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

Rheological and hydration characterization of calcium sulfoaluminate cement pastes

Calcium sulfoaluminate (CSA) cements are currently receiving a lot of attention because their manufacture produces less CO2 than ordinary Portland cement (OPC). However, it is essential to understand all parameters which may affect the hydration processes. This work deals with the study of the effect of several parameters, such as superplasticizer (SP), gypsum contents (10, 20 and 30 wt%) and w/c ratio (0.4 and 0.5), on the properties of CSA pastes during early hydration. This characterization has been performed through rheological studies, Rietveld quantitative phase analysis of measured x-ray diffraction patterns, thermal analysis and mercury porosimetry for pastes, and by compressive strength measurements for mortars. The effect of the used SP on the rheological properties has been established. Its addition makes little difference to the amount of ettringite formed but strongly decreases the large pore fraction in the pastes. Furthermore, the SP role on compressive strength is variable, as it increases the values for mortars containing 30 wt% gypsum but decreases the strengths for mortars containing 10 wt% gypsum.This work has been supported by Spanish Ministry of Science and Innovation through MAT2010- 16213 research grant, which is co-funded by FEDER, and Ramón y Cajal Fellowship (RYC-2008- 03523)

Hydration studies of calcium sulfoaluminate cements blended with fly ash

The main objective of this work is to study the hydration and properties of calciumsulfoaluminate cement pastes blended with fly ash (FA) and the corresponding mortars at different hydration ages. Laboratory X-ray powder diffraction, rheological studies, thermal analysis, porosimetry and compressive strength measurements were performed. The analysis of the diffraction data by Rietveld method allowed quantifying crystalline phases and overall amorphous contents. The studied parameters were: i) FA content, 0, 15 and 30 wt.%; and ii) water addition, water-to-CSA mass ratio (w/CSA = 0.50 and 0.65), and water-to-binder mass ratio (w/b = 0.50). Finally, compressive strengths after 6 months of 0 and 15 wt.% FA [w/CSA = 0.50] mortars were similar: 73 ± 2 and 72 ± 3 MPa, respectively. This is justified by the filler effect of the FA as no strong evidences of reactivity of FA with CSA were observed. These results support the partial substitution of CSA cements with FA with the economic and environmental benefitsThis work has been supported by Spanish MINECO through MAT2010-16213 research grant, which is co-funded by FEDER. I. Santacruz thanks a Ramón y Cajal fellowship, RYC-2008-03523

Effect of calcium sulfate source on the hydration of calcium sulfoaluminate eco-cement

The availability of cements, including eco-cements, with tailored mechanical properties is very important for special applications in the building industry. Here we report a full study of the hydration of calcium sulfoaluminate eco-cements with different sulfate sources (gypsum, bassanite and anhydrite) and two water/cement ratios (0.50 and 0.65). These parameters have been chosen because they are known to strongly modify the mechanical properties of the resulting mortars and concretes. The applied multitechnique characterization includes: phase assemblage by Rietveld method, evolved heat, conductivity, rheology, compressive strength and expansion/retraction measurements. The dissolution rate of the sulfate sources is key to control the hydration reactions. Bassanite dissolves very fast and hence the initial setting time of the pastes and mortars is too short (20 min) to produce homogeneous samples. Anhydrite dissolves slowly so, at 1 hydration-day, the amount of ettringite formed (20 wt%) is lower than that in gypsum pastes (26 wt%) (w/c = 0.50), producing mortars with lower compressive strengths. After 3 hydration-days, anhydrite pastes showed slightly larger ettringite contents and hence, mortars with slightly higher compressive strengths. Ettringite content is the chief parameter to explain the strength development in these eco-cements.Universidad de Málaga. Campus de Excelencia Interncaional Andalucía Tech

Effect of different retarders on the hydration of calcium sulfoaluminate eco-cement pastes

VERSIÓN PRE-PRINTThe manufacture of Calcium SulfoAluminate (CSA) cements is more environmentally friendly than that of OPC [1] as their production releases up to 40% less CO2 than the latter. The main performances of CSA cements are fast setting time, good-chemical resistance properties and high early strengths. CSA cements are prepared by mixing CSA clinker with different amounts of a calcium sulfate set regulator such as gypsum (CaSO4•2H2O), bassanite (CaSO4•½H2O), or anhydrite (CaSO4), or mixtures of them. It is possible to modify the hydration process of CSA cements not only by its composition, but also by the selection of different quantities or sources of calcium sulfate [2,3]. The dissolution rate of the sulfate source is a key point to control the reactions during the hydration of CSA cements, and hence the mechanical properties of the corresponding pastes and mortars. The solubility of bassanite in water (0.88 g/100 mL) is 3-4 times larger than that for gypsum or anhydrite and hence, all the reactions start quickly, showing initial setting times as short as 20 min, which do not allow the preparation of homogeneous samples, with the consequent dramatic effect onto their mechanical strength values. However, the setting time can be controlled through the addition of small amounts of different additives/retarders. The objective of this work is to control the hydration, including setting time, of CSA cements prepared with bassanite and different retarders to obtain tailored CSA cements and mortars for different applications. The addition of these additives reduced considerably the viscosity of the bassanite-pastes to a minimum value which depends on the properties of the additive. The mechanical strength values of selected mortars have been correlated to those variables.- Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. - Spanish MINECO through MAT2010-16213 and MAT2010-15175 which are co-funded by FEDER. Junta de Andalucía through P11-FQM-07517 and FQM-1656 research projects. I. Santacruz thanks a Ramón y Cajal fellowship, RYC-2008-03523

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

In-situ hydration study of sulfobelite eco-cements

ALBA User Meeting and VI AUSE Conferenc

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

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

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