Reactivity of modified iron silicate slag as sustainable alternative binder

A possible solution to decrease the CO2 footprint caused by cement industry and to enhance the transition to circular economy is to use slags as Supplementary Cementitious Materials (SCM). The study presented here focuses on valorizing and investigating the reactivity and mechanical properties of blended binder systems combining modified iron silicate (MFS) slag and Ordinary Portland cement (OPC). MFS slag is a fumed by-product synthesized during the production of copper metal (Cu). This slag can be used as possible alternative SCM due to its pozzolanic behavior. To study the replacement level in relation to reactivity and strength development, replacement levels of 15, 30 and 50 wt% of MFS-slag in OPC are analyzed. The work can be divided into two categories, 1) assessing the reactivity through thermogravimetric analysis (TGA) and 2) evaluating the compressive strength (as a function of time) of mortar with MFS-slag after 2, 7, 28 and 90 days. TGA at 7, 15, 28 and 90 days allows to determine the reduction of portlandite (CH) content which gives an indication of the pozzolanic reactivity. Reactivity of the MFS-slag blended systems is also determined relative to inert filler blended systems to discern between the reactive behavior of the MFS-slag and the filler effect

Reactivity assessment of Modified Ferro Silicate slag by R-3 method

Traditional methods to track the reactivity of supplementary cementitious materials (SCMs) and their contribution to the hydration mechanism mostly use Portland Cement (PC) as an activator. Alternatively, a novel method to assess the reactivity of SCMs called R-3 was recently presented. This novel method uses lab grade chemicals such as portlandite (CH), K2SO4, KOH, and CaCO3 to activate the SCM by resembling the pH of the alkaline pore solution created by PC. By using this method, the reactivity of the SCM can be easily quantified from measured heat release, bound water content, and CH consumption. The primary objective of the current study is to apply the novel methodology to analyze the reactivity of Modified Ferro Silicate (MFS) Cu slag benchmarked against siliceous fly ash (FA), ground granulated blast-furnace slag (GGBFS), and inert quartz filler. GGBFS showed the highest cumulative heat release and bound water content due to its latent hydraulic behavior. Determination with XRD analysis of the major phase of the R-3 model MFS slag paste showed the participation of Fe in the hydration mechanism by forming Fe-AFm. R-3 paste with GGBFS showed the presence of hydrotalcite/Al-AFm, whereas FA showed the presence of ettringite (AFt) as their crystalline reaction products. The experiments also indicate that the MFS slag acts as a reactive pozzolanic material with an acceptable performance in heat release, bound water content, and CH consumption, and can be used as SCM to make concrete. With the possibility of using MFS slag as SCM to replace part of PC, sustainability and circular economy can be fairly well achieved

Reactivity Assessment of Modified Ferro Silicate Slag by R3 Method

Traditional methods to track the reactivity of supplementary cementitious materials (SCMs) and their contribution to the hydration mechanism mostly use Portland Cement (PC) as an activator. Alternatively, a novel method to assess the reactivity of SCMs called R3 was recently presented. This novel method uses lab grade chemicals such as portlandite (CH), K2SO4, KOH, and CaCO3 to activate the SCM by resembling the pH of the alkaline pore solution created by PC. By using this method, the reactivity of the SCM can be easily quantified from measured heat release, bound water content, and CH consumption. The primary objective of the current study is to apply the novel methodology to analyze the reactivity of Modified Ferro Silicate (MFS) Cu slag benchmarked against siliceous fly ash (FA), ground granulated blast-furnace slag (GGBFS), and inert quartz filler. GGBFS showed the highest cumulative heat release and bound water content due to its latent hydraulic behavior. Determination with XRD analysis of the major phase of the R3 model MFS slag paste showed the participation of Fe in the hydration mechanism by forming Fe-AFm. R3 paste with GGBFS showed the presence of hydrotalcite/Al-AFm, whereas FA showed the presence of ettringite (AFt) as their crystalline reaction products. The experiments also indicate that the MFS slag acts as a reactive pozzolanic material with an acceptable performance in heat release, bound water content, and CH consumption, and can be used as SCM to make concrete. With the possibility of using MFS slag as SCM to replace part of PC, sustainability and circular economy can be fairly well achieved

