Effect Of Compound Doping On Recycled Cement Clinker Production And Property

Recycled cement production, utilizing all components of the waste concrete without separation treatment as raw materials, provides an effective method for the high value-added resource recovery of waste concrete. Low mechanical property is one of main problem in the former study of recycled cement, which is mainly caused by two possible factors. High quartz content of fine aggregate in waste concrete is one reason accounting for low mechanical property, which passively affects the burnability of raw meal and formation of silicate minerals. Another factor is inappropriate modulus in former study. Hardened cement paste and silicious materials with high quartz content are the characteristic component of recycled cement and significantly influence clinker production, which results in the difference of the modulus design between Portland cement and recycled cement. Hence, the optimal modulus for recycled cement production and effective measure to improve reactivity of raw meal should be studied to optimize recycled cement property, especially enhance mechanical property.In this paper, burnability test, Rietveld quantitative phase analysis and mechanical property test were utilized to study the effect of compound doping of industrial residue and mineralizer on recycled cement clinker production and property. First of all, different silicious materials were utilized to prepare the Portland cement raw meal with high quartz content to analyze the effect of modulus on raw meal burnability, which provided theoretical basis for modulus design in further recycled cement study. What's more, the effect of quartz crystallinity on recycled cement clinker production and property were studied. On this basis, fluorine-sulphur compound mineralizer were added into recycled cement raw meal with steel slag or blast furnace slag in different content to study the effect of compound doping on recycled cement clinker production and property. This paper attempts to confirm the optimal modulus for recycled cement with high quartz content of sand as silicious materials, analyze the effect mechanism of quartz crystallinity on recycled cement clinker production and property and explore optimal compound doping combination to improve raw meal burnability and clinker property.The main conclusions are as follows:(1) The raw meal with higher crystallinity quartz in silicious materials presented worse burnability that was dramatically affected by modulus. The Portland cement raw meal containing sandstone presented better burnability with free lime content under 1.0%, while the raw meal containing sand presented worse burnability and modulus had a significant influence on its burnability. The appropriate clinker modulus for the Portland cement raw meal with high quartz content were SM=2.2, IM=1.2, or KH=0.96, SM=2.8, IM=1.4, or KH=0.94, SM=2.2, IM=1.0?1.4.(2) Recycled cement raw meal with high quartz content presented poor burnability and mechanical property and its clinker contained less C3S content, but more C2S content. Quartz crystallinity significantly affected burnability and C2S formation, while it had little impact on C3S formation. Low C3S content and high free lime content were the main reasons for low mechanical property.(3) Compound doping of steel slag and fluorine-sulphur mineralizer dramatically improved recycled cement raw meal burnability, and were beneficial for C3S formation with a similar C3S content (>50%) to industrial clinker. Compound doping enhanced the mechanical property with 28d compressive strength of recycled cement paste exceeding 50MP. In addition,7d compressive strength of recycled cement paste of compound doping combination with 1.5% and 3.0% steel slag were close to industrial clinker. Fluorine-sulphur mineralizer had the dominant effect in compound doping of steel slag and fluorine-sulphur mineralizer. The optimal doping combination for steel slag and fluorine-sulphur mineralizer under experimental condition of this paper was 3% steel slag,0.5% CaF2 and 1.31% CaSO4·2H2O.(4) Compound doping of blast furnace slag and fluorine-sulphur mineralizer evidently improved recycled cement raw meal burnability, and it's conductive to C3S formation. However, with blast furnace slag content rising in raw meal, recycled cement clinker contained less C3S content and more C4AF content. The recycled cement clinker with compound doping had a good mechanical property with 28d compressive strength of recycled cement paste exceeding 50MPa, but inferior to industrial clinker. In addition, the blast furnace slag content had little impact on paste compressive strength. The optimal doping combination for steel slag and fluorine-sulphur mineralizer under experimental condition of this paper was 3% blast furnace slag,0.5% CaF2 and 1.31% CaSO4·2H20.