Mortar Testing

Until 1970-1980, characterization of masonry mortars were mostly based on traditional wet chemical analysis (Jedrzejewska, 1960, Stewart and Moore, 1981), where interpretation of results were often difficult if not impossible without a good knowledge of the nature of different ingredients. The majority of later characterization proposed optical microscopy (Erlin and Hime 1987, Middendorf et al. 2000, Elsen 2006) as the first step in identification of different components of mortar based on which other analytical techniques including wet chemistry are performed. Many advanced instrumental analyses e.g., scanning electron microscopy and X-ray microanalysis, X-ray diffraction, X-ray fluorescence spectroscopy, atomic absorption, thermal analysis, infrared spectroscopy, etc. play significant roles in examinations of masonry mortars (Bartos et al. 2000, Elsen 2006, Callebaut et al. 2000, Erlin and Hime 1987, Goins 2001, 2004, Groot et al. 2004, Doebley and Spitzer 1996, Chiari et al. 1996, Middendorf et al. 2000, 2004, 2005, Leslie and Hughes 2001, Martinet and Quenee 2000, Valek et al., 2012, and Jana 2005, 2006). The choice of appropriate analytical technique depends mainly on the questions that have to be addressed, and, on the amount of material available.

Methodologies

Purposes of laboratory testing of mortar are:

To document a historic or modern masonry mortar by examining its sand and binder components, proportions of various ingredients, and their effects on properties and performance of the mortar,
Evidence of any chemical or physical deterioration of mortar from unsoundness of its ingredients to effects of potentially deleterious agents from the environment (e.g., salts),
Records of later repointing events and their beneficial or detrimental effects on the performance of the original mortar and masonry units, and finally,
An assessment of an appropriate restoration mortar to ensure compatibility with the existing mortar.

Currently there are two standardized procedures available that describe various laboratory techniques for analyses of masonry mortars with special emphasis on historic mortars.

One is ASTM C 1324 "Standard Test Method for Examination and Analysis of Hardened Masonry Mortar," which includes detailed petrographic examinations, followed by chemical analyses, along with various other analytical methods to test masonry mortars as described in various literatures, e.g., XRD, thermal analysis, and infrared spectroscopy.
The second one is the RILEM method described in a series of publications from Middendorf et al. (2004, 2005).

Mortar samples are tested by following these established methods of ASTM C 1324, and RILEM, which include detailed petrographic examinations, i.e., optical and scanning electron microscopy and X-ray microanalyses (SEM-EDS), followed by chemical analyses (gravimetry, acid digestion), X-ray fluorescence (XRF), X-ray diffraction (XRD), and thermal analyses (TGA, DTG, and DSC). Mortar sample was first photographed with a digital camera, scanned on a flatbed scanner, and examined in a low-power stereomicroscope for the preliminary examinations, e.g., to screen any unusual pieces having different appearances, e.g., representing contaminants from prior pointing episodes or remains of host masonry units.

Representative subset pieces of interest are then selected for:

Optical microscopy,
Scanning electron microscopy and X-ray microanalysis for chemical and mineralogical compositions, and microstructures of sand, paste, and overall mortar,
Acid digestion, preferably from un-pulverized or lightly pulverized sample for extraction of siliceous sand by acid digestion for grain size distribution,
Loss on ignition from ambient to 950°C temperatures for free and hydrate water, and carbonate contents,
Acid digestion for determination of insoluble residue content,
Cold acid and hot alkali digestions for determination of soluble silica content from hydraulic binder if any, after pulverizing a subset to finer than 0.3 mm size, and,
Ultra-fine pulverization (<44-micron) of a subset for XRD, XRF, and thermal analysis.
Any additional analyses, if needed, e.g., water digestion of mortar for determination of water-soluble salts by ion chromatography, or, Fourier-transform infrared spectroscopy of mortar for determining any coatings or organics added, etc. are done on the as-needed basis from the remaining set.

Information obtained from petrographic examinations is crucial to devise appropriate guidelines for subsequent chemical and other analytical methods, and, to properly interpret the results of chemical analyses. For example, detection of siliceous versus calcareous versus argillaceous components of aggregates in sample, or, the presence of any pozzolan in the binder (slag, fly ash, ceramic dusts, etc.) from petrography restricts which chemical method to follow, and how to interpret the results of such analyses, e.g., acid-insoluble residue contents.

Therefore, a direct chemical analysis e.g., acid digestion of a mortar without doing a prior petrographic examination to determine the types of aggregates and binder used could lead to highly erroneous results and interpretation.

Armed with petrographic and chemical data and based on assumed compositions and bulk densities of the sand and the binder(s) similar to the ones detected from petrographic examinations, volumetric proportions of sand and various binders present in the examined sample can be calculated. The estimated mix proportions from such calculations can provide only a rough guideline to use as a starting mix for mock-up mixes during formulation of a pointing mortar to match with the existing mortar.

