Right additive to make cement waterproof

Hi everyone,
Happy New year.

I moved into a house recently and noticed that the area around the doors and windows were filled with foam and I now see water seeping inside and wetting the walls. I want to put a layer of cement around the door and window to block this seepage.
I'm a first time DIYer. 2 points I need advice on:
1. What is the ratio of cement to sand I should use
2. What additive should I add to this mixture to make it waterproof.
I've bought this cement: https://www.toolstation.com/blue-circle-mastercrete-cement-handy-bag/p57285

I saw the below products regarding waterproofing:
1. https://www.toolstation.com/everbuild-202-integral-liquid-waterproofer/p80460
Is this the right additive? or can someone kindly recommend the right additive to make the cement waterproof.

Thanks a lot.

 

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image of doors from outside. I want to put a 10cm cement border around the door. So want this cement to be waterproof.

 

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When you say "sbr the area before applying cement", you mean, I first chip the dash coat off, then on the base, I first apply the sbr liquid, right? And once this dries, then apply the cement mixture, right?

Also, do I need to add the sbr to the sand/cement mixture too? If yes, what proportion?

I did a search and would you recommend this one? https://www.screwfix.com/p/cementone-sbr-admixture-white-5ltr/91566

Thanks.

 

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