Slag Cement Pastes Research Articles

Microstructure, Mass Transport and Densification of Slag Cement Pastes

AbstractThe kinetics and mechanism of the hydration reactions in slag cements were explored in blends containing either 40 or 50 wt% ground granulated blast furnace slag, made to a water:solid ratio of either 0.50 or 0.60 and aged 7d to 885d at 20°C. The microstructure of the slag rapidly differentiates into zonal structures. Analytical electron microscopy has been used to analyze the composition and indirectly determine the mineralogy of the zones. It is shown that considerable chemical matter is transported out of the slag grains, resulting in porosification of the former slag grains and densification of the cement hydrate matrix. This is supported by mass balance calculations. The driving force for these migrations is accounted for by the existence of a chemical potential gradient of water; hydrophyllic species such as Ca, Si, etc. migrate into regions of high chemical potential for water. The densification of the phase matrix is probably responsible for the low permeabilities of matured slag-cement blends.

Densification And Migration Of Ions In Blast Furnace Slag-Portland Cement Pastes

AbstractWell-cured slag cement blends are known to give rise to paste matrices of low permeability. Electron microscopy has been used to follow microstructural development in slag cement pastes. An initially dense layer of hydrated product develops around the slag grains. Its chemistry and microstructure are not constant but evolve with time. These variations have been explored by analytical electron microscopy, X-ray diffraction, etc. Mg is concentrated in this zone and serves as a useful chemical marker. However an outer matrix of mixed hydration products also develops, to which slag and cement both contribute. The additional contribution of Ca, Al and Si derived from slag to this matrix is considered to explain its gradual densification and resultant low permeability.

Densification and Migration of Ions in Blast Furnace Slag-Portland Cement Pastes

ABSTRACTWell-cured slag cement blends are known to give rise to paste matrices of low permeability. Electron microscopy has been used to follow microstructural development in slag cement pastes. An initially dense layer of hydrated product develops around the slag grains. Its chemistry and microstructure are not constant but evolve with time. These variations have been explored by analytical electron microscopy, X-ray diffraction, etc. Mg is concentrated in this zone and serves as a useful chemical marker. However an outer matrix of mixed hydration products also develops, to which slag and cement both contribute. The additional contribution of Ca, Al and Si derived from slag to this matrix is considered to explain its gradual densification and resultant low permeability.

The Effect of Bond Characteristics between Steel Slag Fine Aggregate and Cement Paste on Mechanical Properties of Concrete and Mortar

ABSTRACTThe ordinary fine aggregate in concrete has been replaced by ground and sieved steel slag fine aggregate, treated and exposed to air for three months. Compared with concrete made from natural sand, properties such as compressive strength, flexural strength, elastic modulus, permeability and abrasion resistance are considerably improved. The improvement increases with a decrease in w/c ratio, an increase in curing time and an increase in the replacement weight of sand. These results are due to the fact that the steel slag contains some active minerals such as C3S, C2S, C4AF, etc., and shows favorable surface physical characteristics that improve the bond between steel slag particles and cement paste. The results of XRD, SEM and EPM microhardness showed that there are heavier concentration of ions, with finer crystals and a lower degree of CH orientation at the interfacial zone between steel slag particles and cement paste. The study also found small cementitious and fibrous C-S-H crystals growing from the fine aggregate, which are linked with hydrated products from cement paste making the bond and structural characteristic more favorable with cement. The steel slag fine aggregate is an active mineral similar to cement. The bond between the aggregate and cement paste is strengthened both physically and chemically.

The effect of Bond Characteristics Between Steel Slag fine Aggregate and Cement Paste on Mechanical Properties of Concrete and Mortar

ABSTRACTThe ordinary fine aggregate in concrete has been replaced by ground and sieved steel slag fine aggregate, treated and exposed to air for three months. Compared with concrete made from natural sand, properties such as compressive strength, flexural strength, elastic modulus, permeability and abrasion resistance are considerably improved. The improvement increases with a decrease in w/c ratio, an increase in curing time and an increase in the replacement weight of sand. These results are due to the fact that the steel slag contains some active minerals such as C3S, C2S, C4AF, etc., and shows favorable surface physical characteristics that improve the bond between steel slag particles and cement paste. The results of XRD, SEM and EPM microhardness showed that there are heavier concentration of ions, with finer crystals and a lower degree of CH orientation at the interfacial zone between steel slag particles and cement paste. The study also found small cementitious and fibrous C-S-H crystals growing from the fine aggregate, which are linked with hydrated products from cement paste making the bond and structural characteristic more favorable with cement. The steel slag fine aggregate is an active mineral similar to cement. The bond between the aggregate and cement paste is strengthened both physically and chemically.

