A TRIAC will switch DC on, but then you've got a problem -- it won't turn off. TRIACs are made to switch alternating current. The device will turn off when the voltage and current across the device is sufficiently low.
If you try something like this, the TRIAC will never turn off (view in fixed font or M$ Notepad):
.------o---------. | | | | | | | | | | .-. | | | | .-. | | | ( X ) --- '-' '-' - | | | | T _|_ | | --- V_A | '---o o-/ | | | | | '----------------' created by Andy´s ASCII-Circuit v1.24.140803 Beta
If you want to make a SSR to switch DC, use an optoisolator and a transistor or power FET. And the data sheet is right about 99.999% of the time, if it's not a preliminary data sheet. Always look at your wiring and circuit first.
Good luck Chris