Effects of air voids on ultrasonic wave propagation in early age cement pastes

The objective of this paper is to investigate effects of air voids on ultrasonic wave propagation in fresh cement pastes, and relate ultrasonic wave parameters to cement setting times. First, Biot's theory was used to analyze wave propagation in poroelastic media containing air bubbles. Then, in the experimental study, both the compressional (P) and shear (S) waves were monitored in cement pastes with different water/cement ratios (w/c = 0.4 and 0.5) and various air void content (0.1%–5.3% by cement paste volume). Experimental results indicated that existence of air bubbles in cement paste significantly decreases the P wave velocity, but has little effect on the shear wave propagation. Further analysis shows that the shear wave velocity corresponding to the Vicat initial setting times is a relatively constant value for the investigated air content range. This study shows the potential of using shear waves to monitor setting and hardening process of cement.     

Rheometric and ultrasonic investigations of viscoelastic properties of fresh Portland cement pastes

The paper investigates the possibility of using a shear wave reflection technique to monitor the viscoelastic behavior (represented by storage shear modulus and viscosity) of Portland cement paste at very early age. Three cement pastes with water/cement ratios equal to 0.4, 0.5 and 0.6 cured under water at a constant temperature of 25 °C were studied. By measuring the wave reflection coefficients and the phase angles of reflected ultrasonic waves, the dynamic storage shear moduli and the viscosity of the cement paste can be calculated. The calculated results of the storage modulus were compared with the results obtained directly from the oscillatory rheometric measurement. In addition, the viscosity calculated from the wave reflection measurements was compared with results obtained directly from the step rheometric method and a qualitative agreement was found. The results show that as a non-destructive method, the ultrasonic wave reflection method provides useful information about both the elastic and viscous behavior of cement pastes at very early age.     

Possibilities of using the ultrasonic wave transmission method to estimate initial setting time of cement paste

In this paper, the applicability of the ultrasonic wave transmission method to estimate the initial setting time of an arbitrary cement paste is discussed. Ultrasonic pulse velocity measurements were fully automated and measured continuously. The Vicat Needle Test was used in order to determine the initial setting time of cement pastes. Different cement pastes were prepared in order to check the influence of the water/cement ratio, type of cement, curing temperature, cement fineness, and some clinker compositions, on the relationship between the initial setting time and ultrasonic pulse velocity. It was found that the initial setting time of an arbitrary cement paste can be estimated very accurately by the time the first inflection point appears on the ultrasonic pulse velocity curve. Moreover, it can be estimated quite accurately by the time the ultrasonic pulse velocity reaches a fixed value, close to the value of the ultrasonic pulse velocity in water.     

Early age microstructure of Portland cement mortar investigated by ultrasonic shear waves and numerical simulation

This paper investigates the ability of a shear wave reflection (WR) method to monitor microstructural changes of Portland cement mortar during hydration. The wave reflection method measures the reflection loss of shear waves at an interface between a steel plate and mortar. Mortars with water/cement ratios of 0.35, 0.5 and 0.6 were tested at isothermal curing conditions of 25 °C. The numerical model HYMOSTRUC3D was used to simulate the evolution of microstructural properties of the cement paste phase of the tested mortars. The parameters obtained from the simulations were the volume fraction of the total and connected solid phase and the specific contact area of the hydrated cement particles. The investigations have shown that the wave reflection measurements are governed primarily by the degree of the inter-particle bonding of the cement particles as calculated from the specific contact area of a simulated microstructure.     

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.     

Effect of treatment temperature on the early hydration characteristics of superplasticized silica fume blended cement pastes

In this investigation, two mixes were used: ordinary Portland cement (OPC) and a blended cement prepared with the partial substitution of OPC by 10 mass% silica fume (SF). The setting and hardening characteristics were monitored by the aid of electrical conductivity as a function of curing time. The shear stress and electrical conductivity were studied at different temperatures, namely, 20, 35, 45 and 55 °C. As the temperature increases, the shear stresses decrease with the increase of shear rate. The height of electrical conductivity peaks of superplasticized cement pastes increases due to the increase of the paste fluidity. In the presence of 1.0% polycarboxylate (PC), the electrical conductivity of cement pastes decreases from 1 to 28 days. PC retards the hydration of cement pastes. The presence of PC extended the setting times of cement pastes at 35 °C than at 20 °C due to the increase in the adsorption capacity at this temperature. PC extends the dormant stage of the hydration process and delays the onset of the accelerating stage, without affecting its rate.     

