3rd Historic Mortars Conference
11-14 September 2013, Glasgow, Scotland
How to identify a natural cement : case study of the Vassy
Church, France
M. Bouichou1, E. Marie-Victoire2, A.Texier3, T. Blondiaux4
1.
2.
3.
4.
Laboratoire de recherche des monuments historiques, [email protected]
Laboratoire de recherche des monuments historiques, [email protected]
Laboratoire de recherche des monuments historiques, [email protected].
Bureau d’étude Brizot-Masse, [email protected]
Abstract
Natural or Roman cements were the first modern cements to be industrially produced at the beginning of
the 19th century in Europe. Used equally by engineers for their hydraulic properties, and by architects for
their aesthetic qualities, they were massively employed for façade decoration or as cast-stone elements for
masonries. This cultural heritage, even if it is abundant, is relatively unknown and needs now to be
clearly identified and restored. Because of lack of knowledge and data on these cements, they are often
identified as Portland cement or hydraulic lime, or hydraulic lime mixed with gypsum or Portland cement.
Furthermore, heterogeneous calcination and varied quarried stone, which strongly modify their properties,
make their characterizations more difficult. This paper presents a case study of the Vassy Church, with a
focus on the binder identification of mortars. The Vassy church, built in 1859 by Gariel, a Vassy cement
producer, is located in Burgundy, France. On the church, ochre-coloured mortars, were used either
indoors, outdoors, as renders, as pointing mortars, as “run in situ” mortars, applied on stone or brick.
Different types of mixes were employed, from rich to poor mortars. The testing protocol of those analysis
firstly consisted in clinker grains analysis on polished sections (with and without Borax etching) by
optical (OM) and scanning electron microscopy (SEM) observations, coupled with EDS analysis.
Secondly, hydrated phases were characterised on mortar fractures, through SEM observations coupled
with EDS analysis. Finally, crystallised phases were identified by XRD analysis of binding powder
samples. Results of these analysis show several clinker morphologies and compositions, and different
types of hydrates phases, according to the carbonation state. These results show important differences in
terms of microstructure between natural cements and the other common binders like Portland cement or
lime.
Keywords: natural cements, clinkers, ettringite, vaterite.
1. Introduction
Natural cements were the first cements to be industrially produced at the beginning of the 19th
century. They were fabricated by extracting a single quarried stone, a limestone containing clay, and by
burning it at a low temperature in vertical kilns. Starting from England, the production of these cements
expanded then throughout Europe : France, Austria, Poland and Russia (Hughes 2007a, Weber 2007a).
Because of the diversity of limestones used and manufacturing processes, natural cements present
different chemical compositions and properties (Gosselin 2009, Weber 2007b) .
In architecture, natural cements were used for façade decoration: mouldings cast or run in situ in the
mortar (Hughes 2007b, Weber 2007b), and rendering; but also as cast-stone elements for masonries
(Cailleux 2005, Cailleux 2006). They were also used, thanks to their hydraulic properties, by enginners
for maritime works, railways, and water and gas networks.
1
The identification of these materials and their characterisation is an essential preliminary step toward
their restoration. Relatively unknown, these binders are often confused with Portland cement or hydraulic
lime. In France, natural cement mortars were also identified as gypsum-lime mortar, French natural
cements containing sulfates. This paper presents a case study of the Vassy Church, in the town of Etaules
located in Burgundy, where Vassy cement was produced. Vassy cement is known for its use as restoration
materail in several cathedrals during the 19th century (Gosselin 2009). This study aimed at characterising
and identifing the binder of mortars used in the construction of the Vassy Church, in order to be able to
advise compatible restoration mortars.
2. Visual Assesment
The Vassy-lès-Avallon church was built between 1859 and 1862 by the architect Tircuit, at the
initiative of the Gariel family, producers of the Vassy cement.
Indoors, the walls are covered with an ochre mortar, with a white paint finition (Fig. 1). The arches
and the foundations which are made of stone, are coated with an ochre mortar up to the intersecting ribs.