Send us an email at info@cmc-concrete.com with any questions, or call us at 724-834-3551

Download : Application of Petrography in Restoration of Historic Masonry Structures

Sand Extraction by Acid Digestion and
Sieve Analysis of Extracted Sand

For mortars containing siliceous sand (e.g., containing quartz, quartzite, granite, sandstone, siltstone, feldspar, etc.), sand can be extracted by digesting a few representative as-received mortar fragments in (1+3) dilute hydrochloric acid to dissolve away all binder fractions and extract, wash, and dry the acid-insoluble component of mortar, which is mostly the siliceous (and acid-insoluble argillaceous) component of sand.

The mortar fragments are first gently broken down into small pieces in a porcelain mortar and pestle making sure not to reduce inherent grain-size of sand during this size-reduction process of bulk mortar. Subsequent smaller pieces are then placed in a 250-ml glass beaker completely immersed in dilute hydrochloric acid and stirred with a magnetic stirring rod over a stirrer for a period of at least 24 hours to several days depending on the binder type for complete digestion of binder fractions and settlement of siliceous sand at the bottom of beaker to be filtered out for sieve analysis.

Sand particles thus extracted are washed, oven-dried, and sieved in an automatic mini sieve shaker through various U.S. Sieves from No. 4 (4.75 mm) through 8 (2.36 mm), 16 (1.18 mm), 30 (0.6 mm), 50 (0.3 mm), 100 (0.15 mm), and 200 (0.075 mm) for determination of the size, shape, angularity, and color of sands retained on various sieves. Grain-size distribution of sand is then compared with ASTM C 144 specifications for masonry sand.

Photomicrographs of sand retained on each sieve are then taken with a stereomicroscope to record the sand size, shape, and color variations.

For low amount of sample, or, for sample having calcareous sand, image analysis (e.g., Image J) on stitched photomicrographs of thin sections taken from multiple areas can be done to determine the sand-size distribution (Elsen et al. 2011).

Optical Microscopy

The main purposes of optical microscopy of masonry mortar are characterization of:

Aggregates, e.g., type(s), chemical and mineralogical compositions, nominal maximum size, shape, angularity, grain-size distribution, soundness, alkali-aggregate reactivity, etc.;
Paste, e.g., compositions and microstructures to diagnose various type(s) of binder(s) used;
Air, e.g., presence or absence of air entrainment, air content, etc.;
Alterations, e.g., lime leaching, carbonation, staining, etc. due to interactions with the environmental agents during service, and effects of such alterations on properties and performance of mortar; and
Deteriorations, e.g., chemical and/or physical deteriorations during service, cracking from various mechanisms, salt attacks, possible reasons for the lack of bond if reported from the masonry unit, etc.

Fragments selected from preliminary examinations for microscopy are sectioned, polished, and thin-sectioned (down to 25-30 micron thickness) preferably after encapsulating and impregnating with a dyed-epoxy to improve the overall integrity of the sample during precision sectioning and grinding, and to highlight porous areas, voids, and cracks. Prepared sections are then examined in a high-power stereo-zoom microscope up to 100X magnifications having reflected and transmitted-light, and plane and crossed polarized-light facilities, and eventually in a high-power petrographic microscope (up to 600X magnifications) equipped with transmitted, reflected, polarized, and fluorescent-light facilities. Capturing high-resolution micrographs from these microscopes via high-resolution high frame rate digital microscope cameras with appropriate image analyses software are an integral part of documentations during petrographic examinations.

Therefore, the essential steps followed during optical microscopy are:

Visual examination of as-received, fresh fractured, and sectioned surfaces of mortar on a flatbed scanner and in a stereo-microscope;
Preparation of clear epoxy-encapsulated block of mortar for subsequent sectioning and lapping for examinations of sand and binder in a stereo-microscope;
Preparation of a blue or fluorescent dye-mixed epoxy-impregnated large-area (50 by 75 mm) thin section of mortar of uniform thickness of 25-30 micron across the section;
Observation of thin section in a transmitted-light stereo-zoom microscope from 5X to 100X preferably with polarized-light facilities to observe large-scale distribution of sand and mortar microstructure in plane polarized light and sand type and carbonation of paste in crossed polarized light; and finally
Observation of thin section in a polarized-light (petrographic) microscope from 40X to 600X equipped with transmitted and reflected, polarized and fluorescent-light facilities for examinations of sand and binder compositions and microstructures.

For thin section preparation, representative fragments are oven-dried at 40 to 60°C to a constant mass and placed in a flexible (e.g., molded silicone) sample holder, then encapsulated with a colored dye-mixed (e.g., blue dye commonly used in sedimentary petrography, or, fluorescent dye, Elsen 2006) low-viscosity epoxy resin under vacuum to impregnate the capillary pore spaces of mortar, improve the overall integrity of sample during sectioning by the cured epoxy, highlight porous areas of mortar, alterations, cracks, voids, reaction products, etc. The epoxy-encapsulated cured solid block of sample is then de-molded, sectioned if needed, and processed through a series of coarse to fine grinding on metal and resin-bonded diamond grinding discs with water or a lubricant, eventually a perfectly flat clean ground surface is glued to a frosted large-area (50 by 75 mm) glass slide. Careful precision sectioning and precision grinding of the sample is then done in a thin-sectioning machine till the thickness is down to 50 to 60 micron. Final thinning down to 25 to 30 micron thickness is done on a glass plate with fine (5-15 micron) alumina abrasive. Thin section is eventually polished with various fine (1 micron to 0.25 micron size) diamond abrasives on polishing wheels suitable for examinations in a petrographic microscope, and eventually in SEM-EDS. Sample preparation steps are described in detail in Jana (2006).