磷渣对水泥浆体水化性能和孔结构的影响

通过对水泥浆体凝结性能、水化放热、力学性能和孔结构的测定,以及扫描电镜分析和差热-热重分析,研究了不同掺量磷渣对水泥浆体水化性能和微观结构的影响.结果表明:随着磷渣掺量的增加,浆体的凝结时间延长,水化热减少,早期抗压强度下降.但掺磷渣水泥浆体的后期抗压强度已接近或超过了纯水泥浆体的,磷渣掺量的增加对水泥浆体的后期抗压强度影响不显著.浆体中的Ca(OH)2量随龄期的延长而增加并随磷渣掺量的增加而降低.磷渣的活性效应和填充效应的发挥有效地改善了浆体水化后期的微观结构和孔结构,从而使浆体的力学性能有所提高.     

Influence of superplasticizers on the evolution of ultrasonic P-wave velocity through cement pastes at early age

The paper discusses a possibility of using an ultrasonic wave transmission method to study the influence of superplasticizers on the formation of structure of cement pastes at early ages. When compared to mixtures without additives, lower P-wave velocity was found through superplasticized cement pastes, indicating that superplasticizers prevent formation of a solid network frame. Comparing to sulfonate naphthalene-formaldehyde superplasticizers, polycarboxylate ether (PCE) admixtures retarded the solid network frame development more intensively, resulting in a plateau on a P-wave velocity curve during the setting period. The length of the plateau is proportional to the dosage of the PCE and inversely proportional to the specific surface area of the hydration products developed, proving that the specific surface area of a solid phase affects the performance of the PCEs. Validation of ultrasonic results was determined on the basis of the temperature evolution of the material in time.     

Early hydration and setting of oil well cement

A broad experimental study has been performed to characterize the early hydration and setting of cement pastes prepared with Class H oil well cement at water-to-cement ratios (w/c) from 0.25 to 0.40, cured at temperatures from 10 to 60 °C, and mixed with chemical additives. Chemical shrinkage during hydration was measured by a newly developed system, degree of hydration was determined by thermogravimetric analysis, and setting time was tested by Vicat and ultrasonic velocity measurements. A Boundary Nucleation and Growth model provides a good fit to the chemical shrinkage data.Temperature increase and accelerator additions expedite the rate of cement hydration by causing more rapid nucleation of hydration products, leading to earlier setting; conversely, retarder and viscosity modifying agents delay cement nucleation, causing later setting times. Lower w/c paste needs less hydration product to form a percolating solid network (i.e., to reach the initial setting point). However, for the systems evaluated, at a given w/c, the degree of hydration at setting is a constant, regardless of the effects of ambient temperature or the presence of additives.     

The pozzolanic reactivity of monodispersed nanosilica hydrosols and their influence on the hydration characteristics of Portland cement

This study reveals that the nanosilica hydrosols with higher specific surface areas had faster pozzolanic reactivity, especially at early ages; moreover, the results are indicative of the accelerating influence of nanosilicas and silica fume on the hydration of cement. Faster initial and final setting times observed for cement pastes containing nanosilicas are consequence of these mechanisms. However, less hydration degree of cement compared to the plain paste was observed at age of 7 days and after. This can be attributed to the entrapment of some of mix water in the aggregates of nanosilicas formed in cement paste environment, making less water available for the progress of cement hydration. The same mechanism is believed to be responsible for the reduction of flowability of cement pastes.     

