Influence of character of clinker on the early hydration process and rheological property of cement paste

Influence of characteristics of alite and interstitial phase in five Japanese clinkers on early hydration of cement and rheological properties of fresh cement paste was investigated. The yield value of fresh cement paste increased rapidly at first due to the formation of ettringite, later due to the formation of C-S-H until it reached setting. The setting time was strongly influenced by alite intense hydration, which started when Ca(OH)₂ saturation ratio in the liquid phase approached a maximum value. That position mainly was dependent upon the free CaO content in clinker and was partially dependent upon water soluble alkali content in clinker and hydraulic reactivity of alite itself. Changing the burning degree of clinker and the hydraulic modulus of raw mix, and intergrinding of clinker with quick lime are practical ways to reduce slump loss and to set an optimum setting time for concrete. All of these methods are related to free lime content for which optimum amounts are varied with burning degree of clinker and quick lime.     

An investigation of microstructure evolution in cement paste through setting using ultrasonic and rheological measurements

The response of hydrating cement paste through setting are monitored using rheological measurements and ultrasonic reflection measurements. Increases in the elastic modulus and yield stress of cement paste with time are obtained from the rheological measurements. Ultrasonic measurements are performed using horizontally polarized shear waves (SH) reflected off of the hydrating cement paste. Changes in the ultrasonic signal through setting are related with changes in the porosity and stiffness of an equivalent water-filled poroelastic material, which provides identical acoustic impedance. The measured changes in the shear modulus obtained from ultrasonic measurement are shown to correlate well with increase in elastic modulus obtained from rheological measurements. The increase in the shear modulus of the porous material obtained from the ultrasonic measurements is shown to correspond well with the observed increase in the yield stress of the cement paste. By combining the information from rheological and ultrasonic measurements, it is found that even in the fluid stage there is sufficient structural integrity in the arrangement of cement grains to support low-amplitude shear stress and the evolution of a continuously connected network of cement particles within the paste is coincident with a rapid increase in the shear modulus of the porous skeleton.     

锂渣掺量对水泥-减水剂浆体流变特性和混凝土性能的影响

本文研究了锂渣掺量对水泥-减水剂浆体流变特性、新拌混凝土工作性及硬化混凝土强度的影响。从紧密堆积理论和固体颗粒体积分数角度解释了锂渣掺量对水泥-减水剂浆体流变性能的影响。结果表明：锂渣水化初始的屈服应力和塑性黏度均随锂渣掺量的增加而增大,流变性能劣化。新拌混凝土的工作性与锂渣掺量呈负相关;随锂渣掺量的增加,7 d抗压强度不断降低,28 d抗压强度呈现出先增后降的趋势。     

Hollow shell hydration of cement particles in bulk cement paste

Investigations on bulk cement paste specimens made from different cements confirm the occurrence of hollow-shell or “Hadley grains”, similar to those previously established as occurring near interfaces with glass slides. The existence of such grains in previous microstructural investigations of cement pastes has apparently been overlooked. Since few grains in bulk cement pastes fracture to reveal details of their interiors, the relative frequency of occurrence of hollow-shell grains and grains showing inner hydration product in contact with the exterior shell can not be assessed unambiguously, but almost all fractured grains observed seem to be of the hollow-shell variety.     

Performance of new viscosity modifying admixtures in enhancing the rheological properties of cement paste

The use of viscosity modifying admixtures (VMA) has proved to be very effective in stabilizing the rheological properties and consistency of self-compacting concrete (SCC). SCC is known for its excellent deformability, high resistance to segregation and use, without applying vibration, in congested reinforced concrete structures characterized by difficult casting conditions. Most of the commercial VMAs currently available in the market are costly and increase the price of such a concrete. Identification or production of new low-cost VMA is then essential. This paper presents the performance of four new polysaccharide-based VMAs in enhancing the rheological and consistency properties of cement paste. The study of the rheological properties and consistency of cement paste to screen the dosage and type of new VMA to be used in SCC is a promising approach. Investigation was carried out on cement pastes with combinations of various dosages of new VMAs and of a superplasticizer (SP) to study the influence on rheology, consistency and washout mass loss. A commercial VMA designated in this paper as “COM” was tested for comparison. The study on new VMAs is encouraging and confirms that pastes with satisfactory rheological and consistency properties comparable with or even better than commercial VMA can be developed. The combined use of proper dosages of VMA and SP is shown to clearly contribute to securing high-performance cement pastes that is highly fluid yet cohesive enough to reduce water dilution and enhance water retention. Attempt has also been made to correlate rheological properties (yield stress) to consistency (slump) of pastes.     