Some elements, such as the columns, are also covered with an ochre mortar that can be made out under
the white paint. There is no evidence of the implement technologies used such as with run-in-situ mortar
making, or molding marks.
Figure 1 : Vassy church,
indoors.
Figure 2 : Vassy church façade
(photo Thierry Leynet).
Figure 3 : Stone columns covered with an
ochre run-in-situ mortar.
This fourteenth century Gothic style church (Fig. 2) is 45 meters long and has three naves and five
spans. Its bell tower is completed by a spire. The structure and the tower were primarily made of a bricks
masonry, covered with a render. The bricks are joined with an ochre mortar. Some elements, like the bell
tower, the door and the pinnacles are made of stone. The columns which decorate the tower are also made
of stone, but covered with a run-in-situ ochre mortar (Fig. 3). Under the balustrade at the base of the spire,
a slab made of pieces of stone coated with an ochre mortar surmounts a run-in-situ mortar cornice. The
pointing mortars of both the cut stone masonry and the stone hook of the spire, are made of an ochre
mortar, with a high binder content. The bell tower is decorated with a clock, with roman numerals and
decorations made of mosaics, with ochre mortar between the pieces of mosaics.
The worst deteriorations were observed on the façades that was the most exposed to wind and rain,
that is to say, the northern and the western façades. These degradations essentially consisted in an erosion
of the mortars skin, and in cracking and pattern cracking of the rendering mortars, specially noticeable in
the areas the most exposed to the wind and rain. Underneath the balustrade at the base of the spire, an
abundant spalling of the cement slab was also visible, probably linked to a waterproofing problem of the
lead sheet protection.
2
2. Mortars analysis
2.1 Analysis protocol
Several types of analysis were carried out in order to characterise the different types of mortars
present on the building, focusing on the identification of the binders that were used.
Phenolphthalein tests were undertaken to assess the carbonation depth of the mortars, bearing in mind
that only the presence of non carbonated areas in mortars enables to recognize the original hydrated
phases of the hydraulic binder.
Then, the crystallized phases of the sampled mortars were identified using X-ray diffraction The
residual anhydrous binder grains were identified on polished sections using optical microscopy (OM),
before and after borax attack to reveal the chemical compositions of clinker phases (Campbell 1999) and
using scanning electron microscopy (SEM) coupled to elemental analysis (EDS). Finally, the hydrated
phases of the mortars, were observed by SEM+EDS on freshly broken samples.
Three mortars sampled from the bell tower were analyzed :
- a low-binder-content render mortar covering the brick walls, named M1,
- a rich-binder-content render mortar called M2, which certainly helped to set the stone hooks in the
spire. It should be noted that no metallic reinforcement was observed,
- and a mortar from a run-in-situ element, named M3.
2.2 Macroscopic observations and phenolphthalein tests
The three samples were light-ochre coloured. The M1 render mortar had a high aggregate-content,
when the rich-binder-content M2 mortar was very compact and only contained a few aggregates. The M3
run-in-situ mortar very much looked like the the render mortar M1, but was exhibiting a white crust on its
outer surface.
Concerning the measured depths of carbonation, only a few areas were not carbonated in mortars M1
and M2, from a depth of 3 cm to the surface. However, carbonation fronts were completely
discontinuous. Finally, white crystallization were noticed in pores, in non-carbonated areas.
2.3 The crystallised phases : X-ray diffraction (XRD)
The X-ray diffraction analysis (Table 1) lead to the determination of the nature of the binder used in
the different mortars.
The presence of gehlenite and wollastonite (anhydrous phases typical of a moderate firing of natural
cements), the absence of portlandite in the non-carbonated samples and the presence of gypsum in small
amounts (in relation to the calcite amount), indicated that the binders used in mortars M1 , M2 and M3
were natural cements. The white crust on M3 mortar outer surface is Gypsum.