Mix development and performance of concrete with treated slag from copper production as cement and sand replacement

Eco-efficiency of concrete is essential for sustainable development given the huge amounts of concrete used every year by mankind. Three aspects are fundamental in this regard: low carbon emission, limited use of non-renewable resources, and extended service life. First, concerning carbon emissions, reducing the clinker factor in concrete is essential due to the high carbon intensity of this constituent. Even though alternative binder materials have successfully proven to be sustainable alternative binders, the versatility of Portland cement (PC) makes it a hard-to-fully-replace binder. Production of Portland clinker is however responsible for about 6 to 8% of the global CO2 emissions, so more environmentally friendly alternative binders are still needed to take action on climate change. Interesting alternatives are binders based on industrial by-products, e.g. ferrous slags (FS) such as blast furnace slag (BFS), or fly ash (coal combustion by-product). A wide range of studies have already discussed in detail the reactivity and reaction products of binders based on fly ash and BFS but these industrial by-products are not available in sufficient quantities to cover the needs. An interesting alternative to investigate is the applicability of non-ferrous slags (NFS). NFS are industrial by-products synthesized during the production of non-ferrous metals such as copper (Cu), lead (Pb), Zinc (Zn) and others. There is a large production of NFS and usually, these slags are stockpiled or used in low-value applications. NFS can find a high-value application as a sustainable binder for concrete. In this research, Modified Ferro Silicate (MFS) slag, one kind of cleaned NFS is used as a PC replacement. These MFS slags can be used as a PC replacement, whereby in this work three different binder systems have been applied: binder systems with MFS as supplementary cementitious material (SCM), hybrid binder systems and quaternary binder systems. In the SCM binder series, for instance, MFS slags are added together with PC in a mortar or concrete mix, and the amorphous slag phase reacts with the alkaline pore solution (enriched in calcium hydroxide CH, resulting from the PC hydration) precipitating hydrates of calcium silicates (C-S-H). These MFS slags can be used as SCM without modification and can satisfy the requirements of demanding construction applications. However, these binders are slow in reactivity and do not allow for high PC replacement levels. In the second set of series, hybrid systems are a solution that can provide higher strengths at an early age, and also at higher PC replacement levels. In such a case, the MFS slag is mixed with a small proportion of PC and an additional alkali-activator is applied, allowing for the production of concrete with a similar performance to more conventional concrete types. By adding activators in combination with PC the slag hydration is more complete via a complex mechanism, which involves increased pH from the equilibration of portlandite in the presence of reduced Ca2+ activity. Reduced Ca2+ activity appeared to promote slag dissolution by increasing the under-saturation of the slag. The third set of the series is a novel quaternary system where MFS slag is mixed with PC, as well as with additives. In this thesis, the combined usage of BFS and dolomite as additives was tested. By adding dolomite and BFS into the system, the synergetic effect of latent hydraulic and dedolomitization tend to increase the hydration and reactivity of the MFS slag. Since all these binders contain also a large concentration of Fe in the system, the role of Fe is also a vital factor in influencing its reactivity. XRD analysis of the MFS slag model paste indicated the participation of Fe in the hydration mechanism, as well as the formation of Fe-AFm, besides other hydrated phases in the SCM binder system. Second, related to the extended service life of structures, good durability performance is required for eco-efficient concrete to prevent the early need for intervention or replacement of structures, hence extended service life. Amongst other, important durability properties of concrete are related to carbonation, frost scaling and chloride ingress. Usually, the durability properties such as frost scaling and carbonation of the PC replacement binders might be less good, due to their different pore structure and CO2 buffer capacity and also possible coupling of the deterioration mechanisms. Thus, as an important scope of the thesis, for the three different binder systems (SCM, hybrid and quaternary binder) concrete compositions were designed and further tested, including durability. The durability properties such as carbonation resistance, chloride resistance and frost scaling of the concrete were thoroughly assessed. All series of concrete showed positive chloride binding capacity, this is likely due to the binding of Cl ions in calcium alumina silicate hydrates (C-A-S-H), ettringite and Fe-AFm phases. However, SCM and hybrid concrete showed an increased carbonation rate compared to quaternary binder concrete. This positive result of the quaternary system is believed to be due to the synergetic effect of BFS and dolomite. As expected, all series also showed poor frost scaling resistance, when no air entraining agent was applied. Third, depletion of non-renewable resources is also a growing environmental concern globally. Aggregates are a major ingredient of concrete. Depending on its geographic location, the construction industry mainly uses non-renewable sources, such as marine sand, dredged gravel or crushed rocks, often granite or limestone, as aggregates. By using recycled aggregates as a primary source for the inert skeleton in concrete, a contribution can be made to the development of a circular economy. Promoting the recycling of waste concrete into high-value applications is key to developing such a circular economy. However, the attached mortar in recycled aggregate particles is a specific feature that limits structural applications. The water absorption and porosity of the recycled aggregates are higher than for most natural aggregates. The durability performance of recycled aggregate concrete can be affected unless transport properties are controlled by embedding the recycled aggregate in a compact matrix that isolates their pore structure. In this scope, novel hybrid binder concrete with MFS slag (as partial replacement of PC, rather or not in combination with BFS) and alkali activator was successfully produced containing 50% recycled aggregates. The presence of recycled aggregates had an adverse effect on early age strength whereas after 91-day no difference could be observed between concrete with or without recycled aggregates. A positive effect on chloride binding capacity could be observed in the BFS/MFS slag system with recycled aggregates. However, BFS/MFS slag concrete with recycled aggregates showed an increased carbonation rate and frost scaling compared to the system with virgin aggregates. Durability properties such as sorptivity and water penetration were positively affected by a longer curing time for the BFS/MFS system. Finally, the usage of the MFS slag for the production of large-scale elements under realistic conditions was investigated and proven to be successful. The PC replacement by binders such as BFS and MFS slag could be easily introduced in the silos of the concrete plants without the need for significant changes in the mixing protocol. Thus, large-scale reinforced concrete (RC) slabs applying MFS slag-based concrete were manufactured to test their flexural performance. Flexural behaviour was investigated in terms of deformations under increasing load, crack patterns, load bearing capacity and failure aspect

