Autogenous shrinkage of concrete containing granulated blast-furnace slag

This paper presents the experimental results and prediction model for the autogenous shrinkage of concrete made with various water-to-cementitious materials ratios (w / cm) ranging from 0.27 to 0.42 and granulated blast-furnace slag (BFS) in the range of 0% to 50% by mass of the total cementitious materials. Test results showed that BFS concrete exhibited greater autogenous shrinkage than ordinary concrete with no BFS with the same w / cm, and that the higher the BFS content, the greater the autogenous shrinkage. At the same content of BFS, the increasing rate of autogenous shrinkage is affected by the w / cm; the lower the w / cm, the smaller the increasing rate of autogenous shrinkage. Based on the test results, a prediction model for autogenous shrinkage was proposed. In particular, an effective autogenous shrinkage that is a realistic shrinkage strain responsible for stress development was introduced in the model. It was determined by taking into account the characteristics of ultrasonic pulse velocity evolution in concrete. This prediction method for autogenous shrinkage may be effectively used to estimate the stress induced by autogenous shrinkage.     

Characterization of the Tunisian blast-furnace slag and its application in the formulation of a cement

The Tunisian blast-furnace slag has been characterized by several physicochemical methods to evaluate its hydraulic reactivity. It has been noted that nearly all the slag is glassy, so its use as a replacement of cement is possible.This result has been confirmed by different physical tests applied to blended cements as specific surface, normal consistency, setting time, stability to expansion and the minislump.Finally, a slag cement composition has been formulated and optimized using a mixture design. The optimized formula giving the maximum of compressive strength at 7 and 28 days was 61% clinker, 35% slag, 3% gypsum, and 1% limestone.     

A study on the alkali-aggregate reaction in high-strength concrete with particular respect to the ground granulated blast-furnace slag effect

The alkali-aggregate reaction (AAR) in high-strength concrete and the effect of ground granulated blast-furnace slag (GGBFS) were studied in this paper. From the results of this study, following conclusions can be drawn:

(1)

In high-strength concrete, because of high alkali content, the possibility of alkali-aggregate reaction is much higher than conventional concrete.

(2)

The occurrence of large expansion can be prevented by using nonreactive aggregate, which has been judged according to the mortar bar and chemical method's as specified in JIS A 5308, in high-strength concrete.

(3)

The replacement of cement by 30% of blast-furnace slag and using low-alkali cement can prevent the alkali-aggregate reaction from causing large expansion in high-strength concrete.

     

Investigation of hydraulic activity of ground granulated blast furnace slag in concrete

Ground granulated blast furnace slag (GGBFS), a by-product of the steel manufacturing industry, being used as an effective partial cement replacement material, has already been proven to improve several performance characteristics of concrete. The reactivity of GGBFS has been found to depend on the properties of slag, which vary with the source of slag, type of raw material used, method and the rate of cooling. The present work aims at bringing out a novel relationship between the Hydraulic Index (HI) of slag at 7 and 28 days (HI7 and HI28) and the influencing properties of slag, namely, glass content, fineness and chemical composition by employing multiple regression analysis on 37 slag samples from various sources. HI7 and HI28, thus obtained, have been mapped onto a Slag Activity Index (SAI) plot, giving an indication of the ranges of strength of slag.     

Strength optimization of “tailor-made cement” with limestone filler and blast furnace slag

The use of cements made with portland clinker and two or three additions has grown because they present several advantages over binary cements. Production of composite cements has produced a necessary shift in the manufacture process used in the cement industry. Now, it is known that the separate grinding and mixing technology is more convenient in order to produce these cements, called market-oriented or tailor-made cements. However, their optimum formulations require the help of methods of experimental design to obtain an appropriate performance for a given property with the least experimental effort.In this study, the interaction between limestone filler (LF) and blast-furnace slag (BFS) is analyzed in mortars in which portland cement (PC) was replaced by up to 22% LF and BFS. For this proposition, a two-level factorial design was used permitting to draw the isoresponse curves. Results show that compressive and flexural strength evaluated at 2, 7, 14, 28, 90 and 360 days are affected in different ways by the presence of mineral additions.     

Effect of curing temperature and type of cement on early-age shrinkage of high-performance concrete

This paper presents the results of an experimental study on the influence of curing temperature and type of cement [Portland cement and blast-furnace slag (BFS) cement] on the autogenous deformations and self-induced stresses in early-age concrete. It was found that higher temperatures do not lead to higher deformations in the observed period, but generally cause a faster shrinkage and a faster development of self-induced stresses. Another experimental finding is that, at the temperatures tested, concrete made with BFS cement shows higher shrinkage in the first days than concrete made with Portland cement.     

