A Study On The Composition, Structure And Performance Of Slag-Fines Cementing Materials Alkali-Activated

Nowadays, exploitation of blast furnace slag from metallurgical industry for manufacture of high quality cement substituting for a portion of clinker has become a trend of building material development. This approach is of great importance in the development of society with economical consumption of resources and of environmental friendship. For decades, massive efforts have been carried out to investigate the application of blast furnace slag to cement and concrete resulted in significant advancement. Up to now, the slag powder production lines using vertical mill, in total of twenty-one have been commissioned and others are under construction in China. The production ability of slag powder has reached more than fifteen million tons per year. So far, there exist two main applications of slag powder. In the first approach, slag fines substitute a portion of Portland cement clinker in slag cement production. And in the other approach, the fines are used as a mineral additive of concrete. In the dissertation, the slag chemical composition, the slag microstructure, the hydration mechanism, effects of various activators, the derivatives of hydration, the pore structure hardening body have been investigated in details. Furthermore the main performance of slag fine cementing material was investigated systematically and an optimized burden ratio was recommended.By considering the process of water quenching of slag formation and the microstructure of the slag glass phase, the hydration-hardening activity of slag fines is understood thoroughly. It is known that the slag consists of glass phase, as the major constituent, and crystal phase. In the slag glass phase two distinct microstructures have been recognized: silicon-poor phase and silicon-rich phase forming microstructures. Experiment showed that after the slag fine dissolved in aqueous solution of strong acid or strong alkali, Ca²⁺ and Mg²⁺ ions were ionized first from the poor-silicon phase, and then the [SiO₄]^4- released from the rich-silicon phase at a relatively slow rate. It was observed that in a strong acid (H⁺) solution [SiO₄]^4- ionized from slag formed silicane alcohol without cementitious properties. Therefore strong acid can not be used as activator for preparation of slag fine cementing material. Whereas in alkali solutions [SiO₄]^4- and Ca²⁺ ions released from slag fine were combined to form C-S-H gel. Thus alkalis are commonly employed in the manufacture of slag fine cementing material as activators. Since ions of [SiO₄]^4- are released from slag fine at a relatively slow rate, concentration of [SiO₄]^4- in the solution is minor in the early stage of hydration, so alkalis with silicon constituents are preferable for the activation of slag fines.A series of experiments of alkali activation of slag fine employing NaOH and KOH as activators showed that an increase in the content of alkali, to a certain extent, resulted in better cement strength. In case Ca(OH)₂ was utilized as activator, early stage strength of the derived cement tended to be low. When water glass was used as a liquid activator the cement yielded could develop better bending and compressive strength. The theoretical consequences were strongly justified by experimental results.On the basis of investigations mentioned above, two types of solid activators, W1 and W2, were developed. It manifested that grinding mixing slag and solid alkali activator eliminated the shortcomings associated with the application of liquid activators.The solid activators W1 or W2 in combination with slag fines resulted in slightly low value of compressive strength of the derived cement, while in the middle and later stages of curing it developed satisfactorily. The major hydration products of alkali-slag cement were gel mineral of sodium and calcium sillico aluminate like zeolite and C-S-H gel, these substances increased with the curing time increase. A series of orthogonal experiments had been carried out for an optimized portioning formula for slag fines cement, by rejecting the worst.When a kind of combined sulphate and clinker activator was used in the experiment, the experimental results showed that the bending and compressive strength of the samples could achieve that of SC 42.5 grade with low strength at early stage, at the middle and later curing period, the bending and compressive strength developed to a desirable level, the compressive strength of the cement in 28 days was developed to a level beyond Portland cement of 52.5 grade, after 60 days, compressive strength decreased in minor degree while the bending strength increased continuously, and that in the case of clinker activated slag cement, the main hydration product was C-S-H gel and Aft crystal.Analysis on microstructure of the hydration showed that after hydration of the slag fine cement, there existed, attached to the slag particle, granular and spherical particles of sodium and calcium sillico aluminate minerals like zeolite and C-S-H gel. These cementation substances gradually increased in quantity with increased age, the interstitial sites between the slag particles so as to cause the diminution of inter porous diameters. By examining the porosity in the hydration samples of slag powder cement at various curing periods, it was found that detrimental pores with size greater than 100nm diameter eventually decreased in concentration, at the same time the harmless pores with size less than 50nm increased in concentration with increased curing periods. Thus the harden block of slag fine cement become more compactive.Based on the principle of packing modes of spheres and fractal theory, a fractal dimension model of hardened cement body was developed. Furthermore, the model was justified by porosity data determined by mercury injection apparatus. The model confirmed to be correct and was employed to calculate the fractal dimension of pores configuration in the body of hardened cement. This provided a new approach for characterizing pore configurations in hardened cement body.Three kinds of slag fines ground by using vertical-mill and ball-mill and vibrating-mill respectively were examined. Those yielded by vertical mill possessed flaky grain with relatively narrow size distribution, and those produced by ball-mill and vibrating-mill showed quasi spherical shape with wider sizes distribution. Mechanics performance results showed when the three kinds of fines with same specific surface area were activated with the activators mentioned above, the slag cements made up of the fines of those three kinds resulted in standard strength of PC 52.5 grade. Apparently, slag cement prepared by vibrating mill showed the highest strength in comparison with those by both the vertical- mill and ball-mill grinding. And relatively high bending strength resistance was resulted from the vertical mill grinding, while its compressive strengthes were lower than those from ball mill. The shape of slag fines definitely influences early performance of those slag cement.Under the same experimental conditions, an investigation into the relationship between the strength and fineness of a given sort of slag indicated that the highest bending and compressive strength was resulted from the fines of 550m²/kg in specific surface area among those of 550 and 820m²/kg respectively. Contrarily, a lower bending and compressive strength resistance was resulted from fines of 820m²/kg in specific surface area.These observations depicted that the slag fines were not the finer the better for use in the blended cecent.The shrinkage property of the slag fines cement was determined and optimized by applying the expansion additive prepared by the author. With the mass ratio of the additive ranging from four to six percent, the specimen shrinkage was compensated without degradation in cement strength detected. Although addition of the expansion additive more than the range described compensated the cement shrinkage, it caused an appreciable decrease in cement strength at the same time.Research of resistance to chemical attack properties of the slag fines cement showed a superior performance to that of ordinary Portland cement, especially the resistance to sulphate attack.