The origin of cement, employed as a binding material, can be attributed to Romans who found that a mixture of lime and crushed volcanic ashes was able to set under water, the resistance being increased along the time, in a way completely different to any other material. Since that age, a huge amount of different kind of cements have been produced to satisfy the request of different mechanical behaviors. To deeply understand the mechanisms that lead to the development of mechanical strength, reaction kinetics that occur during the hydration process must be known. Nowadays we can affirm that cement research has set many important results but despite of this “long-time story”, a lot of improvements are required to better understand the mechanisms of kinetics.
Cements mixed with water are complex systems undergoing critical chemical and physical changes during the hydration process. A unique hydration model able to explain the controlling mechanisms is the main purpose of cement research, but the physical-chemical parameters involved are actually too many. To partly overcome the chemical complexity of common cement materials, simplified cement systems are often used for research purposes. A project has been set to investigate the fundamental reactions occurring during the hydration process and has been divided within 3 different partners: NIST (National Institute of Standards and Technology), W.R. GRACE and University of Padua. Our part of the project was to collect x-ray powder diffraction patterns on the hydrating suspensions, using Rietveld refinement for quantitative phase analysis.
Three simplified cement systems formed by the synthetic phases tricalcium silicate Ca3SiO5 (C3S), tricalcium aluminate Ca3Al2O6 (C3A) and a varying amount of gypsum CaSO4∙2H2O (CŠH2) were investigated by means of in-situ x-ray powder diffraction (XRPD) and isothermal calorimetry (IC) in order to evaluate dissolution-precipitation kinetics. The main hydration products detected by means of XRPD were ettringite, hemicarboaluminate, portlandite, Ca-Si hydrates (C-S-H): the same occurring in real cements.
The Avrami nucleation and growth model successfully fits the degree of hydration data, confirming that C-S-H should have a layered structure as well as the phases resulting from the decomposition of ettringite. The mass balance method was used to calculate the exact amount of C-S-H formed during hydration, which is not directly accessible from Rietveld refinement. The comparison between the degree of hydration calculated from isothermal calorimetry data and the degrees of hydration calculated from x-ray diffraction has revealed how much the reactant phases are responsible for heat release. In particular, it was seen that the study of C3S-C3A-Gy systems is not a simple sum of the investigations of C3S-Gy and C3A-Gy systems, which are two further simplified model cements. The synthetic materials suffered a loss on reactivity despite of the under-vacuum sealing, leading to a continuous and unpredictable change of the materials features (particle size, degree of reactivity) during time.
The obtained experimental data should be necessary to proof the effectiveness of software modelling (HydratiCA). The software has been tested and returned satisfactory results for further simplified systems, such C3S-Gy. Nevertheless, the software is still under a development stage and improvements has to be planned for C3A-Gy systems before testing more complex blend
