Study of the hydration of a cement paste by automatic image analysis usingscanning electron microscopy

For the investigation of civil engineering concrete, the hydration of the c ement paste is important to know. The parameter investigated for that is th e hydration degree, a. Different techniques are used to study this paramete r: In this work, we have chosen to determine ct using scanning electron mic roscope (SEM) and automatic image analysis. On a SEM image of a cement paste, two phases can be observed : hydrated pha se which appears in grey and anhydrous phase (NH) which appears in white. S o, after an image processing, the surface area of the anhydrous phase can b e calculated: it is equal to the volume fraction of this phase, V-V(NH). In this case, one has : alpha = 1 - weight of non hydrated cement / initial weight of anhydrous cem ent = 1 - rho anhydrous cement * V-V(NH)* V-test tube / initial weight of anhyd rous cement For this work, two cement pastes were prepared. They differ by their water/ cement ratio (w/c), in weight, which are respectively 0.5 and 0.3. 50 images per specimen were acquired by SEM using back scattering electron mode. This number requires to use an automatic process. It is made of three stages : - a filtering of the grey image, - a segmentation of the anhydrous phase, - a binary process to perform the final image. The kinetic of hydration is principally studied with the hydration degree. But it is interesting to analyse the stereological parameters, like the spe cific connexity (N-A(NH)) and intercept (N-L(NH)) numbers. They allow to ca lculate the specific surface area (S-V(NH)), the mean free path ((L) over b ar(3)(NH)), the mean curvature ((H) over bar(3)(NH)), and the mean surface area ((A) over bar(NH)) of the anhydrous phase. So, we note that, for a given time the hydration degree of a cement with ra tio w/c = 0.3 is lower than the one of a cement with w/c = 0.5 For w/c = 0. 3, the hydration evolves very slowly rather than the hydration of the cemen t with w/c = 0.5 increases rapidly between 2 to 14 days, before taking an a symptotic value. To validate our results obtained by automatic image analysis, they were com pared them with results obtained by thermogravimetric analysis (TGA) : the evolution of the hydration degree by TGA gives similar results as those obt ained by automatic image analysis. The analysis of the stereological parameters shows that the specific surfac e area of the anhydrous phase decreases during the first days, and then tak es an asymptotic value. It can be interpreted like an evolution of the surf ace energy: during the first days, there is a minimization of the superfici al energy between phases, then this interface takes a stationary value. The evolution of ((L) over bar(3)(NH) and (H) over bar(3)(NH) of the anhydr ous phase shows that the smaller anhydres disappear in the first days, and after their mean size stays constant. Finally the study of (A) over bar(NH)/((L) over bar(3)(2)(NH) shows that, a fter 4 days, the shape of the anhydres stays constant: hydration is an isot ropic phenomenon.