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Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Geología y Geoquímica. Fecha de lectura: 05 de febrero de 2016

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.

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.

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En port.: Instituto de Ciencias de la Construcción "Eduardo Torroja" (CSIC), Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química Inorgánica. Fecha de lectura: 13-12-2007

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