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Volume instability of porous solids: Part I/ Instabilite en volume de
solides poreux
Litvan, G. G.
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Ser
T H ~
National Research
Conseil national
N21d
*
Council Canada
de recherches Canada
no.
981
c. 2
VOLUME INSTABILITY OF POROUS SOLIDS
:
PART
I
by G.G. Litvan
ANALYZED
Reprinted from
7th International Congress bn the
Chemistry of Cement
VoL
111, Paris
1980
p. VII-46 VII-50
DBR Paper No.
981
Division of Building Research
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.
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VII
-
46
Volume Instability of Porous Solids
:
Part I
lnstabilite en volume de solides poreux
G.G. LITVAN (Senior Research Officer),
Division of Building Research, National Research Council of Canada, Ottawa, Ontario, Canada.
A N A L Y Z E D
RESUME : L'adsorption d e NO Na en solution a q u e u 5 e fait subir i un verre poreux d e s i l i c e sa- 3
turd d'eau une variation de longyeur d e 2 7 x 1 0 -
.
L'dchantillon a s u b i u n e variation irrd-versible d e longueur d e 5 0 x 1 0 - AL/P.. L q i s o t h e r m e ressemble beaucoup
i
c e l l e o b t e n u e pourl'adsorption d e la vapeur d ' e a u par l e v e r r e sec e t p r d s e n t e u n e hystdrdsis sur l'ensemble
d e s ' c o n c e n t r a t i o n s . L'expansion d e 325 x l O - - ~ g / e a dtd t r o u v d , - f o r s d e l'attaque du verre
poreux par NaOH 0.2 N; et un changement d e longueur d e 1 2 5 x 1 0 A&/% a eu lieu durant l a
dissolution en C L H 1.0 N d'un dchantillon d e c i m e n t hydratk (e/c = 0.6).
Lorsqu'on a lessivd la chaux d Q u n e 5 p i t e d e c i m e n t ( e / c : 0.6) avec du glycol d'dthylkne, o n a
observd u n e variation d e 1 2 0 x 10- ~p,/p.
.
C e s r d s u l t a t s introduisent u n e donn$e additionnellequi pourrait expliquer l e processus d e ddtdrioration d e s s o l i d e s poreux e x p o s e s c e r t a i n s
produits c h i m i q u e s , par exemple l'attaque par l e sulfate et par l'eau d e m e r , e t la rdaction alcali-agrdgat.
SKMARY: A 27 r lo-' fractional length change occurred in r watef-saturated porous silica glass on adsorption of Nah'O- from aqueous solution. The speclmen suffered a 50 x 10-
& P I E
irreversible length change after deter- minatioi of the extension adsorption isotherm. The isotherm greatly resembles that obtained on water vapouradsorption by dry glass, and exhibits marked hysteresis in all concentration regions. An expansion of
325 x 10'~ A t
1
L was found while porous glass was under attack from 0.2 .V NaOM; and 125 xlo-'
A R ] L length changeoccurred during the dissolution in 1.0 N HCk of a 0.5 H'/C hydrated cement specimen. On leaching lime from
0 . 6 W/C cement paste with ethylene glycol, 120 x A L ~ L was observed. These results introduce an additional factor that may explain the deterioration process of porous solids exposed to some chemicals, for example, in alkali aggregate reaction, sulfate attack and seawater attack.
VII:
47
INTRODUCTION to 400°C. '1'0 restore the hydroxyl groups on the surface, specimens were stored for five days in Mechanical breakdown is the most common and most simmering dlstillea water. Cement specime;ns were cut
important form of concrete deterioration usually from a 31 mm diameter cylinder that had been cured in associated with volume change. Several causes of lime water for two years. The specimens, when placed volume change are known. Thermal expansion does not in the cell, were fully saturated with water and care normally result in damage, but under certain con- was taken to maintain this condition during the ditions, as when large temperature gradients are transfer procedure. After attainment of constant created, it can be disruptive. Stresses, usually non- length (fluctuation less than f0.5 x A L ~ L in uniform in nature, are created by secondary effects 24 h) water was replaced by the appropriate solution of the temperature change, including change in through the filling tube (e) with the aid of a moisture content and phase change. syringe.
