A段落

Concrete is the second most used substance in the global economy, after water – and one of the world’s biggest single sources of greenhouse gas emissions. The chemical process by which cement, the key ingredient of concrete, is created results in large quantities of carbon dioxide. The UN estimates that there will be 9.8 billion people living on the planet by mid-century. They will need somewhere to live. If concrete is the only answer to the construction of new cities, then carbon emissions will soar, aggravating global warming. And so scientists have started innovating with other materials, in a scramble for alternatives to a universal commodity that has underpinned our modern life for many years.

B段落

The problem with replacing concrete is that it is so very good at what it does. Chris Cheeseman, an engineering professor at Imperial College London, says the key thing to consider is the extent to which concrete is used around the world, and is likely to continue to be used. ‘Concrete is not a high-carbon product. Cement is high carbon, but concrete is not. But it is the scale on which it is used that makes it high carbon. The sheer scale of manufacture is so huge, that is the issue.’

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Not only are the ingredients of concrete relatively cheap and found in abundance in most places around the globe, the stuff itself has marvelous properties: Portland cement, the vital component of concrete, is mouldable and pourable, but quickly sets hard. Cheeseman also notes another advantage: concrete and steel have similar thermal expansion properties,  so steel can be used to reinforce concrete, making it far stronger and more flexible as a building material than it could be on its own. According to Cheeseman, all these factors together make concrete hard to beat. ‘Concrete is amazing stuff. Making anything with similar properties is going to be very difficult.’

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A possible alternative to concrete is wood. Making buildings from wood may seem like a rather medieval idea, but climate change is driving architects to turn to treated timber as a possible resource. Recent years have seen the emergence of tall buildings constructed almost entirely from timber. Vancouver, Vienna and Brumunddal in Norway are all home to constructed tall, wooden buildings.

E段落

Using wood to construct buildings, however, is not straightforward. Wood expands as it absorbs moisture from the air and is susceptible to pests, not to mention fire. But treating wood and combining it with other materials can improve its properties. Cross-laminated timber is engineered wood. An adhesive is used to stick layers of solid-sawn timber together, crosswise, to form building blocks. This material is light but has the strength of concrete and steel. Construction experts say that wooden buildings can be constructed at a greater speed than ones of concrete and steel and the process, it seems, is quieter.

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Stora Enso is Europe’s biggest supplier of cross-laminated timber, and its vice-president Markus Mannstrom reports that the company is seeing increasing demand globally for building in wood, with climate change concerns the key driver. Finland, with its large forests, where Stora Enso is based, has been leading the way, but the company is seeing a rise in demand for its timber products across the world, including in Asia. Of course, using timber in a building also locks away the carbon that it absorbed as it grew. But even treated wood has its limitations and only when a wider range of construction projects has been proven in practice will it be possible to see wood as a real alternative to concrete in constructing tall buildings.

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Fly ash and slag from iron ore are possible alternatives to cement in a concrete mix. Fly ash, a byproduct of coal-burning power plants, can be incorporated into concrete mixes to make up as much as 15 to 30% of the cement, without harming the strength or durability of the resulting mix. Iron-ore slag, a byproduct of the iron-ore smelting process, can be used in a similar way. Their incorporation into concrete mixes has the potential to reduce greenhouse gas emissions.

But Anna Surgenor, of the UK’s Green Building Council, notes that although these waste products can save carbon in the concrete mix, their use is not always straightforward. It’s possible to replace the cement content in concrete with waste products to lower the overall carbon impact. But there are several calculations that need to be considered across the entire life cycle of the building – these include factoring in where these materials are being shipped from. If they are transported over long distances, using fossil fuels, the use of alternative materials might not make sense from an overall carbon reduction perspective.’

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While these technologies are all promising ideas, they are either unproven or based on materials that are not abundant. In their overview of innovation in the concrete industry, Felix Preston and Johanna Lehne of the UK’s Royal Institute of International Affairs reached the conclusion that, ‘Some novel cements have been discussed for more than a decade within the research community, without breaking through. At present, these alternatives are rarely as cost-effective as conventional cement, and they face raw-material shortages and resistance from customers.’

