concrete 2.03 y 4.00 Flashcards
components of microstructure of cement stone
*solids:
-rest of unhydrated cement
-C3, S2, H3 and other hydration products
-calcium hydroxide
*pores:
-capillary pores
-gel pores
on what does the volume of the cement stone components depend?
on the water-to-cement ratio
composition of standard concrete
-Air
-water
-cement
-sand
-coarse aggregates
what other elements can you add to the concrete?
-additives
-supplementary cementing materials
what happens if you add superplasticizer and silica fume?
you increase the packing density of concrete, witch means:
-denser cement stone structure
-increase in strength
-improvement of durability
pozzolanic additives
-fly ash
-silica fume
latent-hydraulic additives
slag sand
behavior of concrete under tensile strength
1-. approximate linear elastic behavior
2-. formation of micro cracks
3-. formation and opening of one of main cracks in the cement stone matrix
behavior under compressive loads
depends on the specimen geometry
what’s the behavior of the concrete in conditions up to 100 C
micro cracks caused by:
- thermal expansion
-cement stone shrinkage
mostly negligible strength change
what’s the behavior of the concrete in conditions from 100 to 300 C
-Quartzite concrete: minor decrease in strength
-Calcite concretes: pronounced decrease in strength, up to 50%
name deterioration mechanisms of concrete due to high temperatures
a) strength and stiffness decrease (formation and propagation of micro cracks)
b) spalling
c) cross section weakening and acceleration of heating due to Spalling in connection with further decrease in strength and stiffness.
degradation mechanism due to frost
-ice formations will increase in volume
-cracks if the concrete tensile strength excided
how do you increase the the frost resistance?
you have to add more air entrainment into concrete by:
-reduction of pore pressure if water can move to NOT completely fill pores
-interruption of the capillary pore system and reduction of the capillary suction
examples of wearing attacks that makes abrasion of concrete surfaces
- grinding or rolling traffic
-rolling, slipping or bounding bulk materials
-fitful movements of heavy objects
-strongly gutting waters, which carry solids.
technological actions to prevent or reduce abrasion
-reducing of the W/C ratio, increase in strength
-limiting the fine grain content
-use of wear resistance aggregates
-strengthening of the surface zone
name examples of concrete corrosion due to chemical attacks
-sulphate attack
-alkali-silica reaction
how to increase resistance to chemical attacks?
improvement of the bond zone structure using additives like micro silica
what is carbonation corrosion?
reaction of basic calciumhydroxide due to the absorption pf carbonic acid from air to chemical neutral limestone
how do you classify a zero risk concrete?
X0
what does a concrete class XC means?
reinforcement corrosion due to Carbonation
what does a concrete class XD means?
name: Deicing-salt
reinforcement corrosion due to Chloride
what does a concrete class XS means?
name: Seawater
reinforcement corrosion due to Seawater
how do you classify a concrete corrosion due to Frost and de-icing salt?
XF (Frtost)
how do you classify a concrete corrosion due to chemical attack?
XA
how do you classify a concrete corrosion due to Mechanical abrasion?
XM (wear)
what is clinker?
marble sized nodules which come out of cement kiln, which is then ground to make the grey powder we know as “cement”. (Portland cement is made of 95% clinker)
Types cement:
-CEM 1: Portland cement 95% clinker
-CEM 2: Portland-composite cement 65-94% clinker
-CEM 3: Blast furnace cement 5-64% clinker
-CEM 4: Pozzolanic cement 45-89% clinker
-CEM 5: Composite cement 20-64% clinker
Name supplementary cementitious materials (SCMs)
-limestone
-fly ash
-slag
-natural pozzolan
-calcined clay
what are pozzolanic materials?