Valorising an industrial residue towards a calcium sulfoaluminate cement

Calcium sulfoaluminate (CSA) cements and the formation of the ye’elimite (C4A3$) phase was first synthesised and studied by Ragozina1 in 1957, and its composition was identified as 3CaO·3Al2O3·CaSO4 by Fukuda2 in 1961. CSA cements can be a feasible alternative binder with major benefits such as quick setting, fast strength development, and less shrinkage, while achieving lower CO2 emissions during production.3 CSA cements consists predominantly of C4A3$, which react with H2O in the presence of anhydrite (CaSO4) to form a binder phase of monosulfate (AFm), ettringite (AFt) and aluminium hydroxide (AH3).4 The sulphate-bearing phase C4A3$can be synthesised in the temperature range between 1200 and 1450°C depending upon the initial chemical composition used.5 However, Khessaimi et al.6 stated that optimal solid state synthesis condition for C4A3$ was at 1300°C for 3 h. In general, any oxide source of Al, Ca and S and industrial by-products, such as fly ash, lime kiln dust and scrubber sludge might be a potential candidate for the synthesis of CSA cements. Sahu et al.7 showed that fly ash can be used to produce belite – CSA–based cements. Investigations were carried out by firing different kinds of fly ash (class C and F) along with limestone and gypsum at 1200°C. In this paper a rather unexplored industrial residue generated as a slurry during the acid leaching of Zn metal (Jarosite) is studied. Due to the presence of the high sulphur content, this residue might be a suitable candidate in making CSA cements. The primary objective is to produce therefore a CSA clinker from these industrial residues, by following the necessary thermal treatment in order to transform the unreactive industrial residue into a CSA cement. The reactivity of the clinker was assessed by thermogravimetric analysis (TGA) and heat flow calorimetry. The compressive strength of the 100% CSA mortar was also evaluated

