George Osborne's spending plans are expected to receive more detail when he delivers his Autumn statement this afternoon.
But only £5bn of the touted £30bn will be injected into the economy by 2015, with a further £5bn to follow in the next parliament.
The government hopes the remaining £20bn will come from National Association of Pension Funds and the Pension Protection Fund, though no deals have been confirmed yet.
Separately it is also seeking more investment in infrastructure from insurance companies and from China.
Osborne said: "We are finding the resources in difficult times to build the railways, to build the roads. Britain's got to get away from the quick-fix debt solutions that got us into this mess.
"We have got to weather the current economic storm and we have got to lay the foundations for a stronger economic future.
"We have got to make sure that British savings in things like pension funds are employed here and British taxpayers' money is well used."
But Paul Johnson, director of the Institute for Fiscal Studies, said the £5bn of government investment over three years was a "pretty small number", as capital spending was expected to be cut from £40bn in 2010-11, to £24bn in 2013-14.
The money raised from private investors appeared to be "aspirational" rather than guaranteed, he added.
- Full details of the Chancellor's Autumn Statement will be published on The Construction Index this afternoon.