A段落

混凝土是全球经济中使用的第二多的物质，仅次于水，同时也是世界上最大的温室气体排放来源之一。水泥作为混凝土的关键成分，在化学过程中产生大量二氧化碳。联合国估计到本世纪中叶，全球将有98亿人口，他们需要居住的地方。如果混凝土是建设新城市的唯一答案，那么碳排放将激增，加剧全球变暖。因此，科学家已经开始探索其他材料的创新，寻找替代这种多年来支撑我们现代生活的普遍商品的方法。

B段落

替代混凝土的问题在于它在所做之事上非常出色。伦敦帝国学院的工程学教授Chris Cheeseman指出，关键要考虑的是全球使用混凝土的程度，以及未来可能继续使用的程度。“混凝土并不是高碳产品，水泥是高碳的，但混凝土不是。但是它的大规模使用使得它具有高碳的特点。制造规模如此巨大，这才是问题所在。”

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不仅混凝土的成分相对便宜并且全球大部分地区都富含这些物质，它本身具有神奇的性能：混凝土的关键组成部分波特兰水泥具有可塑性和可流动性，但很快就会固化。Cheeseman还注意到另一个优势：混凝土和钢铁具有类似的热膨胀特性，因此可以使用钢铁加固混凝土，使其比单独使用时更具强度和灵活性。根据Cheeseman的说法，所有这些因素使得混凝土变得难以超越。“混凝土是一种了不起的东西。制造出具有类似性能的任何东西将变得非常困难。”

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替代混凝土的一个可能选择是木材。使用木材来建造建筑物可能看起来有点中世纪的想法，但气候变化正在驱使建筑师转向经过处理的木材作为一个可能的资源。近年来，出现了几乎完全由木材建造的高楼大厦。温哥华、维也纳和挪威的布鲁蒙达尔都有建造的高大木质建筑。

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然而，使用木材来建造建筑物并不简单。木材会随着吸收空气中的湿气而膨胀，并易受害虫以及火灾的侵害。但对木材进行处理并与其他材料结合可以改善其性能。交叉层压木材是一种工程木材，通过胶粘剂将多层实木交叉粘合在一起以形成建筑块。这种材料重量轻但具有混凝土和钢铁的强度。建筑专家表示，与混凝土和钢铁相比，木制建筑可以更快地建造，而且这个过程似乎更加安静。

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Stora Enso公司是欧洲最大的交叉层压木材供应商，其副总裁Markus Mannstrom表示，该公司看到全球对木材建筑的需求不断增加，其中关键驱动力是对气候变化的担忧。芬兰，拥有大片森林资源的地方，是引领这一领域的国家，但该公司也看到全球范围内对其木材产品的需求增长，包括亚洲地区。当然，使用木材建筑还能储存生长过程中吸收的碳。但即使经过处理的木材也有其局限性，只有在更广泛的建筑项目实践中证明其有效性后，才能真正将木材视为在建造高楼大厦中替代混凝土的可行选择。

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在混凝土配方中，粉煤灰和铁矿石炉渣可以作为水泥的替代品。粉煤灰是燃煤电厂的副产品，可在混凝土配方中占水泥的15%至30%，而不会影响配方的强度和耐久性。铁矿石炉渣是铁矿石冶炼过程的副产品，可以以类似的方式使用。将它们纳入混凝土配方中有潜力降低温室气体排放。

然而，英国绿色建筑委员会的Anna Surgenor指出，尽管这些废弃物在混凝土配方中可以节省碳排放，但它们的使用并不总是简单的。可以用废弃物替代混凝土中的水泥成分以降低整体碳影响，但需要考虑整个建筑生命周期的几个计算因素，包括这些材料的运输距离。如果它们通过化石燃料进行长距离运输，从整体减少碳排放的角度来看，使用替代材料可能并不合理。

H段落

虽然这些技术都是有前景的想法，但它们要么是未经验证的，要么是基于不充足的材料。在英国皇家国际事务研究所的Felix Preston和Johanna Lehne对混凝土行业的创新进行综述时得出结论：“在研究界，有些新型水泥已经讨论了十多年，但没有突破。目前，这些替代品很少有与传统水泥一样具有成本效益，并且它们面临原材料短缺和客户的反对。”