How to fill this gap : internal concrete floor

[Steve Campbell]

Hi folks. See below. In taking up carpet to get new floor fitted, this section turned out to be very weak mortar that was falling to pieces (the carpet grippers just fell out). Looks like this patio door may once have been a window and the builders just filled the floor space after the concrete slab with crap weak mortar. I’ve dug it out (not difficult !). Now need to refill….but….what with and how ?

Basic DIY’r here so looking for advice. I presume I can blank off the underneath and door frame with some damp course material as per half of it already so whatever I put in doesn’t stick to door ?

I don’t have a mixer…how much “stuff” will it take….presume some kind of concrete mix ?

Total dimensions = 4cm deep, 17cm wide & 180cm long
Thanks