Table 1: Synthesis of the XRD diffractograms obtained on the different samples.
Mortar M1
Carbonation
Calcite
Vaterite
Ettringite
Gypsum
Monocarboaluminate
Hydrocalumite
Quartz
Feldspar
Gehlenite
Wollastonite
Yes
++++
+
Ø
Ø
Ø
Ø
++++
+++
++
Ø
No
+++
++
+++
+++
Ø
Ø
++++
+++
++
+
Rich Mortar M2
Yes
+++
++++
++
++
++
+
+++
Ø
+++
++
No
+++
+
++++
+
+++
++
+++
Ø
+++
++
Molding
Mortar M3
Yes
++++
+
Ø
+++
Ø
Ø
++++
+++
++
Ø
Mortar
M3 crust
+
Ø
Ø
++++
Ø
Ø
++
Ø
Ø
Ø
Very significant presence ++++, significant presence +++, average presence ++, traces +, absence Ø
3
2.4 Identification of the anhydrous grains
As a reminder, in the cement field, a particular notation is used to designate the components : C=
CaO, SiO2 = S, Al2O3 = A, Fe2O3 = F, CO2 = C (Taylor 1997).
Four types of anhydrous grains were encountered on the polished sections of the rich-binder-content
M2 mortar, and therefore containing a high amount of residual anhydrous grains.
The first type of grain was large and well crystallized, with small round beige C2S crystals enclosed in
a very clear single-phase matrix (Fig. 4). After borax attack, some crystals turned to brown while others
remained beige, revealing differences in the C2S crystallographic structures (Fig. 4). The SEM
observations of type 1 grains, coupled with EDS confirmed the presence of C2S (Fig. 5). While some
crystals showed parallel striations and others did npt, they all exhibited the same elemental chemical
composition. The matrix phase was composed of Ca, Al, Si and Fe, indicating a mixture of
brownmillerite (C4AF) and gehlenite (C2AS).
Figure 4 : OM view using polarized light (Z x 500)
after borax attack, type 1 clinker grain, M2 mortar.
Figure 5 : BSE -SEM view (Z x 1800), type 1
clinker grain details, M2 mortar.
The second type of grain was well crystallized, with large beige C2S crystals enclosed in a bright
white and gray two-phases matrix (Fig. 6). After borax attack, some crystals turned to brown while others
remained beige. The SEM observations of type 2 grains coupled with EDS, confirmed the presence of
C2S. Again, while some crystals showed parallel striations and others did not, they all showed the same
elemental chemical composition. The dark matrix phase was composed of Ca, Al, Si, which corresponds
to gehlenite (C2AS) (fig. 7). The clear matrix phase was a mixture of brownmillerite (C4AF) and gehlenite
(C2AS) (fig. 7).
Figure 6 : OM view using polarized light (Z x 500),
type 2 clinker grain, M2 mortar.
Figure 7 : BSE -SEM view (Z x 1800), details of
type 2 clinker grain, M2 mortar.
The third type of grain was large and well crystallized, with large beige crystals enclosed in a clear
white single-phase matrix. After borax attack, all the crystals turned to blue (Fig. 8), suggesting the
presence of C3S with an occasional brown coloration of these C3S crystals.
4
The SEM observations of the type 3 grains, coupled with EDS, confirmed the presence of C3S (Fig.
9). The crystals observed within the C3S (corresponding to the brown coloration) were C2S (pointer - Fig.
9). The matrix phase was mainly composed of Ca, Al, Fe, which corresponds to a mixture of
brownmillerite (C4AF) and bi-calcium aluminate (C3A).
Figure 8: OM view, using polarized light (Z x 500)
after borax attack, type 3 clinker grain, M2 mortar.
Figure 9: BSE-SEM view (Z x 2000), details of
type 3 clinker grain, M2 mortar.
The fourth type of grain was very little-crystallized (Fig. 10). It was composed of residual quartz
grains inserted in a matrix with a high content of Si, Ca and Al (Fig. 11). It should be noted that the Ca
and Al content of the quartz grains was higher towards the outer part of the grains (pointer - Fig. 11).
