Leslie J. Struble

Viscosity of Portland cement paste as a function of concentration

Abstract The flow behavior of Portland cement paste was studied as a function of its concentration. The relationship between viscosity and concentration was found to be well described by the Krieger-Dougherty equation. For pastes dispersed using a superplasticizer, the maximum volume fraction was found to be approximately 0.7, and the intrinsic viscosity was found to be approximately 5. Pastes that were not dispersed were considerably higher in viscosity; such flocculated pastes fit the Krieger-Dougherty equation, but gave a much lower maximum volume fraction, indicating fairly open flocs even at the high strain rate used for viscosity measurements (500 seconds−1).

Use of oscillatory shear to study flow behavior of fresh cement paste

Abstract Low-amplitude oscillatory shear was used to study the flow behavior of fresh cement paste. Three cements were studied, slightly different in their particle size distributions, at w/c levels of 0.40, 0.45, and 0.50. The results indicated that the suspensions were flocculated and behaved as viscoelastic solids. The storage modulus values for cement paste in its low-strain linear-viscoelastic region ranged from 14 to 24 kPa. As strain was increased above some critical level, the storage modulus values decreased; this critical strain for cement paste was approximately 10−4. As w/c was increased, the storage modulus tended to decrease and the critical strain decreased slightly.

Rheological changes associated with setting of cement paste

Abstract A method is described in which the yield stress of portland cement paste is determined from its creep/recovery behavior measured using a constant stress rheometer. As long as the applied stress does not exceed the yield stress, yield stress can be determined as a function of time on a single specimen without breaking down the microstructure. The early hydration is seen to strengthen the interparticle bonding within the flocculated structure, hence increasing the yield stress. Time evolution of the measured yield stress shows two regions, the first in which the yield stress increases slowly, and the second in which the yield stress increases rapidly. These correlate with the induction period and the acceleratory period, and the transition to a rapid increase in yield stress corresponds to the initial setting time. Reducing the water:cement ratio caused this initial setting time to be reduced.

RHEOLOGY OF CEMENT PASTE AND CONCRETE

Much has been learned in recent years about the rheology of cement paste and how it relates to microstructure. Such progress has been made possible in part by the use of specialized instruments for measuring dynamic rheological properties, developed to characterize viscoelastic materials. This paper reviews recent studies of cement paste in our laboratory in which dynamic rheological properties are used to follow the effects of cement hydration. Not as much has been learned about the rheology of concrete. Measuring flow behavior of concrete presents interesting challenges; concrete rheometers have been developed, but they are not widely used. It appears likely that concrete rheology is controlled by rheology of its paste, although such links have not yet been directly established. This paper also presents preliminary results from our laboratory on concrete flow behavior. Concrete rheology provides important information about its workability, and rheological parameters have important advantages over slump when characterizing workability.

Adiabatically cured, alkali-activated cement-based wasteforms containing high levels of fly ash. Formation of zeolites and Al-substituted C-S-H

Abstract Cementitious binder compositions with high fly ash contents proposed for immobilization of highly alkaline low-level radioactive waste solutions have significantly different chemistry and behavior than normal Portland cement pastes. In this paper, we investigate the mineralogy of a simulated wasteform proposed for the immobilization of the tank wastes stored at the Hanford Reservation, Washington State, USA. These solutions have very high (multimolar) concentrations of sodium, aluminate, phosphate, carbonate, nitrate, and nitrite ions. The cementitious blend proposed to solidify this solution contains a high proportion of fly ash to avoid the large heat output that would be associated with the hydration of a neat Portland cement. In addition to calcium silicate hydrate (C-S-H), the binding phase normally formed by hydrating cements, zeolites also formed during adiabatic curing up to 90°C. The zeolites, sodalite and Na-P1 (gismondine framework), immobilize part of the sodium in the system. Compared to C-S-H in ordinary Portland cement (OPC), the C-S-H in this system, which had a layer spacing of 1.1 nm, was more crystalline, and had a longer chain length and a higher aluminum content. Changing the curing conditions had a large effect on early age mineralogy, for example, traces of chabazite are formed on isothermal curing at 90°C. The long-term mineralogy depended mainly on the final curing temperature.

