Aleena, is your variation systematic? That is to say, for the peak in question, after normalization, does it increase (decrease) consistently over the cause of the 7 day period? If the change is systematic, then it would amount to a change in concentration or in your specific case (hydration) - a chemical or physical incorporation of water to the sample.

On the other hand if the peak is not present at the onset of the experiment (Day 1) but rather appears over the cause of the experiment and perhaps increases over time in intensity, this would infer formation of a new product as can be seen in the example reaction below. (Dicalcium silicate hydrates to form calcium silicate hydrates and heat)

Dicalcium silicates + water  = calcium silicate hydrate + lime

C2S + 4H = C3S2H3 + CH, D H = 62 cal/g

I hope this is helpful.

Absolutely I confirm opinion Theresah Zu. There is a chemical linkage of molecules of water, it is prominent feature of such systems.This phenomenon necessarily should be reflected in results of measurements of properties.

The peak is present in day1. It is not a new peak indicating product formation. But rather, the peak height increases w.r.t. all the other peaks when tested at day 7. 

Well Raman gives chemical bond information. Have you looked up in literature or a Raman database what possible chemical bond information is linked to this peak? What is the peak number in question? And did you take any other measurements other than day 1 and day 7? Also I see you normalized to the maximum intensity - I would suggest you normalize the data to one distinct peak you know for sure does not change over time (you can add an internal standard if you are not sure and normalize the dataset to this standard). Best,