LIU Yanni¹ ，WU Xiaorong¹ ，WANG Gang² ，ZHAO Mingyu¹ ，TONG Yu¹

1. School of Materials Science and Engineering， Shenyang Jianzhu University， Shenyang 110168， China；

2. The 2nd Construction Limited Company of China Construction Eighth Engineering Division， Jinan 250022， China

Abstract

Objective Recycled fine powder （RFP） is microscaled granules generated in the preparation of recycled aggregates by a process of crushing of construction or demolition wastes. Some feasible ways have been developed to realize the resourced utilization of RFP， of which with the most adaptable one being as the active admixture for the production of Portland cement or commercial concrete. The higher the pozzolanic activity of RFP is， the greater the utilization rate that can be achieved. The process of calcination at a temperature not exceeding 1 000 ℃ is generally esteemed as one of the most effective ways to enhance the economic and technological results of RFP as concrete admixture. However， the activation mechanism of RFP in the process of high temperature calcination may not consistently explain the experimental results in any cases， since the the efficiency of calcination is apparently depending on the source of RFP material and the temperature regime of the calcination. A strong requirement arises subsequently to clarify the activization mechanism of RFP in the process of high temperature calcination.

Methods In this paper， RFP specimens were calcined at a certain temperature ranging from 600 to 900 ℃， while the calcination time was adjusted in the range of 0. 5 to 3. 0 hours. The as-prepared RFP specimens were used to partly replace the Portland cement at a weight ratio of 30% for the preparation of standard mortar with a water-to-cement （W-C） ratio of 0. 5 by weight and a weight ratio of 3∶1 for the standard sand to Portland cement. Changes in the compressive strength of such mortar blocks after the standard curing of 7 d and 28 d were investigated in details to reveal the effect of high temperature calcination on the pozzolanic activity of RFP， followed by the measurements and discussion of the microstructural characteristics of RFP by means of comprehensive thermal analysis，i. e. thermal gravity （TG） and differential scanning calorimeter （ DSC ）， and X-ray diffraction （XRD） to demonstrate the phase changes of RFP during the high-temperature calcination.

Results and Discussion When the calcination temperature increased from 600 to 900 °C step by step， the activity index of calcined RFP was found to increase evidently with the increasing temperature for calcination， but further elevation of calcination temperature higher than 600 °C was not proposed due to the lowering of the activity index. Similar results was observed from the activity index tests of RFP calcined at 600 °C with different time， in which the activity index of RFP was found to increase apparently when the calcination time was not longer than 1. 5 hours， but further increase of calcination time gave rise of a relative low activity index from the obtained RFP. Therefore， the suggested calcination conditions for RFP were to be 600 °C in temperature and 1. 0~1. 5 h in time， which could bring forth of a maximum activity index of 90. 19% at the age of 28 d. It is also found that the as-prepared RFP resulted in a relative high index of pozzolanic reactivity at the age of 7 d compared to that at 28 d. Furthermore， microstructural characterizaion under TG-DSC and XRD showed that the calcination being carried out at 600 °C brought forth of an appreciable change of microscaled structure in RFP，i. e. the dehydration of clay minerals such as kaolinite， as well as the hydrated products of Portland cement including calcium hydroxide and calcium silicate hydrate （ C-S-H）， which must be helpful to improve the pozzolanic activity of RFP. However， high temperature calcination at 800 to 900 °C resulted in the generation of low-activity products such as spinel （Al₂O₃ ·SiO₂ ） or gehlenite （2CaO·Al₂O₃ ·SiO₂， C₂AS）， and thus made a negative effect on the pozzolanic activity of RFP. The calcination of hydrated Portland cement at 900 °C did give rise to the generation of C₂S rather than C₂AS.

Conclusion The processing of high-temperature calcination is evidently helpful to upgrade the pozzolanic activity of RFP， but the calcination regime must be coordinated with the chemical composition and mineral components of RFP. As a result， the calcination of RFP in laboratory is proposed to be carried out at 600 °C in temperature and 1. 0 to 1. 5 h in time to realize an optimized improvement of its pozzolanic activity， which can be attributed to the dehydration and amorphization of clay minerals， as well as the thermal decomposition of hydrated Portland cement， especially C-S-H.

Keywords：concrete； recycled fine powder； calcination； activity index； microstructural characteristics