Microstructure and Microchemistry of Slag Cement Pastes

ABSTRACTPastes of a portland cement (60%) blended with a granulated blastfurnace slag (40%) were examined, principally by SEM with EDX analysis. Reaction rims around slag particles and relicts of fully reacted slag particles had compositions compatible with mixtures, in varying proportions, of C-S-H having Si/Ca -0.62 and a phase of the hydrotalcite family having Al/Mg -0.38. Calculations taking into account relevant densities and water contents. indicated that replacement of the slag by its in situ hydration products entails little or no change in the numbers of Mg and O atoms per unit volume, but that substantial proportions of the Ca, Si and Al are released and an equivalent amount of H gained. In other respects, the microstructures qualitatively resembled those of pure portland cement pastes of similar ages, but less CH was formed and the C-S-H not formed in situ from the slag had a Si/Ca ratio of 0.56, higher than that of 0.50 to 0.53 found in the absence of slag. None of the individual phases in the slag cement pastes showed significant compositional variation with time in the 28 day to 14 month period studied. The relative amounts of Ca, Si and Al expelled from the slag are such that, in order to form C-S-H and AFm phase, more Ca is required. It is obtained partly at the expense of CH formation, and partly through increase in the Si/Ca ratio of the C-S-H formed from the clinker phases. Mass balance, volume composition and bound water content were calculated for the 14 month old paste and compared with corresponding results for the pure portland cement.

Microstructure and Microchemistry of Slag Cement Pastes

ABSTRACTPastes of a portland cement (60%) blended with a granulated blastfurnace slag (40%) were examined, principally by SEM with EDX analysis. Reaction rims around slag particles and relicts of fully reacted slag particles had compositions compatible with mixtures, in varying proportions, of C-S-H having Si/Ca -0.62 and a phase of the hydrotalcite family having Al/Mg -0.38. Calculations taking into account relevant densities and water contents indicated that replacement of the slag by its in situ hydration products entails little or no change in the numbers of Mg and 0 atoms per unit volume, but that substantial proportions of the Ca, Si and Al are released and an equivalent amount of H gained. In other respects, the microstructures qualitatively resembled those of pure portland cement pastes of similar ages, but less CH was formed and the C-S-H not formed in situ from the slag had a Si/Ca ratio of 0.56, higher than that of 0.50 to 0.53 found in the absence of slag. None of the individual phases in the slag cement pastes showed significant compositional variation with time in the 28 day to 14 month period studied. The relative amounts of Ca, Si and Al expelled from the slag are such that, in order to form C-S-H and AFm phase, more Ca is required. It is obtained partly at the expense of CH formation, and partly through increase in the Si/Ca ratio of the C-S-H formed from the clinker phases. Mass balance, volume composition and bound water content were calculated for the 14 month old paste and compared with corresponding results for the pure portland cement.

Effects of slag content, fineness and additive to cement pastes on the corrosion behaviour of embedded reinforcing steel

AbstractPrevious studies proved that the slag content of the blast furnace slag‐Portland cement mix could be increased, while retaining the engineering properties of the produced slag cement pastes within the normal range and increasing the fineness of the mix. The corrosion behaviour of reinforcing steel embedded in the cement pastes giving the optimum mechanical properties was studied using the galvanostatic polarisation technique. The most corrosion‐resistant mix has been determined. The effect of adding CaCl2 to a paste of this mix on the corrosion behaviour of embedded reinforcing steel has been investigated. For the purpose of comparison, anodic polarisation tests were carried out using pastes having the composition of the most corrosion‐resistant mix but at the Blaine area of the ordinary slag cement. Portland and normal slag cements were also tested. The threshold concentration of CaCl2, below which breakdown of steel passivity did not occur, has been determined. The results of this study might have practical implications in the field.

A reply to B. Mather's discussion on “air entrainment in portland blast furnace slag cement pastes: Effects on strength and pore structure”

A reply to B. Mather's discussion on “air entrainment in portland blast furnace slag cement pastes: Effects on strength and pore structure”

A discussion of the paper “air entrainment in portland blast furnace slag cement pastes: Effects on strength and pore structure” by R. Sh. Mikhail, A. M. Youssef and M. Shater

A discussion of the paper “air entrainment in portland blast furnace slag cement pastes: Effects on strength and pore structure” by R. Sh. Mikhail, A. M. Youssef and M. Shater

Air entrainment in portland blast furnace slag cement pastes: Effects on strength and pore structure

The compressive strength of air entrained blast furnace slag cement pastes were compared with the neat cement pastes of the same water/cement ratios. Air entrainment leads in all cases investigated to a decrease in the degree of hydration and this effect besides the increase in the porosity could lead to a decrease in compressive strength. A third factor, namely the changes taking place in the pore size distribution of micro- and mesopores, and their bearing on the strength results at different water/cement ratios is discussed.