Composition and microstructure of 20-year-old ordinary Portland cement-ground granulated blast-furnace slag blends containing 0 to 100% slag

The morphology of outer-product (Op) C-S-H in 20-year-old slag-cement pastes appeared in most blends to be finer than at younger ages. The Ca/Si and Ca/(Si + Al) ratios of the Op C-S-H decreased with increasing slag content, and the Al/Si ratio increased. The Ca/Si ratio of C-S-H in the slag-containing pastes was lower at 20 years than at 14 months and the amount of Ca(OH)₂ was reduced indicating that additional slag must have reacted. The mean aluminosilicate chain length of the C-S-H was very long in all the samples and would be expected to have increased with age. The TEM-EDX and NMR data are consistent with nanostructural models for C-S-H. The Mg/Al ratio of the Mg-Al layered double hydroxide phase (LDH) was lower at 20 years than at 14 months in all cases except for the neat slag paste; aluminium hydroxide-based structure might be interstratified with those of the Mg-Al LDH.     

Effect of shrinkage-reducing admixtures on the properties of alkali-activated slag mortars and pastes

The effect of a shrinkage-reducing admixture (SRA) based on polypropylenglycol on the dimensional stability of waterglass-activated slag mortars was studied. The analysis also showed the effect of the admixture on pore structure of the mortars as well as on the mineralogical composition and microstructure of the alkali-activated slag pastes.The SRA reduced the shrinkage by up to 85 and 50% when the alkali-activated slag mortar specimens were cured at relative humidities of 99 and 50%, respectively. The mechanism primarily involved in shrinkage reduction is the decrease in the surface tension of pore water prompted by the admixture. The SRA also modified the pore structure - under both curing conditions - increasing the percentage of pores with diameters ranging from 1.0 to 0.1 μm. Capillary stress is much lower in these pores than in the smaller capillaries prevailing in mortars prepared without admixtures.Microstructurally, the SRA occasioned a slight increase in the proportion of Si units Q² in the CSH gel and a decrease in the percentage of Al replacing the Si in the gel structure. The admixture did not, however, modify the mineralogical composition of the pastes.Finally, the SRA admixture retarded the alkaline activation of the slag, more intensely at higher admixture dosages. While the admixture did not significantly alter the degree of reaction in pastes cured for 7 days at RH = 99%, the value of this parameter dropped by 7% in the presence of the admixture in pastes cured at 50% relative humidity.     

Hydration mechanisms of super sulphated slag cement

The hydration and the strength evolution of supersulphated cements (SSC) produced by the activation of two different ground granulated blast furnace slags with anhydrite and small amounts of an alkaline activator have been investigated. The main differences between the two mixtures are found to be the strength development, the dissolution rate of the slags, the amount and volume of the individual hydration products formed and the growth mechanisms of the ettringite. The chemical composition of the slag had a large influence on the amount of the hydrates formed and thus on the volume of the hydrated slag.Advancement of the amount of hydrates of a slag with low reactivity by adding aluminium sulphate and calcium hydroxide did increase the amount of ettringite. Nevertheless, the early compressive strength was not increased, but late strength shows a slow increase. It was concluded that the early compressive strength of an SSC using low reactive slag cannot be overcome by adding stoichiometric amounts of constituents which are used for the formation of a specific hydration product. The best way to increase early compressive strength is to increase the intrinsic dissolution rate.     

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.     

Prediction of compressive strength of fly ash concrete by new apparent activation energy function

A new prediction model using apparent activation energy is proposed to estimate the variation of compressive strength of fly ash concrete with aging. After analyzing the experimental result with the model, fly ash replacement content and water-binder ratio influence on apparent activation energy was investigated.According to the analysis, the model provides a good estimation of compressive strength development of fly ash concrete with aging. As the fly ash replacement content increases, limiting relative compressive strength and initial apparent activation energy increase. Concrete with water-binder ratio smaller than 0.40 gives nearly constant limiting relative compressive strength and initial apparent activation energy when analyzed with various water-binder ratios. However, concrete with water-binder ratio larger than 0.40 increases limiting relative compressive strength and initial apparent activation energy.     

Study on the flexural fatigue performance and fractal mechanism of concrete with high proportions of ground granulated blast-furnace slag

The flexural fatigue performance of concretes with 50% and 80% proportions of ground granulated blast-furnace slag (ab. ggbs) by mass of total cementitious materials in concrete has been investigated. The effect of different proportions of ggbs on concrete fatigue performance was investigated by experiments and was estimated by the fractal theory from five aspects, i.e. the 1D fractal dimensions of critical surface cracks, the prediction area of fractured profiles, the ratios between the area of debonded coarse aggregates and the Euclidean area of fractured profile, fracture energy modified by fractal theory, and the brittleness index. In order to estimate these fractal parameters on-line, the grey model GM (1, 1) was employed. Those experimental and numerical results show that the brittleness of concrete is impaired by the incorporation of ggbs, which contribute to higher fracture energy and more complicated characteristics on fractured profiles of concrete. Therefore, potential hydration and pozzolanic effect of ggbs in matrix prolong the fatigue life of concrete comparing with those of concrete without them. The fractal theory and grey model are novel approaches to present quantitatively the flexural fatigue performance of concrete.     

