Quantitative study of hydration of C3S and C2S by thermal analysis

Abstract

This research is part of a European project (namely, CÓDICE project), main objective of which is modelling, at a multi-scale, the evolution of the mechanical performance of non-degraded and degraded cementitious matrices. For that, a series of experiments were planned with pure synthetic tri-calcium silicate (C3S) and bi-calcium silicate (C2S) (main components of the Portland cement clinker) to obtain different calcium-silicate-hydrate (C-S-H) gel structures during their hydration. The characterization of those C-S-H gels and matrices will provide experimental parameters for the validation of the multiscale modelling scheme proposed. In this article, a quantitative method, based on thermal analyses, has been used for the determination of the chemical composition of the C-S-H gel together with the degree of hydration and quantitative evolution of all the components of the pastes. Besides, the microstructure and type of silicate tetrahedron and mean chain length (MCL) were studied by scanning electron microscopy (SEM) and Si magic-angle-spinning (MAS) NMR, respectively. The main results showed that the chemical compositions for the C-S-H gels have a CaO/ SÍO2 M ratio almost constant of 1.7 for both C3S and C2S compounds. Small differences were found in the gel water content: the H 2 0/Si0 2 M ratio ranged from 2.9 ± 0.2 to 2.6 ± 0.2 for the C 3 S (decrease) and from 2.4 ± 0.2 to 3.2 ± 0.2 for the C 2 S (increase). The MCL values of the C-S-H gels, determined from 29 Si MAS NMR, were 3.5 and 4 silicate tetrahedron, for the hydrated C3S and C2S, respectively, remaining almost constant at all hydration periods. Keywords C-S-H gel • Chemical composition • Thermal analysis • C3S • C2S

Caracterización textural y mecánica de geles C-S-H formados en la hidratación de muestras sintéticas T1-C₃S, β-C₂S y sus mezclas

“…These percentages have been quantitatively calculated from TGA measurements (24). As can be seen, the aforementioned synergy is clearly visible, since both the 70-30 and 30-70 mixtures give percentages of C-S-H gel much higher than those presumably expected in view of the percentages of the pure samples.…”

“…To this end we have analyzed the textural and mechanical properties of C-S-H gels coming from the hydration of pure T1-C 3 S, β-C 2 S and their blends. Recent works of some of us (24,25) proposed the existence of a synergic effect of T1-C 3 S and β-C 2 S hydration reactions. In essence these works found a strong increase of the hydraulic activity of the β-C 2 S due to the mere presence of T1-C 3 S, reducing its inactive induction period.…”

“…Los detalles del procedimiento de síntesis se pueden consultar en las referencias (24,25). Las pastas se prepararon mezclando el polvo de C 3 S y β-C 2 S en diferentes proporciones en peso, con agua destilada y descarbonatada en una relación agua/sólido de 0,4.…”

“…Estos porcentajes han sido cuantitativamente calculados a partir de los resultados de TGA (24). Como puede verse, la sinergia, anteriormente citada, es claramente visible, ya que tanto las mezclas de 70-30 y 30-70, dan porcentajes de gel C-S-H mucho más altos que los esperados en vista de los porcentajes de las muestras puras.…”

“…En recientes trabajos de algunos de nosotros (24,25) se propone la existencia de un efecto sinérgico de las reacciones de hidratación del T1-C 3 S and β-C 2 S. En esencia, estos trabajos encuentran un fuerte incremento de la actividad hidráulica de la fase β-C 2 S, que reduce su periodo de inducción inactivo, debido a la mera presencia de T1-C 3 S. Este comportamiento sinérgico también se espera que se refleje en la estructura porosa del gel C-S-H. Aunque los mecanismos subyacentes de la sinergia no se entienden completamente, este trabajo pretende aportar evidencia sobre la existencia de este interesante tema. Con este fin, se han llevado a cabo medidas de la superficie específica y la distribución de tamaño de poro por medio de isotermas de sorción de gas nitrógeno y el método BET, mientras que las propiedades micro-mecánicas han sido evaluadas mediante experimentos de nanoindentación.…”