More elaborate steps followed during optical microscopy include:

Visual examinations of sample as-received to select fragments for detailed optical microscopy; initial digital and flatbed scanner photography of sample as-received;
Low-power stereo-microscopic examinations of saw-cut and freshly fractured sections of sample for evaluation of variations in color, grain-size and appearances of sand, and the nature of the paste;
Examinations of oil immersion mounts for special features and materials in a petrographic microscope;
Examinations of colored (blue or fluorescent) dye-mixed epoxy-impregnated polished thin sections in a transmitted-light stereo-zoom microscope for determination of size, shape, angularity, and distribution of sand, as well as abundance and distribution of void and pore spaces that are highlighted by the colored dye-mixed epoxy;
Image analyses of micrographs of thin sections for estimations of pores, voids, intergranular open spaces, and shrinkage microcracks by using Image J or other image analysis software, where multiple micrographs are collected in plane polarized light mode by using a high-resolution stereo-zoom microscope equipped with transmitted and polarizing light facilities and stitched to get an adequate representative coverage;
Examinations of colored (blue or fluorescent) dye-mixed epoxy-impregnated polished thin sections in a petrographic microscope for detailed compositional, mineralogical, textural, and microstructural analyses of aggregates and binders, along with diagnoses of evidence of any deleterious processes and alterations (e.g., lime leaching, precipitation of secondary deposits and alteration products, salts);
Examinations of polished thin or solid section in reflected-light (epi-illumination) mode of petrographic microscope after etching the surface with acids to identify various non-hydrated hydraulic phases (e.g., C2S, C3S, C3A, etc., Middendorf et al., 2005);
Examinations of any physical or chemical deterioration or signs of improper construction practices from microstructural evidences;
Stereo-microscopical examinations of size, shape, and color variations of sand extracted after hydrochloric acid digestion; and finally,
Selection of areas of interest to be examined by scanning electron microscopy.

Image Analysis of Thin Section Micrographs to Determine Sand Size Distribution and Volumetric Proportions of Sand and Voids

Scanning Electron Microscopy & Microanalysis by Energy-Dispersive X-ray Spectroscopy (SEM-EDS)

Methods followed during SEM-EDS studies include:

Secondary electron imaging (SEI) to determine the microstructure and morphology of the examined surface of sample,
Backscatter electron (BSE) imaging to determine compositions of various phases from various shades of darkness/grayness/brightness from average atomic numbers of phases from the darkest pore spaces to brightest iron minerals (e.g., thaumasite, periclase, ettringite, quartz, dolomite, monosulfate, gypsum, calcite, C-S-H, aluminate, calcium hydroxide, belite, alite, free lime, and ferrite having progressively increasing average atomic numbers and brightness in BSE image),
X-ray elemental mapping (dot mapping) of an area of interest to differentiate various phases,
Point-mode or area (raster)-mode analysis of specific area/phase of interest on a polished thin or solid section, and
Average compositional analysis of a specific phase or an area on a polished thin or solid section or small subset of a sample.

The main purposes of SEM-EDS examinations of masonry mortars are to:

Observe the morphologies and microstructures of various phases of sand and binder,
Characterize the typical fine-grained microstructure of hydrated, carbonated, and hydraulic components of binder that are too fine to be examined by optical microscopy and are not well crystallized to be detected by XRD;
Determine major element oxide compositions, and compositional variations of paste, and from that determine the type of binder(s) used, especially to differentiate non-hydraulic calcitic and dolomitic lime mortars from hydraulic lime varieties (e.g., from silica contents of paste), natural cements (e.g., from silica and magnesia contents), pozzolans, slag cements, Portland cements, etc. all from their characteristic differences in compositions and hydraulicities (e.g., cementation index of Eckel 1922);
Determine composition of residual hydraulic phases to assess the raw feed and calcination processes used in manufacturing of binder;
Assess hydration, carbonation, and alteration products of binders,
Investigate effects of various alterations of paste during service and its role on properties and performance of mortar,
Detect salts and other potentially deleterious constituents,
Detect pigments and fillers,
Examine compositional variations across multiple mortars installed, etc.; and eventually
Complement and confirm the results of optical microscopy.

Due to characteristic difference in compositions of pastes made using various binders, e.g., non-hydraulic lime (CaO dominates over all other oxides), variably hydraulic lime (CaO with variable SiO2 contents depending on degree of hydraulicity), dolomitic lime (high CaO and MgO), natural cement (CaO, SiO2, Al2O3, and MgO contents are high, high MgO and FeO contents are characteristic), and Portland cement (CaO and SiO2 contents are higher than all other oxides), SEM-EDS analysis of paste is a powerful method for detection of the original binder components in the sample. Effects of chemical alterations and various chemical deteriorations of a mortar (e.g., lime leaching, secondary calcite precipitates, gypsum deposits, etc.) can also be detected by SEM-EDS.

