement paste setting time and early hydration characteristic of cement.And the early strength mechanism of TIPA at low temperature of 5 ℃ was also discussed.The results showed that,at 5 ℃,the incorporation of TIPA promoted the condensation of cement paste,shortened the initial and final setting time,and accelerated the strength development of specimens at all ages,among which the strength after 3 d increased significantly.The 1,3,7,and 28 d compressive strength ratios of the mortars mixed with 1% TIPA could reach 196%,179%,160% and 110% respectively,and the mortar strength after 3 d exceeded that of the contrast sample cured at 20 ℃.Under low temperature condition,TIPA could promote the hydration reaction of cement,shorten the induction period and advance the acceleration period.Furthermore,the maximum heat release rate and cumulative heat release quantity would be all increased,and the cumulative heat release of the cement mixed with TIPA hydrated for 12 h and 7 d increased 73% and 38% respectively.TIPA could shorten the nucleation and crystal growth (NG) stage and increase its hydration degree significantly,so it promoted cement hydration reaction.Additionally,the hydration reaction rates in phase boundary reaction (I) phase and diffusion reaction (D) phase were increased,and the duration of I process was prolonged,thereby the development of specimen strength would be accelerated.TIPA did not obviously change the types of hydration products,but increased the content of Ca(OH)_2 in the samples and the degree of cement hydration.After hydration to 7 d,large amounts of hydration products,whose surface was smooth,were formed and bonded into sheets,and the structural density of samples improved significantly.     

HYDRATION PHASES IN MAGNESIUM OXYCHLORIDE CEMENT PASTE AND THEIR INFLUENCE ON THE STRENGTH INDEX

The study is for the purpose of synthesis-ing the pure phases of magnesium oxychloridecement and investigating the basic hydrationphases and the patterns of phase changes.Theresults suggest that the artificial synthesisbased on the correct formulations can yieldthe basic hydration phases of magnesiumoxychloride,i.e.5.1.8 and 3.1.8.The patternsof change between the two phases are described.The hydration phases in magnesiumoxychloride cement prepared with MgCl_2 solu-tions of different concentrations are mainly5.1.8 and Mg(OH)_2.The relationship betweenthe ratio of the hydration phases and thestrength of the cement was calculated using asemiquantitative method.Based on thisrelationship,the cement paste with theoptimum strength can be obtained by choosingthe proper ratio of the hydration phases.     

Effects of citric acid on hydration process and mechanical properties of thermal decomposed magnesium oxychloride cement

In order to make full use of salt lake magnesium resources and improve the strength of the thermal decomposed magnesium oxychloride cement(TDMOC), the effects of citric acid on the hydration process and mechanical properties of TDMOC was studied. The hydration heat release at initial 24 h and strengths at 3, 7, and 28 days of TDMOC specimens were conducted. The hydration products and paste microstructure were analyzed by XRD, FT-IR and SEM, respectively. The results showed that citric acid can not only reduce the 24 h hydration heat release and delay the occurring time of second peak of TDMOC, but also produce more 5Mg(OH)2·MgCl 2·8H2O and less Mg(OH)2 in hydration process of TDMOC. More perfect and slender crystals were observed in the microstructure of the TDMOC pastes with citric acid. The results demonstrated that citric acid as an additive of TDMOC can decrease the hydration heat release and increase the compressive strength and flexural strength of TDMOC. The possible mechanism for the strength enhancement was discussed.     

ON THE UTILIZATION OF BAUXITE OF POOR QUALITY

One kind of energy saving and high early strengthcement can be produced by using bauxite of poorquality which is refuse before. Through a series ofexperiments proper chemical compositions of the cementare obtained, and the cement clinker is sintered at alow temperature (1250℃) by the use of the bauxite tothe utmost limit. The results show, the cement notonly has high early strengths but also has a goodcharacteristic of resisting corrosion by sulphate.So it will be of great practical use. The hydraulicproperties of C_4 AF are always problematical, it isstudied again in this paper, showing that C_kAF hasgood hydraulic properties exactly; Cr~(3+), Ni~(3+) and Ti~(4+)can promote the formation of C_4AF and increase itsearly strengths; CaSO_4·2H_20 can also improve its 3days' strength but retard its hydration rate espe-cially in early ages.     