By far the largest group of causes of excessive vol- RESULTS ume instability in concrete comprises phenomena in-
volving chemical reactions. These may involve inter- I. Length changes of the porous 96% silica glass action of one or more constituents of the concrete, (immersed in aqueous NaN03 solution) in response to or some component or components of the concrete and a a change in solute concentration are shown in Fig.2. substance originating from the environment. Hydration The reference length, i.e., the origin of the plot, during setting and alkali aggregate reaction are is the equilibrium state of the glass fully saturated examples of the former; sulfate attack, carbonation, with water. Its length is approximately 200 x seawater attack, and corrosion of the reinforcing AL~!?, longer than in the dry state. Each point indi- steel belong to the latter category. In character- cates constancy of dimensions attained in 14 days, a~ izing these expansions Calleja (1) employs the terms the average. The run, not yet complete, has lasted intrinsic and extrinsic, depending on the type of 254 days.
reaction.
The shape of the adsorption branch of the isotherm Examination of the pertinent literature makes it clear resembles the well-known extension isotherm of the that the mechanism of expansion is far from understood, porous glass-water system (5,6), shown in Fig.3. although the causal relation between volume instabi-
lity and the chemical processes is well established. A review of alkali aggregate reaction by Diamond (2) clearly indicates that no consensus concerning the mechanism of expansion exists. Similarly, as dis- cussed by Hansen (3), several explanations have been put forward with regard to sulfate attack. Mather too has stated (4) that in spite of extensive studies fundamental aspects of the sulfate attack on cement are poorly understood, including the relation between the physical properties of the material surrounding the newly formed hydrated sulfate and the amount of expansion, and the effect of restraint on expansion. Further work to elucidate the mechanism of expansion of hydrated cement paste due to chemical attack is warranted and desirable.
The present contribution deals with the dimensional changes of water-saturated porous solid in response to change in concentration of-non-aggressive aqueous NaNO solution with which it is in contact. In add-
3
ition, length changes have been observed during leaching and dissolution.
EXPERIMENTAL
Dimensional changes were measured in a stainless steel cell (Fig.1). The 28 by 6 mm specimen (c) was clamped to the base (b) and connected to the transformer core
(g) through a floating jaw (d) and extension rod (f). The' Trans-tek #240 ,000 linear variable differential transformer (h) was housed in a double-walled section of the cell so that temperature could be maintained to
t0.05'C with the aid of a Colora thermostated circula- t ing pump.
The transformer, which has a t1.27 mm working range with t0.5% linearity, was supplied with 6 V dc froma Hewlett Packard Model 6102 power source (stability better than 0.01%).
Corning glass code 7930, porous 96% silica glass specimens were purified by continuous heating in air
Fig.1 - Schematic diagram of extensometer. For explanation see text.
I I 1 I
-
-
-
A D S O R P T I O N-
D E S O R P T I O N-
-
--
Fig.2 - Extension isotherm of the porous 96% silica glass
-
aqueous NaN03 solution system.A D S O R B E D W A T E R , g l g
irom the water extension isotherm (Fig.3), which undergoes a large expansion of approximately
100 x A 9 . l ~ in the final stages of saturation (i.e. between points C and D).
Amberg and McIntosh (5) attribute the expansion bet- ween C and D (Fig.3) to flattening of the menisci, increasing spreading force of the adsorbed film in filled pores, and further adsorption in wide channels. Because no menisci are present in adsorption from solution, no expansion at high concentrations is to be expected in the present experiments. Its absence supports the meniscus flattening hypothesis postulated for the vapour adsorption method.
The desorption branch of the NaN03-H20-glass system differs from that of the water-glass system in at least two major aspects: there is no contraction on diminishing saturation, and the volume changes are not reversible in any concentration region. Most significantly, hysteresis and continous expansion were observed on decreasing concentrations.
11. The dimensional changes observed during dissolu- tion of porous glass in 0.2 N NaOH are shown in Fig.4. The 5 mm thick glass disintegrated in 82 h. Except for the last 3 min, the sample expanded continuously. The total fractional expansion was 326 x A a l L .
Feldman and Sereda (7) reported expansion at a rapid but decreasing rate on treating porous silica glass with alkali, but their experiment was not carried to complete dissolution. The present findings appear to be consistent with the results obtained on leaching of sodium borosilicate glasses with sulfuric acid. Krasikov et a1 (8) found 0.16 to 0.2% expansion on complete leaching of 2 mm thick glass specimens. The observed 0.326% ultimate increase in length on dis- solution of glass, though very substantial, is merely a fraction of that obtained in other similar experi- ments (to be reported elsewhere). The magnitude of expansion depends on several factors, but the weight of the extension rod and transformer core assembly in the present experimental arrangement is considered to have decreased it.
