Monitoring the setting of concrete containing blast-furnace slag by measuring the ultrasonic p-wave velocity

Ultrasonic transmission measurements allow the continuous monitoring of the setting of both mortar and concrete samples, which is important to determine for instance the formwork removal time. However, aspects such as the cause of the low initial velocity, the relation between the velocity and the setting times and the effect of cement type or cement replacing additives are still under discussion. Therefore, different concrete compositions with blast-furnace slag were tested by traditional as well as ultrasonic measurements.The ultrasonic method gives a more complete picture of the setting. The change of ultrasonic velocity in time is sensitive to the differences in setting behaviour of the tested mixtures. The initial setting seems to correspond with the inflection point of the velocity-vs.-time graphs and the final setting with the point at which the velocity increase levels off.     

Modeling the hydration of concrete incorporating fly ash or slag

Granulated slag from metal industries and fly ash from the combustion of coal are industrial by-products that have been widely used as mineral admixtures in normal and high strength concrete. Due to the reaction between calcium hydroxide and fly ash or slag, the hydration of concrete containing fly ash or slag is much more complex compared with that of Portland cement. In this paper, the production of calcium hydroxide in cement hydration and its consumption in the reaction of mineral admixtures is considered in order to develop a numerical model that simulates the hydration of concrete containing fly ash or slag. The heat evolution rates of fly ash- or slag-blended concrete is determined by the contribution of both cement hydration and the reaction of the mineral admixtures. The proposed model is verified through experimental data on concrete with different water-to-cement ratios and mineral admixture substitution ratios.     

Early age strength enhancement of blended cement systems by CaCl2 and diethanol-isopropanolamine

The enhancement of the 1 day strength of cementitious systems by a combination of calcium chloride (CaCl₂) and diethanol-isopropanolamine (DEIPA) was studied, particularly in blended cement systems. A combination of quantitative X-ray diffraction with Rietveld refinement (QXRD), scanning electron microscopy (SEM)/backscattered electron image analysis, thermogravimetric analysis (TGA), and isothermal calorimetry were used to investigate the mechanism of strength enhancement by the additives. The additives were found to increase the early age mortar strength by enhancing the cement hydration, with the DEIPA enhancing primarily the aluminate hydration. DEIPA also affected the morphology of portlandite which was formed as thin plates. In parallel, the calcium-to-silica ratio of the C-S-H was found to increase with the use of DEIPA, possibly because of the inclusion of microcrystalline portlandite. After 48 h DEIPA was found to directly enhance the rate of reaction of granulated blast-furnace slag and fly ash.     

The encapsulation of Mg(OH)2 sludge in composite cement

A range of magnesium hydroxide waste sludges arising from the re-processing of nuclear fuel exist in the UK and require safe long-term disposal. Similar wastes undergo a cementation process in order to immobilise radioactive material prior to disposal. Simulant magnesium hydroxide sludges have been prepared and their subsequent interactions with composite cement systems based on the partial replacement of ordinary Portland cement with blastfurnace slag and pulverised fuel ash have been studied. This work has concluded that there was little reaction between the sludge and any of the composite cements during hydration. Apart from a small quantity of a hydrotalcite-type phase containing magnesium from the sludge, the main phases detected were C-S-H and unreacted brucite. This indicates that the magnesium in the sludges is encapsulated by the cement, rather than being immobilised or chemically bound within the hardened matrix.     

A study on the relationship between porosity of the cement paste with mineral additives and compressive strength of mortar based on this paste

Mercury intrusion porosimetry study was carried out on ordinary Portland cement (OPC) pastes with 10% to 40% mineral additives, such as steel-making slag, granulated blast furnace slag and fly ash. For all samples, the porosity of paste and compressive strength of mortar based on this paste were determined at 3, 7, 28, 90 and 180 days. Relationship between the porosity and strength was investigated and some equations for the strength-porosity relationship were presented according to Balshin multiplicative model. Results show that mineral additives delayed process that micropore structure of OPC paste developed and strength development of sample with mineral additives was faster than that of OPC sample. Balshin equation fits the results of strength and porosity of all samples and there is a strongly quantitative relationship between strength and porosity. After being mixed with mineral additives, the intrinsic strength σ₀ and power n both increased and the sequence of σ₀ and n for different mineral additives was fly ash>steel-making slag>blast furnace slag.     

