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

In-situ hydration studies of ye’elemite at early ages for understanding eco-cement performances'

Póster; ALBA User Meeting and VI AUSE Conferenc

Rietveld quantitative phase analyses of SRM 2686a: a standard Portland clinker

SRM 2686a is a NIST reference Portland clinker with reported mineralogical analysis from powder diffraction and electron microscopy. This sample is used in ASTM C1365 test method for Rietveld quantitative phase analysis validation procedure. Here, we have analysed SRM 2686a by using three state-of-the-art powder diffraction configurations: i) strictly monochromatic CuKα 1 radiation in flat reflection geometry; ii) strictly monochromatic MoKα 1 radiation in flat transmission geometry; and iii) synchrotron radiation in rotating capillary transmission geometry. The silicate and aluminate enriched residues have also been studied by CuKα 1 powder diffraction. All the powder patterns were analysed by Rietveld method with the best available protocols. The results indicate that belite in SRM 2686a is composed of two polymorphs (- and α’H-) that must be included in the analyses. The use of a unique phase for describing belite (-polymorph) and improper peak shape modelling could explain the problems found for implementing ASTM C1365 in some cement manufacturing plants. Furthermore, all the patterns are deposited as open data access at Zenodo, so interested laboratories can analyse these data to verify their protocols.This work has been supported by Spanish MINECO through BIA2014-57658-C2-2-R, which is co-funded by FEDER, and BIA2014- 57658-C2-1-R. We also thank ALBA synchrotron for providing beam time at BL04-MSPD beamline

Clinkering and early age hydration characterization of Belite-Alite Calcium Sulfoaluminate (BACSA) cements

In this work, four BACSA clinkers with different compositions (belite ~ 70-55 wt %, alite ~ 10-15 wt % and ye’elimite ~ 10-15 wt %) were synthesized (at laboratory scale). Lime, gypsum, kaolin and sand were used as raw materials and clinkering temperatures from 1280 to 1300 ºC were studied. Every BACSA clinker was chemically and mineralogically characterized through X-ray fluorescence and laboratory X-ray powder diffraction (LXRPD), the latter in combination with the Rietveld methodology to obtain the full phase assemblage including amorphous contents. The clinker with the targeted composition (belite ~ wt 60 %, alite ~ 13 wt % and ye’elimite ~ 10 wt %) was chosen to perform the scaling-up (5 kg). To do so, the processing parameters (milling time and clinkering conditions) have been optimized. Finally, the hydration of the corresponding BACSA cement pastes (prepared with the scaled-up clinker and gypsum) was studied through rheological measurements (at very early hydration time), calorimetry and impedance spectroscopy (first 24 h), and LXRPD in combination with the Rietveld methodology and G-factor method, at 1 and 7 hydration days to determine degree of hydration.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Hydration of Belite-Ye’elimite-Ferrite(BYF) cements with different calcium sulfate sources

Belite-Ye’elimite-Ferrite cements, BYF, are a sustainable alternative to OPC to reduce CO2 emissions. The aim of this research is to understand the influence of the sulfate source on the hydration of two laboratory-prepared BYF-cements. One studied clinker contained β-belite and orthorhombic-ye'elimite (non-active), and the other one α'H-belite and pseudo-cubic-ye'elimite (activated with borax during clinkering). Pastes were mainly characterized through Rietveld-quantitative-phase-analysis of powder patterns, thermal analysis and scanning-electron-microscopy. Active-mortars developed higher compressive strengths than non-active-mortars, independently of the sulfate source. The highest values for active-mortars (w/c=0.55) were 40±1MPa (28d) with anhydrite, and 68±1MPa (120d) with gypsum.This work is part of the PhD work of G. Álvarez-Pinazo funded by MINECO BES-2011-044690 grant. Funding from Junta de Andalucía (P11-FQM-7517), Spanish MINECO (BIA2014-57658-C2-2-R, which is co-funded by FEDER, and BIA2014-57658-C2-1-R research grant

