Cementizi e ceramici appunti

Cementitious and ceramic materials engineering

Introduction to concrete

Concrete is characterized by:

• Good resistance to compression
• Good resistance to water and atmospheric agents
• Good compatibility with steel rebars, to make it work under tensile conditions
• Easy to produce, even with complex geometry
• Low cost: raw materials, easy to find on earth. The only expensive ingredient is cement, a binder, but it’s used in a small amount compared to the full concrete volume

Aims to follow: reduction of emission production and use of recycled materials.

Rebars are characterized by:

• Very good mechanical resistance to tensile stresses
• Very good compatibility with concrete: steel remains passive in alkaline environments, in this way rebars are protected from the external environment.

The use of concrete with metallic rebars leads to the use of reinforced concrete. Concrete is a composite material, made of aggregates (minerals, stones, sand) kept together by the cement paste (cement and water), in this sense we can say that the matrix is the cement paste and particles are the aggregates. There’s the formation of a needle-like structure connected by bridges, the connection is obtained by hydration products; porosity is reduced, and the material is able to sustain loads.

• Cement paste = cement + water
• Mortar = cement + water + sand (d < 4 mm) (used for concrete repair and surface finishing)
• Concrete = mortar + gravel
• Reinforced concrete = concrete + rebars

Portland Cement: it’s the most diffuse type of cement, during its production a lot of CO₂ emission is produced, for this reason blended cements, which keep properties but with a reduced environmental impact and costs, are produced, too. It is produced mainly by calcium carbonate 2→ + ( ), and by burning coal in the furnace, too. It’s a hydraulic binder: after it has reacted with water and assumes its own structure by the formation of hydration products, it can also work in contact with water without dissolution of cement paste (there are some non-hydraulic binders like gypsum which in contact with water dissolve).

Portland cement is a binder made of fine inorganic mineral powders (submicrometric granulometry to avoid too high porosity), which when are mixed with water, form a paste that sets and hardens due to hydration reactions. Once hardened, it’s stable also in direct contact with water. , , , , Portland ingredients: +clay gives the formation of which by hydration 3 2 3 3 3 form CSH, CAH and Ca(OH)₂: calcium silicate hydrates, calcium aluminate hydrates and release of calcium oxide which is present in excess and isn’t stable in water producing calcium hydroxides.

Gypsum is added to slow down the initial reactions between water and cement, giving time to mix and cast in a proper shape. Blended cements are based on amorphous silica, which is a component also of cement (bound to calcium oxide); blended cement = Portland cement + pozzolanic materials (don’t contain CaO, only silica) like natural pozzolana, fly ash, silica fume, ground granulated blast furnace slag. In presence of calcium hydroxides, which are reaction products of Portland cement hydration, there’s the formation of the same reaction products of cement. + + () → 2.

Blended cements have smaller porosity, durability is higher but mechanical resistance is a bit lower.

Hydration of Portland cement

Mixing water and cement we get the cement paste, which at the beginning is plastic and workable, not rigid, it can flow. After some minutes (less than 1 hour) hydration reactions become relevant and we lose plasticity: this is the starting point of “setting”, which is the stage in which the material can’t change shape anymore a certain level of hydration is reached, after which material’s shape can’t be modified.

Generally, time of setting is about 12 hours; after setting, the form work can be removed, but mechanical resistance has still to be achieved, since hydration reactions require more time to confer sufficiently high mechanical properties. To have the perfect formation of good quality cement paste (or concrete), we need to keep the material isolated from environment (otherwise equilibrium with atmosphere would lead to loss of water content). The hardening period is important to reach high mechanical resistance, depending also on relative humidity conditions and temperature (curing process).

Aluminates react first and are the main responsible for setting, their hydration leads to the formation of calcium aluminum hydrates. Hardening is governed by silicates hydrates: and hydration leads to calcium silicate hydrates forming the C-S-H gel 50-60% of completely hydrated cement paste volume. CSH is made of very small particles with a layer structure that tends to aggregate in formations a few microns in dimension, characterized by interlayer spaces of small dimensions and by a large surface area, giving considerable strength to the cement paste.

Hydration of calcium silicates produces also crystals of portlandite () , 20-25% of volume of completely hydrated cement paste, which don’t contribute to the strength of cement paste but promote an alkaline pH up to 13.5, useful for the reinforcement protection.

Big particles are not very good: there’s a larger core which doesn’t react waste of cement. During hydration there’s the formation of an ettringite layer over the cement core, as it grows, it slows down the approaching of water and the hydration reaction rate.

Porosity: curing for a longer time reduces the total number of pores and their dimension, too. The number of bridges increases, which together with a lower porosity, leads to a higher-strength material.

