Analysis
MACROSCOPIC ANALYSIS
AGGREGATE SHAPE AND SIZE
Aggregates come in various shapes and sizes, depending mainly on the source and physical processing of these materials. Any variation of these properties may have an effect to the overall performance of concrete.
In terms of size, aggregates are generally divided into coarse and fine fractions. The presence of both is equally important and proper size distribution or grading must be followed to produce workable concrete mixes, reduce the amount of cement paste used and minimize the amount of voids, which may affect the overall performance of concrete. Aggregate particle size and grading may be quantified using sieve analysis.
Aggregate shape and angularity also play an important role in concrete; particularly with the aggregate-cement paste bonding. Rounded aggregates typically sourced from rivers, often have poorer bonding than angular aggregates sourced from crushed quarry rocks, due to less surface area in contact with the cement paste. Under fluorescent light, adhesion cracks are observed surrounding aggregates with weak bonding. Uniaxial compressive strength test may also be used to show the bond strength of aggregates and cement paste. In terms of shape, flaky aggregates are least desirable due to larger specific surface area and high cement demand.
References:
Alexander, M.G., & Mindess, S. (2005). Aggregates in concrete. London: Taylor & Francis Group.
Poole, A. B., & Sims, I. (2016). Concrete Petrography: A Handbook of Investigative Techniques (2nd ed.). CRC PRESS.
MICROSCOPIC ANALYSIS
Analyzing the concrete sections under the microscope enables a petrographer to see the different phases of concrete deterioration.
Know more about these deterioration phases here.
WHAT ELSE CAN WE SEE UNDER THE MICROSCOPE?
Air Content and Distribution
Most concrete, even though compacted by vibration or ramming, will still retain entrapped air. It becomes air voids once the concrete has hardened. A problem may occur once the concrete is under-compacted or the matrix proportion is insufficient compared to the coarse aggregates used. What may happen is the interconnection and clustering of large air voids. This is called honeycombing and can be seen through concrete petrography in detail. In concrete petrography, the air content is highlighted by impregnating the sample with fluorescent dye and epoxy solution.
Water/Cement Ratio
The cement paste that holds the concrete components together is made up of a mixture of water and cement.
Achieving the optimal ratio between water and cement in the cement paste is an important factor that will influence the strength, durability and overall quality of concrete. Adding water is generally done to increase the workability of concrete. High water-cement ratios however, may result in a weaker, dilute and more porous material. This may cause a number of problems such as the increased vulnerability to chemical reaction from external agents, cracks, shrinkage and weaker adhesion of components to the cement paste.
Although water-cement ratios may range from low (0.3) to very high (0.8), the optimal amount to be used will depend on the exposure conditions, type of structure to be built and requirements for construction. For buildings using ordinary Portland cement, around 0.4 to 0.6 are typically used.
Petrography may be used for concrete thin-sections impregnated with fluorescent dye to estimate the water-cement ratio used in the cement paste. The higher the water-cement ratio, the brighter green color will be observed for the cement paste under the fluorescent light setting. Numerical value may be estimated by comparing with a standard concrete thin-section with known water-cement ratio. In plane-polarized light, it may also be observed that concrete with higher water-cement ratio will have widely spaced clinkers or cement minerals.
References:
What is the Right Water-Cement Ratio for Mix Design? (2018, September 25). Retrieved August 13, 2020, from https://theconstructor.org/practical- guide/water-cement-ratio-mix-design/5874/
What is Water Cement Ratio? - Guide & Calculation. (2018, April 02). Retrieved August 13, 2020, from http://www.civilology.com/water-cement-ratio/
Poole, A. B., & Sims, I. (2016). Concrete Petrography: A Handbook of Investigative Techniques (2nd ed.). CRC PRESS.
Aggregates Used
Aggregates comprise the majority of the concrete sample. It is important to note their texture, type, size, shape, presence of alterations and other properties. These can be identified under the microscope in detail. Knowing the types of aggregates used can lead to identifying its provenance or even the quarry where it is sourced.
Andesite
This aggregate is composed mainly of plagioclase, biotite, amphibole and/or pyroxene. It is usually light to dark gray in color. The crystals are set in a fine grained groundmass. Quarries of this aggregate can be found in Rizal, Batangas and Bataan.
Diorite
This aggregate is coarse grained and has a "salt & pepper" appearance. Large crystals of plagioclase, amphibole, biotite and some pyroxene makes up this rock. Some of its quarries are in Rizal and Cebu.
Basalt
This aggregate is dark colored and is composed of fine grained crystals of plagioclase and pyroxene with or without olivine. It can also have magnetite as an accesory mineral. It underlies most areas within the Earth's ocean basins. Quarries of this aggregate can be found in Bulacan, Rizal and Bataan.
Gabbro
This aggregate is made up of large interlocking crystals of pyroxene, plagioclase and some olivine. It is dark colored due to the abundance of ferromagnesian minerals. Its density is 2.7–3.3 g/cm3 and is the most abundant rock in the deep oceanic crust.