煅烧气氛对水泥浆体流变性的影响

用旋转黏度计和微型坍落度仪测定水泥浆体的流变性,又利用岩相分析、XRD、水化热等测试方法分析相关的作用机理,研究探讨煅烧气氛对水泥浆体流变性的影响。结果表明,在还原气氛下煅烧的熟料,其中的Fe2O3部分被还原为FeO、FeS,因而减少了C4AF含量,增大了C3A含量,使得水泥的水化速度加快,浆体的流变性降低。     

煅烧时间对水泥浆体流变性的影响

采用电炉模拟预分解窑热工制度煅烧水泥熟料,并用旋转黏度计和微型坍落度仪等测定水泥浆体的流变性,又利用扫描电镜、水化热等近代测试方法,分析相关的作用机理,研究熟料煅烧时间对水泥浆体流变性的影响。研究结果表明：煅烧熟料的保温时间过短,矿物晶体来不及充分长大,颗粒细小,发育不完善,加快水泥的水化速度,使水泥浆体的流变性变差。     

水泥浆体早期收缩的试验研究

开发了一套采用高精度LVDT位移传感器测定水泥浆体长度变化的仪器,并用该仪器对不同水灰比和不同混合材的水泥浆体的收缩行为进行了研究。结果表明,该仪器能够准确、连续地测量水泥浆体终凝后的自收缩和干燥收缩的发展过程。早期自收缩和干燥收缩随着水灰比降低而明显增加,在20℃相对湿度50%的环境下,水泥浆体的自收缩占干燥收缩的10%左右;掺粉煤灰可以少量降低水泥浆体的自收缩和干燥收缩,而掺入石灰石和矿渣则不同程度的增大了水泥浆体早期收缩率。     

The effect of gyrolite additive on the hydration properties of Portland cement

The influence of gyrolite additive on the hydration properties of ordinary Portland cement was examined. It was found that the additive of synthetic gyrolite accelerates the early stage of hydration of OPC. This compound binds alkaline ions and serves as a nucleation site for the formation of hydration products (stage I). Later on, the crystal lattice of gyrolite becomes unstable and turns into C–S–H, with higher basicity (C/S ~ 0.8). This recrystallization process is associated with the consumption of energy (the heat of reaction) and with a decrease in the rate of heat evolution of the second exothermic reaction (stage II). The experimental data and theoretical hypothesis were also confirmed by thermodynamic and the apparent kinetic parameters of the reaction rate of C₃S hydration calculations. The changes occur in the early stage of hydration of OPC samples and do not have a significant effect on the properties of cement stone.     

Modeling of hydration reactions using neural networks to predict the average properties of cement paste

This paper presents a hydration model that describes the evolution of cement paste microstructure as a function of the changing composition of the hydration products. The hydration model extends an earlier version by considering the reduction in the hydration rate that occurs due to the reduction of free water and the reduction of the interfacial area of contact between the free water and the hydration products. The BP Neural Network method is used to determine the coefficients of the model. Using the proposed model, this paper predicts the following properties of hardening cement paste: the degree of hydration, the rate of heat evolution, the relative humidity and the total porosity. The agreement between simulation and experimental results proves that the new model is quite effective and potentially useful as a component within larger-scale models designed to predict the performance of concrete structures.     

Effect of polysaccharides on the hydration of cement paste at early ages

This work deals with the relative efficiency of polysaccharides and their influence on cement hydration. Several parameters such as the structure, concentration, average molecular weight, and soluble fraction value of polysaccharides were examined. Cement hydration was monitored by isothermal calorimetry, thermogravimetry (TGA), and Fourier transform infrared (FTIR) spectroscopy. Results clearly show that retardation increases with higher polysaccharide-to-cement weight ratio (P/C). Low-molecular-weight starch showed enhanced retarding effect on the hydration of cement. The retardation effect of polysaccharides is also dependent on the composition of cement.     

An investigation on microstructure of cement paste containing fly ash and silica fume

In this study, high-calcium fly ash (HCFA) and silica fume (SF) were used as mineral admixtures. The effect of these admixtures on the microstructure of cement paste was investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The reaction of HCFA and SF with portlandite, which occurs in Portland cement (PC), forms a new calcium-silicate-hydrate (C-S-H) gel.     

Effects of polycarboxylate-type superplasticizer on fluidity and hydration behavior of cement paste

Polycarboxylate (PC)-type superplasticizers were synthesized with different average molecular weight of polyethylene oxide (PEO) graft groups, the molar ratios of graft group to carboxylic group, and then the chemical structure, polymerization condition, and physical and chemical properties were analyzed. In order to evaluate the effects of PC-type superplasticizers in cement paste, the adsorption, the initial plasticity and slump retention characteristics and the degree of hydration reaction were investigated. As the average molecular weight of graft group decreased and molar ratio increased, the conversion rate to copolymers, the adsorption amount of PC-type superplasticizer on cement particles improved; on the other hand, the hydration reaction was delayed.     

Modeling the coupled effects of temperature and fineness of Portland cement on the hydration kinetics in cement paste

This paper revisits the coupled impacts of fineness and temperature on the kinetics of Portland cement hydration. The approach consists in i) modeling the impact of fineness on cement dissolution through the hypothesis that the surface dissolution rate of cement particles is independent of their size, in order to, in a second step, ii) model the impact of temperature on the kinetics of cement dissolution. The analysis of the experimental results shows that the effect of cement fineness on the hydration kinetics can be captured by a simple hypothesis: for any age, the reacted thickness of cement grains can be considered independent of the initial cement particle size. In addition, the analysis of the results at different temperatures shows that a constant activation energy can account for the effect of temperature on the hydration kinetics, with an Arrhenius equation applied to the kinetics of surface dissolution. The results from the model give a good agreement with the experimental results in a significant number of combinations (different Portland cements, water/cement ratio from 0.5 to 0.6, cement Blaine fineness from 3500 to 6600 cm²/g, temperature histories between 20 and 60 °C).     

