| Steel slag is an industrial solid waste generated in the steelmaking process, the amount of which produced annually is huge, but its utilization is very low. Currently, steel slag is basically in abandoned state, causing some problems such as environmental pollution, land use and waste of resources. As the chemical and mineral composition of steel slag is similar to Portland cement clinker, it will has a great potential for using in the cement and concrete industry. However, the poor grindability, low hydration activity and undesirable stability of steel slag are three major shortcomings or deficiencies, greatly limiting the application of steel slag in the cement and concrete industry. In addition, the systematic theoretical research on steel slag is less, which is also an important reason caused by low utilization of steel slag. Currently, most of steel slag is the converter steel slag in our country, and with the upgrading of steel slag treatment technology(especially, hot stew treatment technology has been widely used) in recent years, the factors(such as f-Ca O and f-Mg O) caused by undesirable stability of steel slag are greatly controled and improved, which provide favorable conditions for subsequently processing and use of steel slag. In this paper, with the converter hot stew steel slag as the research object, the grinding property of steel slag, hydration and hardening characteristics of powder and comoposition, structure and properties of the compsite cementitious materials are deeply systematically studied.The mineral characteristics of steel slag are studied from the chemical composition, iron phase, mineral composition, microstructure morphology and hardness and so on. The results show that the mineral phases of the hot stew steel slag is composed of silicate mineral phase, RO phase, ferrite salt phase, merwinite and andradite phase and a small amount of metallic iron phase. The microstructures morphology of steel slag mineral phases are varied, specifically:(a) black hexagonal-shaped, leaf-shaped and irregular shaped phases are the silicate minerals phases composed by oxygen, silicon and calcium elements;(b) black circular shaped phases is the mixed phase of the silicate phase, merwinite and andradite composed by oxygen, magnesium, silicon, calcium and iron elements;(c) the irregular shaped gray minerals filled in other minerals are the calcium ferrite phase composed by oxygen, calcium and iron elements;(d) the irregular light gray or nearly white mineral phases in continuous distribution are the RO phases composed by oxygen, magnesium, calcium, manganese and iron elements;(e) the white or light colored round granular minerals among other minerals are metallic ion phases.The grinding property of steel slag is studied from the change reules of fineness, specific surface area, angle of repose and particle size distribution under different grinding time. The results show that the relationship between surface specific area of steel slag powder and the natural logarithm of grinding time is linear, and the relationship between characteristics particle size of steel slag powder and the secondary natural logarithm of grinding time is linear. In the early grinding period, the phases that is hard to grind(denote as PHG) of steel slag are ferrite phase, RO phase and metallic iron phase, while the PHG mainly is the metallic iron phase in the latter grinding period.The effects of many kinds of organics on the grinding property of steel slag are studied. The results show that the grinding effects of organics are closely related to their dosage, functional group number and type, carbon chain length and molecular structure. Specific rules are as follows:(a) the greater the dosage of organics, the stronger their overall grinding effect, not a single molecular layer theory dosage is the best dosage. When the dosage of organics is the bimolecular molecular layer or more molecular layer theory dosage, its effect also slow growth, but lead to a higher costs, in practice rarely used.(b) The relationship between the grinding effects of organics and their function groups number basically follow these rules: when hydroxyl number is less than 3, the effects of organics containing the same functional group of the same series are gradually strong with the increase of functional groups increase number; and for 3-hydroxy greater than organics, the greater the number of functional groups, the weaker the grinding effects of organics.(c) Comparison of grinding effects of different functional groups: hydroxylamine > hydroxyl & hydroxy ether group > hydroxy aldehyde.(d) The releationships between grinding effects of organics and thier carbon chains length: when the grinding effect of each series of organics is best, basically corresponding to a certain carbon chain length, for example, the optimum carbon chain length of a monohydric alcohol or glycols is three.(e) For the organics with the spatial three-dimensional heterogeneous structure, it is conducive to improve the grinding effect of orgincs, which is better than ethanolamines. For example, the grinding effect of isopropyl alcohol amine organics is better than ethanolamines orgranics cotaining same function group type and number.(f) Comparison of grinding effects of different series organics: alcoholamine organics > diol & triol organics > monoalcohol organics > multi-hydroxy organics, the first two types of organics is suitable for steel slag grinding aids, the latter two types of organics is not suitable for steel slag grinding aids.