Early-age acoustic emission measurements in hydrating cement paste: Evidence for cavitation during solidification due to self-desiccation

In this study, the acoustic emission activity of cement pastes was investigated during the first day of hydration. Deaired, fresh cement pastes were cast in sealed sample holders designed to minimize friction and restraint. The majority of acoustic emission events occurred in lower water to cement ratio pastes, while cement pastes with higher water to cement ratios showed significantly less acoustic activity. These acoustic events occurred around the time of setting. A layer of water on the surface of the cement pastes substantially reduced acoustic emission activity at the time of setting. According to these experimental results, the acoustic emission measured around setting time was attributed to cavitation events occurring in the pores of the cement paste due to self-desiccation. This paper shows how acoustic emission might be used to indicate the time when the fluid–solid transition occurs in a cement paste, often referred to as time-zero. Knowledge of time-zero is fundamental for determining when mechanical properties develop and in calculations of residual stresses.     

Influence of hydration on the fluidity of normal Portland cement pastes

The rheological properties of cement paste strongly influence the workability of concrete. It is known that early hydration processes alter phase composition and microstructure of cement pastes. These processes affect fluidity and setting behaviour of cement paste. While many studies tried to measure and model rheological properties of cement pastes, only a few studies assessed the influence of the hydrate morphology on the fluidity of cement pastes.Results of the present study compare the influence of long prismatic hydrates (i.e. syngenite, secondary gypsum) on the fluidity of cement pastes with the effect of other hydrates (AFm).To induce the formation of certain hydration products the cement composition was modified by addition of set regulators and alkali sulphates. Furthermore a combination of various analytical methods such as fluidity (viscometric) testing and microstructural analysis (phase quantification by XRD-Rietveld analysis, investigation by Environmental SEM, BET analysis etc.) was performed. Results are implemented into a fundamental discussion on the influence of various hydration products on the fluidity of the paste.     

Air void morphology in fresh cement pastes

Two different procedures are used in conjunction with low-temperature scanning electron microscopy to image the air voids in cement pastes at very early ages. The first procedure isolates the air voids from cement paste after less than 30 min of hydration, and allows them to be imaged apart from the paste. The second procedure involves quenching the fresh cement paste specimens in liquid nitrogen after 5 min of hydration. In both cases, a distinct air void shell is apparent even at these short hydration times. The shell appears to be made up of small (1-5 μm) mineral particles. The second method confirms the presence of a water-rich transition zone around the air voids in the quenched pastes, consistent with earlier studies. Foam stability studies show that sodium oleate gives more stable foams than sodium dodecyl benzene sulfonate, but is more sensitive to the presence of calcium ions.     

Influence of spraying on the early hydration of accelerated cement pastes

In practice, most of the studies about the interaction between cement and accelerators is performed with hand-mixed pastes. However, in many applications mixing occurs through spraying, which may affect accelerators reactivity and the microstructure of the hardened paste. The objective of this study is to analyze how the mixing process influences the early hydration of accelerated cement pastes. Isothermal calorimetry, X-ray diffraction, thermogravimetry and SEM imaging were performed on cement pastes produced by hand-mixing and by spraying, using equivalent doses of an alkali-free and an alkaline accelerator and two types of cement. Results showed a great influence of the spraying process on the reactivity of accelerators and on the morphology of the precipitated hydrates. Variations in hydration kinetics caused by the mixing method are explained and the results obtained might have a significant repercussion on how future research on the behavior of accelerated mixes will be performed.     

The use of frequency spectrum of ultrasonic P-waves to monitor the setting process of cement pastes

In this paper, a possibility of using a frequency spectrum of ultrasonic P-waves to monitor the formation of structure of cement pastes at early ages is studied. A new parameter, labeled as a TG parameter, is defined as a dimensionless ratio between maximum amplitudes of two dominant frequency ranges that appear in a frequency spectrum of received ultrasound signals. Four stages and three characteristic points can be identified on the TG-vs.-time graphs, indicating that the development of the frequency spectrum is closely related to the setting phenomena. By comparing the TG parameter with the P-wave velocity and temperature evolutions in time, important milestones in the process of formation of microstructure were identified, such as the time of reduced workability and a period of intensive setting. The combined use of P-wave velocity and TG parameter results in a comprehensive ultrasonic method that gives a more complete picture of setting.     