Selection of an effective ASR-prevention strategy for use with a highly reactive aggregate for the reconstruction of concrete structures at Mactaquac generating station

Mactaquac Generating Station was constructed in the mid 1960's and is located in the province of New Brunswick in Eastern Canada. The effect of ASR expansion on the concrete structures of the station were first noticed approximately 10 years after construction and, ASR was conclusively diagnosed in 1986. Since 1985, various remedial measures have been undertaken to mitigate the effects of concrete expansion. Eventually reconstruction of the concrete structures will be necessary and current projections are that replacement should be complete by 2030. Due to the lack of any suitable locally-available non-reactive aggregate, consideration is being given to using the same source of reactive aggregate for reconstruction.This paper describes a research study to determine the optimum strategy for preventing deleterious ASR expansion with this aggregate. The options being evaluated include the use of pozzolans and slag, limiting the alkali content of the concrete, and the use of chemical admixtures. Methods of evaluation include accelerated laboratory tests and field exposure of large blocks.     

Impact of time-dependant thermal expansion coefficient on the early-age volume changes in cement pastes

Volume changes and time-dependent thermal expansion coefficient were determined at early stages and the measured total strain was separated into thermal strain and autogenous strain. Cement paste specimens were subjected to temperature histories that imitated hydration-induced temperature rise of the mass concrete. It was shown that the thermal expansion coefficient increased significantly with the development of hydration and became more conspicuous when the ground granulated blast furnace slag was added. The time-dependant increase of thermal expansion coefficient, due to self-desiccation, could result in considerable shrinkage strain at the end of the temperature history. The impact of the time-dependant increase of thermal expansion coefficient might be taken into account as one of the necessary factors in the crack control design from now and cannot even be neglected within the range of the water to binder ratio of this study, because the shrinkage originated in that effect sometimes exceed the autogenous shrinkage.     

Influence of nucleation seeding on the hydration kinetics and compressive strength of alkali activated slag paste

Addition of pure calcium silicate hydrate (C–S–H) to alkali-activated slag (AAS) paste resulted in an earlier and larger hydration rate peak measured with isothermal calorimetry and a much higher compressive strength after 1 d of curing. This is attributed to a nucleation seeding effect, as was previously established for Portland cement and tricalcium silicate pastes. The acceleration of AAS hydration by seeding indicates that the early hydration rate is controlled by nucleation and growth. For the experiments reported here, the effect of C–S–H seed on the strength development of AAS paste between 1 d and 14 d of curing depended strongly on the curing method. With sealed curing the strength continued to increase, but with underwater curing the strength decreased due to cracking. This cracking is attributed to differential stresses arising from chemical and autogenous shrinkage. Similar experiments were also performed on Portland cement paste.     

Properties of fresh and hardened concrete

The present paper reviews the literature related to the properties of fresh and hardened concrete published after the previous (12th) International Congress on the Chemistry of Cement held in Montreal in 2007.Workability and fundamental rheological properties, reversible and non-reversible evolution, thixotropy, slump loss, setting time, bleeding, segregation and practical issues related to formwork filling and pressure, are addressed among the properties of fresh concrete.Among hardened concrete properties compressive strength and other mechanical and physical properties of hardened concrete, such as tensile strength, elastic properties, shrinkage, creep, cracking resistance, electrical, thermal, transport and other properties are covered. Testing, interpretation, modeling and prediction of properties are addressed, as well as correlation with properties of fresh concrete and durability, effects of special binders, recycled and natural aggregates, fiber reinforcement, mineral and chemical admixtures. Special attention is given to the properties of hardened lightweight and self-compacting concrete.     

A model to predict the amount of calcium hydroxide in concrete containing mineral admixtures

This study examines in detail the degree of reactivity of admixtures, such as fly ash and blast furnace slag, and their effect on the levels of calcium hydroxide in cement paste. Experimental results indicate that reactivity between calcium hydroxide and mineral admixture is dependent on the amount of calcium hydroxide and the degree of hydration of mineral admixtures.From these results, a model was formulated to predict the reaction between calcium hydroxide and mineral admixtures, and its validity verified by comparing calculated data with the data from the tests with cement mortar specimens. The calculated values of calcium hydroxide agree well with the test results. The parameters of the prediction model are dependent on the physical and chemical characteristics of mineral admixtures.     

Integration approach of the Couette inverse problem of powder type self-compacting concrete in a wide-gap concentric cylinder rheometer: Part II. Influence of mineral additions and chemical admixtures on the shear thickening flow behaviour

The influence of mineral additions and chemical admixtures on the shear thickening flow behaviour of powder type self-compacting concrete (SCC) is studied by means of a wide-gap concentric cylinder rheometer. The Couette inverse problem is treated by means of the integration method in order to derive the flow curve τ(γ?) from the torque measurements.According to the experimental results, the shear thickening effect is found to be strongly influenced by the addition of the chemical admixture (a polycarboxylate ether based superplasticizer), whereas mineral additions were found to modify the intensity of shear thickening. The limestone, quartzite and fly ash addition used in this research project, respectively increase, unalter and decrease the shear thickening intensity. The powder volume and the available amount of free water proved to have a major impact on the viscosity of the powder type SCC mixes. Increasing the powder volume or decreasing the amount of free water results in an increased viscosity of the SCC mix.     

