Differential hydration in rendering mortars

CEMENT and CONCRETE RESEARCH. Vol. 16, pp. 429-439, 1986. Printed in the USA. 0008-8846/86 $3.00+00. Copyright (c) 1986 Pergamon Press, Ltd.

DIFFERENTIAL

HYDRATION

IN RENDERING

MORTARS

C.H. ~trich~ and J.C. Maso Laboratoire de G~nie Civil de l'Institut National des Sciences Appliqu~es et de 1'Universit~ Paul Sabatier de Toulouse.

(Communicated by F.H. Wittmann) (Received March 3, 1986) ABSTRACT The disorders which affect the renderings arise from the inconsistent deformations that appear in the multilayer system rendering-substrate and from the differential hydration of the binder. The latter finds its origin in the heterogeneity of the mortar in place, resulting from the variations of characteristic parameters of mortar, substrate and placing conditions. These variations yield local modifications of the mode of rendering mortar desiccation and lead to juxtaposition of materials which rapidly acquire different behaviours. Introduction The disorders which affect the rendering mortars are generally attributed to the existence in the multilayer rendering-substrate system of inconsistent deformations. The potential contraction, free shrinkage, of hydraulic binder mortars is affected by the adherence to an almost indeformable substrate (I). Certain disorders find their origin in the heterogeneity of the mortar in place and in the hydration locally delayed according to the utilised binder. The parameters that govern the behaviour of rendering mortars can be classified into five categories according to the rendering, substrate, rendering/ substrate connection, placing conditions and finally the environmental conditions. These specific parameters are characterized by factors likely to be affected by fluctuations which are occasionally uncontrolable and often inevitable. These fluctuations lead to modification in the desiccation mode and therefore disturbance of the setting kinetics and the hardening conditions of the binder. Finally they provoke the juxtaposition of materials of different character, as much in the development of potential shrinkage as in the acquisition of mechanical characteristics. This results in risks of disorders (2) which are much more difficult to quantify than those arising only from the obstacle of the endogeneous deformation of the material. This paper presents a description of the water movements that occur in the rendering applied against masonry. The desiccation, its effects on the behaviour of renderings as well as the risks of material heterogeneity that cause the variations of the parameters governing the desiccation mode are also analysed. 429

430

Vol. 16, No. 3 C.H. D~trich~ and J.C. Maso

Desiccation of rendering mortars and their effects Right from their application, the rendering mortars are subjected to desiccation by evaporation and capillary absorption of part of their mixing water (3). This desiccation is due to the exposing of a large free surface to drying climatic conditions and to the presence of open porosity within the substrata. Afterwards, the desiccation is characterized by the ratio AE/E between the loss in water in a given instant and the initial quantity of mixing water. The desiccation causes a mechanical tightening of the solid particles by a capillary depression combined with a simultaneous increase in the concentration of the dissolved ions which accelerates the crystallization of the hydrates. This brings about a rapid increase in mortar stiffness (4) and a considerable volume contraction in the first hours of placing. The evolution of the mechanical characteristics during the hardening phase is also influenced by t h i s i n i t i a l desiccation as has been previously shown (5). Desiccation of rendering mortar by evaporation The equilibrium of a porous solid, with a continuous porosity d i s t r i b u t i o n and a given r e l a t i v e pressure is characterized by a value r, radius of c a p i l lary tubes such that a l i the c a p i l l a r y tubes of radius i n f e r i o r to r are f u l l whereas a l l capillary tubes of radius superior to r are empty. The evaporation velocity of mortar water is closely linked to the p o s s i b i l i t i e s of d i f fusion of the saturated a i r of the empty c a p i l l a r y tubes in the ambient mil i e u . Air movement through the c a p i l l a r y tubes is assimilable to a flow under the effect of motor pressure, the difference between the p a r t i a l pressure of saturated vapour and that of the ambient milieu, at the curing temperatures. T h i s flow is defined by a discharge of saturated a i r which rapidly increases with the i n i t i a l average radius of c a p i l l a r y tubes of mortar and which decreases on the other hand as this radius decreases and as the length covered increases, that is to say as the evaporation front progresses inside the material mass. The retention of mixing water during the evaporation mechanism w i l l be therefore linked to : - the climatic conditions of conservation which regulate the hygrothermic equilibrium parameters ; - the physical nature of the mixture (proportion of binder, fineness of the fine fraction) which determines the initial characteristics of the porous milieu. (From this point of view the role of the ratio sand/cement (S/C), (figure la) is particularly significant) ; - the chemical nature of the mixture (especially the binder composition) which will be responsible for the kinetics of the capillary network evolution owing to the more or less rapid structuring of the paste (The difference between a Portland cement -CPA 55R- of a stronger reactivity and a natural hydraulic lime-XHN 60- of a greater initial specific surface is illustrated (figure Ib) ). All the results of figure I concern the mortar layers of thickness e M = 12 mm. conserved at e = 20°C and H = 60 %. - the thickness of the layer e M (figure 2). In practice local variations of rendering thickness (correcting the defects of the substrate planeness is one of the functions of the rendering), variations in the time of mortar dosage (ratio C/S) and variations in the binder nature (linked to the way of storing or to the variation of the cement-lime proportion) express themselves in the form of variations in the desiccation mode.

