Calcium aluminate cement (CAC) is less commonly used as ordinary Portland cements (OPCs) for structural concrete, is relatively expensive, but may have certain advantages when used for solidification of wastes; it introduces rapid strength gain and has a higher resistance to chemical attack than OPC. However, the most widely identified degradation process suffered by CAC is the so‐called conversion of hexagonal calcium aluminate hydrate to a cubic form. Mixes of CAC with silica fume (SF) or fly ash (FA) represent an interesting alternative for the stabilization of CAC hydrates, which might be attributed to a microstructure based mainly on aluminosilicates. This paper deals with the microstructure of cement pastes fabricated with binders of CAC–SF and CAC–FA, and their evolution over time. Mid infrared and near infrared spectroscopy have been used to assess the microstructure of these formulations. Microstructural characterization was completed by backscattering electron microscopy observation and microanalysis.

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Low-pH cements are designed to be used in underground repositories for high level waste. When they are based on Ordinary Portland Cements (OPC), high mineral admixture contents must be used which significantly modify their microstructure properties and performance. This paper evaluates the microstructure evolution of low-pH cement pastes based on OPC plus silica fume and/or fly ashes, using Mid-Infrared and Near-Infrared spectroscopy to detect cement pastes mainly composed of high polymerized C-A-S-H gels with low C/S ratios. In addition, the lower pore solution pH of these special cementitious materials have been monitored with embedded metallic sensors. Besides, as the use of reinforced concrete can be required in underground repositories, the influence of low-pH cementitious materials on steel reinforcement corrosion was analysed. Due to their lower pore solution pH and their different pore solution chemical composition a clear influence on steel reinforcement corrosion was detected.