The influence of blast-furnace slag hydration products on microcracking of concrete

New hydration products of ground granulated blast-furnace slag are formed during the hydration process of Portland slag cement concrete. Spatial distribution of microcracks in concrete is related also to newly formed slag hydrates. The chemical composition of hydration products is variable and unstable. The Si/Ca ratios rise in hydration products near the unhydrated slag core significantly. There is also a certain increase of Mg and Al content in the central parts of hydration rims. The scope of the paper is to find a relation between slag hydration products and the process of microcracking. The amount of microcracks was reduced in concretes with a lower content of vitreous fraction where the slag basicity was high. Neoformation of hydration products is accompanied by volume changes, which can lead to concrete microcracking.     

Compressive strength and electrical properties of concrete with white Portland cement and blast-furnace slag

Electrical resistivity is an important characteristic of concrete because it allows evaluation of the accessibility of aggressive agents prior to the beginning of the corrosive process and estimation of the corrosion propagation. This study investigated the apparent electrical resistivity of concrete mixes with white Portland cement and with and without blast-furnace slag using Wenner’s four-electrode method. The compressive strength of concrete cylinders and the electrical conductivity of the pore solution were tested. Examined slag contents were 50% and 70% by mass and the results were compared to reference mixtures of 100% white Portland cement and 100% grey Portland cement, as well as to mixtures with equal percentages of slag and grey Portland cement. Larger amounts of slag resulted in increased electrical resistivity and decreases in the electrical conductivity of the pore solution, when compared to the reference concretes. The mixture made of 50% slag and 50% white Portland cement showed, on average, compressive resistance levels between 35 MPa and 60 MPa, electrical resistivity values that were approximately five times greater, costs that were 14.6% less per m³, and whiteness similar to the reference concrete. These results indicate that white Portland cement can be partially substituted by blast-furnace slag.     

Properties of concrete incorporating fly ash and ground granulated blast-furnace slag

This paper presents a laboratory study on the influence of combination of fly ash (FA) and ground granulated blast-furnace slag (GGBS) on the properties of high-strength concrete. A contrast study was carried out for the concrete (GGFAC) incorporating FA and GGBS, control Portland cement concrete and high-volume FA high-strength concrete (HFAC). Assessments of the concrete mixes were based on short- and long-term performance of concrete. These included compressive strength and resistance to H₂SO₄ attack. The microstructure of the concretes at the age of 7 days and 360 days was also studied by using scanning electron microscope. The results show that the combination of FA and GGBS can improve both short- and long-term properties of concrete, while HFAC requires a relatively longer time to get its beneficial effect.     

Concretes made of efficient multi-composite cements with slag and limestone

The present paper deals with the performance of concretes made of multi-composite cements with granulated blast furnace slag (GBFS) and limestone (LL) contents beyond the limits of current EN 197-1. The combined application of ground limestone (up to 50%) and GBFS in multi-composite cements can enable a significant reduction in the Portland cement clinker content of cements for the same concrete strength and durability. The lower the w/c-ratio the higher is the potential for the efficient use of Portland cement clinker together with limestone and slag.A comprehensive experimental study was conducted to analyze the mechanical properties and durability of concrete made of multi-composite cements at both laboratory and plant scale. The efficient use of Portland cement clinker together with GBFS and limestone is analyzed for different cement compositions and w/c-ratios. An approach for the optimization of the slag efficiency is proposed. It was concluded that the efficiency of GBFS increases significantly with decreasing w/c-ratio and slag content.Due to the limited availability of slag, the slag content was limited to 30 wt.-% for multi-composite plant cements. The performance of concretes made of such cements containing 50%, 35% and 20% clinker and 20%, 35% and 50% limestone respectively were experimentally analyzed. The results revealed that concrete made of cement with 50 wt.-% clinker, 30 wt.-% GGBFS and 20 wt.-% limestone and a w/c-ratio of 0.50 could exhibit comparable hardened properties compared to reference concrete made of slag cement CEM III/A 42.5 N (with 50 wt.-% slag) with the same w/c-ratio. Concrete made of cements with clinker content of 35 wt.-% and a reduced w/c-ratio of 0.40 could result in a performance similar to the reference concrete made of CEM III/A 42.5 N. Further reduction of clinker content to 20 wt.-% was possible only at a low w/c-ratio of 0.35. Life cycle assessment (LCA) analysis revealed that the application of such multi-composite cements can lead to a concrete with a remarkable lower global warming potential up to about 35% compared to the concrete made of German average cement with a similar performance.     

