Volume 63 (2020): Edition 2 (December 2020)

Bauxite residue is a major waste stream available in large volumes globally that can cause risks to the surrounding environment (e.g. ecotoxicity) when disposed and stored by conventional methods. There is yet no large-scale application and the utilization as supplementary cementitious material might be the best way to re-use bauxite residue. The main obstacle for the utilization of bauxite residue in the construction industry is the high alkalinity. This paper presents first results of a study on alkali reduction of bauxite residue by acetic acid treatment and the potential application of this alkali reduced bauxite residue as pozzolan in cementitious binders. A process of alkali reduction is presented that can help solving waste management problems of alumina refineries by transforming bauxite residue to a less hazardous waste, while producing a reactive pozzolan and Na-acetate that can find application in the construction and infrastructure market. 90% alkalinity reduction of bauxite residue could be achieved by simply washing with diluted acetic acid. Alkali-reduced bauxite residue showed good pozzolanic reactivity regarding portlandite consumption, bound water and 28-day compressive strength of mortars with 20% replacement of OPC.

Wet sprayed concrete quality is affected by more production factors than cast concrete, particularly due to the propulsion through the nozzle and the flash set caused by the set accelerator. Practitioners often use the term “sprayability” to describe these factors. We propose a definition of “sprayability” that relates the application to the final properties of the hardened sprayed concrete and review factors affecting it: concrete constituents, proportioning, and application mechanics. These factors affect the hardening and the structure of the hardened sprayed concrete – the porosity, permeability and durability. We consider improving sustainability through proportioning with increased share of supplementary cementitious materials, calculate the placed composition and focus on factors that affect water transport, and hence durability. Due to the spray application and flash-set, irregular compaction voids dominate the macro pore structure of sprayed concrete. Studies of permeability of sprayed concrete have shown that it is possible to obtain low permeabilities given adequate composition and curing. Presumably these samples have been well-cured, uncracked and with non-percolating macro voids. Given observations of cracks in sprayed concrete linings and the macro voids, important further studies will be on the effect of accelerator, compaction porosity and cracking on permeability.

Air-entraining agents (AEA) are normally used to improve the frost resistance of concrete. However, it is not possible to accurately control the air void system in concrete with AEA. Thus, a significant loss of concrete strength is caused by over-dosing voids, and this increases the environmental impact from concrete structures. Superabsorbent polymer (SAP) can also be used to produce frost-resistant concrete. Compared to AEA, it can be used to precisely engineer the air void structure of concrete, promote cement hydration, and mitigate self-desiccation cracks. In this study, life cycle assessment methodology is applied to evaluate the overall environmental impact of frost-resistant concrete based on AEA and SAP, respectively. The results illustrate that frost-resistant concrete with SAP has a lower environmental impact than frost-resistant concrete with AEA if the strength and durability of concrete are considered in the defined functional unit. In addition, frost-resistant concrete with SAP reduces the environmental burdens of the vertical elements such as columns, but it increases the environmental load of the horizontal elements such as slabs, where the strength increase cannot be utilized. Moreover, the inventory data for AEA and SAP can affect the impact assessment results.

Worldwide increased concern of the CO₂ emissions has led to the replacement of coal by biomass in combustion-based power plants. However, this would cause the scarcity of fly ash, one of the most well-known rest products from coal combustion, which is used as supplementary cementitious materials (SCM) in construction sector to reduce the large environmental footprint of cement production. Seeking to find alternative SCMs, this article aims to demonstrate the viability of using bio ashes in Sweden as SCM, which, due to lack of studies validating their value, are landfilled today. According to the obtained results, bio ashes produced at pulp and paper industries have a considerably consistent chemical composition and exhibit a satisfactory pozzolanic behaviour. Nevertheless, according to the conclusions of this study, the pozzolanicity of these alternative binders is not reflected equally with respect to the most known reactivity tests for common SCMs. The results imply that although “R3” tests method infers the pozzolanic characteristics of the bio ashes in focus of this study, the “activity index test” as well as “calcium consumption test” indicate otherwise.

Concrete production, especially the cement production, stands for 5-8 percent of the global CO₂ emissions. Since concrete is the most frequently used man-made construction materials, this fact is not surprising. Concrete is also the only realistic alternative in order to improve the living circumstances in many countries around the world. Due to its size, the concrete sector has a great responsibility for limiting the consequences of the on-going climate change. The Swedish cement producer Cementa has an ambitious zero vision stating zero CO₂ emissions in year 2030. The measures include energy efficiency, bio mass instead of fossil fuels, blended cements, CO₂ uptake through carbonation and Carbon Capture Storage (CCS). This paper discusses these measures but also others such as optimization of the concrete mix, optimization of the structural geometry and prolongation of the service life. The paper is ended by a section on adaptation since concrete will also have an important role concerning protection of the built environment for climate change. Protection structures against flood, reconstruction of dams, new waste-water systems and bright permeable concrete pavements reflecting sunlight and improving drainage after heavy rain constitute some examples.

According to Swedish experience the slab method in CEN/TS 12390-9 is successful in predicting the salt-frost resistance of Portland cement concrete. However, doubts have been raised whether the same can be said when used on concrete with supplementary cementitious material, e.g. fly ash or ground granulated blast furnace slag (GGBS). Test results from concrete mixes with up to 35 % fly ash 65 % GGBS, with two different Portland cements and a water-to-binder ratio of 0.45 are presented in this paper. The tests were carried out with the standard method and with five modifications concerning the pre-conditioning of the specimens before freeze-thaw cycling. The age of the specimens at sawing was increased, the time in 65 % RH was prolonged and exposure to 1 % CO₂-environment was used. The results show that for air-entrained concrete with fly ash or GGBS both prolonging the exposure to 65 % RH and exposure to CO₂ diminishes the salt-frost resistance. The influence increases with increasing amount of fly ash or GGBS. However, the type of cement also has a certain influence. The influence of exposure to CO₂ on the salt-frost resistance of concrete without entrained air was totally different from the influence on concrete with entrained air.