In order to gain a deeper understanding of the rheological development of hydrating ordinary Portland cement (OPC) pastes at initial state, and to better understand their underlying processes, quantitative X-ray diffraction (XRD) analysis and rheological measurements were conducted and their results combined. The time-dependent relation between phase development and flow behavior of cement paste was investigated at two different temperatures (20 and 30 °C), over a period of two hours. Regarding the phase development during hydration, ettringite precipitation was identified as the dominant reaction in the first two hours. For both temperatures, the increasing ettringite content turned out to correlate very well with the loss of workability of the reacting cement paste. An exponential relationship between ettringite growth and flow behavior was observed that could be explained by applying the Krieger-Dougherty equation, which describes the influence of solid fraction on the viscosity of a suspension.

Exposure to elevated temperatures has detrimental effects on the properties of cementitious composites, leading to irreversible changes, up to total failure. Various methods have been used to suppress the deterioration of concrete under elevated temperature conditions. Recently, nanomaterials have been introduced as admixtures, which decrease the thermal degradation of cement-based composites after exposure to high temperatures. This paper presents a comprehensive review of recent developments related to the effects of nanoparticles on the thermal resistance of cementitious composites. The review provides an updated report on the effects of temperature on the properties of cement-based composites, as well as a detailed analysis of the available literature regarding the inclusion of nanomaterials and their effects on the thermal degradation of cementitious composites. The data from the studies reviewed indicate that the inclusion of nanoparticles in composites protects from strength loss, as well as contributing to a decrease in disruptive cracking, after thermal exposure. From all the nanomaterials presented, nanosilica has been studied the most extensively. However, there are other nanomaterials, such as carbon nanotubes, graphene oxide, nanoclays, nanoalumina or nano-iron oxides, that can be used to produce heat-resistant cementitious composites. Based on the data available, it can be concluded that the effects of nanomaterials have not been fully explored and that further investigations are required, so as to successfully utilize them in the production of heat-resistant cementitious composites.