Rietveld quantitative phase analysis with molybdenum radiation

Building materials are very complex samples of worldwide importance; hence quantitative knowledge of their mineralogical composition is necessary to predict performances. Rietveld quantitative phase analysis (RQPA) allows a direct measurement of the crystalline phase contents of cements. We highlight in this paper the use of laboratory X-ray powder diffraction (LXRPD) employing high-energy radiation, molybdenum (Mo), for attaining the RQPA of cements. Firstly, we evaluate the accuracy of RQPA employing a commercial calcium sulfoaluminate clinker with gypsum. In addition to MoKα1 and MoKα1,2 radiations, Cu and synchrotron patterns are also analyzed for the sake of comparison. Secondly, the assessment of the accuracy of RQPA results obtained using different radiations (synchrotron, Mo, and Cu) and geometries (reflection and transmission) is performed by analyzing two well-known commercial samples. As expected, for LXRPD data, accuracy in the RQPA results improves as the irradiated volume increases. Finally, three very complex aged hydrated cements have been analyzed using MoKα1-LXRPD and Synchrotron-XRPD. The main overall outcome of this work is the benefit for RQPA of using strictly monochromatic MoKα1 radiation. Best laboratory results arise from MoKα1 data as the effective tested volume is much increased but peak overlapping is not swelledUniversidad de Málaga. Campus de Excelencia Internacional. Andalucía Tech

Quantitative phase Analisis: A comparative study of Mo and Cu strictly monochromatic radiations

A comparison of the Rietveld quantitative phase analyses (RQPA) obtained using Cu-Kα1, Mo-Kα1, and synchrotron strictly monochromatic radiations is presented. The main aim is to test a simple hypothesis: high energy Mo-radiation, combined with high resolution laboratory X-ray powder diffraction optics, could yield more accurate RQPA, for challenging samples, than well-established Cu-radiation procedure(s). In order to do so, three set of mixtures with increasing amounts of a given phase (spiking-method) were prepared and the corresponding RQPA results have been evaluated. Firstly, a series of crystalline inorganic phase mixtures with increasing amounts of an analyte was studied in order to determine if Mo-Kα1 methodology is as robust as the well-established Cu-Kα1 one. Secondly, a series of crystalline organic phase mixtures with increasing amounts of an organic compound was analyzed. This type of mixture can result in transparency problems in reflection and inhomogeneous loading in narrow capillaries for transmission studies. Finally, a third series with variable amorphous content was studied. Limit of detection in Cu-patterns, ~0.2 wt%, are slightly lower than those derived from Mo-patterns, ~0.3 wt%, for similar recording times and limit of quantification for a well crystallized inorganic phase using laboratory powder diffraction was established ~0.10 wt%. However, the accuracy was comprised as relative errors were ~100%. Contents higher than 1.0 wt% yielded analyses with relative errors lower than 20%. From the obtained results it is inferred that RQPA from Mo-Kα1 radiation have slightly better accuracies than those obtained from Cu-Kα1. This behavior has been established with the calibration graphics obtained through the spiking method and also from Kullback-Leibler distance statistic studies. We explain this outcome, in spite of the lower diffraction power for Mo-radiation (compared to Cu-radiation), due to the larger volume tested with Mo, also because higher energy minimize pattern systematic errors and the microabsorption effect.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Hydration Reactions and Mechanical Strength Developments of Iron- Rich Sulfobelite Eco-cements

Belite calcium sulfoaluminate (BCSA) cements are low-CO2 building materials. However, their hydration behavior and its effect on mechanical properties have still to be clarified. Here, we report a full multitechnique study of the hydration behavior up to 120 days of nonactivated and activated BCSA laboratory-prepared clinkers, with β- or αH-belite as main phase, respectively. The effects of the amount of gypsum added were also studied. The hydration and crystallization processes are reported and discussed in detail. Finally, shrinkage/expansion data are also given. The optimum amount of gypsum was close to 10 wt %. Our study has demonstrated that β-belite reacts at a higher pace than α′H-belite, irrespective of the gypsum content. The hydration mechanism of belite determines the development of the mechanical strengths. These are much higher for activated BCSA cement, ∼65 MPa at 120 days, against ∼20 MPa for nonactivated BCSA cement, with the latter having larger amounts of stratlingite.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

The use of mo and cu monochromatic radiations for quantitative phase analysis: study of the accuracy