A pozzolan is a siliceous or siliceous and aluminous material that in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties
information about fly ash
- Incombustible inorganic material in coal
- Recovered from exhaust gases by electrostatic or mechanical precipitator (baghouses)
- Mainly consist of spherical glassy particles (1-3000 µm)
- Less reactive than slags
- Very heterogeneous material and variable material
- Typical substitution level: 30% in blends
information about ground granulated Blast-furnace slag
- In the process of reduction of iron from ore
- Flux is added to assist the separation of impurities from the iron
- This forms slag, which is molten liquid in the blast furnace
- 90-100% amorphous
- Sharp edge and irregular glassy materials
- High CaO content (30-50%): latent hydraulic material
- 30-70% replacement of cement typical (even 90% possible)
- Limited availability
name the hydration phases of cement:
- Hydration involves reaction of cement with water
- This increases volume of solids, bridging space between grains and forming a solid
- Essential aspect is volume change
- Silicates: C3S and C2S form C-S-H and CH
- Aluminate and Ferrite form AFt and AFm phases
hydration: silicates
Calcium hydroxide: Ca(OH)2 or CH:
-Crystalline hexagonal morphology
-Usually referred to as “Portlandite”
~10-20% of hydrated paste
Calcium Silicate Hydrate (C-S-H)
-Nano crystalline
-Multiple morphologies
-Usually referred to as “C-S-H gel”
~50-65% of hydrated paste
Hydration phases: AFt
Ettringite/AFt:
-Good Crystallinity
-Long column needles
-Low density
Hydration phases: AFm
AFm-Ms or Ms:
-Some Crystallinity
-Thin hexagonal plates
water cement ratio:
-w/c ratio is a number directly related to the average distance between cement particles within a cement paste after it is mixed with water just before it begins to hydrate.
-This distance influences hydration conditions and microstructure of hardened cement paste, consequently its mechanical and durability performance
how does temperature affects hydration rate?
- Fast dissolution
- Shorter dormant/induction
periods - Accelerated reaction rates
- Enhanced reaction
how does chemicals admixtures (retarders) affects hydration rate?
- Delayed reactions, longer induction/dormant period
- Slight decrease in maximum peaks
how does particle size affects hydration rate?
Fine cement – smaller grain size
* Fast dissolution
* Shorter induction/dormant period
* Accelerated reaction rates
* Enhanced reactions
how does W/C ratio affects hydration rate?
High w/c ratio
* Fast dissolution
* Shorter induction/dormant period
* Comparable reaction rates
* Comparable reaction peaks
Why do we add calcium sulfate in cements?
Sulfates are added to control the reaction of aluminate clinker phase and avoid flash setting.
Furthermore, we can control the reaction kinetics to optimize the early reactions and control induction
period by adjusting sulfate quantities or types of sulfate.
How we can measure the hydrated phases?
- Thermogravimetric Analysis (TGA)
- X-ray Diffractometer (XRD)
- Scanning Electron Microscopy- Energy Dispersive X-ray Analysis (SEM-EDX)
- Mercury Intrusion Porosimetry (MIP)
- Nuclear magnetic resonance spectroscopy (NMR)
Explain Thermogravimetric Analysis (TGA):
stages:
Dehydration
Decarbonation
Decomposition
test process:
* Sample crushed and dried using solvent exchange method to remove free water
* Sample holder Alumina crucible
* Initial mass of sample ~ 50-60 mg
* Temperature range 40 °C – 1000 °C
* Rate of temperature increase 20 °C/min
* He or N2 as purging gas at a rate of 30-50 mL/min.
Quantification:
1-. bound water: degree of reaction Portland cement
2-. portlandite: decompose 400-550 *C
3-. calcite: decompose above 600 *C
explain X-Ray diffraction (XRD)
- X-rays are scattered by the crystal and produce scattered X-rays:
- crystalline pattern= sharp peaks
-amorphous pattern= broader humps
- crystalline pattern= sharp peaks
explain SEM-EDX
- SEM
- Secondary electrons (SE)
- Backscattered electron (BSE)
- Characteristic X-rays
- SEM-EDX
- Characteristic X-rays
- Polished section
- Under vacuum condition
explain Mercury Instrusion Porosimeter (MIP)
-mercury has a non-wetting property
- fill the voids with mercury to measure the pores diameters and volume.
porosity in hydrated cement:
-Air voids: measure from micro-m to mm, largest pores in cement matrix and result from imperfect placing.
-capillary pores: measure from micro-m to nano-m, space after hydration products volume reduces as hydration consume capillary water
-gel pores: nm, all intrinsic pores in C-S-H
explain Nuclear magnetic resonance spectroscopy (NMR)
- Uses magnetic fields and electromagnetic frequencies to study molecular structures
- Crystalline and amorphous phases can be studied