Effect of curing time on granulated blast-furnace slag cement mortars carbonation

Currently, ground granulated blast-furnace slag cements use in cement-based materials is being increasing because perform well in marine and other aggressive environments. However, mortars and concretes made of this type of cement exhibit high carbonation rates, particularly in badly cured cement-based materials and when high blast-furnace slag contents are used. Concrete reinforcement remains passive but can be corroded if the pore solution pH drops as a result of the carbonation process promoting the reinforced concrete structure failure during its service life. Results show the very sensitive response to wet-curing time of slag mortars with regard to the natural carbonation resistance. Then, a minimum period of 3–7 days of wet curing is required in order to guarantee the usual projected service life in reinforced concrete structures. In this work, estimation models of carbonation depth and carbon dioxide diffusion coefficient in ground granulated blast-furnace slag mortars as a function of the curing period and the amount of ground granulated blast-furnace slag are proposed. This information will be useful to material and civil engineers in designing cement-based materials and planning the required curing time depending on their ground granulated blast-furnace slag content.     

Carbonation – comparison of results for concretes containing PFA,cementitious slag,or alternative aggregates

Abstract

Depths of carbonation on specimens of concretes aged for up to 10 years are compared. Both good and poor curing conditions, with either indoor or outdoor exposure are considered. It is demonstrated that the carbonation depth is related systematically to the standard cube strength in dense concretes containing up to and including 40% replacement of Portland cement by pulverized fuel ash (PFA) or up to and including 60% by ground granulated blast furnace slag (GGBFS) or ground pelletized blast furnace slag (GPBFS). The main implications of these findings to methods of specifying concrete (BS 5328: 1981) are that designed mixes may give higher rates of carbonation for low-heat Portland blast furnace cement, but not for other permitted cement replacements, and that prescribed mixes will usually give higher rates if Portland cement is partially replaced by any permitted quantity of cement replacement. Depths of carbonation in concrete containing porous aggregate are more closely related to total water/cement ratio than to standard cube strength. This is probably a result of a closer relationship between total water/cement ratio and pore structure, which controls the rate of carbonation. A comparison has been made between concretes containing porous aggregate and those containing cement replacements by introducing an efficiency factor (k) for cement replacements similar but not always identical to the cementing efficiency factor.

MST/690     

Performance characteristics of concrete based on a ternary calcium sulfoaluminate–anhydrite–fly ash cement

This paper reports an assessment of the performance of concrete based on a calcium sulfoaluminate–anhydrite–fly ash cement combination. Concretes were prepared at three different w/c ratios and the properties were compared to those of Portland cement and blast-furnace cement concretes. The assessment involved determination of mechanical and durability properties. The results suggest that an advantageous synergistic effect between and ettringite and fly ash (Ioannou et al., 2014) was reflected in the concrete’s low water absorption rates, high sulfate resistance, and low chloride diffusion coefficients. However, carbonation depths, considering the dense ettringite-rich microstructure developed, were higher than those observed in Portland cement concretes at a given w/c ratio. It was concluded that the amount of alkali hydroxides present in the pore solution is as important factor as the w/c ratio when performance of this type of concrete is addressed.     

