Analytical investigation of hydration mechanism of a non-Portland binder with waste paper sludge ash

The development and production of new materials requires advanced analytical characterisation to explain the relation between the physico-chemical structure of the material and its properties. Highly integrated microelectronic structure analysis of surfaces with laser beams and X-ray fluorescence aided devices are found to be helpful for providing important information, including the interrelationships between physical, chemical, mechanical and durability characteristics of the new developed products. In most instances no single technique provides all the needed information and hence simultaneous application of several techniques becomes necessary. This study was aimed for hydration analysis, characterization and evaluation of a new novel non-Portland binder (NPB) with waste paper sludge ash (PSA) using FTIR and TG/DTA. The progressive formation of hydration products within the non-Portland binder was identified and their microstructural characteristics were analysed. The stable and non-expansive nature of secondary ettringite formation was also identified after a period of 365 days curing

Active Sulpho-belite cements. Hydration mechanisms and mechanical properties

The threat of climate change is considered one of the major environmental challenges for our society, where carbon dioxide (CO2) is one of the main Greenhouse gases (GHGs). Every ton of ordinary Portland cement (OPC) produces about one ton of CO2. The design of new formulations of cements is advisable to provide a solution. One alternative consists on Belite Calcium Sulpho-Aluminate (BCSA) cement, that can release ~0.22 tons of CO2/ton of clinker less than OPC. The most common formulation of BCSA clinkers consists on beta-C2S, orthorrombic-C4A3S and C4AF. Due to the presence of the latter, these cements are usually called iron-rich-BCSA (BCSAF). These cements are less limestone demanding and need less clinkering temperature, but compromise the early-age strength development because beta-C2S reacts slowly. This problem may be overcome by the activation of belite and the presence of high amounts of C4A3S. Although BCSAF are promising alternatives, before implementation in Europe, all the steps evolved in the process need to be under control [clinkering (activation/composition; temperature), hydration (rheological behaviour; phase assemblage), and final performances (mechanical strength; dimensional stability)]. This Thesis is focused on the study and optimisation of those parameters to improve the final performances of BCSAF mortars. One of the main objectives was to perform the "medium-scale" synthesis (2kg) of two BCSAF clinkers in our laboratory (50wt% C2S, 30wt% C4A3S, 20wt% C4AF). One of the clinkers was “activated” by adding borax. The aim of the activation has been obtaining clinkers with different belite (-C2S/'H-C2S) and ye'elimite (orthorhombic/pseudo-cubic) polymorphs to understand the effect of the polymorphism on the paste hydration mechanism and mechanical performances of the mortars. X-ray diffraction coupled with Rietveld analysis is a suitable methodology to obtain quantitative phase analysis of these materials including the amorphous/sub-cooled and/or non-crystalline phases. The quantification of the amorphous content is performed using two approaches: i) external standard procedure (G-factor method) with reflection geometry; and ii) internal standard procedure (ZnO) with transmission geometry. Other objective of this Thesis was to understand the influence of calcium sulphate source (type and amount) on the hydration of BCSAF-cements. BCSAF clinkers were mixed with different types and amounts of calcium sulphate sources (gypsum, anhydrite, bassanite) and prepared at a w/c=0.55. Two studies were carried out to better understand the hydration behaviour: i) an in-situ synchrotron X-ray powder diffraction (SXRPD) study for the first hours of