SEM-EDS analysis is done in a CamScan Series 2 scanning electron microscope equipped with a high-resolution column 40Å tungsten, 40 kV electron optics zoom condenser 75° focusing lens operating at 20 kV, equipped with a variable geometry secondary electron detector, backscatter electron detector, EDS detector for observations of microstructures at high-resolution, compositional analysis, and quantitative determinations of major element oxides from various areas of interest, respectively. Revolution 4Pi software was used for digital storage of secondary electron and backscatter electron images, elemental mapping, and compositional analysis along a line, or on a point or an area of interest.

Portion(s) of interest on the polished 50 mm by 75 mm size thin section used for optical microscopy were subsequently coated with carbon or gold-palladium film and placed on a custom-made aluminum sample holder to fit inside the large multiported chamber of CamScan SEM equipped with the eucentric 50 by 100 mm motorized stage. Usually, features of interest from optical microscopy are marked on the thin section with a fine-tipped conductive marker pen for further observations in SEM. Alternately, solid polished section or grain mount from phases or areas of interest can also be examined. Procedures for SEM examinations are described in ASTM C 1723 and Sarkar, Amin, and Jana (2000).

Chemical Analysis
(Gravimetry and Instrumental Analysis)

Following petrographic examinations, chemical analyses of the mortar are done to determine the:

Hydrochloric acid-insoluble residue content to determine the siliceous sand content;
Losses on ignition due to release of free water, hydrate water, and CO2;
Soluble silica contents contributed from hydraulic binders; and,
Bulk oxide contents, e.g., lime, silica, alumina, magnesia, alkalis, and others.

Chemical analyses are done by using various methods outlined in ASTM C 1324 and Middendorf et al. 2005a, e.g., by wet chemistry (gravimetry) and various instrumental techniques, e.g., atomic absorption spectroscopy (AAS), inductively-coupled plasma atomic emission spectroscopy (ICP-AES), and X-ray fluorescence spectroscopy (XRF).

Acid Digestion

Acid digestion is perhaps the most commonly used test of masonry mortar, which is done to:

Extract sand from sample by dissolving out the binder fractions so that grain-size distribution of sand can be done by sieve analysis, and
Assess insoluble sand content in the sample.

Sand content after acid digestion is determined both from:

1.00 gram of pulverized sample (finer than 0.3 mm size) digested in 50-ml dilute (1+3) HCl (heated rapidly but below boiling), and,
From digesting a representative bulk sample per se (for harder mortars or mortars perhaps with light pulverization) in multiple fresh batches of (1+3) HCl at ambient temperature.

The former usually gives better result due to small amount, pulverization to easily remove the binder fraction for digestion, and use of rapidly heated acid, whereas latter method requires multiple episodes of digestion in fresh acid and is time-consuming. Acid digestion is also done as the first step to determine soluble silica content in a sample as described below, which is contributed from the hydraulic components in binder.

All these goals of acid digestion depend on the assumptions that:

Sand is siliceous in composition and does not contain any acid-soluble constituents (e.g., carbonates), and,
Binder entirely dissolves in acid and does not contain any acid-insoluble constituents (gypsum, clay, etc.).

Applicability of acid digestion to assess these tasks should therefore be first verified by optical microscopy to confirm the siliceous nature of sand without any appreciable acid-soluble constituents, and calcareous nature of binder, and none without any appreciable argillaceous (clay) constituents.

For grain-size distribution of sand (for sample found from optical microscopy to contain siliceous sand), a few representative fragments of (preferably not pulverized or lightly pulverized in a porcelain mortar and pestle for harder mortars to break down to smaller size fraction without crushing the sand to retain the original sand size) are selected for digestion in multiple fresh batches of (1+3) dilute hydrochloric acid to dissolve away all binder fractions and extract, wash, and oven-dry the acid-insoluble component of aggregate. Usually multiple episodes of acid digestion in fresh batches of acid and filtration of residues are needed to entirely remove the binder fractions without losing the finer fractions of sand.

Soluble Silica
From Cold Acid & Hot Alkali Digestion

Digestion of a pulverized sample of mortar in a cold acid followed by further digestion of residue in a hot alkali hydroxide solution are done to determine the soluble silica content contributed from the hydraulic component of binder, where cold acid digestion usually dissolves most of the binder without affecting the sand, followed by hot alkali hydroxide digestion to dissolve remaining soluble silica from calcium silicate hydrate component of paste or in mortars containing hydraulic binders. The soluble silica content corresponds to the silica mostly contributed from the hydraulic binder components (and a minor amount from any soluble silica component in the aggregates).

