I found that this did exactly what I wanted with great results.I was using it to repair old tatty grouting in between grey granite patio slabs.
The job was done quickly once the vacuum sealed bag was opened - a must as it starts to set when the air gets to it, so you only have 45 minutes to complete while the geo-fix is still workable. The hard work was in doing the preparation, as the gaps need to be 25mm deep x 5mm wide(minimum), but the actual application of geo-fix was easy - just brush into the gaps and compact down (I used the edge of my hand) - repeat, when compacted until the gap, is filled with packed down geo-fix and then simply slide along with a brick mortar tool or use a short piece of hose pipe to give a nice u shaped smooth finish - then stand back and admire! 4 hours later it is set hard but is still water permeable so you won't get standing water on your patio when it rains.
Stops the weeds coming through the patio. Wonderful!