Figure 10: BSE-SEM view (Z x 190), type 4 clinker
grain, M2 mortar.
Figure 11: BSE-SEM view (Z x 600), details of
type 4 clinker grain, M2 mortar.
2.5 Hydrated phases
a) Non-carbonated areas
Observations in non-carbonated areas were undertaken on the M2 rich-binder-content mortar sample.
In non-carbonated areas, the binder phase was mainly composed of hydrated calcium-silico-aluminates
(C-A-S-H), with variable sulfur concentrations depending on the area, but sometimes equivalent to that of
aluminum. This compact binder phase was generally low-crystallized (Fig. 12).
In some areas, small crystals were observed, which did not, however, revealed major differences in
chemical composition (higher carbon concentration, presence of potassium).
Ettringite, or tri-sulfo-calcium-aluminates, were also observed with different shapes :
- abundant acicular Ettringite crystallization, completely filling up the pores (fig. 13) ;
- less crystallized Ettringite along the edges of the pores, where crystals were agglutinating in a
compact mass ;
- and small thin Ettringite needles in the bottom of the pore.
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Figure 12: SEM view (Z x 2000) of M2 mortar,
non-carbonated low-crystallized binder matrix.
Figure 13: SEM view (Z x 550) of a pore in noncarbonated mortar M2, acicular Ettringite.
b) Carbonated areas
The cement paste in carbonated areas was compact with occasional large pores. It consisted of a
carbonated binding matrix, mainly made of calcium carbonates, originating from the carbonation of
calcium silico-aluminates, of Ettringite and calcium aluminate, leading to the formation of Calcite,
Vaterite and amorphous carbonates. In mortar M2, the binding matrix was mainly composed of numerous
and large trigonal calcium carbonate crystals (Fig.14). They did not have the typical facies of Calcite
(rhombohedral), nor of Vaterite, which usually crystallizes as thickset needles. However, the shape of the
Vaterite crystallization strongly depends on the pH and the water intake (Tai 1998), and the results of Xray diffraction showed that this was the main phase in carbonated areas. It is very likely that these crystals
were a special type of Vaterite.
Figure 14: SE-SEM view (Z x 2200) of M2 mortar,
details of presumably Vaterite crystals.
Figure 15: SEM view (Z x 3300) of M2 mortar,
amorphous calcium carbonates.
Figure 16: SE-SEM view (Z x 1800) of the M2
mortar, Ettringite and tablet-shaped calcium
aluminate at the bottom of the pore.
Figure 17: SEM view (Z x 430) of the M3 mortar,
pore covered with a layer of massive gypsum.
6
Rhomboedral Calcite crystals were also observed, coming from the carbonation of hydrated calcium
silico-aluminate phases with a high silicon content. Indeed, the carbonation of this phase, which produces
Calcite, depletes it of its calcium and thus increases its proportion of silicon.
Finally, amorphous calcium carbonates were filling the pores and crevices of the cement paste (Fig.
15). Surprisingly, needle-shaped Ettringite crystals in the pores and crevices of the concrete, as well as
tablet-shaped calcium aluminate were observed in the M2 mortar carbonated (Fig. 16).
In mortars M1 (poor mortar) and M3 (run-in-situ mortar), tablet-shaped gypsum and massive Gypsum
(pointer - Fig. 17) crystallizations were observed in the pores, Gypsum being one of the by-products
resulting from the carbonation reaction of Ettringite. It should be noted that during carbonation, Ettringite
decomposes into Calcite, Gypsum and alumina gel (Nishikawa 1992).
3. Conclusion
In the Vassy church, ochre mortars were used both inside and outside, for rendering, pointing mortar
or run-in-situ mortar, purposes. They were applied onto stone or brick. Different types of mixtures were
used, from rich-binder-content mortars (almost cement paste) to poor-binder-content mortars. The main
degradations that were observed outdoors were a generalized erosion phenomenon, pattern cracking and
localized spalling under a defective lead sheet protection.
