For example, the Architectural Service Department and the Housing Authority did not have purchasing policies for green products. 'We can see this from the case of the Hong Kong government's new headquarters which has not opted for any environmentally friendly construction materials,' Tsang said.

He called on the government to follow others' example of promoting the use of eco-friendly construction materials in public buildings.

'The government should formulate policies on adopting green products,' he said. 'A grace period of, say, three to five years should be set for industry players to apply for green labels during which green products would be given priority for use in government projects. This serves as a motivation and incentive for the industry and to reflect the government's commitment to green buildings.'

Tsang added: 'Being accredited with a green label is a must for such a scheme. Without a green label, a product would not be considered fit for government projects.'

Linda Ho, chief executive of Green Council, a non-profit organisation that approves green labels for products that meet international environmental standards, said Hong Kong lagged behind overseas markets.

'Taiwan, Japan and South Korea have laws requiring companies to give preferential treatment to green-labelled products. While it's not required by law in Singapore, the government offers many incentives for companies to use environmentally friendly construction materials,' she said.

'The Hong Kong government needs to follow suit with incentives for using environmentally preferable construction materials - such as introducing a green purchasing law.

'There are many sustainable construction materials suppliers in the city. They should form an alliance to promote the importance of choosing such products.

'The Green Council encourages consumers, developers, architects and interior designers to specify eco-friendly products in their projects. There should be more assistance and education for corporations to establish green-purchasing policies.'

The government, however, denied claims it was doing little to encourage sustainability in construction and manufacturing.

Since January, the government had issued a circular to encourage greater use of green and recycled materials in public works projects, a spokeswoman for the Development Bureau said.

'When the technical performance of a new green product proves satisfactory, there are adequate supplies and the price is comparable with conventional materials, the new product will be proposed for wider application in public works projects,' the spokeswoman said.

She added that all new government buildings more than 100,000 square feet must comply with international environmental performance standards.

In October, the Highways Department began using paving blocks made from recycled glass instead of concrete in road maintenance contracts, she said.

The new government headquarters at the Tamar site will feature many green elements such as a double-layer ventilated facade for better cooling and recycled timber or wood from sustainable forests, the spokeswoman said.

The Housing Department introduced environmental guidelines for contractors, suppliers and service providers in 1999, a spokesman said.

'For our construction works, we specify the use of processes, practices, materials and products that can avoid, reduce and control pollution so as to minimise the impact on the environment,' he added.

'Other green designs and construction techniques have also been adopted in new estates in past years to improve environmental quality, reduce energy consumption, reduce water usage and enhance waste management both during construction stage and after occupation.'

For example, wall tiling in lift lobbies and corridors had been replaced by the use of acrylic paint to reduce the carbon footprint and construction waste.

Ho, however, said the government tended to concentrate on building design when considering environmental issues.

'The government focuses on whether the design of the public buildings can use less lighting and air conditioning. But it has not gone far enough in using green construction materials,' Ho said.

CaSO is one of a growing number of trading companies specialising in eco-friendly construction materials in the city, Tsang (pictured) said. 'I believe that increasing public awareness of environmental issues means that eco-friendly construction materials have a bright future. That's why I spent my life savings setting the company up.'

Tsang, who studied building services engineering at Polytechnic University, said he had always wanted a construction career.

'There is always a demand for construction. Developers need to erect new buildings and many old buildings need to be redeveloped,' he said. 'Home or office owners also need to refurnish their buildings from time to time.'

After graduating in 1993, Tsang worked in marketing and sales of building materials when an increasing awareness of environmental issues led him in 2004 to set up his own firm selling environmentally friendly construction materials.

Tsang said his Augreen block wall system had won seven green label awards from Hong Kong and international associations. The blocks are lighter and dry quicker than their traditional concrete counterparts, allowing builders to use less than half the tonnage of conventional bricks. The product also has a longer life than traditional materials.

He said the key component of Augreen used limestone to absorb sulphur dioxide discharged from electricity plants. CaSO had linked up with several mainland electricity producers in the project.

'Sulphur dioxide from power plants is a main cause of air pollution and acid rain,' he said. 'We're turning a hazardous material into a green building product.'

Tsang said Augreen blocks generated less construction waste and made some procedures like plastering redundant.

Starting with capital of just HK$100,000 and five staff in 2004, Tsang became the Asia-wide agent for a mainland company that made the green construction material.

'It was a tough start-up back in 2004. Not many companies paid much attention to green concepts,' he said. 'Many construction firms stuck to the traditional way of using concrete blocks and plaster.'

Tsang gradually built up a client base. 'Some of our early customers were companies like MTR Corp and developers who wanted to promote eco-friendly concepts,' he said.

Today the company occupies an 80,000 sq ft factory and has completed 100 projects in buildings such as The One and iSquare, both in Tsim Sha Tsui. The Cova coffee chain is using Augreen blocks in the renovation of its Lee Garden shop. Michael Tsui Fuk-sun, co-owner of Cova, said his company was keen to support environmental protection concepts.

Other property developers have adopted a wait-and-see approach. An executive at Sun Hung Kai Properties, who did not want to be identified, said his company was keen on promoting environmental protection and was using energy saving lighting in its projects. But, he stressed, the company had to keep a lid on costs.

'In general, green construction materials are usually more expensive than conventional materials,' he said. 'In addition, the government has yet to offer any incentives to promote companies to use green-labelled construction materials. Government buildings themselves don't use environmentally friendly construction materials either. We would like to see the government take the lead.'

He said the government should offer policies and incentives to encourage the private sector to embrace green-labelled products.

Tsang said more companies today were focusing on green concepts than six years ago. 'If we offer two products for construction companies to choose, the first thing they will consider is the price. But if both products are a similar price, they prefer the green product.'

But cost control is still a big issue. Tsang is working with his mainland partner to move production from Shandong to Guangdong to be closer to Hong Kong to save transport costs and time.

'The outlook for green business is good. People are more concerned about air quality, pollution and climate change. Green-labelled products are easier to sell,' Tsang said.