hydration at ALBA synchrotron (Barcelona); and ii) ex-situ studies at later ages of hydration by laboratory X-ray powder diffraction (LXRPD). The in-situ study showed important differences in the hydration process. In non-active-BCSAF-cement, gypsum and ye'elimite dissolves (completely) earlier than in active one, and then, the AFt content was higher (after 1h). Moreover, under our experimental conditions, β-C2S reacts faster than α'H-C2S to yield stratlingite, and this behaviour may well be justified with the formation of high amounts of ettringite at early hours which implies a concomitant large quantity of amorphous aluminium hydroxide. The availability of amorphous-AH3 promotes the precipitation of stratlingite, from belite reaction. Then, the hydration behaviour of C2S is more dependent on the chemical environment than on its polymorphism. At late ages of hydration (>24h), the same behaviour was found: β-belite reacts at a higher pace than α′H-belite. Ye'elimite reaction kinetics showed a small dependence on the amount of added gypsum. Finally, the hydration of C4AF was strongly retarded by increasing the gypsum content in both (active and non-active) cements. In all cases the main crystalline hydrated compounds were ettringite, stratlingite and katoite. The amount of crystallised ettringite in active-cements resulted higher than that in non-active-cements, irrespective of gypsum content. The in-situ SXRPD study of BCSAF cements with different calcium sulphate sources showed that the dissolution kinetic of anhydrite is much slower than that for gypsum or bassanite, and as a consequence the precipitation of ettringite is the lowest. Moreover, the reactivity of ye'elimite with water to form AFm as main hydrated phase has not taken place. At late ages of hydration (>24h), the sulphate source was always consumed before 3 days of hydration to form ettringite (main crystalline hydrated phase), and variable amounts of AFm and stratlingite. Independently of the sulphate source, ettringite seems to be more stable in active cements, as it is almost constant with time of hydration. At latter ages, the analysis of the data indicates that the phase assemblage is slightly sensitive to the initial sulphate source. Since our objective is to study the effect of the calcium sulphate source (including compressive strengths of the corresponding mortars) similar rheological behaviour at very early hydration ages are desired. In this case a small amount of a commercial polycarboxylate-based superplasticizer was added to water to prepare bassanite-containing pastes. They exhibited a considerable diminishing in viscosity and similar rheological behaviour to those prepared with gypsum or anhydrite. Mechanical properties of standard mortars were prepared with a cement/sand/water ratio of 1/3/0.55. The most important result is that all mortars prepared with the active-BCSAF cement developed higher compressive strengths than non-active mortars, independently of the type and amount of sulphate source. Within the non-active mortars, anhydrite-mortar presented the highest value, which may be explained/justified by the higher BET area of the particles and the slightly higher stability of AFt when compared to the gypsum-mortar. For bassanite-mortar, although the addition of a small amount of SP improved the workability, the delay in the setting time was not enough to develop comparable mechanical strength values to other mortars. Within the active-mortars, at 120 days, gypsum-mortar developed the highest mechanical strength value (68±1 MPa), even when the amount of ettringite in other pastes was slightly larger. Therefore, we are forced to conclude that the amorphous contents are playing a key role in the strength development at late ages. Moreover, the active gypsum-cement has the highest BET area value and the pastes shows the lowest porosity values (10%) at that age (120 days); this behaviour also helps to justify the measured mechanical strengths