Hardened slag–cement pastes of various porosities. II. Water and nitrogen adsorption

AbstractSpecific surface areas and pore structure studies were carried out on hardened slag‐cement pastes. Water and nitrogen adsorption isotherms were measured, and the derived parameters were critically compared. Nitrogen areas and total pore volumes were found to be much smaller than water values. For water vapour adsorption, the specific surface areas derived from the adsorption branches are found to be lower than those derived from the resorption isotherms. Some evidence is presented in this paper that the surface area values derived along the adsorption branches are underestimated values, and that the resorption values might be considered to be nearer to the actual areas of the various pastes. Calculations of the hydraulic radii refute the hypothesis that the areas inaccessible to nitrogen are interlayer areas.

Hardened slag–cement pastes of various porosities. III. Surface area in relation to pore structure

AbstractThe total surface areas, total pore volumes, and the distribution of pore surface and pore volume in pores of different sizes were determined on the hardened slag‐cement pastes. Adsorption isotherms of four different adsorbates, namely water, methanol, nitrogen and cyclohexane, were used for surface area and pore structure determinations. The results obtained indicated that water can penetrate into the total pore system, namely both the micropores and the wide pores; whereas methanol can measure wide pores and some of the micropores; moreover, nitrogen and cyclohexane molecules tend to measure only wider groups of pores. The indication gained from this result is that the molecular sieve property plays the most significant role in surface area and pore structure determinations of the hardened cement pastes. The polarity of the molecules seems to be important at low relative vapour pressures, and at high pressures it causes expansion of the pastes.

Hardened portland blast-furnace slag cement pastes III. Corrosion of steel reinforcement versus pore structure of the paste matrix

The pore structure of the slag cement paste matrix seems to affect to a sensible extent the corrosion behavior of embedded steel. For both additive free, and additive containing slag cement pastes, the pore structure data were discussed in paper I of this series, whereas the corrosion behavior of embedded steel was discussed in paper II. In this paper III, the correlation between papers I and II is established, and the concluding remarks presented.

Hardened Portland blast-furnace slag cement pastes II. The corrosion behavior of steel reinforcement

The corrosion behavior of reinforcing steel embedded in various slag cement pastes was studied using the galvanostatic polarization technique. The corrosion resistance is appreciably affected by the degree of fineness of the dry slag cement. In pastes produced from high Blaine area cement, the behavior of embedded steel was very close to that in normal or type I portland cement paste, and is much better than a low Blaine area cement. W/C ratios of 0.25 and 0.40 provided a better passivating medium as compared with W/C ratios of 0.18 and 0.70. Effects of lime or gypsum addition were also investigated and comparatively studied for their action on the corrosion of embedded steel. The results obtained were supported by corrosion rates obtained using the linear polarization technique.

Hardened portland blast-furnace slag cement pastes: I. Effects of additives on surface area and on pore structure

Portland blast-furnace slag cement pastes were prepared with various water/cement ratios. Specific surface areas and pore structures of the hardened pastes were investigated by nitrogen adsorption. The “accessibility” of the nitrogen molecules to the pore structure is discussed in terms of degree of hydration and total porosities of the pastes. Effect of presence of CaCl 2, a typical steel reinforcement corrosive agent, was also studied, and results indicated that it alters the area and pore structure extensively, to a more “open structure,” thus facilitating its own accessibility. Lime and gypsum addition was also studied in presence and in absence of CaCl 2, and the effect of the Blaine surface area of the unhydrated cement is particularly emphasized in this investigation.

Studies on a Method to Determine the Amount of Granulated Blastfurnace Slag and the Rate of Hydration of Slag in Cements

A method of quantitative determination of granulated blastfurnace slag not only in unhydrated slag cement but also in hydrated paste as an unreacted part was established by using a salicylic acid, aceton-methanol solution. In this solution slag hardly dissolves, but most of the hydrated slag, clinker minerals and their hydrates dissolve. Utilizing this principle, with some corrections if necessary, it is possible to determine the amount of unreacted slag.The experiment to determine the amount of unhydrated slag on the hydration of various types of slag cement pastes were carried out.The amount of unreacted slag contained in these cement pastes was determined by using the salicylic acid, aceton-methanol solution. The accuracy of this measurement was found to be satisfactory. Unreacted C3S was quantified and the hydration products were detected by X-ray diffraction. Non-evaporable water, free Ca(OH)2, and free CaSO4 were also determined by chemical analysis, and the hydrated phases were observed under a microscope.The chemical compositions of the products and the rate of reaction of each component such as slag, C3S and CaSO4, and the depths of the reacted layer on the particles of slag and C3S were calculated from the experimental data.In every type of slag cement, the rate of reaction of slag contained was unexpectedly slow. Further, after hydration for about 3-7 days, the rate of reaction of slag and C3S showed a remarkable decrease.