(g) Effect of grinding time on the grinding effect of organics is relate to their dosage, when grinding effect of organics in a certain dosage reach to the best, corresponding to a optimal grinding time.The hydration and hardening characteristics of steel slag powder are studied from hydration heat, hydration products, hydration degree, strength, paste mircrostructure and hydration kinetics equation. The results of hydration heat show that the hydration process of steel slag paste also has five periods that are similar to cement, but it has a longer early hydration(pre-induction period, induction period) and lower second exothermic peak, both the heat release ratio and total heat volume of hydration is much lower than cement. The results of hydration products show that the hydration products of steel slag and gypsum are amorphous C-S-H gels, bar shaped ettringite and a small amount of hexagonal plate Ca(OH)2, and the products structure is very loose. While there is scarcely ettringite in hydration products of pure steel slag paste. The results of hydration degree and strength show that both the nonevaporable water content and Ca(OH)2 content of steel slag paste and its curing age conform to expontial relationship, while the strength of steel slag paste and its curing age conform to logarithmic relationship. The results of hydration kinetics equation of steel slag paste show that the relationship between the hydration depth and time in the early, mid and late hydration conform to linear, exponential and logarithmic relationship, respectively. Steel slag particle, particle size of which is greater than 4.39?m, can't fully hydrate in 360 days. And the contribution rates of early, mid and later hyration on hydration degree are 37.59%, 33.61% and 28.80% in 360 days, respectively.The properties of supplementary cementitious material composite powders such as steel slag-slag, seel slag-fly ash, steel slag-quartz and steel slag-silicon fume are studied. The results show that steel slag-slag composite powder has a “1+1>2” superimposed composite effect. Steel slag and slag can promote mutual hydration reaction and improve hydration degree. The steel slag-slag ratios are 2:3 and 1:1, corresponding to the optimal early and later property of compostite powder, respectively.The effects of steel slag powder on the properties of composite Portland cement are studied. The results are as follows:(a) Compared with pure Portland cement, blended cement containing steel slag powder has a long hydration induction period, low hydration degree and strength and poor structure. The higher the mixing amount, the greater the effect of steel slag powder on the properties of blended cement. And the strength of blended cement and the mixing amount of steel slag powder conform to a polynomial function relationship.(b) The steel salg powder(particle size of which is lesss than 32?m) and 3d, 7d and 28 d flexural strength and 7d, 28 d compressive strength of blended cement are positive association, respectively, while the relationship between steel salg powder and 3d compressive strength is negative association. And 16-24?m particle diameter of steel slag is the best match with the reference cement.(c) The filling effect of steel slag powder on blneded cement is related to its particle size, the smaller its particle size, the stronger its physical and chemical filling effects.(d) The contribution rate of steel slag powder on the hydration of blended cement is enhanced with increasing of its mixing amount, and the smaller its particle size, the stronger its chemical contribution.(e) The contribution ratio of steel slag powder on the strength of blended cement is negative at 3 days, while it is positive at 28 days.The composition and properties of steel slag-sulfur aluminate cement(CS—A-SS) and steel slag-aluminate cement(CA-SS) composite binder are studied. The results show that the decline of strength and hydration degree of CS—A-SS and CA-SS is small when mixing amount of steel slag powder is low, conversely, it has a significantly decline. Compared with CS—A, the porosity of CS—A-SS is higher, pore size distribution is poor. While the pore size distribution of CA-SS containing small mixing amount of steel slag powder can be optimized. Ie., harmless pore(<20nm) content is increased and harmful pore(20-50nm) content is reduced. But the overall porosity of CA-SS paste is still higher than that of CA paste.The roles of a small amount of different cements on the hydration and hardening properties of steel slag are studied. The results show that a small amount(20%) of Portland cement, sulfur aluminate cement and aluminate cement can significantly improve the early cementitious property of steel slag, the roles oder is as follow: aluminate cement > sulfur aluminate cement > Portland cement. While the improvement effects of sulfur aluminate cement and aluminate cement on the later property of steel slag is small, and the effect of Portland cement is significant. The effects oder of cements is as follow: Portland cement > sulfur aluminate cement > aluminate cement.Effects of ultrafine grinding, chemical activator and thermal curing on the properties of steel slag and its composite cementitious materials are studied. The results are as follows:(a) Ultrafine steel slag powder can obviously improve the cementitious activity of steel slag, reduce the impact of steel slag content on the properties of blended cement and improve the pore structure of paste.(b) Seven kinds of chemical activators selected can significantly improve the hydration and hardening properties of steel slag, the best effects of which are Na2SO4 and TEA. Inorganic activators can improve the early properties of blended cement containing steel slag powder, but it will reduce the later properties. While both the early and later properties of blended cement can be improved by organic activators.(c) Effect of thermal curing on the properties of steel slag powder and its composite cementitious materials are closely related to thermal curing time. The longer the curing time, the better the early properties of cementitious materials, but it may be unfavorable to the development of later properties. It should be considered early and later properties needs to select the appropriate thermal cruing time.On the basis of Fuller distribution, the composition model of steel slag-slag-cement three components blended cement(“Fuller-SS-S-C” blended cement) is proposed. The characteristics of the model is as follows: the overall particle size distribution of blended cement is conformed to Fuller distribution, but the particles of 0-15?m, 15-30?m, 30-45?m and 45-80?m fractions are composed of SS-S-C, C, SS-S and SS, respectively. And the properties of “Fuller-SS-S-C” blended cement can be adjusted by changing the composition ratio of SS-S-C in range of 0-15?m. The “Fuller-SS-S-C” blended cement prepared accoding to this model has good physical and mechanical properties and dense paste structure. |
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