Cement hydration is a very complex process in which crystalline phases are dissolving in water and after supersaturation hydrated crystalline and amorphous phases precipitate. Great efforts are being made to develop analytical tools to accurately quantify these processes and X-ray Powder Diffraction (XRPD) combined with Rietveld methodology is a suitable tool to quantify these complex mixtures and their time evolutions. However, some problems/drawbacks should be overcome to fully apply it to cement pastes characterization in order to get accurate phase analyses. In order to tackle this issue, a comparison of the Rietveld quantitative phase analyses (RQPA) obtained using Cu-Kα1, Mo-Kα1, and synchrotron strictly monochromatic radiations of three set of mixtures with increasing amounts of a given phase (spiking-method) is presented. The main aim is to test a simple hypothesis: high energy Mo-radiation, combined with high resolution laboratory X-ray powder diffraction optics, could yield more accurate RQPA, for challenging samples, than well-established Cu-radiation procedure(s). Firstly, a series of crystalline inorganic phase mixtures with increasing amounts of an analyte was studied in order to determine if Mo-Kα1 methodology is as robust as the well-established Cu-Kα1 one. Secondly, a series of crystalline organic phase mixtures with increasing amounts of an organic compound was analyzed. This type of mixture can result in transparency problems in reflection and inhomogeneous loading in narrow capillaries for transmission studies. Finally, a third series with variable amorphous content was studied. Limit of detection in Cu-patterns, ~0.2 wt%, are slightly lower than those derived from Mo-patterns, ~0.3 wt%, for similar recording times and limit of quantification for a well crystallized inorganic phase using laboratory powder diffraction was established ~0.10 wt%. From the obtained results it is inferred that RQPA from Mo-Kα1 radiation have slightly better accuracies than those obtained from Cu-Kα1. The results obtained in the previous comparison have been taken into account to obtain accurate RQPA, including the amorphous component with internal standard methodology, of hydrating cement pastes. The final goal of this second study was understanding the early-stage hydration mechanisms of a variety of cementing systems (Ordinary Portland Cement or Belite Alite Ye’elimite cement) as a function of water content, superplasticizer additives and type and content of sulfate source. In order to do so, X-ray powder diffraction data were taken in-situ with the humidity chamber coupled to the Mo-Kα1 powder diffractometer. Some results of this ongoing investigation will be reported and discussed.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

In-situ early age hydration of cement-based materials by synchrotron X-ray powder diffraction

Cement based binders are building materials of worldwide importance. Since these samples are very complex, the knowledge/control of their mineralogical composition are essential to design and predict materials with specific/improved performance. Rietveld quantitative phase analysis (RQPA) allows the quantification of crystalline phases and, when combined with specific methodologies, as the addition of an internal standard or the external standard approach (G-factor), amorphous and non-crystalline phases can also be quantified. However, to carry out a proper RQPA in hydrated cementitious-materials, a good powder diffraction pattern is necessary. In this work, synchrotron X-ray powder diffraction (SXRPD) has been used, allowing in-situ measurements during the early-age hydration process. This work deals with the early hydration study of cement-based materials. The studied samples were: a laboratory-prepared belite calcium sulphoaluminate (BCSAF) clinker (non-active) mixed with 10wt% gypsum, labelled G10B0; two active laboratory-prepared BCSAF clinkers (activated with 2wt% borax), one mixed with 10wt% gypsum and the other one with 10wt% monoclinic-bassanite, hereafter named G10B2 and B10B2, respectively; and an environmentally-friendly cement sample from Henkel, composed of bassanite mixed with 15wt% Portland cement and 10wt% Metakaolin, labelled H1. Anhydrous G10B0 contains beta-belite and orthorhombic-ye'elimite as main phases, while alpha'H-belite and pseudo-cubic-ye'elimite are stabilized in G10B2 and B10B2, with the corresponding sulphate source. Anhydrous H1 contains monoclinic and hexagonal bassanite and alite as main phases. Ye'elimite, in the non-active BCSAF cement pastes, dissolves at a higher pace than in the active one (degree of reaction is α~25% and α~10% at 1 h, respectively) (both prepared with gypsum), with the corresponding differences in ettringite crystallisation (degree of precipitation is α~30% and α~5%, respectively). Moreover, the type of sulphate source has important consequences on the hydration of the active BCSAF cement pastes. Bassanite is quickly dissolved and it precipitates as gypsum within the first hour of hydration (in B10B2). At that time, ettringite starts to crystallize, and after 12 hours is almost fully crystallized, similar to G10B2. In H1, bassanite transforms into gypsum within the first hour, being the principal hydration product; ettringite starts to be formed just after few hydration minutes. These results are crucial in the understanding and development of improved cement materials.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech