Particle size distribution and suspension stability in aqueous submicron grinding of CaCO₃ and TiO₂

Abstract During the past decade submicron and nanoparticles have aroused a wide interest and gained new applications due to their high surface area and strength. Grinding with a wet stirred media mill is usually the last process step before the submicron or nanoparticles are added to an application, and the step where the final particle size distribution is achieved. Since stirred media milling is an energy-intensive process, energy efficiency should be optimized. This can be done by determining the optimum operational parameters for the mill and using the highest possible solids concentration. The solids concentration can be increased by controlling particle-particle interactions with stabilization chemicals, e.g. polymers. This thesis concerns parameters and grinding aids affecting the particle size distribution and suspension stability of the aqueous submicron grinding of calcium carbonate (CaCO3) and titanium dioxide (TiO2) in stirred media mills. TiO2 particles are aggregates produced via a bottom-up method, while CaCO3 are primary mineral particles produced by a top-down method. The most energy efficient grinding of TiO2 to a 300 nm particle size with the narrowest possible particle size distribution was obtained with the lowest stress energy, implying the smallest grinding medium size. It was observed that electrosteric stabilization with sodium polyacrylates was effective for TiO2, and sodium polyacrylate with a molecular weight of 12500 g/mol was found to be the most effective for reducing the viscosity of the suspension. As with TiO2, electrosteric stabilization with sodium polyacrylates was also found to be effective for CaCO3, but in this case sodium polyacrylate with a lower polydispersity index was more effective, showing a better stabilization potential in micron and submicron grinding and reducing the viscosity and particle size to a greater extent. Nanogrinding experiments were performed for a CaCO3 suspension with low PDI sodium polyacrylate and it was found to be possible to obtain a particle size of 26 nm, smaller than any size previously reported when grinding CaCO3.Tiivistelmä Viimeisen kymmenen vuoden aikana alle yhden mikrometrin partikkelit ovat herättäneet kiinnostusta ja niille on kehitetty uusia sovelluksia niiden suuren pinta-alan ja lujuuden ansiosta. Ultrahienojauhatus märkähelmimyllyllä on useimmiten viimeinen prosessivaihe ennen partikkelien lisäämistä sovelluskohteeseen ja siinä saavutetaan partikkelien lopullinen partikkelikokojakauma. Helmimyllyjauhatuksen energiankulutus minimoidaan etsimällä optimioperointiparametrit kullekin jauhatusprosessille ja käyttämällä korkeinta mahdollista suspension kuiva-ainepitoisuutta. Suspension kuiva-ainepitoisuutta voidaan nostaa hallitsemalla partikkelien välisiä vuorovaikutuksia stabilointiaineilla, kuten polymeereillä. Tässä väitöskirjassa tutkittiin operointiparametrien ja jauhatusapuaineiden vaikutusta titaanidioksidin (TiO2) ja kalsiumkarbonaatin (CaCO3) partikkelikokojakaumaan ja lietteen stabiilisuuteen submikronijauhatuksessa. Tutkitut TiO2-partikkelit olivat aggregaatteja, jotka oli valmistettu sulfaattiprosessilla saostamalla, ja tutkitut CaCO3-partikkelit olivat primäärisiä mineraalipartikkeleita. TiO2-partikkeleille saavutettiin energiatehokkain jauhatus ja samalla toivottu partikkelikokojakauma, eli mediaani 300 nm ja mahdollisimman kapea jakauma, pienillä helmillä, jotka aiheuttavat partikkeleihin pienimmän puristusenergian. Elektrosteerinen stabilointi käyttämällä natriumpolyakrylaatteja stabilointiaineena havaittiin tehokkaaksi menetelmäksi hallita TiO2-partikkelien välisiä vuorovaikutuksia. Natriumpolyakrylaatti, jonka molekyylimassa oli 12500 g/mol, oli tehokkain TiO2-partikkeleille alentaen suspension viskositeettiä eniten. Myös CaCO3-partikkeleille elektrosteerinen stabilointi natriumpolyakrylaatteja käyttäen oli tehokkain stabilointimenetelmä. Myös natriumpolyakrylaattien polydispersiteetti-indeksin vaikutusta tutkittiin CaCO3-suspensioille. Tulokset osoittivat matalan polydispersiteetti-indeksin olevan tehokkaampi alentaen viskositteettia ja pienentäen partikkelikokoa tehokkaammin kuin natriumpolyakrylaatti, jolla oli korkeampi polydispersitetti-indeksi. Tämän vuoksi natriumpolyakrylaatti, jolla oli matala polydispersiteetti-indeksi, valittiin nanojauhatuskokeisiin. Kokeissa CaCO3-partikkelit saatiin jauhettua 26 nm kokoon, joka on pienin koskaan aiemmin jauhamalla saavutettu koko CaCO3-partikkeleille