Processing and characterisation of calcium sulphoaluminate (CSA) eco-cements with tailored performances

Climate change mitigation usually involves the reduction of greenhouse gases emissions, such as carbon dioxide (CO2). Every tonne of Ordinary Portland Cement (OPC) produces about one tonne of CO2. Consequently, OPC accounts for 5-6% of anthropogenic CO2 emissions and for 4% of total global warming. Due to these environmental problems the industry of building materials is under increasing pressure to reduce the energy used in the production of OPC and the greenhouse gas emissions. Hence, there is a growing interest in developing alternatives to OPC, such as, calcium sulphoaluminate cements (CSA), that will release 0.25-0.40 tons of CO2 less than OPC per tonne of clinker, depending on their composition. Although CSA cements are very promising, their use is strongly limited in Europe. At the present state of European standard, CSA cements cannot be used in structural concrete according to the EN 206-1; only three formulations ,produced by Buzzi Unicemin Trino (Italy), obtained in 2013 a CE mark, also allowing the use for structural application. Hence, the general aim of this PhD Thesis is to better understand the behaviour of these eco-cement during their hydration. Moreover, I am a member of a working group which has a large experience in anhydrous cements and clinkers characterisation by X-Ray powder diffraction combined with Rietveld methodology, and in the processing of materials. So, I would like to highlight my contribution in the processing and characterisation of hydrated CSA pastes, including the quantification of ACn (Amorphous and Crystalline not-quantified) content through laboratory X-ray powder diffraction (LXRPD), and the measurement of mechanical properties of the corresponding mortars. Moreover, another objective was to establish a methodology to prepare, store and stop the hydration of cement pastes and mortars. Although anhydrous cements are mainly crystalline materials, they may contain a non-negligible amount of ACn; in addition part of the hydration products can be amorphous. This is the reason why the quantification of the ACn is an important issue. In previous studies, we used the internal standard methodology, measuring the sample in reflection geometry, to calculate the ACn content with quite accurate results. However, the addition of an internal standard may alter the cement hydration, dilute the phases in the pastes, and produce microabsorption problems. Hence, both external standard in reflection geometry and internal standard in transmission geometry methodologies have been compared. With that study, we can conclude that the ACn content in a mixture of powders can be calculated using both, internal and external standard, where the latter is of even greater utility in the study of hydration of cement pastes. That methodology has the inherent benefit of using common experimental requirements of LXRPD (knowing the diffractometer constant) and moreover, the sample is not altered/diluted. Moreover, the internal standard method is useful to corroborate the obtained values. However, the transmission approach is not very suitable for following ACn evolution in a process because it is experimentally tedious. Once we trust the methodology to control the hydration phases including ACn content, the effect of the following parameters on the hydration of CSA pastes and mortars was studied: gypsum content, sulfate source, water/cement (w/c) ratio, superplasticizer and the possible addition of fly ash (FA). The optimum w/c ratio must be high enough to have a high degree hydration of cement phases, and to provide workability, but in turn, it should be low enough to yield good mechanical strengths. The use of the optimum type and amount of additives improves the workability of cement pastes, allowing the use of lower w/c ratios and, consequently, higher compressive strength values. The optimised amount of gypsum to prepare CSA cements has been determined to be close to 25wt%. Therefore, we were able to correlate the phase assemblage of CSA pastes with compressive strength values of corresponding mortars. Another aim was the cost reduction of CSA cement and the study of the possible pozzolanic effect when the cement is partially replaced by FA. Although the hydration did not show enough evidences of pozzolanic effect even after 6 months of study, we found that the partial substitution produces: i) filling and ii) diluting effects. And we concluded that the partial substitution of cement (15wt% of FA) involves economic and environmental benefits. Moreover, the phase assemblage evolution is the same in the presence or not of FA, suggesting that durability is not compromised. Furthermore, the construction industry often requires cements with tailored properties. The knowledge and control of CSA hydration and properties of their mortars make possible the production of tailored cements and in the future, to comply all European standards. Through the use of different kinds of sulphate sources, w/c ratio and additives, the rheological behaviour and setting time of cement pastes have been controlled and modified while high compressive strength values were achieved. Each sulphate source (gypsum, bassanite or anhydrite) provides mortars with different setting times, as a consequence of their different rate of solubility. Hence, the use of different sulphate sources is a key point to control the rate of cement hydration. Bassanite solubility in water is very high thus, when added to cements/mortars it yields to short setting times (20min) that produces heterogeneous mortars with low mechanical strengths. However, the use of the optimum amount and type of additive, delays the setting time and allows the preparation of mortars with compressive strength values up to 80MPa at 7 days of hydration. Anhydrite dissolves slowly so, at 1 hydration-day, the amount of ettringite formed (~20wt%) is lower than that in gypsum pastes (~26wt%), producing mortars with lower compressive strengths (40.7 and 21.2MPa, respectively). However, after 3 hydration-days, similar ettringite contents are produced but mechanical strengths of anhydrite-pastes are slightly higher. This behaviour is mainly due to a higher plasticity of anhydrite-paste. Moreover, it must be noted that mortars with gypsum-w/c0.50 or anhydrite-w/c0.50 gave compressive strengths higher than OPC mortars at early ages. At this point, we are able to obtain tailored CSA eco-cements and mortars for diverse applications through the control of their hydration and also with the consequent economic and environmental benefits

Evaluation of the leachate chemistry and contaminants attenuation in acid mine drainage by fly ash and its derivatives

