Prática de Shadowing: Introduction 2, Why "Portland" cement - Aprenda a falar inglês com vídeo
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Okay, so welcome back.
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In this second unit of this introductory module, I've called it Why Portland Cement?
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And what we're going to look at in this module is why the composition of cement is as it is.
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Why Portland cement?
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99.99% of cement made today is based on what we call Portland cement or more precisely Portland cement clinker.
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So clinker is these nodules you see in the picture here and that's the material that comes out of the cement kiln.
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And the name Portland, this is purely a marketing name,
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this was chosen by Joseph Aspin who was the person who patented the term Portland cement in 1824
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and what he wanted to do, he he wanted to highlight the appearance of his new material to Portland stone
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which was then regarded as the best building material in England.
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So don't worry about this word Portland, it's purely a marketing term.
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So the dominance of this type of cement is really not by chance.
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It's a direct consequence of the raw materials which we have available on earth.
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So here on the left, we see the composition of the Earth's crust.
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And it's really remarkable to see that we have just eight elements, that's to say oxygen, silicon, aluminium, iron, calcium, sodium, potassium, magnesium.
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These together make up more than 98% of the Earth's crust.
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This is quite remarkable, but those of you who may have done some astronomy
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will realise that it's a direct consequence of the formation of elements in stars.
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and it also means that this is not just the composition of the Earth
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but will be the similar composition for all the rocky planets we can imagine in the universe.
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What this means is that even elements we regard as quite common,
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like for example copper, are very very much less abundant than these eight rock forming elements,
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like for example silicon.
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And this relative abundance of 1 to 1000 will translate in terms of the availability and the cost of these materials.
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So having seen in the last module how the very large amount of cement is produced,
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clearly we can't think about making cement out of all the other elements that's in that remaining 2%.
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So now we can understand that the composition of the earth limits the practical chemistries we can have for cementitious materials.
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But looking on the bright side, it also means that we can explore all possible options.
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It means that there's no kind of breakthrough out there that we haven't discovered yet.
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And what we need to really know in a bit more detail is how cement works
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to understand how these different elements can be used.
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So this is what I've illustrated in the next few slides.
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So this schematic here really tries to explain this.
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When you mix grey cement powder with water, what you have is grains just floating about,
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and that is very convenient because it means you have a flowable material, you can cast into moulds, you can make into different shapes.
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And then we have a chemical process whereby these grey cement grains are dissolving
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and then the ions are reacting with the water to give us this new solid which are called the hydrates.
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So in this image here the red areas are the cement hydrates and these have a higher volume of solid
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and therefore it holds the cement grains together, creating a rigid solid.
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So let's go back and look at this composition of the earth.
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Most of those elements are associated with oxygen as oxides,
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and look at whether these different oxides can react to give us a cementitious material.
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Now if we take the two alkali oxides, sodium and potassium, the problem here is these produce very, very soluble salts.
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So maybe in your chemistry lessons you've used sodium hydroxide
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and you'll know that you can have very strong sodium hydroxide because a very high amount can go into solution without precipitating.
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So clearly this is no good for producing solid hydrates which hold the structure together.
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On the other hand, if we look at iron oxide and magnesium oxide, the problem here is that these oxides have fairly low mobility in alkaline solutions,
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which is the solutions we have in cementitious materials.
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So what we can see in this image here, this is just a piece of concrete taken from a normal concrete wall.
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And in this image we have some very bright areas.
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And these very bright areas are where all the iron oxide is located.
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And in fact, those bright areas are exactly the same as in the original cement grains.
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During that whole 30 years, that iron in the cement grains has just stayed in the same place.
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So it's not doing any harm, but basically it means it's not going out into solution,
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it's not precipitating new solids and therefore making very little contribution to the bonding and the development of strength.
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It's a similar story for magnesium.
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Here in this picture now, we see a blended cement.
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We're going to talk about that in a few modules time.
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And in this blended cement, the darker grains, such as in the center of the picture here,
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you see are now surrounded by a region of dark products.
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And these darker products are because all the magnesium that was in that slag grain is concentrated in that area.
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And same situation, this magnesium has not moved into the space between the grains.
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it's not really contributed to bridging the space between them.
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So this means we end up with just three oxides, which are really the most useful, and are the essential component for the Portland cement we have today.
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So this is really in no way a coincidence that we've ended up with this composition.
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Because if we now look at that ternary system between calcium oxide,
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silicon dioxide and alumina, we see the composition of Portland cement.
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And in fact, Portland cement, as we see it here, this is composed of calcium silicates.
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There's really only one other region in this diagram where we have minerals
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that will react with water in the way we previously saw and give us cementitious materials.
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And that's this region here, which is the region where we find calcium illuminate cement and calcium sulphur illuminate cement.