The physical and chemical characteristics of the shell of air-entrained bubbles in cement paste

Recent research has suggested that the shell of an air-entrained void is important for resisting coalescence between air-voids and diffusion of gas from the surrounding fluid. The current paper describes the physical and chemical properties of an air-void shell during the first 2 h of hydration and chemical characteristics at 60 days. Results from this research suggest that the air-void shells found in air-entrained paste have varied physical properties and the crystalline material of these shells is largely made up of fine cement particles during the first 2 h of hydration. Observations of paste at 60 days of hydration suggest that the shell is made up of calcium silicate hydrate (C-S-H) with a morphology different from that in the bulk paste.     

Evolution of penetration resistance in fresh concrete

The objective of this research was to examine the setting of concrete through its penetration resistance; an experimental device, especially developed for this purpose was used, which consists of a system that lets a sphere fall on concrete from a certain height and then measures the depth of the crater. Forty-five samples were made with four different types of cement varying its quantity, water-cement ratio, at ambient temperature and humidity. A semiempirical model which explains the penetration resistance evolution in fresh concrete was proposed and experimentally demonstrated. The results are compared with the consolidation curves for soils represented by the logarithmic method. The proposed setting time was defined as the elapsed time between the placement of the concrete and the time when the depth of the crater is 18% of the initial one.     

Ultrasound monitoring of the influence of different accelerating admixtures and cement types for shotcrete on setting and hardening behaviour

The possible use of ultrasound measurements for monitoring setting and hardening of mortar containing different accelerating admixtures for shotcrete was investigated. The sensitivity to accelerator type (alkaline aluminate or alkali-free) and dosage, and accelerator-cement compatibility were evaluated. Furthermore, a new automatic onset picking algorithm for ultrasound signals was tested. A stepwise increase of the accelerator dosage resulted in increasing values for the ultrasound pulse velocity at early ages. In the accelerated mortar no dormant period could be noticed before the pulse velocity started to increase sharply, indicating a quick change in solid phase connectivity. The alkaline accelerator had a larger effect than the alkali-free accelerator, especially at ages below 90 min. The effect of the alkali-free accelerator was at very early age more pronounced on mortar containing CEM I in comparison with CEM II, while the alkaline accelerator had a larger influence on mortar containing CEM II. The increase of ultrasound energy could be related to the setting phenomenon and the maximum energy was reached when the end of workability was approached. Only the alkaline accelerator caused a significant reduction in compressive strength and this for all the dosages tested.     

Rheology and vibration of fresh concrete: Predicting the radius of action of poker vibrators from wave propagation

The compaction of fresh concrete by an internal poker vibrator has been analysed using closed-form solutions for the propagation of the shear and compressive waveforms, assuming that concrete conforms to the Bingham model. In the inner liquefied zone around the vibrator the flow is due to shear whereas in the outer unsheared zone propagation is due to compressive waves. The analysis gives a method of predicting the radial position at which the flow changes, which coincides with the radius of action of the vibrator. Theory and experiment agree well and confirm that the peak velocity of the vibration governs its efficacy, with radius of action increasing with increasing velocity. The radius of action increases with decreasing yield stress and with increasing plastic viscosity. The work offers the potential to optimise the design and use of vibrators.     

A volumetric technique for measuring the coefficient of thermal expansion of hardening cement paste and mortar

Knowledge of the coefficient of thermal expansion (CTE) is of paramount importance for the determination of the cracking risk of concrete structures at early ages. This paper presents a novel technique which is suitable to measure the CTE of hardening materials with high accuracy starting from casting time.The technique consists of casting a small amount of cement paste or mortar into flexible membranes. The specimens are immersed in an oil bath, whose temperature is rapidly changed and then kept constant in repeating cycles. By suspending the sample from a high-precision balance and reading the change of mass after each temperature step, the CTE is calculated with high accuracy from the measured temperature and strain.Results on cement pastes and mortars (water/cement 0.3) showed a good repeatability. In particular, a sudden decrease in the CTE at setting time, followed by a gradual increase as the cement paste self-desiccates, was measured.     