Pumice
This aggregate is white or very light gray and is very lightweight. The lava becomes frothy as the dissolved gases are released. This causes tiny bubbles and voids to fill the rock. It is made up mostly of volcanic glass. It also floats on water due to its density which is around 0.25 g/cm3.
Limestone
This aggregate is a sedimentary rock made up almost entirely of CaCO₃ or calcite. It usually forms in clear, warm, shallow marine waters. It consists of shell remnants and precipitated calcite deposits. This rock also reacts with diluted hydrochloric acid. Bulacan, Rizal, Cebu and Pangasinan are some of the location of its quarries.
Chert
This aggregate is a sedimentary rock which is extremely hard and is composed of microcrystalline silica. It often breaks with a conchoidal fracture and can produce very sharp edges.
Mudstone
This aggregate is a sedimentary rock made up of clay and silt-sized particles. They are very fine grained (< 0.06mm) and varies in color. Most are black, white, gray and brown but others can be red, green or blue depending on its composition.
Sandstone
This aggregate is a sedimentary rock composed of sediments with sizes less than 2 mm but greater than 62.5 μm. The grains are bounded by a cement. It is usually composed of quartz, feldspars and rock fragments.
References:
Geology and Earth Science News, Articles, Photos, Maps and More. https://geology.com/
Lapidus, D. F., Coates, D. R., Winstanley, I., MacDonald, J., Burton, C., & Lapidus, D. F. (2003). Collins dictionary geology. Glasgow: HarperCollins.
Mines and Geosciences Bureau (2019, March 31). MINERAL PRODUCTION SHARING AGREEMENT (MPSA) As of March 31, 2019. Retrieved from http://www.mgb.gov.ph/attachments/article/50/MAR_2019_MPSA_2A.pdf
Images:
alexstrekeisen.it
geologylearn.blogspot.com
homesciencetools.com
pitt.edu
sandatlas.org
thoughtco.com
Type of Cement and Additives Used
It is possible to know the cement type by identifying their mineralogical characteristics. Studying the mineralogy of “clinker” or the product produced by sintering limestone and aluminosilicate materials will lead to identification of the cement type. Presence of mineral additives like flyash, blast furnace slag and natural pozzolans will also help in recognizing the cement type.
Presence of mineral additions can also be seen and identified. Flyash and blast furnace slag can be recognized in the petrographic analysis. While chemical admixtures are very hard to analyze and directly seen, its effect in the concrete can be detected using petrography.
Cement Clinkers
When concrete is observed under a petrographic microscope, phases of the cement paste or the clinker may also be identified. Portland cement namely, has four major phases: alite, belite, ferrite and aluminate.
Portland cement, the most commonly used cement in the world, is produced from heating a mixture of crushed and ground limestone and clay to a temperature of about 1450oC. This process results in gray nodules or lumps known as clinkers which are then cooled, ground and mixed with small amounts of gypsum, to produce commercial cement.
The following below are the primary mineral components of cement and how they are observed under the petrographic microscope:
Alite (tricalcium silicate, Ca3SiO5, C3S)
It makes up 50-70% of the clinker, and is the most abundant constituent. Under a transmitted light microscope, it can be observed as clear, colorless, angular crystals with low birefringence. It reacts quickly with water, and is the most important constituent for early strength development of cement up to 28 days. Surrounding alite are colorless hydration rims, which indicate the overall hydration of the cement paste.
Belite (dicalcium silicate, Ca2SiO4, C2S)
This phase comprises 15-30% of the clinker. These are observed as rounded, turbid, yellowish crystals with moderate birefringence in thin-section. Unlike alite, it reacts slower with water and thus contributes more to the strength development at later stages beyond 28 days.
Ferrite phase (Tetracalcium aluminoferrite, C4AF)
Ferrite makes u 5-15% of the normal Portland cement clinker. These are observed as dark brown, intermediate substances with irregular and ragged forms under transmitted light.
Aluminate phase (Tricalcium aluminate, C3A)
Constitute 5-10% of Portland cement clinker. Under the microscope, these are observed as small, uniform anhedral to rectangular crystals that may be colorless to tan. This phase is isotropic and is best observed as a polished section under reflected light microscopy.
Clinker constituents may be dispersed throughout the cement paste or may clump together as aggregates.
References:
Campbell, D. H. (1999). Microscopical examination and interpretation of portland cement and clinker. Skokie, IL: Portland Cement Association.
Taylor, H. F. (2003). Cement chemistry. London: Telford Publication
How Cement is Made. (n.d.). Retrieved August 11, 2020, from https://www.cement.org/cement-concrete-applications/how-cement-is-made
Other Information
Different Light Settings Used in Concrete Petrographic Analysis
Transmitted plane polarized light (PPL)
Enables the petrographer to identify cement type, aggregate type and dimensions, clinker type and spacing, air void abundance and distribution, and most importantly, identification of concrete deterioration phases.
Transmitted cross polarized light (XPL)
Goes hand in hand with PPL setting. It allows the identification of aggregate type and dimensions, presence of portlandite and its morphology, and identification of concrete degradation such as carbonation of cement paste.