Pozzolanic Activity of Burned Coal Gangue and Its Effects on Structure of Cement Mortar

The pozzolanic activity of coal gangue burned at different burning temperatures was investigated. The burned coal gangue was mixed with portland cement in different proportions （ 20% - 60% ）. The pozzolanic activity of coal gangue burned and the hydration products were examined, the compressive strengths of the pastes of the mixtures were tested, and the mechanism of the reaction was discussed. The experimental results slum, that the coal gangue burned at 750 ℃ has the optimum pozzolanic activity, and the burned coal ganguc with SiO2 and Al2 O3 is in an active form. When the coal gangue burned at 750℃ is mixed into portland cement, the content of calcium hydroxide in paste is significantly reduced, while the contents of hydrated calcium silk.ate and hydrated calcium aluminate are increased accordingly, hence resulting in the improvement of the microstructure of mortar. The compressive strength of cement paste decreases with increasing the content of burncd coal gangue. The decease in strength is small in the range of 20% - 30% coal gangue substitution and significant in 30%- 60% substitution.     

Microstructure Formation and Degradation Mechanism of Cementitious Plugging Agent Slurries

The hydration products and microstructure of class G oil well cement and a newly developed plugging agent (YLD) slurries cured in the simulated temperature and pressure environment, which was of similar temperature and pressure with those at the bottom of oil well in a normal depth, were investigated using XRD, TG and SEM. Severe leakage is confirmed at the interface between hardened slurries and steel tube during the dynamically curing process, which induces the quick loss of cementing property of slurries. This should be the dominating cause of degradation of class G oil well cement slurry. A secondary hydration process can take place at the eroded interface of hardened YLD plugging agent slurry. Newly formed C-S-H gel has a self-healing effect to repair the damaged interface, which unceasingly maintains the cementing property of the YLD plugging agent slurry. Therefore, the effective using period of YLD plugging agent can be prolonged.     

Mierostrueture Formation and Degradation Mechanism of Cementitious Plugging Agent Slurries

The hydration products and microstructure of class G oil well cement and a newly developed plugging agent （YLD） slurries cured in the simulated temperature and pressure environment, which was of similar temperature and pressure with those at the bottom of oil well in a normal depth, were investigated using XRD, TG and SEM. Severe leakage is confirmed at the interface between hardened slurries and steel tube during the dynamically curing process, which induces the quick loss of cementing property of slurries. This should be the dominating cause of degradation of class G oil well cement slurry. A secondary hydration process can take place at the eroded interface of hardened YLD plugging agent slurry. Newly formed C-S-H gel has a self-healing effect to repair the damaged interface, which unceasingly maintains the cementing property of the YLD plugging agent slurry. Therefore, the effective using period of YLD plugging agent can be prolonged.     

INFLUENCE OF ORDINARY ACCELERATORS ON HYDRATION OF BELITE CEMENT

The study revealed that ordinary accelerators for Portland cement also improves early strength of Belite cement remarkably, but one serious defect exists which lead to a high strength reduction at the age of 28 days. The paper analyzes the action mechanism of ordinary accelerators on Belite cement. It shows that these accelerators can not accelerate the hydration of β-C2S directly. The reason that they can increase early strength of Belite cement is that they can promote hydration of C3A and C4AF, through which can make early hydration of β-C2S in clinker advanced relatively. The strength reduction at 28 day age is mainly because these accelerators restrain the further hydration of β-C2S at later age. Considering these results, we obtain basic requirements of ideal accelerators for Belite cement.     

Hydration Characteristics of Sodium Sulfate Slag Cement System

The hydration characteristics by thermal analysis ( DTA ) were determined, and an isothermal calorimeter (IC) was used to study the pastes . The experimental results indicate: (1) The main hydration products of SSC are C-S-H( I ) gel with a low Ca/Si ratio, crystalline Thomsonite-type and AFt-type phases containing certain alkali cations; (2) No phases of the AFm-type and high alkaline Ca( OH)2 in SSC system could benefit the hydrated cements to improve its strength and durability; (3) Crystalline Thomsonite-type and AFt-type phases containing Na will greatly reduce free alkali and alleviate the harmness of alkali aggregate reaction ( AAR) in SSC system; (4) Similar to ordinary Portland cement( OPC), the hydration process of SSC could be classified into five stages : initial, induction, acceleration, deceleration and decay; (5) Regardless of the activator used, the apparent activation energy is higher with the increased slag in cement system, and the rising temperature could promote the hydration of SSC.     

