Alkali-activated slag-blended cements with silica supplementary materials

This paper presents results of detailed studies of composite slag binders with mineral silica additions differing in reactivity. We have identified general trends and assessed the effects of composition, particle size, and binder hardening conditions on the normal consistency, paste setting time, hardening kinetics, strength, average density, and water absorption in composite slag binder stone. Using X-ray diffraction and scanning electron microscopy, we have identified the key features of structure formation in stone with mineral silica additions and examined the influence of additives on the composition and volume of the hardening products of composite slag binders. The results have been used to construct a model of the structure and structural elements of composite slag binder stone with mineral silica additions.     

Comparison of ground waste glass with other supplementary cementitious materials

Finely ground glass has pozzolanic properties that make attractive its recycling as supplementary cementitious material. This paper compares the behaviour of waste glass powders of different fineness with that of natural pozzolana, coal fly ash and silica fume. Chemical analysis, compressive strength measurements and durability tests were carried out to investigate the effect of ground glass on strength and durability performances of mortars. Blended both with Portland cement and lime, ground glass improved strength, resistance to chloride penetration and resistance to sulphate attack of mortars more than natural pozzolana and similarly to fly ash. Mortars with ground glass immersed in water for seven years did not show any sign of degradation and increased their compressive strength. The ranking of ground glass with respect to the other mineral additions was not affected by fineness.     

Evaluating the use of supplementary cementitious materials to mitigate damage in cementitious materials exposed to calcium chloride deicing salt

This paper discusses the role of supplementary cementitious materials (SCM) in reducing damage caused by calcium oxychloride formation. Calcium oxychloride is a destructive product of a reaction between calcium hydroxide (CH) that exists in a cementitious matrix and CaCl₂ that can enter the pores of the matrix when it is used as a deicing salt. Paste samples were prepared where a percentage of ordinary portland cement was replaced with various types of SCM (including fly ash, slag, and silica fume). This paper examined the amount of calcium oxychloride that formed using low-temperature differential scanning calorimetry, and damage development detected using acoustic emission. Thermogravimetric analysis was also performed to determine the relationship between the amount of CH in cementitious materials and the amount of calcium oxychloride formation. The results show that the use of SCM is effective in reducing the calcium oxychloride formation and resulting damage when cementitious materials are exposed to various compositions of solution containing CaCl₂. The explanation of the benefit of using SCM is that it can reduce the calcium oxychloride formation due to a reduction in the amount of CH in the cementitious materials through pozzolanic reaction and dilution of cement. As a result, cementitious materials with SCM exposed to CaCl₂ may experience less damage and have a longer service life.     

Effect of supplementary cementitious materials on shrinkage and crack development in concrete

The autogenous and drying shrinkage of Portland cement concrete, and binary and ternary binder concretes, were measured and compared. The binary and ternary binder concretes were formed by replacing part of the cement with fly ash, very fine fly ash and/or silica fume. Restrained shrinkage test was also performed to evaluate the effect of binder type on early age cracking. After the cracking of the restrained ring samples, crack widths were measured and compared with the results of an R-curve based model, which takes post-peak elastic and creep strains into account.The incorporation of fly ash and very fine fly ash decreased the autogenous shrinkage strain but increased the drying shrinkage strain. Since the total shrinkage strains of both the ternary and the binary concrete mixtures were similar, the strength development became an important factor in the cracking. The lower strength of the concrete with ternary binders led to earlier cracking compared to the binary binder concrete. Portland cement concrete cracked the earliest and had the greatest crack width. Measured crack widths were in accordance with the crack widths calculated with the R-curve model.     

Portland-limestone cements. Their properties and hydration compared to those of other composite cements

The new European Standard EN 197-1 emphasizes the development of composite cements. In Greece a variety of pozzolanic and/or hydraulic materials are used as cement main constituents. Until now, limestone could be used only as a filler (up to 3% w/w), but since 2001 (application of EN 197-1) it can also be used as a main cement constituent. In this work a comparison between limestone and some of the materials that are already used in Greece is presented. An ordinary Portland cement and three Portland-composite cements containing limestone, natural pozzolana or fly ash were produced. The grinding process was designed in order to produce cements of the same 28 day compressive strength. The mechanical and physical properties of the cements were measured and hydrated products, formed after 1–28 days, were identified by means of XRD. The composite cements present significant differences as far as the clinker fineness, the development of the strength, the water demand and the hydration rate is concerned. The production of Portland-limestone cements seems to be very challenging, due to the satisfactory properties of the limestone cements as well as the low cost and the high availability of limestone in Greece.     

