Impact of using different materials, curing regimes, and mixing procedures on compressive strength of reactive powder concrete - A review

Reactive powder concrete (RPC) is an ultra-high performance concrete (UHPC) with an enhanced microstructure. Over the past few years, the demand for RPC has increased due to its superior properties. However, RPC is characterized by its low water-to-binder ratio, high cement and silica fume (SF) content, and absence of coarse aggregates which not only harm sustainable development, but also increase the production costs of RPC and generate shrinkage problems. Within this framework, many studies attempted to use different materials to address these problems and produce eco-friendly RPC with similar performance to that of the traditional RPC. The primary objective of this paper is to present an updated review of the literature on the list of materials used for RPC production and assess their viability as partial and full replacement of cement, SF, and quartz sand/ powder to produce ultra-high strength RPC. The effects of employing different curing regimes and mixing procedures on the compressive strength of RPC will also be reviewed. The results highlight that 1) the use of alternative mineral admixtures (glass powder, limestone & phosphorous slag) can successfully replace cement by up to 50%; 2) replacing SF with mineral admixtures such as slag and fly ash is possible and can yield comparable results by monitoring the molar Ca/Si ratio of the mixes; 3) Quartz sand/powder can successfully be replaced with other types of aggregates/fillers (titanium slag, glass sand, glass powder, rice husk ash, etc); 4) Waste steel fibers can yield comparable strength results to that of steel fibers and the hybridization of glass-steel and polypropylene-steel improves the strength compared to steel or other types alone; and 5) Four-stage mixing yields better strength properties (up to 22% enhancement) compared to three-stage mixing, but further research is required to confirm this finding and establish standard guidelines for the mixing of RP

Impact of Magnesium Sources for Phosphate Recovery and/or Removal from Waste

As the population continues to rise, the demand for resources and environmentally friendly management of produced wastes has shown a significant increase in concern. To decrease the impact of these wastes on the environment, it is important to utilize the wastes in producing and/or recovering usable products to provide for the sustainable management of resources. One non-renewable and rapidly diminishing resource is phosphorus, which is used in several products, the most important being its use in manufacturing chemical fertilizer. With the increase in demand but reduction in availability of naturally occurring mineral phosphorus, it is important to investigate other sources of phosphorus. Phosphorus is most commonly recovered through struvite (magnesium ammonium phosphate) precipitation. The recovery of phosphorus from various wastewater has been well established and documented with recovery rates mostly above 90%. However, one of the major drawbacks of the recovery is the high cost of chemicals needed to precipitate the phosphorus. Since the external magnesium needed to achieve struvite precipitation accounts for around 75% of the total chemical cost, applicability of low-cost magnesium sources, such as bittern or seawater, can help reduce the operational cost significantly. This paper investigates the different magnesium sources that have been used for the recovery of phosphorus, highlighting the different approaches and operating conditions investigated, and their corresponding phosphorus recovery rates. An investigation of the economic aspects of the magnesium sources used for removal/recovery show that costs are dependent on the raw waste treated, the source of magnesium and the location of treatment. A review of published articles on the economics of phosphorus removal/recovery also indicates that there is a lack of studies on the economics of the treatment processes, and there is a need for a comprehensive study on life cycle assessment of such processes that go beyond the technical and economical aspects of treatment processes