Philosophiae Doctor - PhDThe mining industry in South Africa has a huge potential to impact negatively on the environment. Negative impacts include generation of reactive tailings and acid mine drainage (AMD). AMD is highly acidic (pH 2-4), sulphate-rich and frequently carries a heavy metal burden. South Africa uses more than 100 million tonnes of low grade bituminous coal annually to produce cheap electricity. The associated mining operations result in millions of tonnes of polluted water and in turn coal burning power stations produce vast amounts of waste ash such as fly ash. The highly soluble CaO occurring as sub-micron fragments on the fly ash particles is highly reactive and can be utilized in the neutralization of acid mine drainage. Acid mine drainage (AMD) was reacted with two different South African fly ashes in a batch setup in an attempt to evaluate their neutralization and inorganic contaminants removal capacity. The concentrations of major constituents in the AMD were found to determine the final pH attained in the reaction mixture and the reaction time of breakthrough to circum-neutral and alkaline pH. Efficiency of elemental removal in the AMD by the FA was directly linked to the amount of FA in the reaction mixture and to the final pH attained. Most elements attained ≈ 100 % removal only when the pH of minimum solubility of their hydroxides was achieved. In the second part of the study, Acid mine drainage (AMD) was reacted with coal fly ash in a 24 hour equilibration time using 1:3 and 1:1.5 FA: AMD ratios by weight to produce neutral and alkaline process waters. The capacity of the fly ash to remove the major inorganic contaminants from AMD was examined with time. The geochemical computer software PHREEQC and WATEQ4 database were used for geochemical modeling of the process water chemistry at selected reaction times. The collected solid residues were analyzed by X-ray diffraction, scanning electron microscopy (SEM) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX). At both ratios the reaction mixture was at saturation or oversaturated with alunite, basaluminite, jurbanite, boehmite, gibbsite, diaspore, gypsum, barite, K, Na-jarosites, ettringite, amorphous Fe (OH)3 and goethite at specific contact times. The precipitation of the many inorganic contaminants was established in terms of the mineral phases at saturation or over-saturation. Sequential extraction revealed the amorphous fraction to be the most important in retention of the major and minor inorganic contaminants at pH > 6.32 which implies that the concentration of total Fe and Al in the AMD being treated has a direct effect on the clean-up efficiency of the process. In the third part of the study, a column leaching of the solid residues (SR) blended with varying amounts of fly ash (5 %, 25 %, 40 %) and 6 % Ordinary Portland Cement (OPC) was carried out to assess the contaminant attenuation with time. The columns were drained with synthetic acid mine drainage (SAMD) over a period of 165 days. In addition the solid residues were modified with 1-6% OPC and their strength development monitored over a period of 365 days. The column solid cores were observed to acidify in a stepwise fashion, exhibiting three buffer zones. The SR alone and SR blended with fly ash exhibited strong buffering capacity at pH (7.5-9) for an extended period of time (97-110 days). Encapsulation of solid residue particles by the calcium silicate hydrate gels (CSH) in OPC blended solid residues obscured the appearance of the sustained buffering at pH 7-9.5. The fly ash and OPC blend solid residues exhibited decontamination efficiencies of (82-99 %) for Al, Fe, Mn and SO4 2- over the study period. However the OPC blend SR exhibited high attenuation efficiency even as the pH dropped to below 4. SR + 6 % OPC core was observed to be the most efficient interms of retention of highly mobile elements such as B and Mo. pH was observed to be the main determining factor in contaminants attenuation. Geochemical modeling results revealed that pH and SO42- concentrations in the leachate had a significant impact on the mineral phases controlling Fe and Al concentration in the leachates. In the SR + 6 % OPC solid cores, EDX analysis revealed that CSH gels and calcium aluminate hydrate gels were being precipitated. These gels were either incorporating Fe, Mg, Mn in their matrix or encapsulating the solid residue particles that were rich in these elements. Sequential extractions of the leached solid cores revealed the amorphous fraction to be the most important in retention of the major contaminants and were most enhanced in the OPC blend solid residues. The OPC blend solid residue slurries developed unconfined compressive strength (UCS) (2-3 Mpa) comparable to paste formulated from sulphidic rich mine tailings confirming that the solid residues can be used for backfilling. Therefore the solid residues (SR) can successively be applied for a dual purpose in mined out areas namely, to remediate acid mine drainage waters and also provide support for the overburden. Keywords: Acid Mine Drainage; Fly Ash; Neutralization; Sulphates; Metal ions; Solid Residues (SR); Column Leaching; Geochemical Modeling; Sequential Extraction; Buffering.South Afric

Manual for use of Al-containing residues in low-carbon mineral binders

Our society can no longer be imagined without its modern infrastructure, which is inevitably based on the use of various mineral and metallic materials and requires a high energy consumption. Parallel to the production of materials, as well as the production of electricity, huge amounts of various industrial and mining residues (waste/by-product) are generated and many of them are sent to landfill. The European Union (EU) aims to increase resource efficiency and the supply of ”secondary raw materials“ through recycling [1], inventory of waste from extractive industries [2], and waste prevention, waste re-use and material recycling [3]. Much of the industrial and mining waste is enriched with aluminium (Al) and therefore has a potential to replace natural sources of Al in mineral binders with a high Al demand. However, the use of industrial residue in mineral binders requires an extensive knowledge of its chemical composition, including potential hazardous components (e.g. mercury), mineral composition, organic content, radioactivity and physical properties (moisture content, density, etc.). This manual addresses the legislative aspects, governing the use of secondary raw materials in construction products, description of the most common Al-containing industrial and mining residue (bauxite deposits, red mud, ferrous slag, ash and some other by products from industry), potentiality for their reutilisation and its economic aspects, potential requirements/barriers for the use of secondary raw materials in the cement industry and a description of belite-sulfoaluminate cements, which are a promising solution for implementing the circular economy through the use of large amounts of landfilled Al-rich industrial residue and mining waste cement clinker raw mixture. This manual was prepared by partners of the RIS-ALiCE project. It provides a popular content, which targets relevant stakeholders as well as the wider society. Moreover, it offers education material for undergraduate, master and PhD students.Other links: [http://www.zag.si/dl/manual-alice.pdf