The development of high-strength mortars with improved thermal and acid resistance

Granulated blast furnace slag (GBFS) cement, containing up to 60% slag, is sometimes used in repair materials applied at intermediate temperatures of 150-300 °C. Low rate of strength development, especially at early ages, is considered a common disadvantage of repair mortars based on slag cement. The present research was oriented to improving a GBFS-portland cement binder for application as a repair material in the chemical industry when high thermal or acid resistance is required. It was found that the enhancement of GBFS-portland cement-based materials can be achieved with the help of silica fume (SF) and a superplasticizer (SP). The effect of different SPs on the compressive and flexural strength of SF-blast furnace slag-portland cement mortars was investigated. These mortars, in addition to high strength, demonstrate high thermal and acid resistance.     

Determination of the apparent activation energy of one concrete by calorimetric and mechanical means: Influence of a superplasticizer

An instrument has been developed to perform calorimetric tests on concrete in isothermal conditions using 0.11×0.22-m cylindrical samples. The purpose of this article is to determine the apparent activation energy of one concrete as a function of temperature (10, 20 and 40 °C) using this technique and to compare it with that obtained by a mechanical means in order to validate the hypothesis made in maturity measurements, according to which the apparent activation energy values are assumed to be equivalent. The two means give relatively similar results (with differences of the order of 3 kJ/mol). It also seems possible to use a single apparent activation energy value over the entire temperature range (10-40 °C). Lastly, the effect of a superplasticizer on the apparent activation energy is examined, and it appears that its role is relatively small in the case of the concretes studied here.     

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.     

Analysis of mechanism on water-reducing effect of fine ground slag, high-calcium fly ash, and low-calcium fly ash

The addition of ultrafine powder (UFP) to concrete can improve the fluidity of concrete, showing a water-reducing effect. The aim of this article was to analyze the water-reducing mechanism of UFP both experimentally and theoretically. Three UFPs—fine ground slag, high-calcium fly ash, and low-calcium fly ash—were chosen for the study. The contrastive experiments were done to investigate the fluidity of mortars with 30%, 45%, 60%, and 75% equivalent cement replaced by fine ground slag, high-calcium fly ash, and low-calcium fly ash, respectively. The results showed the physical and chemical characteristic of the powders, such as their grain morphology, glass phase activities, densities, specific areas, and their grain cumulating conditions, can strongly affect their water-reducing effect.     

Immobilization of chromium (VI) evaluated by binding isotherms for ground granulated blast furnace slag and ordinary Portland cement

The interaction mechanisms of the Cr(VI) ion in presence of GGBFS and OPC were evaluated by chromium binding isotherms and by pore solution analysis. The chromium in the final leaching solution was measured and the solid samples were investigated by SEM and by XRD. GGBFS was more efficient than OPC in fixing Cr ions at lower initial concentrations. However, from an initial Cr(VI) concentration of 2000 and 5000 mg/L, OPC was more efficient.

For an initial Cr(VI) concentration of 50 000 mg/L, around 145 mg Cr/g was fixed by OPC and only 8 and 55 mg Cr/g were fixed by GGBFS in alkaline and water-based immersion solutions, respectively. The Cr-bearing phases identified by XRD and SEM are: CaCrO₄ and CaCrO₄·2H₂O, C–S–H and calcium aluminate phases. The pore solution chemistry indicates that a value around 92% of chromium was retained by GGBFS and 87% by OPC.     

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.     

Synthesis and characterization of geopolymers containing blends of unprocessed steel slag and metakaolin: The role of slag particle size

The present research deals with the production and characterization of geopolymers prepared by mixing metakaolin with a steel slag from the production of chromium-manganese steel, a commercial sodium silicate solution and a sodium hydroxide solution. Different specimens were prepared by mixing metakaolin with different proportions of steel slag (20, 40, 60, 80 wt%) characterized by different maximum particle size. Specimens containing just metakaolin and steel slag alone were also prepared for comparison. All specimens have been characterized regarding their compressive strength, specific surface area, water absorption and microstructure. It has been observed that the use of fine steel slag powders leads to increases the performances and that the specimens containing 40 wt% steel slag and 60 wt% metakaolin revealed the best overall behaviour.     

The apparent activation energy and dynamic fragility of ancient ambers

According to the literature, different types of materials have different glass transition temperature (T_g) dependences of apparent activation energy (E_g) and dynamic fragility (m). In previous work we found that for different ambers, there were different glass transition temperatures. These same samples provide an opportunity to study the T_g dependence of E_g and m for amber, which has not been reported previously in the literature. In this work, nine pieces of amber from different locations and having different ages were investigated by differential scanning calorimetry. Six cooling rates were used to provide the different thermal histories, and the corresponding limiting fictive temperatures were determined using Moynihan's area matching method. From the cooling rate dependence of the limiting fictive temperature both the apparent activation energy and dynamic fragility were calculated. We find that as glass transition temperature increases, both the apparent activation energy and dynamic fragility increase. The T_g dependence of m for amber shows a similar trend with temperature as do the metallic glass formers and compares favorably with the m–T_g dependence of other aromatic polymers.