For determination of soluble silica content (modified from ASTM C 1324):

5.00 grams of pulverized sample (finer than 0.3 mm size, without excessive fines) is first digested in 100-mL cold (at 3 to 5°C) HCl and filtered through two 2.5-micron filter papers (filtrate #1).
The residue with filter papers is then digested again in hot (below boiling) 75-ml NaOH, and filtered through two 2.5-micron filter papers (filtrate# 2).
The two filtrates from acid and alkali digestions are then combined, re-filtered twice with 2.5-micron and then through 0.45-micron filter paper to remove any suspended silica fines, brought to 250 ml volume with deionized water, and then used for soluble silica determination by an analytical method, such as
atomic absorption spectroscopy (AAS), inductive coupled plasma optical emission spectroscopy (ICP-OES), or X-ray fluorescence spectroscopy (XRF).
Multiple steps of filtrations from 2.5-micron to submicron filter papers are necessary to remove any suspended silica from sand that can skew the result.
Instrument to be used for such determination must be calibrated with several silica standards in matrices similar to the one used in mortar analysis.
An XRF unit calibrated with filtrates from acid-and-alkali-digested series of laboratory-prepared standards of Portland cement and silica sand mortars (moist cured at w/c of 0.50 for 30 days) having various proportions of Portland cements (SiO2 contents of standards ranging from 1 to 10%) are used in our laboratory for determining SiO2 K-alpha X-ray intensities from known stoichiometric silica (cement) contents of standards (using exact 5.00 grams as samples) prepared by the same procedure of cold HCl-digestion/filtration/hot NaOH-digestion/2nd filtration/combination of two filtrates/re-filtration steps as followed for mortars.

Hydraulic binder content is calculated as:

[(Soluble SiO2, weight percent in sample as calculated) divided by assumed soluble SiO2 content in binder] ×100, where assumed SiO2 contents of binders varies with binder types, e.g., 21% in Portland cement, 20% in natural cement, 27% in slag cement, 7 to 10% in hydraulic lime, etc., or, more preferably, from the average paste-SiO2 content determined from SEM-EDS.

Weight Losses on Ignition

Losses in weight of a mortar on step-wise heating from ambient to 110°C, 550°C, and 950°C temperatures liberate free water from capillary pore spaces by 110°C, combined water from dehydroxylation of various hydrous phases (calcium silicate hydrate, calcium hydroxide, etc.) by 550°C, and liberation of carbon dioxide from decomposition of carbonated paste and carbonate minerals by 950°C.
Such losses in weight are measured by following the procedures of ASTM C 1324 by heating 1.00 gram of pulverized mortar (finer than 0.3 mm) in an alumina crucible in a muffle furnace in a controlled step-wise heating at a heating rate of 10ºC/min.
Mortars having hydraulic binders and hydration products of such provide measurable combined water contents after calcination to 550°C, whereas those having high calcareous components (high-calcium lime mortar or mortar having calcareous sand) produce higher weight losses during ignition to 950°C.
Usually, a good correlation is found between weight losses at 550°C from dehydration of combined water, and, soluble silica contents contributed from hydraulic binders amongst series of mortars containing variable amounts of hydraulic phases.
Also, good correlations are found between gravimetric weight losses at 110°C, 550°C, and 950°C and corresponding losses from thermal analysis.

X-ray Diffraction (XRD)

X-ray diffraction is a powerful laboratory technique used during investigation of masonry mortars, for reasons, such as:

Determination of bulk mineralogical composition of mortar, including its aggregate and binder mineralogies; e.g., quartz in sand from major diffraction peaks at 26.65º, 20.85º, 50.14º 2-theta, or calcite in sand or carbonated lime binder from major peaks at 29.41º, 39.40º, 43.15º 2-theta, or Portlandite in binder from major peaks at 34.09º, 18.09º, 47.12º 2-theta;
Individual mineralogy and alteration products of aggregate at various size fractions, and binder phases;
Detection of dolomitic lime binder from brucite in the mortar from major peaks at 38.02º, 18.59º, 50.86º 2-theta;
Detection of lime (Portlandite), gypsum (11.59º, 20.72º, 29.11º 2-theta), or cement binders;
Detection of any potentially deleterious constituents, e.g., deleterious salts, or efflorescence deposits;
Detection of a mineral oxide-based pigmenting component; and,
Detection of components, which are difficult to detect by microscopical methods.

X-ray diffraction can be done on:

Pulverized (to finer than 45 micron size) portion of bulk sample, or
On the sand extracted from mortar by acid digestion, if sand has complex mineralogy, or also
On the binder-fraction by separating sand from the binder from a carefully ground sample (in a mortar and pestle) and passing the ground mass through US 200 sieve (75 micron) to collect the fraction rich in binder.

XRD pattern of a sample containing silica sand typically shows quartz as the dominant phase that surpasses peaks for all other phases (e.g., calcite, dolomite, clay, secondary deposits); hence binder separation is sometimes useful to detect minor minerals of interest (e.g., salts or pigments).

For mortars containing marine shell fragments as sand, aragonite appears with calcite as two calcium carbonate phases from the shell fragments and paste.

For binder mineralogy, sample is first dried at 40°C to a constant mass, then carefully crushed without pulverizing the sand, and sieved through a 75-micron opening screen to retain sand-rich fraction on the sieve and obtain the finer binder-rich fraction for further pulverization down to finer than 45 micron. Salts and other soft components can be analyzed from binder fraction. Efflorescence salts on masonry walls are also analyzed routinely in XRD.

