Portland and white cement pastes of widely-variable water/cement ratios were studied after six years' hydration by means of quantitative X-ray diffractometric analysis of the CH and C-S-H gel contents. The results obtained have led to certain conclusions on the crystallinity and possible crystal structure of the gel. It was also observed that the flexural strength was considerably reduced for the cement pastes of lowest water/cement ratios.
Anomalous XD effects in old cement pastes
In a recent study of the state of crystallisation in the cement matrix of a very old concrete (i), the X-ray diffractometric (XD) effects from various particle-size fractions of the mortar were recorded and compared with those given by a number of pure cement pastes of different compositions, more or less fully-hydrated (moist-hydrated for 5 to 15 years). It was then noticed, in a series of white cement pastes of water/cement ratios (Wo) between 0.32 and 1.32 (stabilized by aerosil addition, maximum Wae 0.05), that the peak heights for the I0"i and 00"i reflexions from CH varied with w o in essentially different ways. The 10"i peaks were of almost constant height, as could be expected, with a low maximum for w o 0.40 to 0.50, whereas the 00"1 peak heights increased continuously and considerably with w o. The strongest gel peak at 20 = 2903 showed a similar increase. Since CH crystal orientation effects are out of the question, the XD samples being cut as 3 x ~20 mm plates from longer cylinders, the reflexion at 20 = 18°0 was interpreted as a CH 00"I peak of about constant height superimposed by the basal reflexion from a C-S-H gel constituent, the content of which increases with wo, for some reason.
In this context, the arrangement of crystal elements, silica chains in a frame of calcium ions, that could possibly satisfy the XD effects observed, was indicated, and a hypothetical structure of the C-S-H gel was given. Some slight evidence for the appearance of a double C-S-H fibre reflexion at about 20:50 ° was also presented.