in case you have amorphous phases in your sample an external or internal standard is recommended. The disadvantage with an internal standard is that the sample is diluted and a smooth homogenisation has to be assured. With an external standard you can overcome this problem.
A method which proved to work very good for cement samples is the G-factor method (external standard). I have attached a publication which should help you.
Kind regards,
Sebastian
Article A Remastered External Standard Method Applied to the Quantif...