Appunti completi Materiali cementizi e ceramici

Introduction to concrete

Concrete peculiarities

Concrete is a material that has some peculiarities:

• Great resistance to compression
• Great resistance to atmosphere

For these first two reasons, it’s used to build constructions that have to stand very large load and be able to survive for a very long time, differently from metals. Made of cheap raw material (crashed stones, sand, water), these aggregates have only one addition that is expensive: cement.

Cement is different from concrete: cement is the powder you put into the concrete, whereas concrete is the final material. For example, the wall is made of concrete that is made of cement. Concrete is resistant to compression but not to tensile and flexural loads. For this reason, in the last decades, we have been exploiting the use of reinforced concrete (concrete + steel rebars inside). This sums up the compression resistance and atmospheric resistance of concrete and the tensile resistance of the steel. At the same time, concrete protects the rebars from the environment like a “shield”. The two materials are compatible together and fit well to provide a nice material for construction. The problem is that the material changes over time due to environmental exposure: concrete modifies itself, which is not dangerous for concrete but for rebars.

Composition of concrete

Concrete is made of:

• The grey part that acts as a matrix; it is cement paste, made up of cement and water. It is made of a mixture of other components, it has a composite aspect, with darker spots (cement particles hydrated) and a white matrix that is like pores made of hydration products. Even more in detail, we see some spikes that are hydration products, which are reaction products between cement particles and water, they create bridges that keep all the material together.
• Within the matrix, the small white dots are aggregates, crashed stones, and sand.

Cement and concrete details

So, concrete is a composite material that is made of cement paste (cement + water) that acts as a matrix, and aggregates that are the reinforcement, made of sand and stones. We also may have some additives.

Mortar is cement paste with the addition of sand of fine aggregates with a diameter of 4 mm, while concrete has bigger aggregates, dimensions between 20 and 40 mm. Reinforced concrete has rebars inside.

Portland cement and alternatives

The first component is Portland cement used as a base material, the very first type of cement invented. It is a hydraulic binder, meaning it glues components together and "hydraulic" means it requires water and, after hardening, can work in contact with water in stable conditions. Very different from other binders like gypsum that produce hydration products soluble in water, so once it is hardened it will be dissolved.

There are other types of cement that are made starting from Portland one, they are called "blended" and are cheaper than Portland and more environmentally friendly. Portland cement generates CO₂ during its production. There is a trend to reduce its quantity in constructions. In addition, it needs very high temperatures like 450°C so a lot of fuel and equipment are required, which are very expensive. To reduce the quantity of Portland cement we use in addition pozzolanic materials, like Romans:

• Natural pozzolana, a mineral extracted from the cave that contains a lot of amorphous silica very reactive
• Artificial pozzolana, made of waste from other productions like silica fume, granulated blast slag from steel production, fly ashes from burning or thermoelectric plants

They are recycled, with a very noble and good ecological aspect. When cement reacts with water, it creates C-S-H gel that is a gel made of calcium and silicates oxides hydrated. The same reaction products can come from pozzolana in presence of calcium hydroxide, which gives the calcium oxide component. Pozzolana is not a binder itself, it becomes one when put in an alkaline environment containing calcium hydroxide Ca(OH)₂. Calcium hydroxide comes from the reaction of cement with water, so if we have pozzolana in the cement it can react with Ca(OH)₂ producing C-S-H gel.

Hydration and setting of cement

The cements that we use have very specific dimensions. Cement in contact with water reacts with the surface of a particle, the hydration reaction starts from the surface and then diffuses through the hydration layer toward the center. This can happen only if the particle size is in the order of tens of µm. Typical diameters are 2-90 µm. If the particle is too large, water will not be able to diffuse in the whole volume and we will be left with some parts not reacting, wasting cement.

If the particle is too small, we don’t want it for these reasons: we have not a good workability because they create a large amount of surface energy and it’s difficult to mix and work this paste, causing problems in the cast process. Moreover, if they are too small they react too fast, they have more surface than volume, all surfaces react with water and reduce the time to cast and shape our paste, solidifying immediately and reducing workability. To avoid this too fast reaction, we add gypsum that delays the moment when concrete becomes solid, called "set". Too small particles also produce heat quickly and when it cools down it will shrink, making them more difficult to achieve and expensive.

Steps of concrete hardening

We see now some details of hydration of Portland cement. We have water + cement at the beginning that gives cement paste that for t = 0 is plastic and workable, so it is simple to give a shape. After a certain time (depending on gypsum and the dimension of particles, typically 45 min) we lose plasticity and have initiation of setting. The material is solid, it can stand its weight but cannot be loaded, as it doesn't yet have the mechanical property to resist a load. However, if we take off the cast it maintains its shape. In this step, the hydration products create bridges and start to bind. The set is complete after some hours (12 h). Afterward, we have the hardening step that lasts some months; we still have a hydration reaction inside the concrete. The moment of mixing and setting we need to cast is when it’s fluid enough to fill the cast without leaving voids.