• C-S-H gel pores: interlayer spacing within the gel, dimensions from few to several nm; they don’t affect durability of concrete and reinforcement protection because they are too small to allow significant transport of aggressive species;
• Capillary pores: micro- or macro-pores of voids not filled by hydration products within the hardened cement paste, they have the dimensions of 10-50 nm and up to 3-5 microns (high w/c ratios or not well hydrated cement) respectively;
• Air voids: entrapped air during mixing which isn’t removed by fresh concrete vibration, larger pores of few mm dimensions. Air bubbles of 0.05-0.2 mm diameter can be intentionally introduced against freeze-thaw cycles.

Capillary pores and air voids are relevant in durability of concrete and reinforcement protection because they determine the penetration resistance to aggressive species.

Water/cement ratio and curing

Curing has a major effect on the capillary porosity; during hydration there’s not a big change in volume; the initial volume of mixed water and cement is equal to the volume of the hardening product: unreacted cement, hydrated cement (CSH gel or solid hydrated products), capillary pores filled by air or water. 70-80% volume is made by aggregates which aren’t shown in the picture because they are inert, don’t react and their volume is unchanged.

The volume of capillary pores, which affects mechanical and transport properties, increases with the amount of water used in the paste (w/c ratio) and decreases with the degree of hydration.

A compromise between high workability (it means high w/c ratio) and mechanical resistance (lower porosity, higher cement fraction) must be reached. As the curing time increases, the volume of capillary pores decreases.

To determine the resistance to degradation in concrete (durability) and its role in protecting the embedded steel, not only the total porosity, but also pore size and interconnection must be considered. A decrease in capillary porosity increases the mechanical strength of cement paste and reduces the permeability of the hydrated cement paste.

The influence of porosity on the transport process can’t be explained simply by the pore volume, the concept of connectivity or the degree of continuity of the pore system has to be taken into account. The influence of geometric structure on transport properties is described by the percolation theory: below a critical porosity (percolation threshold), the capillary pore system is not interconnected, and only finite clusters are present; above that value, the capillary pore system is continuous (infinite clusters).

Curing promotes hydration, reduces capillary pores and leads to capillary pores segmentation (reduction of interconnection). To improve curing, we have to leave the formworks for longer periods, cover fresh concrete with plastic sheet and maintain the cast concrete humid.

Blended cements

They are obtained by inter-grinding or blending Portland cement with mineral substances: natural pozzolana, fly ash (from carbon combustion), silica fume (from silicon industry) and ground granulated blast furnace slag. They are characterized by the formation of very fine hydration products → refinement of pores and reduction of concrete porosity; there’s a slower development of mechanical strength (require a longer wet curing period) and of heat of hydration.

Aggregates

Cement paste always shrinks, at the end it’s full of cracks. The addition of aggregates (60-85% in volume) leads to volume stabilization, their volume is unchanged while only 15-40% shrinks. The presence of aggregates leads to porosity, but at least there are fewer cracks due to shrinkage.

Aggregates are natural or synthetic mineral substances, crushed or not crushed, based on particles with dimension suitable for concrete casting. They allow volume filling up to 85% in volume, reduction of cement paste, of costs and of hydration heat. They confer to concrete dimensional stability against shrinkage and cracking, and affect properties of fresh and hardened concrete.

There’s a size distribution to fill the whole volume.

• Size: fine aggregates (sand) D < 4 mm; coarse aggregates (gravel) D > 4 mm;
• Particle shape and texture: round and smooth improve workability, angular and crushed decrease workability, requiring more water;
• Porosity: they affect porosity, leading to effects on strength, density and water absorption;
• Grading: minimum cement paste volume, maximum workability and minimum segregation;
• Maximum size: higher the maximum size, higher the workability.

Aggregates must be clean: sand from seawater can’t be used due to chlorides contamination (dangerous for reinforcement rebars); other unfavorable elements are organic compounds (delay hydration, increase setting time and reduce mechanical strength), very thin particles (reduce adhesion with cement paste), salts (corrosion), weak aggregates (reduce compressive strength), SiO₂, CaO and MgO (reaction with alkali to produce expansive products).

Water and additives

Water for cement paste must be clean, we need to avoid sulfates, chlorides and impurities in its composition; there could be interference with setting and hardening, reduction in mechanical strength and rebars corrosion. Additives can be used in low amounts to improve properties of fresh or hardened concrete: plasticizers and super-plasticizers, air-entraining mixtures, set-accelerators, set-retarders, shrinkage controllers and corrosion inhibitors.

Fresh concrete properties

It has to be workable, easy to cast without segregation, separation of ingredients and stratification. It should be easy to be produced, transported, casted and not segregating (it has to be cohesive). Mobility, fluidity (rheological properties), compactability (easy to remove voids) are important features.

