Cement hydration is an exothermal process where the reaction between cement and water leads to heat release per time and mass of hydrated cement. In the classical evaluation of a calorimetric test, the total heat leaving the tested hydrated sample is set equal to the heat generation rate of the hydrating sample. This overall heat release rate typically follows a trend characterized by a first peak, an induction period and a main peak. However, such an evaluation is based on the assumption of stationarity, i.e. the internal energy of the individual volume elements making up the hydrating sample is assumed to stay constant over the entire testing period.

The current contribution leaves the aforementioned assumption aside, by focusing on the instationary heat generation and conduction in a cylindrical sample, described by a convolution integral where Green’s functions link heat release rates to temperatures. Hence, the calorimeter needs to be equipped by an additional temperature sensor. The new approach leads to an improved estimate for the heat release rate of cement, with the first peak being twice as large as that estimated by the traditional method, and occurring three times earlier.

Moreover, it is interesting to relate the two peaks in the hydration heat release trend to the two key phenomena governing hydration: dissolution of cement clinker and precipitation of hydration product out of a super-saturated solution (Nicoleau & Nonat 2016). Indeed, the first peak occurs once a temporally constant ion concentration, as measured by coupled plasma – optical emission spectrometry (ICP-OES) is reached. This shows that the first peak, identified here with unprecedented precision, is associated with a strongly exothermic dissolution process.

Nicoleau, L. & Nonat, A. 2016. A new view on the kinetics of tricalcium silicate hydration. Cement and Concrete Research 86, pp. 1–11.

Reactive magnesium oxide cement (RMC) is emerging as a sustainable alternative to ordinary Portland cement (OPC) due to its lower production temperatures, ability to sequester carbon dioxide (CO₂), and recyclability. Despite these advantages, RMC often exhibits low hydration rates, necessitating the addition of hydrating agents, like magnesium acetate or magnesium chloride. Seawater, abundant in magnesium ions, arises as a potential natural alternative to these agents. This study thoroughly investigates seawater's dual impact on both the hydration and carbonation processes of RMC. The results reveal that seawater not only expedites hydration kinetics but also significantly influences the carbonation of RMC, leading to notable enhancements in the mechanical performance of the RMC mortars. Seawater-based mortar samples exhibit a remarkable 60% increase in compressive strength, achieving 81.1 MPa and a 65% increase in flexural strength, reaching 5.3 MPa, after 28-day accelerated carbonation, surpassing RMC mortars mixed with 0.1M magnesium acetate solution. Utilizing X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) techniques, this study unveils a substantial increase in hydrated magnesium carbonates in the seawater samples. The promising results will have wide-ranging effects on concrete production for non-structural applications not only in water-scarce regions that rely on energy-intensive desalination processes, but also globally.

New objectives have been proposed to reduce greenhouse gas emissions and address the ecological impacts of human activities. CO₂ emissions mainly come from burning and decarbonation of limestone in the production of building materials, particularly traditional cement. To lower the emissions, other binders, based on a different chemistry as for example MgO-based cement, are considered. When magnesium binders hydrate, they can result in the formation of different magnesium phases. One of these is magnesium silicate hydrate (M-S-H). Several studies have examined the chemical, and microstructural characteristics and the stability of M-S-H, but their mechanical properties have not been thoroughly investigated. This study investigates the microstructural and elastic properties of M-S-H pastes at the low scale (micrometres scale) after undergoing various production protocols. These protocols were developed to address the high water demand of reactive MgO and microsilica, using various w/b ratios, materials, and adjuvants. Carbonates and phosphates were used as an accelerator and/or superplasticizer in certain mixtures to analyse their effects on the mechanical properties. Chemical (XRD, TGA, SEM/EDS), microstructural (N₂ Physisorption and water saturation), and mechanical characterizations (indentation) were conducted.

Reactive MgO cement (RMC) serves as a sustainable substitute for ordinary Portland cement (OPC), functioning as a low-carbon binder. RMC reacts with water and absorbs environmental CO₂ to precipitate hydrated magnesium carbonates (HMCs). The strength of the ultimate RMC-based composite is influenced by the particular polymorphs of HMCs that precipitate, determined by the CO₂ curing conditions and the composition of the RMC. This study investigates the polymorphism in HMCs by observing the hydration and carbonation reactions of RMC using both ex-situ and specialized in-situ gas cell techniques with Transmission Electron Microscopy (TEM). The first step of the experiments is the sample preparation for both ex-situ and in-situ experiments. For the ex-situ experiments, the sample is first imaged under the microscope, and information about the lattice parameters is obtained before exposure. The sample is then immersed in water for a certain duration to allow complete hydration. The hydrated sample is then imaged in the TEM to obtain information about the formed hydration products. The in-situ experiments are conducted by flowing gases into and out of the gas cell at the required concentration and temperature. Changes in the sample morphology and phases are then captured with videos and images taken using in-situ TEM. Diffraction patterns and lattice images collected from TEM also provide information about the different phases of HMCs formed during the experiments. These results help provide valuable information on the hydration and carbonation of magnesia, which will, in turn, aid in evaluating and enhancing the properties of RMCs.