The presence of Gehlenite and C-A-S-H phases as a matrix in all of the samples, the absence of
Portlandite and the strong presence of Ettringite in non-carbonated areas clearly indicated that the binder
used to manufacture the cement mortars of Vassy church was a natural cement, with a high sulfate and
aluminate content, and an heterogeneous firing process.
Indeed, residual anhydrous clinker grains were characterized on polished sections and 4 types were
distinguished. The most frequently observed grains were type 2, compounds of C2S crystals surrounded
by a two-phase C2AS and C4AF binding matrix. After borax attack of these type-2 grains, C2S crystals
turned to brown while others remained beige. However, no chemical difference between these C 2S
crystals was brought to light through EDS analyzes. On the type-3 grains, C3S crystals (which turned to
blue after a borax etching) were identified, whereas the type-4 grains were low-crystallized and composed
of silica grains with reaction rings and a CSA phase. Moreover, Gehlenite was identified through XRD in
all of the samples and Wollastonite was detected in the sample with the highest cement content.
The coexistence of these four types of grain, and the presence of Gehlenite and Wollastonite which is
typical of a moderate temperature firing (<1100 ° C), could be explained by a very heterogeneous firing
or by a mixture of unburnt, burnt and over-burnt materials (which is frequently mentioned in ancient
documents, for special uses).
In terms of hydrated phases, XRD analyses and SEM observations revealed a significant presence of
high sulfur content compounds : only Gypsum in the carbonated samples and Ettringite and Gypsum in
the non-carbonated samples. Ettringite was thus encountered in the pores in non-carbonated areas. The
crystals had the shape of long well-crystallized needles. This Ettringite may have been primarily formed,
which means that it could be a normal product resulting from the hydration of the cement (Cailleux 2005,
Sommain 2008). It may also have been secondarily formed, resulting from the reaction of external or
internal sulfates with the hydration products of the cement. Aluminate phases, such as hydrocalumite
(C4AH13) and mono-carbonate calcium aluminates (C4ACH11) were also identified through XRD in noncarbonated samples, but Portlandite was never detected. SEM observations of the cement matrix in noncarbonated areas indicated that it was composed of a C-A-S-H type amorphous phase, with traces of Fe,
Mg and S. In carbonated areas, the binding matrix was made of calcium carbonates, probably mainly
Vaterite for the rich-binder-content mortar M2 and Calcite for the lower-binder-content M1 and M3
mortars. Amorphous calcium carbonates were observed on all of the mortars using SEM.
As all of the analyzed mortars had particularly high sulfate contents, in order to be compatible, repair
mortars will have to be resistant to sulfates. The binder to be used will therefore have to meet the
following requirements:
- a low aluminate content to avoid the formation of expansive secondary Ettringite, which could cause
swelling and cracking of the repair mortar,
7
-
and a low alkali content, to prevent the formation of alkali salts such as sodium sulfate (Thenardite,
Mirabilite), potassium-calcium sulfate (Syngenite), or tripotassium natrium sulfate (Aphthitalite),
which are expansive and deleterious salts.
Thus, current Portland cements present alkali and aluminate contents that are incompatible with these
criteria. Contrarily, natural cements, which are still produced today, because their hydration leads to the
formation of small amounts of Portlandite, which does not cause the formation of expansive Ettringite
(Sommain 2008, Gosselin 2010), show a good sulfate resistance. In addition, their color and texture are
similar to those of Vassy mortars, so they represent an interesting restoration material. However, some of
the existing natural cements exhibit a significant alkali concentration, which constitutes a potential risk of
deterioration. Preliminary compatibility tests are therefore necessary.
Acknowledgements
This study was initiated within the frame of the Rocare project (EU 226898) financially supported by
the European Commission (FP7-ENV-2008-1 program).
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