Fresh concrete properties can be evaluated by the slump test, which says how much fluid the mixture is. Once the cone is removed, the table is then vibrated and we measure if the material gets flatted (fluid and workable) or if it remains of the shape of the cone; there are 5 classes of consistence, from S1 stiff (used only when there aren’t rebars, which require to be covered well) to S5 super-fluid (high rebar density, complex geometry).

Workability depends on different factors:

• Dosage of water (l/m³): higher content, higher w/c ratio, higher workability
• Properties of aggregates: shape and texture, fine fraction and maximum size affect workability: a bigger max size, a good distribution and smooth round shape improve workability
• Time from mixing and temperature
• Additive like superplasticizers can be added.

Properties of hardened concrete

Mechanical strength (affected by cracking, voids and pores), resistance to deformation and to cracking depend on w/c ratio, porosity, compaction, curing, type of cement, aggregates, additives. Mechanical tests for hardened concrete are of compressive test using a cube or cylinder, concrete is a viscoelastic material and parameters are temperature and rate of load application.

Trend of mechanical resistance with curing time and w/c ratio: mechanical resistance increases if w/c ratio decreases and if curing time increases. Deformations can occur due to external loads, plastic shrinkage, hygrometric shrinkage, thermal variation; if shrinkage is not free, tensile stresses appear (cracking occurs if they are greater than concrete’s tensile resistance).

Transport phenomena

Concrete can be penetrated through its pores by gases (air and carbon dioxide from the atmosphere, CO must be controlled because it could lead to breakage of passive film of rebars by eliminating alkalinity of concrete) and liquid substances (water with dissolved ions like chlorides and sulphates). Permeability indicates the property of concrete to allow the ingress of these substances, it’s a fundamental factor in the durability of reinforced concrete. Degradation of concrete is related to transport phenomena of substances as water, CO₂, chlorides, oxygen, sulphate ions and electrical current.

The motion of fluids and ions through concrete is due to 4 mechanisms: diffusion (due to concentration gradients), capillary sorption (due to capillary action inside capillary pores of cement paste), ion migration (in the case of applied fields) and permeation (due to pressure gradients). Permeability depends on concrete characteristics (porosity, segmentation of interconnections, dimensions of pores), transport mechanism, chemical reactions (for ex. chlorides tend to bind aluminates of cement paste, lowering the amount of free chlorides) and environmental conditions: relative humidity and temperature variations.

Water in concrete: in the hydrated cement paste, water may be present in many forms:

• Capillary water: in macropores (> 50 nm) water is free to move (no binding forces with walls) and transport ions, with the same transport properties of bulk solution; water from macropores evaporates when RH is below 100%, without causing any significant shrinkage. In micropores (< 50 nm) water is not free but interacts with pore walls, it’s still able to transport ions but with a lower entity due to interactions between liquid and solid; water from micropores evaporates when RH is below 95%;
• Adsorbed water: even when water has evaporated from the capillary pores, a thin molecular layer remains adsorbed on the inner surface of pores, it contributes with a little influence on transport phenomena, has no effects on corrosion and can be removed only when RH < 30%;
• Interlayer water – into CSH gel pores: gel pores are too small to allow transport processes with high rates, interlayer water has no effects on permeability and corrosion, but it affects shrinkage and creep; it can evaporate only when RH < 11%.

Water content can be related to RH of environment: water is first adsorbed on capillary pores’ surface, and as RH increases, it condensates and fills up the pores, starting with the small ones and moving to the larger ones. Considering capillary pores as spherical cavities connected by narrow cylinders, if RH = 100%, all pores and cylinders (which have smaller diameter) are filled up with water, while if RH < 100%, pores and cylinders with diameters under a certain value are filled with water, while larger pores are filled by air.

The presence of water in capillary pores promotes transport phenomena occurring in aqueous solution (like chlorides and ions diffusion) and hinders transport phenomena occurring in gaseous phase (like oxygen and CO₂ diffusion). The presence of air in capillary pores has the opposite effect.

Porous structure can be characterized by total porosity (% in volume), pore size, connectivity and opening to external surface. Total porosity is given by Water content of pores is very important, it affects the transport of ionic species (chlorides, sulphates) which are mobile in water and of gaseous species (oxygen, CO₂) which are mobile in air. Humidity content can be defined as Water absorption is the humidity content of water saturated material. Pore saturation degree ranges from 0-100%:

Capillary condensation: condensation on the external surface of materials can occur when vapor pressure is higher than equilibrium vapor pressure at surface temperature; in porous materials, condensation can occur also if RH < 100%.

Capillary adsorption: when water comes into contact with a porous material like concrete, it’s rapidly absorbed thanks to under-pressure in the pores due to capillary action, which depends on surface tension, viscosity, density of fluid, pore radius and contact angle.