Op-Amp Integrator's virtual ground
Is the question why it reaches 0.65 volts instead of 1 volt or is it about why it stops being a virtual ground?


  The question is why it stops being virtual ground


An op-amp with negative feedback tries to maintain the inverting input at the same voltage as the non-inverting input by adjusting the output voltage so that the potentials are equalized. Once the op-amp output hits the end-stops, it no longer can continue to adjust the output in the correct direction to maintain a virtual earth.