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Reduction of Carbon Dioxide Emissions of Concrete

2020-02-29 来源: 51Due教员组 类别: Essay范文

本文主要讨论如何在混凝土生产中二氧化碳排放量的减少,此篇为范文供大家参考学习。在过去的几十年里,在生产混凝土的同时,已经做出了很大的努力来降低混凝土的碳足迹。硅酸盐水泥、粉煤灰和磨细高炉矿渣作为常用的胶凝材料,常用于生产自密实混凝土。二氧化碳排放主要与波特兰水泥生产有关。用粉煤灰、矿渣、硅灰、天然火山灰等辅助胶凝材料配制硅酸盐水泥混合水泥,取得了许多成功。此外,波特兰石灰石水泥也已被生产,以减少二氧化碳排放量。因此,本文提出并详细阐述了减少二氧化碳排放的几种方法。

 

In the past few decades, significant efforts have been made to lower the carbon footprint of concrete while producing concrete. As common cementing materials, Portland cement, coal fly ash and ground blast furnace slag are frequently used to produce self-compacting concrete. Carbon dioxide emissions are mainly associated with Portland cement production. Many attempts have been made and achievements have been gained by producing blended cements consisting of Portland cement combined with supplementary cementing materials, such as fly ash, slag, silica fume and natural pozzolans. Also, Portland limestone cements have also been produced to reduce the carbon dioxide emissions. Thus, several methods of reducing the carbon dioxide emissions are presented and illustrated in detail in this essay.

The replacement of normal Portland cement with fly ash has been researched and demonstrated by Atiş in his paper Accelerated carbonation and testing of concrete made with fly ash. Since carbonation is a chemical reaction that takes place between portlandite and CO2 and it is known that the process of carbonation is actually is a long-term reaction, to reduce the Emission of Carbon Dioxide, an accelerated carbonation test was carried out in order to assess the carbonation of fly ash (FA) concrete. And the concrete mixtures made with 0, 50 and 70% replacement of normal Portland cement (NPC) with fly ash were prepared with a controlled environment. Comparisons were made to evaluate the influence of FA on the carbonation of concrete.

According to Atiş, the Leeds accelerated carbonation apparatus was constructed to expose the concrete specimens to an environment where four variables could be controlled, i.e. the temperature, the concentration of CO2, the relative humidity and the pressure. And other two devices were added to the apparatus to get it improved.  The accelerated carbonation test was carried out with specimens at different curing ages and two different curing conditions. From the experiments, the following conclusions can be drawn. Fly ash concrete made with 70% replacement ratio showed higher carbonation than that of 50% FA replacement concrete and NPC concrete for both moist and dry curing conditions, while Fly ash concrete made with 50% replacement ratio showed lower or comparable carbonation than that of control NPC concrete for both curing conditions. Thus, high performance fly ash concrete with low potential for carbonation could be produced with the replacement level as high as 50% in weight basis.

Another replacement of SCC containing coal fly ash by crushed limestone dust has been investigated by C. Shi, Y. Wu and C. Riefler in the paper The Use of Crushed Limestone Dust in Production of Self-Consolidating Concrete (SCC). Crushed limestone dust is a waste material from the production of concrete aggregate by crushing quarried rocks. This research was undertaken to investigate the properties of SCC containing crushed limestone dust. Although it consists mainly of elongated particles and is much coarser than coal fly ash or Portland cement, it can be used satisfactorily for production of SCC. SCC containing shows less shrinkage than SCC containing coal fly ash. After the experiment that the SCCs, which were designed by partial replacement of aggregates with limestone dust, were measured, tested, cast into molds, placed back into the fog room, and then some were sealed and measured, while others were cured after demolding then transferred to the conditioned chamber for drying shrinkage testing. After series of experiments and data analysis, the following conclusions were made. Crushed limestone dust collected in a dust collector from crushing operations consists mainly of elongated particles and is much coarser than coal fly ash or Portland cement. However, it can be used satisfactorily for production of SCC. The properties of fresh SCC mixture containing limestone dust are very similar to those of the mixture containing fly ash. However, the former loses its flowability faster. Therefore, crushed limestone dust can be used to replace fly ash or Portland cement. In this way, the reduction of Carbon Dioxide emissions can be achieved.

Two ways of traditionally reductions in the clinker content of cement have been introduced through producing blended cement consisting of Portland cement combined with a supplementary cementing material (SCM). In response to growing pressures to reduce the clinker content in cement and hence the CO2 emissions associated with its production, the Canadian Standards Association (CSA A3001-08) introduced a new classification of cement in 2008, this being Portland limestone cement (PLC) containing up to 15% limestone. Other trials were also conducted at the Brookfield cement plant in Nova Scotia to evaluate the performance of a blended cement containing 15% ground granulated blastfurnace slag (an SCM) with that a blended Portland limestone cement containing the same amount of slag plus 12% interground limestone. Hence, in Thomas’ paper Lowering the Carbon Footprint of Concrete by Reducing the Clinker Content of Cement, the performance of concrete produced with blended Portland cement containing 15% slag and blended Portland limestone cement containing 15% slag and 12% limestone has been studied.

In this paper it has been demonstrated that a blended PLC with 15% slag and 12% limestone (Type GULb) can be produced to provide equivalent performance as a blended cement containing Portland cement and 15% slag (Type GUb). Both these cements gave similar performance to normal Portland cement (Type GU) produced from the same clinker. The Type GULb cement contains approximately 23% less clinker than the Type GU cement from Brookfield cement and this represents a very significant reduction in the CO2 associated with the finished cement. Given that the cements can be produced to provide equivalent performance in concrete it is possible that Type GU cement could eventually be replaced by Type GULb cement. Given that the Brookfield plant produces approximately 300,000 tonnes of cement annually, switching to Type GULb cement could reduce the CO2 emitted from the plant by approximately 70,000 tonnes each year.

Reduction of carbon dioxide emissions with the manufacture of concrete has been investigated by many researchers. Three methods of them have been demonstrated above. First, fly ash concrete could be a replacement of normal Portland cement. And the experiment results was that fly ash concrete made with 50% replacement ratio showed high performance with low potential for carbonation. Second, crushed limestone dust can be used to produce self-consolidation concrete with properties similar to those of SCC containing coal fly ash. SCC containing limestone dust exhibited strengths similar to that containing fly ash during the first seven days, but the former exhibited lower strength than the latter at 28 and 90 days due to the contributions. The former also have lower autogenous and drying shrinkages than the latter. Third, the performance of concrete produced with blended Portland cement of 15% slag and blended Portland limestone cement including 15% slag and 12% limestone has been discussed. Tests revealed that the combined use of blended PLC (Type GULb) and mixer-added SCM can result in very substantial reductions in the CO2 footprint of concrete. It can be seen that the content of Portland cement has been replaced gradually in order to lower the carbon footprint. And new cementing materials has been studied and put into practice to achieve both high performances with low environment impact, which needs more efforts from us.

 

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