A sensor is provided for monitoring cement. An internal reflection is contacted with the cement and a mid-infrared light source directs a beam of mid-infrared radiation into said window for attenuated internal reflection at an interface between the window and the cement. The reflected infrared radiation is passed through a first narrow bandpass filter that preferentially transmits mid-infrared radiation over a band of wavelengths corresponding to an absorbance peak of a species associated with the cement to filter internally reflected mid-infrared radiation received from the window. An infrared detector detects filtered mid-infrared radiation transmitted through the first filter and a processor measures the intensity of the detected mid-infrared radiation transmitted through the first filter, and determines therefrom an amount of the species associated with the cement.