掺入纳米SiO_2对水泥净浆流动性和力学性能的影响

掺入纳米SiO2,采用2种掺入方法和不同掺量,讨论了对水泥净浆的流动度和强度等性能的影响,得出最佳掺量和掺入方法。进一步探究掺入不同粒径的纳米SiO2对水泥净浆性能的影响规律。实验得出,采用将纳米SiO2与水泥先干混再拌合方法更利于拌合分散均匀,掺量越大流动度降低程度越大,而粒径越小流动度明显降低;纳米SiO2掺量的强度影响临界值为1.5%;各个粒径对早、中期强度均有明显增强,30nm的效果最明显。     

Interactions of polymers and organic admixtures on portland cement hydration

Interaction of polymers and other organic admixtures on Portland cement hydration is reviewed. This has been compiled in a systematic way. First hydration of Portland cement is described in short. Later, interaction with 4 important components of Portland cement is discussed. Finally interphase effects in polymer modified hydraulic cement are discussed. It is concluded that polymers and organic admixtures interact with the components of Portland cement when they come in contact with water. This interaction is due to ionic binding, causing cross-links which inhibit the film formation property of polymers and influence considerably the crystallisation process during the hardening of concrete. Some low molecular weight organic substances also have a considerable influence on Portland cement during its reaction with water.     

The hydration of cement paste using the semi-isothermal method of derivative thermogravimetry

The effects of age and water/cement ratio on the products of hydration of cement paste are reported. The products are quantified as weight losses related to five separate peaks isolated by means of the semi-isothermal method of differential thermogravimetry. It is shown that the weight losses for the peaks change in proportion for all ages and water/cement ratios, and hence new measures of the degree of hydration are suggested. Comparisons with other materials indicates that the three peaks occuring at the lowest temperatures consist, at least in part, of calcium silicate hydrates.     

A multi-technique investigation of the nanoporosity of cement paste

The nanometer-scale structure of cement paste, which is dominated by the colloidal-scale porosity within the C-S-H gel phase, has a controlling effect on concrete properties but is difficult to study due to its delicate structure and lack of long-range order. Here we present results from three experimental techniques that are particularly suited to analyzing disordered nanoporous materials: small-angle neutron scattering (SANS), weight and length changes during equilibrium drying, and nanoindentation. Particular attention is paid to differences between pastes of different ages and cured at different temperatures. The SANS and equilibrium drying results indicate that hydration of cement paste at 20 °C forms a low-density (LD) C-S-H gel structure with a range of gel pore sizes and a relatively low packing fraction of solid particles. This fine structure may persist indefinitely under saturated conditions. However, if the paste is dried or is cured at elevated temperatures (60 °C or greater) the structure collapses toward a denser (less porous) and more stable configuration with fewer large gel pores, resulting in a greater amount of capillary porosity. Nanoindentation measurements of pastes cured at different temperatures demonstrate in all cases the existence of two C-S-H structures with different characteristic values of the indentation modulus. The average value of the modulus of the LD C-S-H is the same for all pastes tested to date, and a micromechanical analysis indicates that this value corresponds to the denser and more stable configuration of LD C-S-H. The experimental results presented here are interpreted in terms of a previously proposed quantitative “colloid” model of C-S-H gel, resulting in an improved understanding of the microstructural changes associated with drying and heat curing.     

Low-temperature scanning electron microscope analysis of the portland cement paste early hydration

The use of the frozen hydrated scanning electron microscopy (FHSEM) in the study of cement paste is described. This technique permits analysis of the fractured surface of cement paste in a fully hydrated state with water present as ice in a low temperature scanning electron microscope. At 110 K the paste has a substantial increase in mechanical strength, because water is converted from liquid to a solid state, and this permits the use of bulk specimens at very early hydration. Some preliminary results for 1 hour hydration are presented and future applications of this technique are discussed.     

Influence of organic admixtures on the rheological behaviour of cement pastes

The rheological properties of cementitious pastes used to proportion Self-Consolidating Concretes (SCC) have been examined, in particular, the influence of High range water reducing admixture (HRWRA) with that of Viscosity Modifying Admixtures (VMAs) have been compared. HRWRAs are known to have dispersing effects on the cement particles through steric and/or electrostatic repulsion, while the effects of VMAs are expected to stabilise the paste by increasing the viscosity of the aqueous solution. Both transient and steady state rheological behaviour of the cementitious pastes proportioned with different dosages of HWRA and VMA were considered. Experimental results show that the influence of VMAs on rheological properties is actually minor compared to that of HRWRAs. These results are discussed in the framework of rheology of concentrated suspensions.