| As the largest Steel Slag production country,China accounts for producing more than 55% of the world's steel every year.Industrial by-products such as steel slag,which can be used after grinding as a cementitious material to replace a part of cement,plays a very important role in the development of low-carbon building materials.Therefore,how to improve the utilization rate of steel slag as a cement replacement material is an important issue that can simultaneously improves the sustainable development of both of the steel and the concrete industry.This research studies how to use two high-grade steel slags,such as converter carbon steel slag(CS)and electric arc furnace stainless steel slag(SS),as cement-based cementitious materials.The influence of the high fineness CS and SS on the hydration characteristics of cement in cement-slag complex binder is studied.The standard consistency,the pH value,the setting time and the effect of the slag replacement ratios on the fluidity of cement-slag mortar were determined.Furthermore,this research is divided into two main parts;the first part consisted of a detailed comparison between the hydraulic behavior and cementitious activity of pure CS and that of pure SS,the second part investigated the effect of CS and SS on the hydration of ordinary Portland cement(OPC).The two types of slag were used to replace cement at replacement ratios of 15%(CS15 and SS15)and 30%(CS30 and SS30)by binder weight.The results showed that CS had higher alkalinity and higher pH value and more hydraulic phases than SS.Both types of slag showed water reduction effect due to its high fineness.Neat CS paste showed flash set and acceleration in the initial setting time of blended cement especially at high slag proportions.However,SS prolonged the setting time of blended cement even at low slag proportions.The pH values for blended cement paste contained 50% CS or 50% SS were lower than that of pure cement paste.Compressive strength and pore structure analysis results on pure slag reveals that CS showed higher early hydraulic properties than SS which showed higher rate of activity at later ages than at early ages but still lower than that of CS.Calorimetric analysis showed exothermic reactions in both slag types.The test results of blended cement containing slag demonstrated that the hydration rate of OPC-CS binder is similar to that of OPC-SS binder at 3 days,but higher than the latter at later ages.X-ray diffraction(XRD)and Thermogravimetry(TG)show that the incorporation of SS tends to decrease calcium hydroxide(CH)content more than the case of CS in the cement matrix.The microchemical analysis showed that the average Si/Ca ratio of CS30 and SS30 at 90 days to be 0.41 and 0.45,respectively compared to 0.43 for pure cement.The negative effect of steel slag(CS)on the strength of cement mortar can be neglected when its replacement ratio doesn't exceed 15%.X-ray diffraction(XRD),Thermogravimetry/ differential scanning calorimetry(TG/DSC),Fourier transform-infrared spectra(FTIR)and Scanning electron microscopy(SEM)were used for quantitative microscopic analysis of hardened blended paste and mortar.The test results indicate that the hydration products of CS were mainly C-S-H,C-A-H,C-A-S-H,AH3 and CH;however,it was limited to C-S-H in SS.The average Si/Ca atom ratios for the hydration products in 90 days hydrated CS and SS pastes estimated by means of BSE(back scattered electron)/EDS(energydispersive X-ray spectroscopy)analysis were 0.31 and 0.52,respectively.EDS mapping on hardened blended pastes at 90 hydration days revealed that Si and Ca included in the hydrated matrix surrounding slag particles with the addition of Al inclusion in CS. |
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