One-part alkali-activated blast furnace slag for sustainable construction at subzero temperatures

Abstract The construction season is limited in northern countries due to the severe cold weather conditions and their detrimental impacts on concrete quality. Thus, there are excessive expenses required annually for insulation and energy-intensive heating systems for cold-weather concreting. This experimental study aimed to investigate the potential of using high-strength one-part alkali-activated blast furnace slag (AAS) in cold weather without the need for supplementary heating systems. Therefore, the impacts of different subzero curing temperatures on the hardened properties of one-part AAS mortar in comparison with cement mortar were assessed. After casting, mortars were immediately cured at a temperature of 23, −5, −10, and −20 °C, up to 56d. The results showed that the lower the curing temperature, the lower the UPV and compressive strength of cement and one-part AAS mortar; however when the curing period was fixed, one-part AAS mortar registered higher UPV and compressive strength than cement mortar at all curing temperatures. Owing to additional room temperature curing, the hardened properties of AAS mortar were significantly improved. The findings were further supported by microstructural and thermogravimetric analyses

Optimizing activating solution and environmental leaching characteristics of Fe-rich alkali-activated Zn slag

Abstract In this work, slag from Zn processing was used to produce Fe-rich alkali-activated materials (AAMs) with low environmental impact. The interconnection between activating solution composition, compressive strength, and environmental leaching characteristics was assessed. The reaction products characterised with FT-IR, XRD, and SEM-WDS were represented by Fe-rich C-S-H gel of tobermorite-related structure. The local aggregation of Na and Mg suggests the minor role of these elements in the reaction product. The reaction product seems to be undependable on the alkali cation used in the solution. Besides, the hardening reaction took place fast, and the maximum compressive strength of 70 MPa was determined only after 1 day after mixing with silicate solutions. To decrease the economic and environmental impact, 1) simultaneous decreasing Na₂O/slag and SiO₂/slag ratios or 2) decreasing SiO₂/Na₂O ratio can be applied without prominent deterioration of the strength. Environmental leaching results showed an increase in the leached content of several metal(loid)s (e.g., As, Mo, Cr, Sb, Se, V) as a results of alkali activation, but also some immobilization effect for Ba, Pb and Zn. Also, the presence of liquid silica in the activating solutions or higher water content reduced the leaching of some elements

Improved strength development and frost resistance of Portland cement ground-granulated blast furnace slag binary binder cured at 0 °C with the addition of calcium silicate hydrate seeds

Abstract Low ambient temperatures drastically decelerate the strength development of cementitious materials, which shortens the construction season in cold regions. The use of ground-granulated blast furnace slag (GGBFS) in concreting works is usually avoided in winter because of its slower hydration rate relative to Portland cement (PC). In this study, the impacts of calcium silicate hydrate (C-S-H) seeds on the strength development and reaction rate of PC/GGBFS binders cured at 0 °C were investigated. The results showed that the addition of C-S-H seeds can efficiently compensate for the strength loss caused by replacing PC with GGBFS, and this effect is more obvious at a lower GGBFS content (30%) than at a high content (50%). Better frost resistance was gained in the seeded binder containing 30% GGBFS than in the pure PC binder. The enhanced compressive strength and frost resistance in the seeded binary binder were attributed to the accelerated PC reaction rate, enhanced pozzolanic reaction rate and degree of GGBFS, and increased amount of pore-filling hydration products due to the nucleation effect of the C-S-H seeds

Effect of gypsum content on CSAB cement-based immobilization of Se and SO₄ from industrial filter sludge and sodium–selenium salts

Abstract Release of different heavy metals from various industries is a significant environmental hazard worldwide. In order to reduce the threat from these chemicals, different treatment methods are needed to make them harmless. This study approaches the problem by examining the effect of gypsum content for immobilization of selenium oxyanions selenite and selenate, and sulfate, with calcium sulfoaluminate belite (CSAB) cement-based ettringite binder systems. The study has two experimental sections. In the first section, an industrial filter sludge with high concentrations of selenium and sulfate is immobilized with CSAB and varying amounts of gypsum. The immobilization is efficient and reducing the gypsum amount in the hydrating mixture enhances the incorporation of both selenium and sulfate. The binding occurs through encapsulation and incorporation into ettringite and other hydrated phases. In the second section, two reagent grade sodium–selenium salts, sodium selenite and sodium selenate, are mixed with CSAB and varying amounts of gypsum. The immobilization is not effective and ettringite is not formed in large quantities, most likely due to high sodium content in the reacting system

Production of artificial aggregates by granulation and carbonation of recycled concrete fines