Ultraviolet (UV) light
Ultraviolet (UV) light setting is used for fluorescence microscopy. Samples are impregnated with fluorescent dye to highlight the presence and distribution of air voids, cracks, and most importantly, water/cement ratio which contributes greatly to concrete strength.
Unwanted Aggregate Components
Some constituents in aggregates may be tagged as 'potentially deleterious' as these may result in adverse reactions when mixed into concrete.
Such reactions may include at least one of the following:
i. Chemical interference with the setting of cement (e.g. organic matter, soluble lead)
ii. Poor physical bonding between the aggregate and cement paste (e.g. clay coatings, alkali-reactive constituents)
iii. Poor durability and strength of hardened concrete (e.g. clay lumps, absorptive and microporous particles, organic matter)
iv. Expansion and cracking (e.g. Chlorides, Pyrite, alkali-reactive constituents, releasable alkalis)
v. Weak and poor durability of individual aggregate particles (e.g. Coal and lightweight particles, clay lumps, absorptive and microporous particles, mica, sulphates, pyrite)
Identification and quantification of these constituents is important to avoid unwanted reactions. This is done through a combination of petrographic and various chemical tests, indicated in international and local standards such as the ASTM and PNS, respectively.
References:
Poole, A. B., & Sims, I. (2016). Concrete Petrography: A Handbook of Investigative Techniques (2nd ed.). CRC PRESS.
Sims, I. and Brown, B. (1998) Concrete Aggregates: Chapter 16 in Lea’s Chemistry of Cement and Concrete. 4th Edition, Elsevier, Oxford, 963.
Add-ons to your Cement
Supplementary Cementitious Materials or sometimes called mineral admixtures, are natural materials or by-products added to cement to improve hardened concrete properties.
Supplementary Cementitious Materials or SCMs are fine-grained silica, silicate or aluminosilicate materials that may be used in partial replacement of cement to make concrete mixtures more economical, ecologically friendly, reduce permeability, increase strength and durability etc.
Learn more...
SCMs are usually by-products from other industrial processes or natural materials. These materials may be divided into the pozzolanic materials, which exhibit cementitious properties when mixed with Portland cement, (e.g.fly ash, silica fume, metakaolin), hydraulic minerals which exhibit self-cementing properties in the presence of water (e.g. slag) and other materials showing only small reactivity in cement such as mineral dusts from grinding processes.
Some commonly used SCMs include fly ash, a byproduct of the combustion of pulverized coal; ground granulated blast furnace slag, formed when iron ores are reduced to pig iron or crude iron during smelting and silica fume, a residue of producing elemental silicon or ferro-silicon alloys. Other examples of SCMs are metakaolin,calcined shale or clay, natural pozzolana and limestone. Recent studies have also shown that rice husk ash, diatomaceous earth and bagasse ash may also show cementitious properties.
References:
Pacewska, B., & Wilińska, I. (2020). Usage of supplementary cementitious materials: Advantages and limitations. Journal of Thermal Analysis and Calorimetry. doi:10.1007/s10973-020-09907-1
Portland Cement Association (n.d.). Retrieved August 19, 2020, from https://www.cement.org/cement-concrete-applications/concrete-materials/supplementary-cementing-materials
Nevada Ready Mixed Concrete Association-Supplementary Cementitious Materials - What, why, & how? (n.d.). Retrieved August 19, 2020, from https://www.nevadareadymix.com/concrete-tips/supplementary-cementitious-materials/
Fly Ash
The coal production's waste, is the concrete industry's treasure. Fly ash is one of the most commonly used SCM in concrete. It is mainly mixed in partial replacement of cement, to mainly improve durability.
Fly ash, as previously discussed, is one of the most used supplementary cementitious material or a mineral admixture that is mixed in partial replacement of cement; mainly to improve the durability of concrete. These are unburned residue of coal-fired electric and steam generating plants that are carried away or “flown” from the boiler by flue gases, thus coining the term “fly ash. Another type of ash called “bottom ash” is also produced during coal production, when the particles are too heavy and are instead dropped to the bottom of the furnace; however, unlike fly ash, these are not suitable as cementitious material.
The different types of fly ash are:
Class F
a. Most effectively moderates heat gain during concrete curing
b. Produced from burning anthracite or bituminous coal
c. Has pozzolanic properties or reacts with lime to exhibit cementitious properties
Class C
a. Most useful for “performance” mixes, where early strength is important
b. Produced from lignite or sub-bituminous coal
c. Has cementitious properties
Under the microscope, these are observed as very small, spherical particles that may appear clear, to brown and black, depending on the iron content of the material. They appear as black in cross-polarized light.
References:
Thomas, M. (n.d.). Optimizing the use of fly ash in concrete [PDF]. Illinois: Portland Cement Association.
Pavements. (n.d.). Retrieved August 21, 2020, from https://www.fhwa.dot.gov/pavement/recycling/fach01.cfm
Pozzolanic Philippines, Inc. (n.d.). Retrieved August 21, 2020, from http://pozzolanic.ph/
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