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Small amounts of sulphur is the sort of extra dimension which is not shown in this diagram here.
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So these other compositions, calcium aluminate,
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calcium sulphur aluminate, have a lot of advantage because they have less calcium in.
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You see, they're further away from that calcium oxide corner, means they have less calcium oxide,
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and therefore there's less CO2 produced chemically during the production of these phases.
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So that sounds very good, but the problem is that it's not so easy to make these
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because the raw materials we need are not so widely available.
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And we have to ask the question, what sources of minerals are there which contain much higher amounts of aluminium to silicon?
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Most minerals have about the same ratio of silicon to aluminium as in Portland cement, about two to one.
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To get a composition down here in this alumina rich part of the diagram, we have to go to materials like bauxite.
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Now bauxite is much more localized than the materials used for making Portland cement.
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It's estimated that only 10 countries contain about 90% of the reserves of bauxite.
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And of course bauxite is the first raw material for aluminium production.
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So the consequence of all this means that it's rather expensive
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and it means that you cannot produce calcium aluminate cement
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or calcium sulphur aluminate cement for the same cost as producing Portland cement.
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We can look at this effect of chemical CO2 and how it's related to composition in more detail on this slide here.
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So at the top we see the two calcium silicates which are present in Portland cement.
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These are known as tricalcium silicate or C3S, dicalcium silicate or C2S.
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And many people have proposed that what we should be doing is making Portland cement with more C2S as opposed to C3S.
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But the problem here is that the reduction in chemical CO2 is really very marginal.
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It's only around 10%.
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And that's more than offset by the much slower kinetics of C2S.
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So the strength development is much slower.
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And because the strength development is much slower, this would mean that in practical situations where people need a given strength at a given time,
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then they will use more cement.
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And that use of more cement will completely cancel out this 10% saving.
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So belight-rich clinkers really don't contribute at all to lowering CO2 emissions.
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Down here, we can see the figures for yellow mite.
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That's this complex phase called C4A3S bar.
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and as I said on the previous slide, this has a good reduction potential in terms of lower chemical CO2.
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But we have the problem of the cost of the raw materials.
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And finally on this slide I want to point out this line here
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because many people nowadays are proposing that we should use magnesium-based cements.
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There can be context in which this might be possible,
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not now, but if somebody can invent a way of producing magnesium-based cements from magnesium silicate rocks,
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it might be interesting.
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But what's happening at the minute, most people are using magnesium carbonate.
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And what you can see here very clearly is
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that the chemical CO2 emissions from a breakdown of magnesium carbonate
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are way higher than they are from the calcium silicate phases we have in Portland cement.
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So here we come back to that composition of the earth we've discussed in detail,
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and we compare it with the composition of a typical Portland cement.
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And you can see the elements present are similar.
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The big difference between the Portland cement and the composition of the earth is the dominance of this sector for calcium.
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And this is, as we explained in the last lecture, where the chemical CO2 is coming from.
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But from a practical point of view, it's very easy to make things with this composition because limestone, which is usually very pure calcium carbonate,
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is very widely distributed across the globe.
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And it means that you can make these porthens cements out of limestone.
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The other elements are generally coming from something like clay, but you can make this almost everywhere in the world.
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And therefore, transportation of the final product is minimized.
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So the other important aspect is the pattern of reaction of the Portland cement.
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And this is shown in this diagram here, which plots the heat evolution, which is a signature of the rate of reaction.
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And what we can see is at the beginning of the reaction, we have a burst of reaction,
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and then we have this period here where the reaction remains slow for several hours.
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From a practical point of view this is extremely important as it gives us time to mix the cement
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and transport it to the building site.
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And then after about three hours the reaction takes off again.
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Now this pattern of reaction means that really it's very very simple to use cementitious materials.
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Here we see a picture from India where people are making concrete under really quite rustic conditions.
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But nevertheless, because it's such a robust chemical reaction, it still works, it still gives very good quality building materials.
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Really, what we're going to see is that for these reasons I've explained to you here,
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cements based on Portland clinker will be the most important materials for the foreseeable future.
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They can be made from widely available raw materials, there's an incredible economy of scale, leading to very low cost,
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and they're very easy to use even by unskilled workers.
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Given the composition of the earth, this composition, Portland cement clinker, really is the best option.
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But in the next lecture, what we're going to see, how by replacing part of this Portland cement, we can lower environmental impact.
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So, thank you very much.
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Vocabulário e dicas de fala para esta lição
Esta aula de conversação de nível C1 usa o vídeo “Introduction 2, Why "Portland" cement”. As palavras que mais se repetem: cement, Portland, material, composition, calcium. Este vídeo tem 139 frases e 2024 palavras para praticar shadowing. A fala dura 14:22. O falante mantém um ritmo estável de cerca de 141 palavras por minuto, confortável para o shadowing. Apenas 80% das palavras estão entre as 3.000 mais comuns do inglês, por isso o vocabulário é exigente.