After the cast and the setting, we can take off the formworks. It is not mandatory but if we take them away, they are available to cast new parts. To keep the material hardening properly, we need to keep it saturated with water and insulated from the atmosphere where water would evaporate: we cover concrete with plastic sheets and maintain humidity. The stage of maintaining humidity and removing formworks is called curing. We can’t skip the curing stage because the concrete will be porous and not resistant.

Properties and dangers of concrete components

Calcium silicate hydrate, the second product, is responsible for mechanical resistance. Calcium hydroxide comes from Ca silicates. The hydration starts from the surface of particles forming a layer called “ettringite,” a layer of complex CaO, AlO, Si oxides, sulfates, and water (32 H₂O for each gypsum molecule). Ettringite expands in contact with gypsum, becomes fluffy like a gel: this gel is formed around the particle and forces the water to diffuse through it, slowing down the process. Water after the ettringite layer penetrates into the particles and produces hydrates. If the particle is too large, the center will remain dry. The white hexagons are Ca hydroxide crystals, which do not contribute to mechanical resistance.

Volume changes and concrete porosity

During the reaction, water and cement volumes modify: a fraction of cement will remain non-hydrated, while the rest of the cement reacts with a majority of water producing hydration products, trapping water inside their pores. The remaining water in excess, because we always put more water, remains free in capillary pores, so in the space between particles of cement: if water evaporates the pores remain empty. Less cement at the beginning means more porosity we reach and less durability and mechanical resistance.

The development of strength is related to hydrates formation. The more we keep curing, the higher strength we reach. After some days we have low strength but after 90 days we reach the max. As strength increases, porosity and permeability decrease.

Improving concrete properties

CSH gel forms lamellas spaced by water, interlayer water, which is the reason why it is a gel: this water is not dangerous from the point of view of corrosion because it is immobilized. Aggregates are solid, they have almost zero permeability, the matrix has permeability due to porosity. Between lamellas, we can also have porosity represented by micro and macro pores (size of micrometer) and inside free water, coming from the environment like humidity or rain: this water is dangerous for corrosion, it is called “capillary water,” above 50 nm of pores, this water can transport ions inside and outside of the material because it is free to move between pores.

Compaction and pore size

Macro image that represents huge voids of the size of mm: they are related to bad casting. During the process of casting, the concrete has not been compacted, and some bubbles of air remain entrapped. To compact, we usually vibrate the concrete to release entrapped air. Voids are the most dangerous. Pores are dangerous if their size exceeds 50 nm, they create the risk of free water transport. If voids are all in size of nm the concrete is called well-segmented and the risk decreases even if we have free water. The water to cement ratio W/C describes the max theoretical mechanical resistance. If we represent the mechanical resistance vs. W/C, we draw a decreasing relationship: decreasing W/C means higher resistance.

The plot represents the max mechanical resistance that we can obtain for each corresponding W/C value. Curing affects the resistance, if we perform bad curing we reach a lower value. If we represent a plot of resistance vs. curing time, the more you increase the time, the more we reach the max value of resistance allowed for that specific W/C ratio. W/C and curing time depend both on porosity (volume of pores): if curing increases, porosity decreases; if W/C increases particles are less near to each other, so porosity also increases.

Improving curing and concrete quality

Permeability is related to the transport of water through big macropores, if we are able to do pores segmentation reducing their interconnection, we also improve the resistance of material both mechanical and durability because we decrease permeability.

How to improve curing:

• Decrease W/C, minimum value 0.35, very challenging condition, expensive. Typical values are 0.5-0.6
• Leave formworks for longer periods
• Cover fresh concrete with plastic sheet
• Maintain the cast concrete humid

Blended cements and their benefits

We are not going in detail on blended cements again. We have benefits both from ecological and technological points of view. In general, they have very small particles, very small hydration products that fill the gaps between pores of cement hydrates, doing a very good segmentation cutting on half the cement pores. They develop strength slower, they need first cement to react due to calcium hydroxide, after cement reaction pozzolana starts to react: this means slow production of heat and no overheating. The dark side is that they require longer curing time. For example, Portland needs about 14 days, blended cement needs 18 days, but overall the concrete will have a better quality. Pozzolana reaction consumes Ca(OH)₂, responsible also for alkaline conditions: typical fresh Portland concrete pH is about 13. Rebars passivate in these conditions with freshly produced concrete and reinforce concrete, they don’t need any other coating. Fresh blended concrete has a starting pH lower, about 12, so rebars still passivate but during time concrete will lose its alkalinity.