Abstract There is a growing need to find ways to reuse fine concrete waste from the construction industry. In this study, recycled concrete fines were granulated and used as lightweight aggregates. Ladle slag, a steel industry residue, was used as a co-binder in different ratios (0, 10, 20, and 30%). The materials were blended and granulated, and then the granules were cured in three conditions: ambient condition, humidity chamber, and carbonation chamber. The results showed that the ladle slag content of 30% cured in a humidity chamber produced the strongest granules, with a crushing strength of 127 N, which was 135% greater than a commercial lightweight aggregate. The granules generally had satisfactory density and water absorption with a higher ladle slag content. Carbonation increased the granule strength with a low ladle slag content and decreased the granules’ water absorption. The improved physical and mechanical properties of carbonated granules are attributed to the formation of calcium carbonate during the carbonation process. The granules produced in this study show good potential for use as lightweight aggregates in the construction industry

Direct carbonation of peat-wood fly ash for carbon capture and utilization in construction application

Abstract Carbon dioxide (CO₂) emissions from industrial processes contribute largely to the greenhouse effect and climate change. One of these industries is the cement industry, which contributes around 8% of CO₂ emissions caused by mankind. Two promising and interesting ways to reduce CO₂ emission are the utilization of alternative cementitious materials and carbon capture and utilization through CO₂ mineralization. In this study, peat-wood fly ashes from fluidized bed combustion were used as a construction material for mineral carbonation. A self-hardening characteristic of this type of fly ash was utilized, and simultaneous carbonation and hydration reactions were studied. The study showed that fly ashes from the fluidized bed combustion of peat and wood could be used to capture and mineralize CO₂ during hydration reactions. At the same time, CO₂ could improve the strength of self-hardened fly ashes. One interesting future possibility is fly ash tile production at energy plants: fly ashes can be used to capture CO₂ from flue gases, thus improving the strength of produced tiles

Hazardous industrial filter sludge immobilization with mayenite and gypsum

Abstract Heavy metals escaping from industrial processes and industrial waste are a significant environmental hazard all around the globe. Since they cannot be destroyed, the only way to manage the pollution is to capture and contain the hazardous components. This study focuses on stabilization-solidification of a highly toxic industrial filter slag that contains a range of different heavy metals in extremely high quantities. The stabilization-solidification treatment was executed with a mayenite and gypsum based solid cementitious binder. The major components that were monitored during the study were selenium, lead and sulphate. Different ratios of gypsum addition were tested in order to observe the effect of total sulphate content in the system to the immobilization efficiency of anionic selenium and sulphate. During the experiments, all cationic heavy metals were immobilized efficiently, most of them achieving the immobilization efficiencies of more than 99.9%. By adjusting the sulphate content of the system, the immobilization efficiency of selenium was improved from 95.669% to 99.925% and the immobilization efficiency of sulphate raised from 96.069% to above 99.964%. Controversially, the immobilization of lead was at its highest (99.999%) with high sulphate content and dropped to 98.162% as the sulphate rate decreased. The results show clearly that mayenite has good potential for stabilization-solidification applications

Applicability of fly ash from fluidized bed combustion of peat, wood, or wastes to concrete

Abstract The chemical and physical characteristics of five different fly ashes originating from fluidized bed combustion of peat, wood, or different wastes were investigated to determine whether they fulfilled the requirements for concrete set by the European EN 450-1 standard. Fly ash originating mostly from peat combustion fulfilled all the requirements of the EN 450-1 standard, without any treatments. Some chemical and physical characteristics (i.e., free calcium oxide, sulfate, chloride, and fineness) of the other types of fly ash exceeded the limits in standard, and the sum of the main components (SiO₂, Al₂O₃, and Fe₂O₃) failed to comply with the standard. However, it should be noted that the requirements of the European standard are more restrictive than similar standards in the US; all applications are not needing standardized concrete, and the potential utility of the studied fly ash materials could be improved by mechanical treatments

Enhancing the hardened properties of blended cement paste cured at 0 °C by using alkali-treated ground granulated blast furnace slag

Abstract The use of high-volume ground granulated blast furnace slag (GGBFS) in cement-based materials significantly reduces CO2 emissions. Nevertheless, low curing temperatures are barriers to using environmentally friendly materials in winter construction works. This is mainly attributed to slow GGBFS’s reactivity and blends’ strength development due to the low alkalinity offered by the slow hydration rate of Portland cement (PC) at low temperatures. In this study, sodium hydroxide was employed to produce dry reactive pre-alkali-activated GGBFS (A-GGBFS), with an intention to increase the system’s alkalinity and reactivity. The blended cement paste was prepared with 50% PC replacement with untreated and treated GGBFS, and cured constantly at 0 °C for 28 days. The A-GGBFS accelerated the hydration rate and enhanced the precipitation of hydration products. By adding an optimal NaOH content during the pre-alkali-activation process, the 3 and 28 days compressive strengths of paste increased by 41% and 37%, respectively, gaining a comparable 28 days compressive strength to that measured in a 100% PC-based binder. The microstructural assessments are consistent with compressive strength measurements