Vocabulário principal deste vídeo
As 15 palavras mais avançadas do vídeo, com pronúncia e significado:
| Palavra | Pronúncia | Significado |
|---|---|---|
| cement substantivo | /səˈmɛnt/ | cimento |
| calcium substantivo | /ˈkælsi.əm/ | cálcio |
| oxide substantivo | /ˈɑksaɪd/ | óxido |
| magnesium substantivo | /ˌmæɡˈnizi.əm/ | magnésio |
| silicate substantivo | /ˈsɪlɪkət/ | silicato |
| silicon substantivo | /ˈsɪləˌkɑn/ | silício |
| aluminate substantivo | aluminato | |
| bauxite substantivo | /ˈbɔːksaɪt/ | bauxita, bauxite |
| sulfur substantivo | /ˈsʌl.fə/ | enxofre |
| aluminium substantivo | /ˌæl.(j)uˈmɪn.i.əm/ | alumínio |
| diagram substantivo | /ˈdaɪ.ə.ɡɹæm/ | diagrama |
| sodium substantivo | /ˈsəʊ.di.əm/ | sódio |
| module substantivo | /ˈmɑd͡ʒul/ | módulo |
| carbonate substantivo | /ˈkɑɹbəneɪt/ | carbonato |
| hydrate substantivo | /haɪˈdɹeɪt/ | hidrato |
Phrasal verbs que você vai ouvir
| Palavra | Pronúncia | Significado |
|---|---|---|
| go back verbo | voltar | |
| make up verbo | /ˌmeɪk ˈʌp/ | compensar |
| point out verbo | apontar | |
| take off verbo | tirar |
Gramática neste vídeo
As estruturas que o falante mais usa, com as palavras exatas do vídeo:
| Estrutura | No vídeo |
|---|---|
| Voz passiva be + particípio passado — o foco está no que acontece, não em quem faz | is based · was chosen · is produced |
| Present perfect have/has + particípio passado — uma ação passada que ainda importa agora | I've called · haven't discovered · I've illustrated |
| Orações relativas who / which + oração — informação extra sobre uma pessoa ou coisa | person who patented · solid which are · oxides, which are |
Pronúncia para ficar de olho
O falante usa 17 contrações e formas reduzidas, como we're, I've, don't. Diga-as na forma curta, do jeito que você ouve.
- Os sons de “sh” e “zh”: cementitious /ˌsɛmɛnˈtɪʃəs/, emission /ɪˈmɪʃ.ən/, dimension /daɪˈmɛn.ʃən/
- Palavras longas — acerte a sílaba tônica: magnesium /ˌmæɡˈnizi.əm/, cementitious /ˌsɛmɛnˈtɪʃəs/, aluminium /ˌæl.(j)uˈmɪn.i.əm/, alumina /əˈl(j)uːmɪnə/, illuminate /ɪˈl(j)umɪneɪt/
Sons difíceis para falantes de português:
- Consoante final — sem acrescentar um “i” depois: cement /səˈmɛnt/, oxide /ˈɑksaɪd/, silicate /ˈsɪlɪkət/, bauxite /ˈbɔːksaɪt/, carbonate /ˈkɑɹbəneɪt/
- /l/ final — a língua toca o céu da boca, não vira “u”: module /ˈmɑd͡ʒul/, mineral /ˈmɪn.ə.ɹəl/, foreseeable /fɔɹˈsi.ə.bəl/, nodule /ˈnɑd͡ʒul/, soluble /ˈsɑljəbəl/
- /h/ — um sopro suave, diferente do “r”: hydrate /haɪˈdɹeɪt/, hydroxide /haɪˈdɹɒks.aɪd/
Como praticar com este vídeo
- Ouça o vídeo inteiro uma vez sem falar e anote as palavras que você não conhece.
- Faça shadowing frase por frase na velocidade normal, repetindo cada uma até o seu ritmo ficar igual ao do falante.
- Grave a sua voz e compare com o original, prestando atenção a palavras como cement, calcium, oxide.
O que é a Técnica de Shadowing?
Shadowing é uma técnica de aprendizado de idiomas com base científica, originalmente desenvolvida para o treinamento de intérpretes profissionais. O método é simples, mas poderoso: você ouve áudio em inglês nativo e repete imediatamente em voz alta — como uma sombra seguindo o falante com 1-2 segundos de atraso. Pesquisas mostram melhora significativa na precisão da pronúncia, entonação, ritmo, sons conectados, compreensão auditiva e fluência na fala.