Tobermorite/jennite- and tobermorite/calcium hydroxide-based models for the structure of C-S-H: applicability to hardened pastes of tricalcium silicate, β-dicalcium silicate, Portland cement, and blends of Portland cement with blast-furnace slag, metakaolin, or silica fume

The purpose of this article is to discuss the applicability of the tobermorite-jennite (T/J) and tobermorite-‘solid-solution’ calcium hydroxide (T/CH) viewpoints for the nanostructure of C-S-H present in real cement pastes. The discussion is facilitated by a consideration of the author's 1992 model, which includes formulations for both structural viewpoints; its relationship to other recent models is outlined. The structural details of the model are clearly illustrated with a number of schematic diagrams. Experimental observations on the nature of C-S-H present in a diverse range of cementitious systems are considered. In some systems, the data can only be accounted for on the T/CH structural viewpoint, whilst in others, both the T/CH and T/J viewpoints could apply. New data from transmission electron microscopy (TEM) are presented. The ‘inner product’ (Ip) C-S-H in relatively large grains of C₃S or alite appears to consist of small globular particles, which are ≈4-8 nm in size in pastes hydrated at 20 °C but smaller at elevated temperatures, ≈3-4 nm. Fibrils of ‘outer product’ (Op) C-S-H in C₃S or β-C₂S pastes appear to consist of aggregations of long thin particles that are about 3 nm in their smallest dimension and of variable length, ranging from a few nanometers to many tens of nanometers. The small size of these particles of C-S-H is likely to result in significant edge effects, which would seem to offer a reasonable explanation for the persistence of Q⁰(H) species. This would also explain why there is more Q⁰(H) at elevated temperatures, where the particles seem to be smaller, and apparently less in KOH-activated pastes, where the C-S-H has foil-like morphology. In blended cements, a reduction in the mean Ca/Si ratio of the C-S-H results in a change from fibrillar to a crumpled-foil morphology, which suggests strongly that as the Ca/Si ratio is reduced, a transition occurs from essentially one-dimensional growth of the C-S-H particles to two-dimensional; i.e., long thin particles to foils. Foil-like morphology is associated with T-based structure. The C-S-H present in small fully hydrated alite grains, which has high Ca/Si ratio, contains a less dense product with substantial porosity; its morphology is quite similar to the fine foil-like Op C-S-H that forms in water-activated neat slag pastes, which has a low Ca/Si ratio. It is thus plausible that the C-S-H in small alite grains is essentially T-based (and largely dimeric). Since entirely T-based C-S-H is likely to have different properties to C-S-H consisting largely of J-based structure, it is possible that the C-S-H in small fully reacted grains will have different properties to the C-S-H formed elsewhere in a paste; this could have important implications.     

Method to assess the quality of casein used as superplasticizer in self-levelling compounds

A fast and accurate method of assessing the quality of casein superplasticizer is presented. The method is based on analysis of the content of α-, β- and κ-casein proteins contained in whole casein by ion exchange fast protein liquid chromatography (FPLC). The chromatographic profiles of six commercial casein samples were determined, revealing that the amount of κ-casein present in the biopolymer is the main assessment criteria for the quality of casein. For high dispersing effectiveness, the content of κ-casein needs to be high. The reason is that at pH ~ 12, a high content of κ-casein results in submicelles possessing smaller size (diameter ~ 10 nm), as was proven by dynamic light scattering measurement (DLS). These smaller submicelles are supposed to adsorb on cement in higher amount than large submicelles. Using this FPLC method, the dispersing performance of any casein sample can be determined very quickly without physical testing of mortar.     

Influence of a fine glass powder on cement hydration: Comparison to fly ash and modeling the degree of hydration

This paper reports the results of an investigation carried out to understand the influence of a fine glass powder on cement hydration. The pozzolanicity of the glass powder and a Class F fly ash for comparison was evaluated using strength activity index over a period of time, and a rapid electrical conductivity based method. Flame emission spectroscopy and electrical conductivity tests were used to quantify the alkali release from glass powder, and gain information on the rate of alkali release. It was found that the glass powder releases only a very small fraction of sodium ions into the solution. It was observed that the glass powder modified pastes show higher non-evaporable water contents than the plain paste and fly ash modified pastes, indicating that glass powder facilitates enhancement in cement hydration. An expression has been developed for the change in non-evaporable water content as a result of enhancement in cement hydration and the hydration of the cement replacement material. The efficiency of any cement replacement material with age in the paste system can be quantified using this parameter. Based on this parameter, a 5% cement replacement with glass powder was found to be effective at the chosen water-to-cementing materials ratio (w/cm), whereas at higher replacement levels, the dilution effect dominates. A model to predict the combined degree of hydration of cement pastes incorporating more than one cementing material is outlined. The measured and predicted combined degrees of hydration agree well.