Influence of SiO2 on the pozzolanic activity of vitrified bauxite residue slags

When viewed from a sustainability and circular economy perspective, bauxite residue (BR) would be an interesting supplementary cementitious material (SCM) in view of the high volumes generated. The reactivity of as-is BR is relatively poor, however. This study tried to increase its reactivity by transforming BR into a pozzolanic material by a chemical and thermal modification. In this respect, three different BR slags with varying wt% of SiO2 (10, 25, 30 wt%) and minor C additions were designed and fired at a maximum temperature of 1200 °C for 1 h in an inert atmosphere. After melting, the slags were water-quenched to obtain BR slags with a partially glassy structure. With increasing SiO2, an increasing glass content was observed with a maximum of 83 wt% glass phase for the BR slag with 30 wt% SiO2. EDX point analysis showed that the glass that has formed was rich in Si, Fe and Al. The pozzolanic reactivity of the BR slags were assessed by a standardised test (Frattini test, EN 196:5) in which the calcium hydroxide consumption is assessed and benchmarked with fly ash (FA). Results of the Frattini test showed that an addition of SiO2 content in the altered BR slags increased the pozzolanic reactivity

VALORISING AN INDUSTRIAL RESIDUE TOWARDS A CALCIUM SULFOALUMINATE CEMENT

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Reactivity of modified iron silicate slag as sustainable alternative binder

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Feasibility study on the use of modified copper slag as a sustainable fine aggregate in mortar

Depending on the availability of aggregate sources pertaining to their geographic locations, the concrete industry utilizes conventional aggregates such as marine sand, dredged gravel, or crushed rocks. This method requires high energy and high processing costs for washing and grinding. The objective of this work is to use Modified Ferro silicate slag (MFS), a by-product obtained from the copper industry, as an alternative to the conventional fine aggregates found in mortar. No additional processing such as washing or grinding is required. By using the MFS slag as an aggregate in mortar or concrete, the factors of sustainability and a circular economy are enhanced. The current study focuses on the characterization of the MFS slag, including the mortar mixes with the MFS slag as a fine aggregate, and shows that the MFS slag can be a promising raw material to replace conventional aggregates in mortar. The leaching of its heavy elements such as Sb, As, Cr, Mo, Pb, and Zn was conducted well within limits (VLAREMA 4). The SEM and MIP analyses indicated that the porosity of the MFS slag mortar was higher compared to the standard aggregate mortar. Moreover, the MFS slag mortar showed acceptable resistance toward the alkali-silica reaction and carbonation

Freeze thaw resistance of non-ferrous slag concrete

The objective of this work is to study the freeze thaw resistance of supplementary cementitious materials (SCM) based concrete made from nonferrous slag (NFS) benchmarked with CEMI 52.5 N and CEM III 42.5 B concrete. NFS is synthesized during the production of Cu metal from Cu scraps. The freeze thaw resistance of NFS concrete containing 70% CEMI 52.5 R and 30% NFS (w/b = 0.45) as binder, as well as of CEM I 52.5 N and CEM III 42.5 B concrete was tested following CEN TR 15177 (2006). The analysis was based on a calculation of the relative dynamic elastic modulus determined by ultrasonic measurements and a determination of the water absorption by mass in function of the number of freeze thaw cycles. Furthermore the relative tensile strength loss after 56 cycles was considered and a microstructural analysis was performed. All concrete mixes showed a relative tensile strength after 56 freeze thaw cycles lower than 100% of the initial value, whereas the CEM III 42.5 B concrete showed the highest strength loss of around 15% followed by 11% for NFS concrete. NFS concrete also showed highest water uptake of around 4% whereas CEM I 52.5 N and CEM III 42.5 B concrete showed values of 1.2% and 2.2% respectively