Sulfate resistance and carbonation of plain and blended cements

Results of an experimental investigation on the sulfate resistance and carbonation of plain and blended cement mortars are reported in this paper. In the sulfate resistance test all the specimens were immersed in a 5% Na₂SO₄ solution for 24 months. Two different types of lignite fly ashes and two natural pozzolans were used for the production of 13 blended cements. An ordinary portland cement and a commercially available blended cement were also used for reference. The effect of mineral admixtures on the carbonation depth of mortars was also investigated. Results show that the addition of pozzolanic admixtures in most cases had a positive effect on the sulfate resistance. The carbonation depth in all blended mortars was greater than that in portland cement mortar. However the rate of carbonation of blended mortars was reduced as hydration progressed.     

Examining the pozzolanicity of supplementary cementitious materials using isothermal calorimetry and thermogravimetric analysis

It has recently been proposed that the pozzolanicity of supplementary cementitious materials can be determined by monitoring the heat released when supplementary cementitious materials are mixed with calcium hydroxide at high temperature and high pH. In this study, the heat release is measured using this procedure for a variety of different supplementary cementitious materials. In addition, thermogravimetric analysis is performed on the reacted material to determine the amount of calcium hydroxide consumed. The heat release and calcium hydroxide consumption can be used in conjunction to compare supplementary cementitious materials. Calcium hydroxide consumption can be used to determine the extent of reaction of supplementary cementitious materials in pastes where supplementary cementitious materials are used to replace a portion of cement.     

Thixotropy and structural breakdown properties of self consolidating concrete containing various supplementary cementitious materials

In this study, thixotropy and structural breakdown of 57 self-consolidating concrete (SCC) mixtures containing various supplementary cementitious materials (SCM) were investigated by different approaches. The effects of SCM type and content on high range water reducer demand and plastic viscosity were also studied. For these purposes, various amounts of silica fume (SF), metakaolin (MK), Class F fly ash (FAF), Class C fly ash (FAC) and granulated blast-furnace slag (BFS) were utilized in binary, ternary, and quaternary cementitious blends in three water/binder (w/b) ratios. Results showed that except BFS, use of SCM in SCC mixtures increased thixotropy values in comparison with the mixtures containing only portland cement (PC). Good correlations were established between structural breakdown area and drop in apparent viscosity values for all w/b ratios. The different methods used to evaluate the thixotropy and structural breakdown got more consistent with each other as w/b decreased.     

Micro-structural development of gap-graded blended cement pastes containing a high amount of supplementary cementitious materials

To clarify the strength improvement mechanism of gap-graded blended cements with a high amount of supplementary cementitious materials, phase composition of hardened gap-graded blended cement pastes was quantified, and compared with those of Portland cement paste and reference blended cement (prepared by co-grinding) paste. The results show that the gap-graded blended cement pastes containing only 25% cement clinker by mass have comparable amount of gel products and porosity with Portland cement paste at all tested ages. For gap-graded blended cement pastes, about 40% of the total gel products can be attributed to the hydration of fine blast furnace slag, and the main un-hydrated component is coarse fly ash, corresponding to un-hydrated cement clinker in Portland cement paste. Further, pore size refinement is much more pronounced in gap-graded blended cement pastes, attributing to high initial packing density of cement paste (grain size refinement) and significant hydration of BFS.     