For sample preparation, a Rocklab (Sepor Mini-Thor Ring) pulverizer is used to grind sample down to finer than 100 microns. Usually, a few drops of anhydrous alcohol are added to reduce decomposition of hydrous phases from the heat generated from grinding. Approximately 10 grams of sample is ground first in the pulverizer, from which about 8.0 grams of sample is selected, mixed with an appropriate binder (e.g., three Herzog grinding aid pellets from Oxford Instruments having a total binder weight of 0.6 gram for 8 grams of sample for a fixed binder proportion of 7.5 percent); the mixture is then further ground in Rocklab pulverizer and in a McCrone micronizing mill with anhydrous alcohol down to finer than 44 micron size. Approximately 7.0 grams of binder-mixed pulverized sample thus prepared is weighed into an aluminum sample cup and inserted in a stainless steel die press to prepare the sample pellet. A 25-ton Spex X-press is used to prepare 32 mm diameter pellet from the pulverized sample. The pressed pellet is then placed in a custom-made circular sample holder for XRD and excited with the copper radiation of 1.54 angstroms. Sample holders made with quartz or silicon are best for working with very small quantities of sample because these holders create no diffraction peaks between 2° and 90° 2q (Middendorf et al. 2005).

XRD is carried out either: (a) in a Bruker D2 Phaser benchtop powder diffractometer equipped with a Lynxeye 1D detector, a q-q goniometer, a Cu X-ray tube (Cu k-alpha radiation of 1.54 angstroms), a primary slit of 1 mm, a receiving slit of 3 mm, a position sensitive 1D Lynxeye XE-T detector, generator settings used are 30 kV and 10mA (300 watt, scanned at 2q from 8° to 64° with a step of 0.05° 2q integrated at 0.05 sec. step-1 dwell time, or, (b) in a floor-standing Siemens D5000 Powder diffractometer (q-2q goniometer) employing a long line focus Cu X-ray tube, divergent and anti-scatter slits fixed at 1 mm, a receiving slit (0.6 mm), diffracted and incident beam Soller slits (0.04 rad), a curved graphite diffracted beam monochromator, and a sealed proportional counter. Siemens D5000 is equipped with (a) a horizontal stage (fixed), (b) an X-ray generator with CuKα, fine focus sealed tube source, (c) large diameter goniometer (600 mm), low divergence collimator, and Soller slits, (d) fixed detector slits 0.05, 0.2, 0.6, 1.0, 2.0, and 6.0, and (e) Scintillation detector. Generator settings used are 40 kV and 30 mA. Tests are usually run at 2q from 4° to 64° with a step scan of 0.02° and a dwell time of one second. The resulting diffraction patterns are collected by DataScan 4 software of Materials Data, Inc. (MDI) for Siemens D5000 or Bruker Diffrac.Suite software for D2 Phaser, and analyzed by Jade software of MDI with ICDD PDF-4 database of diffraction data for the Siemens D5000 unit, or Bruker Diffrac.Eva software with COD (Crystallographic Open Database) for the D2 Phaser. Phase identification, and quantitative analyses were carried out with MDI’s Search/Match with Easy Quant, or Bruker’s Diffrac.Eva, and both with Rietveld modules, respectively. A third-party Match! software is also used for transferring raw data from both equipment and processing for phase identification and Rietveld analyses using search/match with the inherent COD database.

Energy-Dispersive X-ray Fluorescence Spectroscopy (ED-XRF)

X-ray fluorescence (XRF) is used for determining: (a) major element oxide composition of sample, and, (b) soluble silica content of filtrate after digestion of sample in cold-HCl and hot-NaOH. Major element oxide compositions provide clues about the siliceous sand content of mortar from silica content, type of binder used (e.g., a dolomitic lime or natural cement based binder gives a characteristically higher magnesia than a calcitic lime or Portland cement based binder), calculation of lime content in a cement-lime mortar from bulk CaO content from XRF, effect of alterations and deteriorations (e.g., salt ingress in a mortar from marine environment can be diagnosed from excessive sodium, sulfate, and chlorine, etc.), etc. A series of standards from Portland cements, lime, gypsum, to various rocks, and masonry cements of certified compositions (e.g., from USGS, GSA, NIST, CCRL, Brammer, or measured by ICP) are used to calibrate the instrument for various oxides, and empirical calculations are done from such calibrations to determine oxide compositions of mortars. For mortars with highly unusual compositions (e.g. severely salt-contaminated or a gypsum-based mortar) a standard-less FP calculation is done to determine the best possible composition.

An energy-dispersive bench-top X-ray fluorescence unit from Rigaku Americas Corporation (NEX-CG) is used. Rigaku NEX-CG delivers rapid qualitative and quantitative determination of major and minor atomic elements in a wide variety of sample types with minimal standards. Unlike conventional EDXRF analyzers, the NEX-CG was engineered with a unique close-coupled Cartesian Geometry (CG) optical kernel that dramatically increases signal-to-noise. By using monochromatic secondary target excitation, instead of conventional direct excitation, sensitivity is further improved. The resulting dramatic reduction in background noise, and simultaneous increase in element peaks result in a spectrometer capable of routine trace element analysis even in difficult sample types. The instrument is calibrated by using various certified (CCRL, NIST, GSA, and Brammer) reference standards of cements and rocks. The same pressed pellet used for XRD for mineralogical compositions is used for XRF to determine the chemical composition.