I
Fig.3 - Extension isotherm of the porous 96% silica glass-water system.
At low concentrations the curve for the NaNOj-glass system rises rapidly to 27 x A!L~P.; at concentra- tions higher than 1 mole/& it decreases slightly, but monotonously, reaching 21 x A!Ll!L at 5 molar concentration. This is the only major difference
I I I I1 I
0 2 0 40 6 0 8 0 100 120 T I M E , h
Fig.4 - Length changes of a porous 96% silica glass immersed in 0.2 N aqueous NaOH, as- a function of time.
VII
-
49
1 1 1 . On immersion of a water-saturated hydrated cement paste specimen in aqueous 1.0 N HCP. solution, monotonous expansion was observed until nearly comp- lete disintegration. The dimensional change versus time of contact plot (Fig.5) is very similar to that of Fig.4, although there are some differences. The magnitude of the expansion is about one third that of the glass-NaOH system. The curve is slightly convex rather than somewhat concave, as in the previous case, and the dissolution occurred after 10.6 instead of 82 h of contact time.
TIME, h
Fig.5
-
Length changes of a hydrated cement paste specimen (W:C = 0.5), immersed in 1.0 N aqueous HCL, as a function of time.IV. Calcium hydroxide can.be leached from portland cement paste with ethylene-glycol (9). The concomi- tant 'ength changes associated with leaching a water- satulated hydrated neat cement specimen(W:C = 0.6) are shown as a function of time in Fig.6.
An over-all expansion of 110 x lo-' ~11111, with respect to the fully water-saturated state, took place after approximately 800 h of leaching at room temperature.
An'unusual feature of the curve is the fairly sub- stantial contraction, -26 x lo-' ALIL, recorded in the first 5 h following replacement of water with ethylene glycol.
Very recently, Midgley (10) reported that ethylene glycol leached out the free lime in cement paste and also attacked the calcium bearing phases. These findings have to be kept in mind when considering t k implications of the present results.
DISCUSSION
Although expansion of active carbon on adsorption. from solution was observed by Pawlow (11) over 50 years ago, it appears that a detailed extension
isotherm has not been reported until now and its existence was not really expected. It is surprising that on immersion in N M O S solution the glass speci- men expanded 25 x All(!? units beyond the length it reached on complete saturation with water (Fig.2). This expansion is about half as much as that which takes place when dry glass becomes 80% saturated with water (see Fig.3, sections A to C).
The relatively large effect of an inorganic salt on dimensions is even more significant in view of the well-documented phenomenon of salt rejection (12,13). In fact, the adsorption isotherm for water on porouJ silica glass containing various amounts of NaCR strongly suggests that salt is excluded from the first monolayer (13,14). The length changes observed in the present work seem to be due to concentration changes o f the liquid held in the centre of the capi-
llaries, separated by at least one water layer from
the glass surface. Further marked decrease in length as the NaN03 concentration increased beyond 1 mole/L
may be caused by decrease in adsorption in the vici- nity of the surface, as has been reported by Mukerjme and Anavil for the ionic surfactants-porous glass system (14).
The pronounced hysteresis of the NaN03-glass isotherm (Fig.2) indicates that on traversing the concentra- tion region that extends from pure water to saturated solution, length changes were probably caused by more
than a mere change in the concentration, or by the stmcture o f the ionic double layer on the surface.
The length in pure water is greater by approximately
SO x 10-5 dR(R after the run. This is a preliminary value only, subject to change when equillbritlm will
have been reached. The possibility t h a t irreversible change was caused by corrosion of the substrate must also be considered.
TIME. h
Fig.6 - Length changes ef a hydrated cement paste specimen (W:C = 0.6). immersed in ethylene glycol, as a function of time.
VII
-
50
The similarity of curves obtained on the dissolution of porous glass in NaOH and hydrated cement paste in HCR in the first instance, and on leaching of porous glass in acid and cement paste in ethylene glycol in the second, suggests that the reported results are of general validity. For the purpose of this discussion, this hypothesis will be accepted and some very brief comments concerning the implications for hydrated cement will be made.