Sustainable iron-rich cements: Raw material sources and binder types

The bulk of the cement industry's environmental burden is from the calcareous source. Calcium is mostly available naturally as limestone (CaCO3), where almost half of the mass is eventually released as CO2 during clinker manufacture. Iron (Fe) is the fourth most common element in the Earth's crust surpassed only by oxygen, silicon, and aluminium; therefore, potential raw materials for alternative cements can contain significant amounts of iron. This review paper discusses in detail the most abundantly available Fe-rich natural resources and industrial by-products and residues, establishing symbiotic supply chains from various sectors. The discussion then focusses on the impact of high iron content in clinker and on ferrite (thermo)chemistry, as well as the importance of iron speciation on its involvement in the reactions as supplementary cementitious material or alkali-activated materials, and the technical quality that can be achieved from sustainable Fe-rich cements

The performance of cement stabilised minestone

This study is concerned with the durability of cement stabilised minestone (CSM). Minestone is dominated by the clay-bearing mudrocks and shales of the Coal Measures. Consequently, engineering problems are often encountered due to the likelihood of these rocks undergoing volume change and degradation when exposed to fluctuations in moisture content. In addition, iron sulphides (chiefly pyrite) are frequently present in minestone as diagenetic minerals which on excavation have the potential to oxidise forming sulphate minerals. The oxidation of sulphides may in itself contribute to volume increase in pyritic rocks and sulphate minerals may combine with the products of cement hydration to produce further expansion. The physical and chemical properties of a wide range of minestones are determined and attempts are made to correlate these with the engineering performance of cement stabilised specimens subjected to short-term immersion in water. Criteria, based on these raw material indices are proposed with a view to eliminating minestones which are unsuitable. A long-term durability study is also described. In this, the geochemical stability of pyrite in CSM was examined together with the role played by the sulphur bearing mineralogy in determining the engineering performance of CSM's exposed to conditions of increased moisture availability. The nature of a number of disrupted CSM pavements which have been examined are also discussed

Controlled Low-Strength Materials Containing Solid Waste from Minerals Bioleaching

Sustainable treatment and disposal of mine waste is a serious environmental issue faced by the mining industry worldwide. Conventional methods of mine waste management predominantly involve indefinite retention in engineered tailings dams. The cost and liability of such surface storage facilities have increased significantly in recent years as an outcome of stringent environmental legislation and mine closure requirements gradually transforming the economics of mine waste disposal. Backfill methods, particularly cemented paste backfill, are increasingly perceived as sustainable, environmentally friendly and cost-effective alternatives as they put waste material to practical use. Controlled low-strength materials (CLSM) offer an effective and practical alternative to similar analogues – requiring minimal compaction, being self-levelling and excavatable in the future if necessary. The aim of this research was to develop and evaluate CLSM, previously un-tested at mines, in which novel utilisation of bioleach waste is maximised and Portland cement content minimised while satisfying performance requirements for classification as CLSM. Leachability of toxic substances was minimised through encapsulating CLSM within a coating of relatively inert CLSM. Formulation and optimisation of CLSM using statistical mixture design and response surface analysis has ensured proper understanding of component interactions and influence on mechanical strength with a minimum amount of experiments. Optimised CLSM formulations were tested for their mechanical, physical, micro-structural, mineralogical and chemical properties. Effects of encapsulation were determined by assessing chemical leaching. The work indicated that bioleach waste could be beneficially reformed as CLSM of appropriate compressive strength for application in groundwork as loadbearing materials. Porosity and hydraulic conductivity were correspondingly high. Leachability of arsenic, barium, chromium, lead and zinc was significant (levels varied depending on waste type). Encapsulation significantly reduced leachability indicating promising potential for implementation of this technology in the mining industry. The research presented in this thesis substantiated the need for, and potential of, sustainable novel alternative technologies such as CLSM to augment future waste management strategies in the mining industry via safe emplacement of solid bioleach waste in the sub-surface