Thermal Analysis

Thermal analyses encompasses: (1) thermogravimetric analysis (TGA), which measures the weight loss in a sample as it is heated, where weight loss can be related to specific physical decomposition of a phase of interest at a specific temperature that is characteristic of the phase from which both the phase composition and the abundance can be determined; (2) differential thermal analysis (DTA, or first derivative of TGA i.e. DTG) measuring temperature difference between the sample and an inert standard (Al2O3) both are heated at the same rate and time where endothermic peaks are recorded when the standard continues to increase in temperature during heating but the sample does not due to decompositions (e.g., dehydration of hydrous or decarbonation of carbonate phases); the endothermic or exothermic transitions are characteristic of particular phase, which can be identified and quantified using DTA (or DTG); and (3) differential scanning calorimetry (DSC), which follows the same basic principle as DTA, whereas temperature differences are measured in DTA, during heating using DSC energy is added to maintain the sample and the reference material (Al2O3) at the same temperature; this energy use is recorded and used as a measure of the calorific value of the thermal transitions that the sample experiences; this is useful for detection of quartz that undergoes polymorphic (a to b form) transitions and no weight loss.

Thermal analyses are done to determine the presence and quantitative amounts of: (a) hydrates (e.g., combined water liberated from paste dehydration during decomposition of calcium-silicate-hydrate component in paste at 180-190ºC); (b) sulfates (gypsum from decompositions at 125ºC, and 185-200ºC, ettringite at 120-130ºC, thaumasite at 150ºC); (c) brucite from its dehydroxylation at 300-400ºC to confirm the presence of dolomitic lime; (d) hydrate water from decomposition of Portlandite component of paste at 400-600ºC; (e) quartz from polymorphic transformation (a to b form) at 573ºC; (f) cryptocrystalline calcite in the carbonated lime matrix from decomposition at 620-690ºC, or magnesite at 450-520ºC, or (g) coarsely crystalline calcite e.g., in limestone by decomposition at 680-800ºC or (h) dolomite at 740-800ºC and 925ºC, and (i) phase transition of belite (C2S) at 693ºC, etc. Phases are determined from their characteristic decomposition temperatures occurring mostly as endothermic peaks or polymorphic transition temperatures as for quartz.

120-150°C = Ettringite decomposition from cement paste (thaumasite at 150ºC) and water release (endotherm);
120, 180-200°C = Gypsum decomposition and water release (endotherm);
100-200ºC = Hydrate water from decomposition of calcium silicate hydrate (CSH);
300-400°C = Brucite decomposition from dolomitic lime mortar (or from soluble magnesium salts in the paste from the use of natural cement) and water release (endotherm);
400-600°C = Portlandite decomposition from Portland cement paste and water release (endotherm);
500-680°C = Magnesite decomposition for dolomitic lime mortar (endotherm);
573°C = Alpha-to-beta polymorphic transformation of quartz the main component of silica sand in mortar;
620-690°C = Calcite decomposition for cryptocrystalline calcite formed during carbonation of lime in mortar;
680-800°C= Calcite decomposition for coarsely crystalline calcite in limestone or marine shells (endotherm);
740-800ºC = Dolomite decomposition (endotherm);
>950°C = Slight exotherm from initial surface reaction of lime and silica, followed by larger endotherm from melting.

Simultaneous TGA and DSC analyses are done in a Mettler Toledo TGA/DSC 1 unit on 30-70 mg of finely ground (<0.6 mm) sample in alumina crucible (70 µl, no lid) from 30°C to 1000°C at a heating rate of 10°C/min with high purity nitrogen as purge gas at a flow rate of 75.0 ml/min. TGA/DSC 1 simultaneously measures heat flow in addition to weight change. The instrument offers high resolution (ultra-microgram resolution over the whole measurement range), efficient automation (with a reliable sample robot for high sample throughput), wide measurement range (measure small and large sample masses and volumes) broad temperature scale (analyze samples from ambient to 1100°C), superior ultra-micro balance, simultaneous DSC heat flow measurement (for simultaneous detection of thermal events, e.g., polymorphic alpha-to-beta transition of quartz and quartz content), and a gastight cell (ensures a properly defined measurement environment).

Fourier Transform Infrared Spectroscopy
(FT-IR)

Fourier-transform infrared spectroscopy (FT-IR) measures interaction between applied infrared radiation and the molecules in the compounds of interest (Middendorf et al. 2005). FT-IR is particularly useful for detection of admixture, additives, and polymer resins, mainly to identify various organic components (functional groups) in mortar (e.g., methyl CH3, organic acids CO-OH, carbonates CO3) from their characteristic spectral fingerprints in FT-IR spectrum. FT-IR can also be used for detection of main mineral phases in a hydraulic binder, CSH, carbonates, gypsum, and clays (Middendorf et al. 2005). Organic compounds such as synthetic (e.g., acrylics, polyesters) and natural resins, carbohydrates, colorants, oils and fats, proteins, waxes as well as inorganic compounds, e.g., corrosion products, minerals, pigments, paints, fillers, stone, glass, and ceramics can be detected by this technique.