Effect of polypropylene fibers on shrinkage and cracking of concretes

The effects of admixed polypropylene (PP) fibers on the drying shrinkage of hardened concrete are presented in this paper. Concrete mixtures made with Ordinary Portland cement (OPC) and OPC/Slag blended cements containing various volume fractions of PP fiber were tested. The results show small but consistently higher drying shrinkages in concretes incorporating PP fibers than that without fiber. The effect is more pronounced in slag concretes and in concretes cured for only 1 day. An attempt to explain this phenomenon was made by water loss, nitrogen adsorption, sorptivity and scanning electron microscopy tests on the same concretes. Additional moisture loss and porosity are proposed as possible reasons. The results of early-age restrained shrinkage tests on slag concretes show that PP fiber concrete had higher cracking tendency than the concrete without fiber. This was found to be due to higher shrinkage and elastic modulus of PP fiber concrete.     

Resistance of concrete containing ggbs to the thaumasite form of sulfate attack

A long-term laboratory study has investigated how cement-type, aggregate-type and curing, affect the susceptibility of concrete to the thaumasite form of sulfate attack (TSA). The cements were Portland cement (PC), sulfate-resisting Portland cement (SRPC) and a combination of 70% ground granulated blastfurnace slag (ggbs) with 30% PC. These were combined with various carbonate aggregates or a non-carbonate control. Initial curing was either in water or in air. Concrete cubes were immersed in four strengths of sulfate solution at 5 and 20 °C. This paper reports the results after up to six years of immersion in sulfate solution.

Deterioration, consistent with TSA, was observed on many of the PC and SRPC concretes that had been made with carbonate aggregate and stored in sulfate solutions at 5 °C, with SRPC providing no better resistance to TSA than PC. Good quality concretes made with 70%ggbs/30%PC showed high resistance to TSA and the presence of carbonate in the mix substantially improved their general sulfate resistance. An initial air-cure, proved beneficial against both the conventional and thaumasite form of sulfate attack.     

The effects of supplementary cementing materials in modifying the heat of hydration of concrete

This paper is intended to provide guidance on the form and extent to which supplementary cementing materials, in combination with Portland cement, modifies the rate of heat evolution during the early stages of hydration in concrete. In this investigation, concretes were prepared with fly ash, condensed silica fume and ground granulated blastfurnace slag, blended with Portland cement in proportions ranging from 5% to 80%. These concretes were subjected to heat of hydration tests under adiabatic conditions and the results were used to assess and quantify the effects of the supplementary cementing materials in altering the heat rate profiles of concrete. The paper also proposes a simplified mathematical form of the heat rate curve for blended cement binders in concrete to allow a design stage assessment of the likely early-age time–temperature profiles in large concrete structures. Such an assessment would be essential in the case of concrete structures where the potential for thermally induced cracking is of concern.     

The co-existence of thaumasite and ettringite in concrete exposed to magnesium sulfate at room temperature and the influence of blast-furnace slag substitution on sulfate resistance

The effects of Type I Portland cement replacement by 45% or 72% blast-furnace slag on the sulfate resistance of laboratory concretes were analyzed by microstructural investigation. The concretes investigated were stored in water or in magnesium sulfate solutions for 23 years under laboratory conditions. For those stored in water only surface layers of carbonation and decalcification were observed. Concretes exposed to sulfate solutions formed brucite, ettringite and thaumasite. Thus, thaumasite was observed to form in concretes stored under laboratory conditions. In all cases both ettringite and thaumasite were found to co-exist in the damaged zones. However, the thaumasite appears to be moving in from the exterior after initial formation of ettringite, and has not resulted in the massive destruction of the hydrated matrix as has been found elsewhere at lower temperature exposures. Slag replacement was observed to be an effective means of conferring resistance to sulfate attack. Although the concretes studied were prepared at a W/cm (water-to-cementitious materials) ratio of 0.50, the depths of attack observed were comparable to those observed in concrete prepared at w/c=0.45 using ASTM Type V (SRPC) cement alone.     

