Investigation On Modifications And Applications Of Recycled Fine Aggregate Prepared From Demolition Concrete

To achieve sustainable development in the construction industry and reduce environmental pollution, there is a need to recycle and reuse demolition concrete as secondary aggregates for producing new building materials. However, due to more microcracks in aggregate surface, higher water absorption and higher content of fine powder, the quality of recycled aggregate has been widely reported to be of inferior to that of natural aggregates. It is essential and important to enhance the properties of recycled aggregate in order to improve the performance of recycled concrete and recycled mortar.By testing performance changes of recycled fine aggregate (RFA) and recycled mortar (RM) before and after modification, including RFA water absorption, water demand ratio and compressive strength ratio of RM, water-cement ratio, water retentivity, toss of consistency and compressive strength and so on, this study evaluated the effect of modification of CO2 curing and microbial mineralization and explored the influence of different test factors.at the same time, the paper used scanning electron microscopy, X-ray diffraction, thermogravimetry, mercury and other micro-analysis method to explore the modified principle. In additon, the study determined the most suitable admixture which consists of several kinds of admixtures according proportion for RM prepared with RFA without any treatment.CO2 curing is a new way to strengthen properties of recycled aggregate, and it is not very clear that the effect of this method and the impact of carbonation conditions. There is little carbonizable substance in RFA prepared from demolition concrete due to the longer storage time, and direct carbonation for RFA is ineffective. Considering the less content of carbonizable substance in RFA, this paper proposed to cure RFA by adding calcium source to enhance carbonation effect while decided the best carbonation condition by exploring the impact of some experiment factors. The results verify that it is feasible to modify RFA by CO2 cuing and determine the optimal conditions, which are as followed:gas concentration of CO2 is 70%, and initial water content of RFA is 5% when CO2 curing is conducted in carbonation test chamber of concrete with temperature of 25 ?, and calcium hydroxide prefers to be the best choice when its concentration is 0.01-0.05mol/kg. Particulate calcium carbonate formed by carbonation can fill microcracks of RFA, which can effectively reduce the aggregate porosity. Compared with RFA without carbonation, Water absorption reduces 62%, and water demand ratio fell to 1.06 from 1.17, correspondingly compressive strength ratio raise to 1.04 from 0.95. The performance of recycled mortar prepared M10 with the RFA with carbonation has been improved remarkably, including improving mortar fluidity, reducing consistency loss. And 28d compressive strength has an increase of nearly 21-26% compared to nature mortar.Microbial mineralization applied in strengthening RFA is as novel as carbonation, so up to now, there no studies on modifying RFA This paper explored the impact on performance of RFA and RM by soaking RFA with microbe cultured solution containing different calcium sources. The results show that microbial mineralization can effectively improve the performance of RFA and RM And the optimal treatment conditions are as followed:Sporosarcina pasteurii is cultured in the media consist of beef extract and peptone for 24h. Then add RFA to the solution for 3-6h and keep oscillating, after which add mixed solution of calcium chloride and urea and continue to save 24h. There are so obvious CaCO3 particles whit sizes of 5-10?m on RFA surface that the aggregate water absorption decreased. And performance of RM M10 prepared with the RFA with modification has been improved remarkably, including a reduction of water-cement ratio, a fall of consistency and an increase in water retentivity.28d compressive strength decreases because significant cracks appear in interfacial transition zone (ITZ) compared to RM prepared with the RFA without modification. But the ITZ still more closely than the one of natural mortar as while as compressive strength are higher than that of natural mortar.Taking into account the complexity of strengthening RFA process, this paper invent the most suitable powder admixture which consists of several kinds of electrolytes according proportion for RM prepared with RFA without any treatment to improve the performance of RM. Considering the effect and cost analysis, the admixture consist of MN, LL and NS uniformly mixed in proportion of 2:6:1. And the optimum dosage is 0.75% by mass of cementitious material. Consistency loss of RM M10 modified greatly reduced, compressive strength increased nearly 66% over the one of previous modification.When the water-cement ratio, water retentivity rate and 28d compressive strength are as the main index, the best modified additive is admixture modification. Under laboratory conditions, CO2 curing RFA processing costs more 1650 yuan per ton mortar, and more cost of microbe mineralization is 8250 yuan per ton mortar. Modifacation RFA by chemical method makes ready-mixed mortar cost plus 30 yuan per ton. When mortar admixture is used to produce ready-mixed mortar, mortar accessional cost is only 4.34 yuan per ton. In view of the effect of modification and economic analysis, this paper concludes that the application of mortar admixture is better than carbonation, and carbonation is better than microbe mineralization.