Experimental study and numerical simulation on the formation of microstructure in cementitious materials at early age

The formation of microstructure in early age cement paste and concrete was examined with an ultrasonic experimental set-up. Research parameters included the influence of curing temperature (isothermal curing at 20, 30 and 40 °C), water/cement ratio (0.40, 0.45 and 0.55) and amount of aggregate. In parallel with the experiments, the cement hydration model HYMOSTRUC was utilized to simulate the formation of the microstructure. In this study, the cement paste was considered as a four-phase system consisting of water, unhydrated cement, hydration products and that part of the hydration product that causes the contact between the hydrating cement grains (so called “bridge volume”). A correlation has been found between the growth of bridge volume calculated with the model and the changes in the pulse velocity. It is believed that ultrasonic pulse velocity (UPV) measurements can represent a valuable tool to investigate the development of the microstructure at early age.     

Monitoring of film formation,curing and ageing of coatings by an ultrasonic reflection method

During film formation, curing and ageing of coatings changes in physical properties and chemistry of the material occur. For these processes, complex interactions of different factors have to be taken into account. In order to get a more detailed insight in mechanisms of film formation, curing and ageing, it is desired to monitor the changes of the mechanical properties in real time. Therefore, in this work various modifications of an ultrasonic reflection technique enabling such analyses are described.The ultrasonic method measures the complex acoustic reflection coefficient at the interface between a substrate and a coating for longitudinal and/or transversal waves at ultrasonic frequencies. From the complex acoustic reflection coefficients and the density of the sample, the corresponding mechanical moduli and the Poisson ratio can be calculated. The method can be considered as a real time dynamic mechanical analysis at high frequencies. Physical changes as vitrification, swelling and drying, crack initiation and delamination can be monitored. Recently, the ultrasonic reflection technique has been combined with near-infrared analysis for monitoring UV-induced curing reactions within seconds.     

高温管道在线不停输检验技术研究

在压力管道在役检验领域,超声检测技术作为重要的检测手段已得到广泛应用。随着温度的变化,材料中超声波的声速、声压也会发生变化,掌握温度对超声波参数的影响规律,对高温状态下超声检测具有重要意义。讨论了在50~250℃温度之间,超声横波速度随温度变化的规律。研制了专门用于高温检测的超声横波斜探头。通过试验,给出了横波速度随温度变化的关系,用以修正不同温度下缺陷的实际位置和缺陷的尺寸大小。     

基于不同粒径分布的低熟料矿渣水泥水化特征和孔结构特性

低熟料矿渣水泥(LSC)是一种水泥熟料用量低,主要由粒化高炉矿渣和石膏组成的水硬性胶凝材料.本文研究水泥不同粒径分布(对应比表面积分别为358 m2/kg、450 m2/kg和516 m2/kg)对低熟料矿渣水泥的抗压强度、电阻率和水化热、水化产物、孔结构的影响.结果表明,当比表面积从358 m2/kg增加到450 m2/kg可以提高低熟料矿渣水泥浆体的抗压强度,当从450 m2/kg增加至516 m2/kg时,强度提高甚微.低熟料矿渣水泥主要的水化产物是钙矾石和水化硅酸钙,增加水泥细度导致放热速率明显加快,电阻率变化曲线的下降段持续时间明显缩短,因而会产生更多的钙矾石.水泥细度增加,浆体的凝胶孔的体积分数增大,大孔减少,进一步提高浆体的密实度.     

煤矸石-矿渣-水泥体系的水化进程及其性能研究

采用选择性溶解法和非线性拟合法分别测量了煤矸石-矿渣-水泥三元体系中三种原材料（煤矸石,矿渣,水泥）的水化进程,并对含有不同掺量矿物掺合料的复合水泥浆体干燥收缩及抗压强度进行了测量和分析。结果表明：当复合水泥浆体中含有5％煤矸石,25％矿渣时,复合材料的干燥收缩较小,抗压强度较大。