Hydration Kinetics of Phosphorus Slag-cement Paste

Hydration characteristics of Portland cement paste with phosphorus slag powder incorporated and hydration kinetics was investigated with SEM, X-ray diffraction, DTA-TG and calorimeter Ⅱ80. Results showed that phosphorus slag powder could reduce total amount of hydration products yet had little influence on the type of hydration products. The total amount of heat of hydration was decreased by 49.11% and the final setting was postponed by 2.28 h when phosphorus slag powder substituted 35% Portland cement by m...     

Abrasion Resistance of Cement Paste with Granulated Blast Furnace Slag and Its Relations to Microhardness and Microstructure

The abrasion resistance of cement pastes with 30 wt%,40 wt% and 50 wt% granulated blast furnace slag (GBFS),and its relations to microhardness and microstructure like hydration products and pore structure were studied.Results indicated that GBFS decreased the abrasion resistance of paste,and among the pastes with GBFS,the paste with 40 wt% GBFS showed the highest abrasion resistance.The microhardness of GBFS was lower than that of the cement,and the microhardness of the hydration products in pas...     

Effect of calcium aluminate cement variety on the hydration of portland cement in blended system

Two kinds of CACs with different monocalcium aluminate（CA） contents were used in the PC/CAC（PAC） mixtures. Effects of CA and CACs on the properties of PAC were analyzed by setting times and the compressive strength tests, and also by means of calorimetry, XRD, DTA-TG and ESEM. The experimental results show that the compressive strength of the PAC mortars decreases with increasing content of CAC while it declines sharply with a higher content of CA in CAC. Compared with neat PC paste, the content of calcium hydroxide in hydrates of PAC paste decreases significantly, and the hydration time of PC is prominently prolonged. Additionally, the higher the content of CA in CAC, the more obviously the hydration of PC is delayed, confi rming that the CA phase in CAC plays an important role in the delay of PC hydration.     

Model of Coherent Interface Formation in Cement-Based Composites Containing Polyblend of Polyvinyl Alcohol and Methylcellulose

The texture of interfacial zone between cement paste and quartz in the cement-based composites containing polyvinyl alcohol (PVA) ,methylceUulose (MC) and their polyblend in an amount of 10 wt % with respect to cement, as well as the texture of dehydrated bodies of PVA , MC , and the polyblend solutions, were inves-tigated with SEM. The network texture of the dehydrated polyblend is confirmed by comparing the texture of dehy-drated bodies of PVA and MC. The network texture has restrained the movement of polyblend molecules in the ce-ment mortar but is helpful to forming a coherent interface between cement paste and quartz. The key factor of form-ing the coherent inteoCace is not the neutralization reaction between H ^ from hydrolysis of quartz and OH ^- from hydration of cement, but the electrostatic attraction and the chemical reaction between polar groups on the polyblend molecule and cations and anions from hydrolysis of quartz and hydration of cement, respectively. The model of the coherent interface formation is that excessive [ HSiO3 ]^ - and [ SiO3 ]^2- an/ons are bonded tgzh the hydrated cations such as Ca^2 and Al^3 , which is confumed by the gel containing Ca and Si on the quartz surface.     

Hydraulicity of Wet-milling Ultra-fine Grouting Cement

The physical and mechanical properties of wet-milling ultra-fine grouting cement were studied,and its microstructure was abserted through modern instrwnentation analysis such as scanning electronic micoscoty (SEM),X-ray diffraction and Hg-intrusion micromeritics ,The experimertal results indicate that wet-milling ultra-fine cement possesses high rheological properties and groutability,It can be filles densely in cracks of rock and hydrate fully,which may endow hydrated cemetnt with high mechanical strength Main hydration products of wetmilling ultra-fine cement are poorly crystalline G-S-H(I),acicular AFt and plank-shape Ca(OH)2,The dense crystal-netwonk structure can be formed in the rock gaps filled with cement psate ,but some weak regions exist owing to Ca(OH)2,The features of micro-pore structure of hydrated wet-milling ultra-fine cement are few big harmful pores,abundant harmless micro pores and little most possible pore radius.