二氧化碳排放量与混凝土的生产减少的关系
Reduction of Carbon Dioxide Emissions with the Manufacture of Concrete
Since 1988, self-compacting concrete (SCC) has been developed and widely used to make durable concrete structures. And various trials have been conducted to get the properties of self-compacting concrete. The improvement of the concrete and the reduction of carbon dioxide emissions have become increasingly important. Countries like Japan, Sweden, and Switzerland have taken this concrete into practice. In Japan, SCC has been used mainly by large construction companies for tall office buildings and advanced extruded tunnels combined with steel fibers. In Sweden, SCC has been widely used for highway bridges and buildings’ slabs, while it has been mainly used for walls in Switzerland. Use of SCC benefited the surroundings by lowering the noise level and diminishing the effect on the environment.
According to Bertil Persson’s experiments in his “A comparison between mechanical properties of self-compacting concrete and the corresponding properties of normal concrete”, SCC with quartzite filler obtained about 20 MPa higher strength at w/b=0.40 and about 5 MPa higher strength at w/b=0.80 compared with normal concrete(NC) without filler. Also, the two types of concretes corresponded well when the porosity was held constant. What is more notable is that the filler added to the SCC did not at all affect self-desiccation and the filler content of SCC did not delay desiccation. The creep coefficient shows similar tendencies in SCC and NC. The moisture conditions, aggregate content and loading conditions being equal when testing SCC and NC, the results of creep also will be alike.
To improve the performance of SCC, cementing materials are diverse to produce it. Portland cement, coal fly ash and ground blast furnace slag are common cementing materials. Different mixtures contributed to different performance. Atis assessed the carbonation of fly ash (FA) concrete using an accelerated carbonation test. By controlled environment, the concrete mixtures made with 0, 50 and 70% replacement of normal Portland cement (NPC) with fly ash were prepared, with Water-cementitious material ratios ranged from 0.28 to 0.55. This accelerated carbonation test used specimens with two different curing conditions and different curing age, and started at 3,7,28 days with concrete mixes at a 3-month age. The specimens out of curing chamber were immediately put into the accelerated carbonation apparatus. Surfaces of the moist cured concrete specimens were dried before putting them into carbonation chamber. Dry and moist cured specimens were carbonated together. And other strictly controlled conditions were adopted to finish this test. The following conclusions were made from this test. 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. Fly ash concrete made with 50% replacement ratio showed lower or comparable carbonation than that of control NPC concrete for both curing conditions. The influence of the superplasticizer on the carbonation depth was found to be insignificant.
To achieve the environment-friendly effect, crushed limestone dust, as a waste material from the production of concrete aggregate by crushing quarried limestone rocks, was used to produce SCC with properties similar to those of SCC containing coal fly ash, which was found by C. Shi, Y. Wu, and C. Riefler and published in the “The Use of Crushed Limestone Dust in Production of Self-Consolidating Concrete (SCC)”. Similarly, using controlled environment, different material content ratios led to different performance. Several design procedures had been proposed for SCC based on scientific theories or empirical experiences, to achieve required flow ability, passing ability and good segregation resistance. The commonly used powders included coal fly ash, calcined clays, ground glass and blast furnace slag. Different powders could show very significant effects on the requirement of superplasticizer, deformability, filling capacity and strength of concrete. For example, fly ashes had spherical particles and may decrease the water requirement for given flow ability and superplasticizer dosage or decrease superplasticizer dosage for a given water content and flow ability. The other powder materials had irregular particles and their effects on water requirement or flow ability are dependent on their particle size and shape. The addition of limestone powder into cement could affect the hydration of cement, the rheological properties of fresh cement pastes and concrete mixtures and properties of hardened concrete. The use of crushed stone dust in SCC could decrease the materials cost of SCC, eliminate the dust disposal cost and reduce environmental pollutions. This study investigated the properties of SCC using crushed limestone dust as filler in comparison with that using ASTM Class F fly ash, which was widely used in normal concrete and SCC. In this study, the SCCs were designed by partial replacement of aggregates with limestone dust to achieve high flow ability and passing ability without showing aggregation. The following conclusions could be drawn from this study. The properties of fresh SCC mixture containing limestone dust were very similar to those of the mixture containing fly ash. However, the former lost its flow ability faster. SCC containing limestone dust set faster than the mixture containing coal fly ash. The two SCCs exhibited similar strengths during the first seven days. However, the SCC containing limestone dust gained less strength than the SCC containing coal fly ash due to the contributions from the pozzolanic reactions between coal fly ash and lime released from the hydration of Portland cement.
After various investigations people had made to SCC, environment problems were paid serious attention. Significant efforts had been made to reduce the CO2 emissions along with the manufacture of Portland cement. Michael Thomas in his “Lowering the Carbon Footprint of Concrete by Reducing the Clinker Content of Cement” studied how to lower the Carbon footprint by lowering the clinker component of the cement as the pyroprocessing used to manufacture clinker produces approximately 1 tonne of CO2 for every tonne of clinker. One measure for reducing the CO2 footprint of Portland cement was to reduce the clinker content of the cement. Historically this had been achieved by producing blended cements consisting of Portland cement combined with supplementary cementing materials (SCM) such as fly ash, slag, silica fume and natural pozzolans. This study presented data from laboratory and field studies on the performance of concrete produced with blended Portland cement containing 15% slag and blended Portland limestone cement containing 15% slag and 12% limestone. The study showed 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 combined use of blended PLC (Type GULb) and mixer-added SCM can result in very substantial reductions in the CO2 footprint of concrete.
The manufacture of concrete and the reduction of carbon dioxide emissions have drawn people’s attention. First, for concrete’s manufacture, experiments are the basic tool for us to study cement and concrete. In an experiment, keeping the controlling conditions and setting a series of parameters for contrast are essential to get a satisfactory result. Second, the clarity of targets is quite important. We must know what we want to conduct and narrow the targets. There are many types of cementing materials and concretes. Even slightly different material ratios can lead to different concrete performances, so rigorous experiments are essential for this subject. More importantly, with the development of this subject, people tend to pay more and more attention to the environment. They prefer to produce environment-friendly materials or reuse the waste, and new produced cementing material and concrete must benefit the environment. The reduction of carbon dioxide emissions should be one of today’s main intents to study this subject.
REFERENCES
[1] Atiş, Cengiz Duran. "Accelerated carbonation and testing of concrete made with fly ash." Construction and Building Materials 17.3 (2017): 147-152.
[2] Thomas, Michael, et al. "Lowering the Carbon Footprint of Concrete by Reducing Clinker Content of Cement." Transportation Research Record: Journal of the Transportation Research Board 2290.1 (2017): 99-104.
[3] Hooton, R. D., A. Ramezanianpour, and U. Schutz. "Decreasing the clinker component in cementing materials: performance of Portland-limestone cements in concrete in combination with supplementary cementing materials." NRMCA Concrete Sustainability Conference. 2017.
[4] Riefler, Monte. "PARISON OF SELF-PACTING AND CONVENTIONAL CONCRETES." Proceedings of the 5th International Symposium on Cement and Concrete (Volume 2). 2017.
[5] Persson, Bertil. "A comparison between mechanical properties of self-compacting concrete and the corresponding properties of normal concrete." Cement and concrete Research 31.2 (2017): 193-198.
[6] C, Shi, Y, Wu, and C, Riefler. "The use of crushed limestone dust in production of self-consolidating concrete." Recycling Concrete and Other Materials for Sustainable Development 219 (2017): 115-129.
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