The very large expansion during complete dissolution is an important factor with regard to an explanation of the mechanism of volume expansion due to chemical attack. Osmotic pressure, crystallization pressure, and mechanical pressure exerted by the volume require- ment of the reaction product, which is greater than the space available, plus gel formation followed by water ailsorption have all been suggested as causes of volume expansion of cement on chemical attack. It is fair to say, however, that none of these theories is entirely satisfactory.
In searching for understanding of the causes of volume instability of porous solids under chemical attack the solid matrix itself has not been con- sidered responsible for volume change, but was assu- med to expand passively in response to an internal pressure generated in the void space (e.g. c-rystalli- zation or osmotic pressure). The present results, although preliminary, clearly indicate that changes occurring on the surface
-
whether sorption of inert ions or dissolution of either a constituent of a non- homogeneous matrix or a portion of the entire solid - result in expansion of a magnitude that induces cracking. It appears that such alteration of the surface energy is sufficient for the disjoining pres- sure to become dominant. If correct, this assumption offers a simple, credible and unified explanation of a number of deterioration processes leading to exces- sive volume instability and cracking.ACKNOWLEDGEMENT
The author is indebted to H. Schultz for carrying out the experimental work.
REFERENCES
1
.-
J. CALLEJA (1978), "L'expansion des ciments," I1 Cemento, Vo1.75, No. 3, 154-164.2.- S. DIAMOND (1975), "A Review of Alkali-Silica Reaction and Expansion Mechanisms," Cem. Concr. Res., Vo1.5, No. 4, 329-346.
3.- W.C. HANSEN (1968), "The Chemistry of Sulphate- Resisting Portland Cements in Performance of
Concrete," Performance of Concrete, by E.G. Swenson (Ed.), Symposium in honour of Thorbergur Thorvaldson.
4.- B. MATHER (1969), "Sulfate Soundness, Sulfate Attack, and Expansive Cement in Concrete," Vol.11, RILEM Symposium on the, Durability of Concrete, Academia Prague.
5.- C.H. AMBERG and R. MCINTOSH (1952), "A Study of Adsorption Hysteresis by Mea_ns of Length Changes of a Rod of Porous Blass." Can. J. Chem., Vol. 30, NO. 12, 1012-1032.
6.- E.W. SIDEBOTTOM and G.G. LITVAN (1971), "Phase Transitions of Adsorbates; Part 2 - Vapour Pressure and Extension Isotherms of the Porous- Glass and Water System below O°C." Trans. Faraday Soc., Vol. 67, Part 9, 2726-2736. 7.- R.F. FELDMAN and P.J. SEREDA (1961), "Character-
istics of Sorption and Expansion Isotherms of Reactive Limestone Aggregate," J. Amer. Conc. Inst., Vol. 58, No. 2, 203-241.
8.- S.E. KRASIKOV, 0.S.MOLCHANOVA and L.A. ORLOVA (1963), "Volume Changes in the Leaching of Sodium Borosilicate Glasses," Zh. Prikl. Khimii, Vol. 36, No. 7, 1398-1403.
9.
-
F.M. LEA (1970), "The Chemistry of Cement and Concrete," London, Arnold, p. 108.10.- H.G. MIDGLEY (1979), "The Determination of Calcium Hydroxide in Set Portland Cements," Cem. Concr. Res., Vol. 9, No. 1, 77-82.
11.- P.N. PAWLOW (1927), "Uber die Quellung aktiver Kohle," Kolloid Zeitschr, Vol. 42, No. 2, 112-119.
12.- K.A. KRAUS, A.E. MARCINKOWSKY, J;S. JOHNSON, and A. J. SHOR (1966), "Salt Rejection of a' Porous Glass," Science, Vol. 151, 14 Jan. 194-195. 13.- G.G. LITVAN (1973), "Phase Transitions of Adsor-
bates; V . Aqueous Sodium Chloride Solutions Ad- sorbed on Porous Silica Glass," J. Coll.Inter- face Sci., Vol. 45, No. 1, 154-169.
14.- P. MUKRJEE and A. ANAVIL (1975), "Adsorption of Ionic Surfactants to Porous Glass: The Exclusion of Micelles and other Solutes from Adsorbed Layers and the Problem of Adsorption Maxima in Adsorption at Interfaces." (K.L. Mittal, Ed.),