Applicability of rheological models to high-performance grouts containing supplementary cementitious materials and viscosity enhancing admixture

The knowledge of yield stress and plastic viscosity of cement-based materials is of special interest in various applications, including consolidation grouting, post-tensioning systems, and for numerical simulations. These rheological parameters are generally estimated from the shear stress-shear rate data using an empirical model. Highly pseudoplastic systems may not be adequately modeled using flow models typically used for conventional grouts. This paper summarizes the results of a study undertaken to evaluate the applicability of a number of analytical models to fit experimental data obtained on cement grout. In total, 44 grouts containing silica fume replacements of 1.5%, 3%, and 5% by mass of cementitious materials, blast furnace slag substitutions of 20% and 40%, and various combinations of high-range water-reducer and viscosity-enhancing admixture were evaluated. All mixtures were prepared with 0.40 water-cementitious material ratio.Test results highlight the difficulties encountered when using conventional models to fit flow data of highly pseudoplastic mixtures of low yield stress. This is true for mixtures incorporating a viscosity-enhancing admixture, especially when combined with low dosage of high-range water-reducer. In general, the Herschel-Bulkley, Robertson, De Kee, and Casson models were found to be adequate for use with highly pseudoplastic grouts. A new model is proposed to provide better fitting of rheological profiles of highly flowable, yet stable, pseudoplastic mixtures that exhibit particularly low yield stress values.     

Evaluation of bagasse ash as supplementary cementitious material

The utilization of waste materials in concrete manufacture provides a satisfactory solution to some of the environmental concerns and problems associated with waste management. Agro wastes such as rice husk ash, wheat straw ash, hazel nutshell and sugarcane bagasse ash are used as pozzolanic materials for the development of blended cements. Few studies have been reported on the use of bagasse ash (BA) as partial cement replacement material in respect of cement mortars. In this study, the effects of BA content as partial replacement of cement on physical and mechanical properties of hardened concrete are reported. The properties of concrete investigated include compressive strength, splitting tensile strength, water absorption, permeability characteristics, chloride diffusion and resistance to chloride ion penetration. The test results indicate that BA is an effective mineral admixture, with 20% as optimal replacement ratio of cement.     

Use of natural zeolite as a supplementary cementitious material

Natural zeolite, a type of frame-structured hydrated aluminosilicate mineral, is used abundantly as a type of natural pozzolanic material in some regions of the world. In this work, the effectiveness of a locally quarried zeolite in enhancing mechanical and durability properties of concrete is evaluated and is also compared with other pozzolanic admixtures. The experimental tests included three parts: In the first part, the pozzolanic reactivity of natural zeolite and silica fume were examined by a thermogravimetric method. In this case, the results indicated that natural zeolite was not as reactive as silica fume but it showed a good pozzolanic reactivity. In the second part, zeolite and silica fume were substituted for cement in different proportions in concrete mixtures, and several physical and durability tests of concrete were performed. These experimental tests included slump, compressive strength, water absorption, oxygen permeability, chloride diffusion, and electrical resistivity of concrete. Based on these results, the performance of concretes containing different contents of zeolite improved and even were comparable to or better than that of concretes prepared with silica fume replacements in some cases. Finally, a comparative study on effect of zeolite and fly ash on limiting ASR expansion of mortar was performed according to ASTM C 1260 and ASTM C 1567. Expansion tests on mortar prisms showed that zeolite is as effective as fly ash to prevent deleterious expansion due to ASR.     

Sulfate resistance of plain and blended cements exposed to varying concentrations of sodium sulfate

Concrete deterioration due to sulfate attack is the second major durability problem, after reinforcement corrosion. This type of deterioration is noted in the structures exposed to sulfate-bearing soils and groundwater. Though concrete deterioration due to sulfate attack is reported from many countries, the mechanisms of sulfate attack have not been thoroughly investigated, particularly the effect of sulfate concentration and the cation type associated with the sulfate ions on concrete deterioration. This study was conducted to evaluate the performance of plain and blended cements exposed to varying concentrations of sodium sulfate for up to 24 months. Four types of cements, namely Type I, Type V, Type I plus silica fume and Type I plus fly ash, were exposed to five sodium sulfate solutions with sulfate concentrations of 1%, 1.5%, 2%, 2.5% and 4%. These concentrations are representative of the sulfate concentration in highly saline soils. The sulfate resistance was evaluated by visual examination and measuring the and reduction in compressive strength. The maximum deterioration, due to sulfate attack, was noted in Type I cement followed by silica fume and Type V cements. The performance of Type V, Type I plus silica fume and Type I plus fly ash was not significantly different from each other. The enhanced sulfate resistance noted in the Type I cement blended with either silica fume or fly ash indicates the usefulness of these cements in both sulfate and sulfate plus chloride environments.