Suchergebnisse

Concrete is one of the most fire-resistant materials, whose resistance depends on the chemical and structural characteristics of the hydrated calcium silicate (C-S-H) formed in the hydration and hardening process. However, the structure and composition of this C-S-H varies with the time of hydration. The effect of the composition of the calcium silicate on the anhydrous material formed has been studied after subjecting it to an accelerated study of the effect of fire, irradiating it with a CO laser. Changes in the composition of C-S-H can lead to changes in the mechanical properties of the cement. C-S-H samples with different chemical composition (Ca/Si ratios 1 and 2) as well as different synthesis processes (double decomposition and hydrothermal) were studied. The crystalline phases obtained after heating were identified through micro-Raman spectroscopy, which confirmed the formation of anhydrous calcium silicates with the same Ca/Si ratio as the initial one. In C-S-H gels with a Ca/Si ratio of over 1.5, stable Ca (OH) was formed. Scanning electron microscopy/energy dispersive X-ray analysis analysis determined that in the process of heating with the laser, water is lost fast, generating porous structures. Such porosity is higher in materials with a lower Ca/Si ratio. ; Spanish Ministry of Economy and Competitiveness. Grant Number: FIS2017‐84318‐R European Social Fund. Grant Number: S2018/NMT‐4372 Regional Government of Madrid (CAM). Grant Number: S2013/MIT‐2914

4 pages.-- Final full-text version of the paper available at: http://dx.doi.org/10.1111/j.1551-2916.2007.02034.x. ; Fly Ash Belite Cement (FABC) pastes with and without nanosilica additions have been prepared and maintained in sulfate solutions (Na2SO4 0.5M) for 180 days. The mechanical performance and the changes in microstructure have been monitored at 28, 90, and 180 days by compressive strength, X-ray diffraction (XRD), and 29Si MAS NMR measurements. We have found that, unexpectedly, and contrary to what happens in Ordinary Portland Cements (OPC), the addition of nanosilica particles induces an initial decline in the compressive strength of the samples. Only in samples maintained for a long time (180 days) does the nanosilica addition improve the mechanical properties. Our XRD and 29Si NMR experiments have revealed that although nanosilica additions trigger the consumption of Belite phases, this is not always accompanied by formation of longer calcium–silicate–hydrate (C–S–H) gel structures. Only at a long time (180 days), and due to a mechanism that seems to be controlled by the pH of the samples, do the nanosilica additions lead to high-polymerized C–S–H gels. ; This work was financially supported by the Spanish Government (Projects no. MAT 2002-04023-CO1-CO2-CO3 and MAT2005-03890). ; Peer reviewed