The main component of the cement hydration, are both, the calcium silicate hydrate (C-S-H) and calcium silicate hydrate with Al (C-S(A)-H), whose composition is characterized by its calcium to silicon ratio (Ca/Si), which normally varies from 0.6 to 1.6. The theoretical Ca/Si ratios of the synthesized gels were compared with those of the experimental gels, which were determined by inductively coupled plasma atomic emission spectroscopy (ICP-OES). In addition, the microstructure of the gels was studied by spectroscopic techniques: Infrared and Raman spectroscopy and Nuclear Magnetic Resonance. By the double decomposition method used in this work (1 day at 25 ⁰C, inert atmosphere and pH = 12,3), only C-S-H and C-S(A)-H gels with a maximum Ca/Si ratio ranging from 0.8 to 1.0 were synthesized. However, the structures of the gels are slightly different as the Ca/Si ratio increases.

The calcium silicate hydrate gel (C-S-H) was synthesized by the double decomposition method because of the simplicity and the quickness of the procedure. The structure of the C-S-H gels after 1 week and 4 weeks in contact with the formation solution was studied through Micro-Raman, Fourier Transformed Infrared Spectroscopy and 29 Si Nuclear Magnetic Resonance. Simultaneous thermodifferential-thermogravimetric analysis and mass spectrometry (DTA/TG/MS) was used to identify the amount of calcium carbonate formed due to the reaction between the calcium and atmospheric CO 2. With DTA/TG/MS, mass loss due to CO 2 was observed to take place at temperatures below 400ºC, unidentified to date, which might be associated with the CO 2 adsorbed on the C-S-H gel. Thus, in the TG analysis in the 300-430ºC range, both the loss of water due to the decomposition of the amorphous calcium carbonate (ACC) and the loss of CO 2 adsorbed on the gel must be considered. Additionally, polymerization of the gel and a decrease in the Ca/Si ratio was observed from the samples from 1 to 4 weeks.

Fire-induced compositional changes lead to strength loss and even failure in cement and concrete. Calcium silicate hydrate (C-S-H) gel, the main product of cement hydration, dehydrates at 25 C to 200 C, while temperatures of 850 C to 900 C alter its structure. A Raman spectroscopic study of the amorphous and crystalline phases forming after CO 2 laser radiation of cement mortar showed that C-S-H dehydration yielded tricalcium silicate at higher, and dicalcium silicate at lower, temperatures. Post-radiation variations were identified in the position of the band generated by Si-O bond stretching vibrations.

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 CO2 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)2 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.

This study explored the viability of synthesising nanolime at ambient temperature by raising calcium solubility through the formation of complexes with dissolved sugars. Micro-Raman findings confirmed the formation of nanolime particles whilst the percentage of Ca(OH) 2 formed, observed to vary with synthesis conditions, was calculated with thermogravimetry. Nanoparticles were synthesised most productively (77%) in a 5% sugary solution at a temperature of 25 °C and a 4 h reaction time. The hexagonal nanoparticles synthesised ranged in size from 200 to 25 nm. Portlandite formation is related to calcium complex formation with less mannitol that sucrose needed to form similar NPs of Ca(OH) 2. The sugary media also favoured the formation of amorphous calcium carbonate.