Cement classification

Table of ISO of different composition of cement: they give roman numbers, the classification is according to composition and content of species.

• Portland cement I
• Blended cement II, the most important is limestone: it does not contain pozzolana materials, it is only filler, not reactive but inert material. It is composed of very small particles that fill the voids. As resistant as pure Portland cements.
• Ground granulated blast slag III: like section II but with a higher quantity of GGBS
• Pozzolanic IV
• Composite V: fixed quantity of GGBS + pozzolanic materials

Reading cement names

How to read cement name on tables:
CEM II A/L 32.5 R:

• CEM: cement
• Roman number: class of cement, in this case, class two, blended
• Letters: refer to type and quantity of mixture, A/L means limestone in small quantity or B/L limestone in high quantity; fly ashes are indicated with V or W; pozzolana with P or Q.
• Number: class of compressive strength, all materials produced with this mixture with W/C of 0.5 we obtain exactly this number of MPa or higher: min mechanical resistance that we can obtain. If we want higher resistance we add more cement to W/C ratio. To obtain the same R value if we have 32.5 of class of strength we need low W/C or if we have 42.5 we can add more water.
• R: it can be present or not, if not present can show N “Normal”, R “rapid set” react faster with water

The role of aggregates in concrete

Concrete is mainly made of aggregates, almost 60-85% of the interior volume, and 40-15% in volume of cement pastes. This is given by three main reasons:

1. First of all, aggregates are less expensive than cement, and therefore it is easier to build a structure with a cheap material such as aggregates held together by an efficient glue that is cement paste.
2. Aggregates are very resistant to compression, so they contribute very importantly to the resistance of the material. In fact, the weakest part of all the material in general is the interface between cement pastes and aggregates. So in the end, we have a cement paste that is resistant, as well as aggregates while the interface between the two is a discontinuity and this makes it weaker than the rest of the material. The interface is therefore very important to take into account when dealing with the material.
3. Aggregates reduce the content of cement pastes, and this is not only an economic consideration, but is also related to the reduction of the heat of hydration that develops during cement reaction. In addition, they not only reduce heat of hydration but also shrinkage.

Cement paste in fact is reduced (shrinks) for a certain hydration value, so considering an initial volume of 1 that includes both cement and water, we get a final volume of cement that could be as much as 0.7. Shrinkage is therefore very consistent in this case. So if we build a component only with cement paste it will break forming cracks because it will shrink too much. In concrete, shrinkage is balanced by the presence of aggregates that are not reduced because they are solid, shrinkage is therefore limited only to the veil of cement paste that unites all the aggregates together. It is always a problem to consider, but is less relevant than in the case of a material completely in cement paste.

In addition, aggregates also have a hard surface, in other words, they are not only highly resistant to compression but are also hard, which makes concrete more resistant to weather conditions. We have to consider that most of the buildings in which we live are composed of concrete and their surface is constantly exposed to the atmosphere. In particular, the atmosphere can give rise to the phenomenon of erosion given by the sand contained in the wind or even by rain. The presence of a hard component is therefore also very important for this reason.

Aggregates specifications

There are certain conditions to be fulfilled to be defined as aggregated. We will not dwell, but certainly, the most important condition is that of size. In particular, we consider:

1. Fine aggregate (sand), with diameter < 4 mm
2. Coarse aggregate (gravel), with diameter > 4 mm

Shape and composition of aggregates

As far as shape is concerned, aggregates can be:

• Angular (crashed), and we indicate a very irregular surface
• Round (natural), and we indicate a spherical surface

The first type is generally extracted from the caves, in practice, we break a large piece of rock into small pieces through the use of specific tools, which leads us to have compositions of the surface of the irregular aggregates. The second type is very common in the case of the river bed because the water levels the aggregates through a corrosion action.

This fact is very important in the case of wettability, and therefore workability of the material; in practice, everything that is round can help to make the mixture flow in a better way. On the other hand, anything that is irregular makes it much more difficult to flow on the surface. In the end, then we end up with less workability when we use crashed aggregates rather than round aggregates.

Another very important parameter for aggregates is the maximum size, which in turn also influences the workability of the system.

It is also important to consider the composition of aggregates. In general, we work with both carbonates and silicate rocks which means for example silica (a typical sand that can prevent from the sea with some limitations). However, there are also other types of aggregates (stuffed, fumous) that are very porous rocks that have a compression resistance and a very low hardness. They are only used when we want to build a very light concrete, so something very light with a low density for special applications. When we work in typical applications we want aggregates to be at least as resistant as cement paste so as to give enough resistance to all the material.