Nanoscale mechanical properties of concrete containing blast furnace slag and fly ash before and after thermal damage

Portland cement blended with waste products such as blast furnace slag and fly ash are frequently used to create more sustainable concrete, but their nanoscale mechanical behavior, particularly after thermal damage, has not been well-studied. Here, nanoindentation experiments confirm that concrete produced with blended cements contains hydration products with nearly identical nanoscale mechanical properties to the hydration products found in concretes produced with ordinary Portland cement. The volume fractions of the hydration products, particularly calcium-silicate-hydrate (C-S-H) phases, are formed in different proportions with the addition of fly ash and blast furnace slag. After exposure to fire damage, the nanoscale behavior of concretes produced with fly ash and slag also matches the nanoscale behavior of conventional concretes. This suggests that any macroscopic differences between fire damage behavior of blended cement concrete and ordinary Portland cement concrete must have origins in a larger length scale.     

Leaching behaviour of major and trace elements from concrete: Effect of fly ash and GGBS

The leaching of major and trace elements from concrete made with Portland cement, fly ash and GGBS (ground granulated blast-furnace slag) was studied using pH static availability and tank leach tests. The release of substances during the tank leach test occurs by surface dissolution of phases at the concrete surface and diffusion inside the concrete, the amounts depending on the phases controlling solubility and concrete porosity. Alkali release is controlled by diffusion and is thus reduced by lower water/binder ratios and the replacement of Portland cement by fly ash. Ca, Al and S release occurs mainly by surface dissolution of portlandite and AFt/AFm, respectively. The release of V is determined by surface dissolution of V substituted ettringite and/or calcium vanadate. Although fly ash can increase the total V content of concrete, enhancing release, only 2% of the total V content in concrete was available for release.     

Sulfate resistance of limestone cement concrete exposed to combined chloride and sulfate environment at low temperature

Concrete durability was investigated, taking under consideration the limestone content of the cement used, as well as the effect of chlorides on concrete’s deterioration due to the thaumasite form of sulfate attack. A normal Portland cement and two Portland limestone cements (15% and 35% w/w limestone content) were used for concrete preparation. The specimens were immersed in two corrosive solutions (chloride-sulfate; sulfate) and stored at 5 ± 1 °C. Visual inspection of the specimens, mass measurements and compressive strength tests took place for 24 months. Concretes containing limestone, as cement constituent and/or as aggregate, suffered from the thaumasite form of sulfate attack, which was accompanied by brucite and secondary gypsum formation. Limestone cement concretes exhibited higher deterioration degree compared to the concrete made without limestone cement. The disintegration was more severe and rapid, the higher the limestone content of the cement used. Chlorides inhibit sulfate attack on concrete, thus delaying and mitigating its deterioration.     

Suitability of GGBFS as a cement replacement for concrete in hot arid climates

Three ground granulated blast-furnace slag (GGBFS) concretes (30, 50 and 70% cement replacement) together with an OPC control, all designed for equal workability and 28-day water-cured strength, are compared when subjected to a variety of curing methods and exposure in both a temperate and a hot arid climate. The effect of replacement level on cube and core strengths, ultrasonic pulse velocity, surface hardness, water absorption and permeability are reported. The test showed conclusively that the 50%, replacement level was best and that a GGBFS concrete can be superior to an equivalent all-OPC concrete in a hot climate, provided that proper curing is provided.     

Limestone aggregate concrete,usefulness and durability

Inland and on sea coasts, crushed limestone aggregate has been used in France and elsewhere for structural concrete when gravel aggregates were not available. Crushed limestone fillers have, since the oil crisis, been allowed as partial constituent of cement; they are now currently used as additions in concrete mixtures, as partial replacement of Portland cement. Limestone aggregate concretes have not been considered inferior to gravel aggregate concretes, particularly in durability. However, recently some cases of alkali-silica deleterious reactivity and one case of sulphate attack, have questioned their good performance status.     