FT-IR measurements are done in a Perkin Elmer Spectrum 100 FT-IR spectrophotometer running with Spectrum 10 software. Sample is measured using attenuated total reflection (ATR) on a single bounce diamond/ZnSe ATR crystal between a frequency range of 4000 to 650 cm–1. Each run is collected at 4 cm–1 resolution with Strong Beer-Norton apodization. Data are collected with a temperature-stabilized deuterated triglycine sulfate (DTGS) detector by placing the sample in contact with the ATR crystal and by applying force from the pressure applicator supplied with the ATR accessory. The application of pressure enable the sample to be in intimate contact with the ATR crystal, ensuring achievement of a high-quality spectrum. Additionally, more conventional KBr pellet is also sometimes used for samples on as-needed basis.

Ion Chromatography (IC)

Salts can cause various deteriorations from: (a) mere aesthetic issues of surface efflorescence by precipitation from evaporation of leachates on the surfaces followed by atmospheric carbonation of the precipitates where salts deposit as individual crystals or as crust to (b) more serious internal distress in mortar from crystallization inside the pores (sub-fluorescence or crypto-fluorescence) from expansive forces associated with crystallization of salt from supersaturated solutions.

Some common salts are calcium carbonates (e.g., calcite, vaterite), magnesium carbonate (magnesite), sodium carbonate hydrate and bicarbonate (thermonatrite, trona, nahcolite), sulphates (gypsum, thenardite, epsomite, melanterite, mirabilite, glauberite, or ettringite and thaumasite from oxidation of sulfides or cement hydrates), and chlorides (halite, sylvite, calcium oxychloride from deicing salts, salt-bearing aggregates, ground water). X-ray diffraction and SEM-EDS can determine many of these salts as long as they are present in detectable amounts.

Ion chromatography is an established technique used for analyses of various water-soluble anions and cations in salts (e.g., chloride, sulfate, and nitrate anions, and magnesium, calcium, alkali, ammonium cations) to assess magnitude of environmental impacts on masonry units and mortars, and subsequent effects of such salt ingress. Samples are pulverized, digested in deionized water to remove all water-soluble salts, then solid residues are filtered out and the water-digested filtrates are analyzed by an ion chromatograph

Ion chromatography methods are described in ASTM D 4327 “Standard Test Method for Anions in Water by Chemically Suppressed Ion Chromatography.” Briefly, an aliquot of 1 gram of pulverized sample (passing No. 50 sieve) is digested in 50 ml deionized water for 6 to 8 hours on a magnetic stirrer at a temperature below boiling point of water; then the digested sample is filtered through two 2.5-micron filter papers using vacuum, followed by a second filtration through micro-filter (0.45 micron) paper, then the filtrate is either used directly or diluted to 100 to 250 ml with deionized water depending on the concentration of anions, and used for analysis to get ppm-level fluoride, chloride, nitrite, bromide, nitrate, phosphate, and sulfate in the water-digested sample in Metrohm 861 Advanced Compact IC. The instrument is calibrated against ten different custom-made Metrohm anion standard solutions having all these anions from 10-ppm to 100-ppm levels. To check the accuracy of the instrument, a solution of know concentration is run first prior to the analyses of samples. Weight percent concentrations are obtained from (ppm-results times original filtrate volume times dilution factor) divided by sample weight.

Steps Followed During Laboratory Testing

Steps followed during laboratory investigation of masonry mortars are as follows:

From preliminary visual examinations to petrographic examinations of mortars to determine the types of aggregates used and the binders present, based on which
Subsequent chemical analyses were done to determine the chemical compositions of binders and proportions of sand, water, and degree of carbonation. Information obtained from petrographic examinations is useful and form the very guidelines to devise the appropriate chemical methods to follow, and to properly interpret the results of chemical analyses.
For example, detection of siliceous versus calcareous versus argillaceous natures of aggregates in mortar, or the presence of any pozzolan in the binder (slag, fly ash, ceramic dusts, etc.) from petrography restricts which chemical method to follow, and how to interpret the results of such analyses, e.g., acid-insoluble residue contents.
Therefore, a direct chemical analysis e.g., acid digestion of a mortar without doing a prior petrographic examination to determine the types of aggregates and binder used could lead to highly erroneous results and interpretation.
Armed with petrographic and chemical data and based on assumed compositions and bulk densities of the sand and the binder(s) similar to the ones detected from petrographic examinations volumetric proportions of sand and various binders present in the examined mortar can be calculated.
The estimated mix proportions from such calculations can provide at least a rough guideline to use as a starting mix during formulation of mock-up tuck pointing mixes to match with the existing mortar.

Which Technique(s) to Use?

Historic Mortar Testing

A Presentation on Historic Mortar Testing
Laboratory Analysis of Masonry Mortars
From Fort Washington

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Mortar References

Mortar References Consulted in this Page