Using creep and recovery to study flow behavior of fresh cement paste

Abstract A controlled stress rheometer has been used to determine the creep and recovery behavior of flocculated cement pastes. The behavior was found to depend on the level of applied stress. At a lower applied stress, the creep curve of each paste was characteristics of a nonlinear viscoelastic solid, with an instantaneous strain superimposed on an elastic strain; the recovery, on the other hand, was characteristics of a viscoelastic liquid, with little or no instantaneous strain, but some retarded strain and a substantial unrecovered strain. At a higher applied stress the behavior was strikingly different. In this case, the behavior was characteristics of a viscous liquid, with a nearly linear increase in strain throughout the duration of the stress and no recovery when the stress was released. This transition from solid-like behavior to liquid-like behavior occurred over a very narrow stress increment, and the transition stress corresponded to the yield stress estimated from flow curves and from oscillatory shear measurements.

Modeling Static Segregation of Self-Consolidating Concrete

A numerical model was developed based on rheology and fluid dynamics to calculate paste-aggregate interface settling speed in an effort to help prevent self-consolidating concrete static segregation, which occurs when aggregate sinks in at-rest concrete. The model was found to be reasonably accurate in describing static segregation. Density, volume fraction, maximum volume fraction, aggregate size, and paste rheology (viscosity and yield stress) are among the properties seen to affect static segregation.

Setting Time Measurement Using Ultrasonic Wave Reflection

Ultrasonic shear wave reflection was used to investigate setting times of cement pastes by measuring the reflection coefficient at the interface between hydrating cement pastes of varying water-to-cement ratio and an ultrasonic buffer material. Several different buffer materials were employed, and the choice of buffer was seen to strongly affect measurement sensitivity; high impact polystyrene showed the highest sensitivity to setting processes because it had the lowest acoustic impedance value. The results show that ultrasonic shear-wave reflection can be used successfully to monitor early setting processes of cement paste with good sensitivity when such a very low impedance buffer is employed. Criteria are proposed to define set times, and the resulting initial and final set times agreed broadly with those determined using the standard penetration resistance test.

Using ultrasonic wave reflection to measure solution properties

Ultrasonic wave reflection coefficients of aqueous solutions were measured using high-impact polystyrene as a buffer material to provide enhanced sensitivity over metal or ceramic buffer materials. The wave reflection values showed linear reduction when the concentration of chemical species in solution was increased, but a distinct relation between concentration and reflection coefficient was obtained for each solute species tested. However, more unified relationships were observed between reflection coefficient and other solution parameters - solution density, acoustic impedance, and P-wave velocity - that were consistent for all solution species. Based on this behavior an expression to compute solution density solely from reflection coefficient is derived, which can be applied to estimate solution density in solutions of unknown solute species and concentration when other measurements, such as wave velocity, are not possible.

Effects of Air Entrainment on Rheology

This article reports on a study in which the effects of air entrainment on rheological parameters were investigated using cement paste. The addition of air-entraining agent increased the air content up to a saturation level, above which no further increase in air content was observed. The results showed that with increasing air content, the yield stress increased and the plastic viscosity decreased. The authors note that the increase in yield stress was an unexpected result because increasing air is well known to cause an increase in slump, and yield stress and slump are known to be negatively correlated (as yield stress increases, slump decreases). The authors propose two explanations for the effects of entrained air bubbles on rheological parameters: the attraction of cement particles and bubbles to form bubble bridges, and a fluid response of air bubbles due to their deformability. The authors contend that bubble bridges dominate in the yield stress and the fluid response will dominate when the sample is flowing.