Freezing and de-icing salt resistance of blast furnace slag concretes

This paper presents the results on investigations made into a concrete containing cement rich in granulated blast furnace slag (57%). Whereas slag cement concretes have proved successful in structures subjected to chemical attack, their use in structures subjected to freezing and de-icing salt attack is a problem of numerous investigations. The results concerning water/cement ratio and air content in concrete mixtures are presented in this paper. The effect of polypropylene microfibre addition to the concrete was also analysed. The research shows that air entraining the concrete mix up to the level of 5–6% guarantees obtaining high resistance to the action of de-icing agents, even at relatively high values of water/cement ratio. Apart the air content, the addition of microfibre to the concrete mixture was highly effective. For these samples scaling was the lowest. Phase composition investigations confirm that calcite and aragonite (as the carbonation products) were present on the surface of concrete.     

Particle size effect on the strength of rice husk ash blended gap-graded Portland cement concrete

Rice husk ash (RHA) has been used as a highly reactive pozzolanic material to improve the microstructure of the interfacial transition zone (ITZ) between the cement paste and the aggregate in high-performance concrete. Mechanical experiments of RHA blended Portland cement concretes revealed that in addition to the pozzolanic reactivity of RHA (chemical aspect), the particle grading (physical aspect) of cement and RHA mixtures also exerted significant influences on the blending efficiency. The relative strength increase (relative to the concrete made with plain cement, expressed in %) is higher for coarser cement. The gap-grading phenomenon is expected to be the underlying mechanism. This issue is also approached by computer simulation. A stereological spacing parameter (i.e., mean free spacing between mixture particles) is associated with the global strength of the blended model cement concretes. This paper presents results of a combined mechanical and computer simulation study on the effects of particle size ranges involved in RHA-blended Portland cement on compressive strength of gap-graded concrete in the high strength/high performance range. The simulation results demonstrate that the favourable results for coarser cement (i.e., the gap-graded binder) reflect improved particle packing structure accompanied by a decrease in porosity and particularly in particle spacing.     

Factors contributing to early age shrinkage cracking of slag concretes subjected to 7-days moist curing

The objective of this study was to investigate all the factors contributing to early age shrinkage cracking in concrete, namely, shrinkage, tensile creep, tensile elastic modulus, tensile strength of concretes, and to study the effect of slag as a binder on these factors. The above-mentioned factors were measured in early age concretes made with 0, 35, 50 and 65% level replacement of ordinary Portland cement by slag. All the concretes studied were moist cured for 7-days. It was found that, at lower slag replacement levels (0, 35 and 50%), the tensile strength decreased with increasing slag replacement. However, this is more than compensated by decreasing tensile elastic modulus and shrinkage. There was no significant change found in tensile creep with the changing slag levels. The study shows that the influence of the tensile elastic modulus is a major consideration for early age cracking of slag concretes.     

The influence of ground granulated blastfurnace slag (GGBS) additions and time delay on the bleeding of concrete

Bleed tests were performed in accordance with ASTM C232-92 on concretes in which up to 85% of the cement was replaced with ground granulated blastfurnace slag (GGBS) obtained from a number of different sources. The time at which the bleed test was started was varied from 30 to 120 min to simulate site conditions. The addition of up to 55% slag increased the bleed capacity by 30% (compared to the plain Portland cement (OPC) mix) but had little effect on bleed rate. Increasing the slag content to 85% had no further significant effect on bleeding. The source of slag was also found to have little effect on the bleeding but comparisons made with results from 10 years ago suggest that now the present day slags have a much less marked effect on bleeding probably because they are ground finer. Delaying the start of the bleed test from 30 to 120 min reduced the bleed capacity of the OPC mix by more than 55% compared with 32% for the slag mixes. The reduction in bleed rate was similar for all mixes at about 45%.