Vol. 16, No. 3

431 DEFORMATIONS, MULTILAYERS, MORTARS, RENDERINGS, HYDRATION

0

60

f Y

"-~" 102in 1 day I

40

20

~

60

//÷7

40

0

20

I

!

3

5

(a)

I

I

I

I

3h

6h

12h

ld

I

?d

(b) FIG. 1

Desiccation by evaporation (a) - influence of the r a t i o sand/cement (b) - influence of binder nature

of

_.......~__._o---o +/

e H = ?.5 mm.

!+5~m. 0--0

50

40

30

20

10

lh

6h

ld

?d

FIG. 2 Desiccation by evaporation in terms of the thickness eM of the layer

lOOd

432

Vol. 16, No. 3 C.H. D~trich~ and J.C. Maso

Desiccation of rendering mortar by capillary absorption Porous materials (mortar and substrate) are assimilated as media having a network of parallel capillary tubes of different diameters. The contained water in the mortar capillary tubes (of average radius r M) will progressively sweep through the capillary tubes (of average radius ~ ) of the substrate under the motor pressure effect AP, the difference b~ween the substrate suction capacity and that of mortar retention (rps< rM). The volume of absor,,

5/2,.

.

~1/~.,~

bed water at an instant t is expressed in the form ~ -~I rp5 ~]-rps/rM) L in which k I characterizes the interstitial liquid by its surface tension, connecting angle and viscosity. Role of porometric characteristics of substrates The previous expression indicates that the volume of absorbed water at an instant t increases with r . The porous substrates characterized by capillary tubes of considerableP~iameters should determine therefore the initial high velocities of desiccation. However if the size of the substrate capillary tubes can be considered as fixed during the suction phase, the average radius of mortar capillary tubes is, on the contrary, a decreasing function with time. This is on the one hand because of the tightening caused by capillarity depression and as a result on the other hand of the hydration progress. During each short interval of time At i = ti+ I - ti, only the substrate capillary

tubes of radius

rps <

rMi are impregnated.

The total volume absorbed

between the contact and the instant t T can be finally written in the folio-

wing form

V T = K ] P~ ~ nj ~

Z

number of substrate c a p i l l a r y

.- ( l - ~ / r. H i ) ~

ALL 1/2

where nj denotes the

tubes of radius j such t h a t r j g rMi. The index

j=p is r e l e v a n t to the smaller r a d i i of substrate c a p i l l a r y The t o t a l quantity are rj

in

volume absorbed at an i n s t a n t t .

J

~p nj ~ =

each time

~/~

1

relating

interval

tubes.

is t h e r e f o r e p r o p o r t i o n a l to the

to only the substrate c a p i l l a r y

inferior

to

rMi.

All

substrate

tubes which

capillary

tubes

> rMi are p r o g r e s s i v e l y eliminated from the suction mechanism. This shows

t h a t the d e s i c c a t i o n mode by c a p i l l a r y absorption is c l o s e l y r e l a t e d to the porometric c h a r a c t e r i s t i c s of the s u b s t r a t e . This is e s s e n t i a l l y by the r e l a t i o n s h i p which e x i s t s , at every i n s t a n t , between the size of the bigger c a p i l l a r y tubes and those of the c a p i l l a r y tubes of the mortar in contact. This is e x a c t l y confirmed by the r e s u l t s of f i g u r e 3 in which the e v o l u t i o n of the 'desiccation w i t h i n the same mortar of two baked clays, 1 and 2 of average r a d i i 0.26 and 2.5 ~m r e s p e c t i v e l y , two mortar specimens, 1 and 28 days old (and t h e r e f o r e of d i f f e r e n t porometric c h a r a c t e r i s t i c s ) , and a specimen of block masonry has been i l l u s t r a t e d . These blocks are obtained by moulding under pressure of a mixture of go % (by weight) of sand and gravel with 10 % of Portland cement. Hence they are c h a r a c t e r i z e d by a very high p o r o s i t y . In p r a c t i c e , the d i s t r i b u t i o n of the substrate porometric c h a r a c t e r i s t i c s is a l o c a l l y v a r i a b l e f u n c t i o n : - owing to the masonry c o n s t r u c t i o n s r e s u l t i n g from assembling d i f f e r e n t and heterogeneous blocks ; - by the presence, between blocks of s i n g u l a r i t i e s which compose the f i t t i n g of j o i n t s .

Vol. 16, No. 3

433 DEFORMATIONS, MULTILAYERS, MORTARS, RENDERINGS, HYDRATION

,~_~,

.

/ /

//

20

J

_

__._____--o

~.o---

//..-a'~ / ° ///7+ . + / /

~//I/'/ 1'

28 days mortar

5'

15'

non- porous substrate

~o

o

1 day mortar

° ~ ° / ° ~ ' ° ~ ° lh

3h

6h

FIG 3 Desiccation by capi}lary absorption for different porous substrates

These local and inevitable variations o f substrate porometric characteristics will appear for recovered mortars in the variable conditions of desiccation from one point to another and hence in different behaviours. Certain of these differences appear on the figure 4 and 5.

/

1000

2B days mortar a

1000 ~

b.c. 1

500

500

100

100 15' 30'

lh

2h

15'

30'

FIG. 4 Evolution of the consistency in contact with different porous substrates

lh

+

434

Vol. 16, No. 3 C.H. D@trich@ and J.C. Maso

b.c. 1

~.~+

28 days mortar A~A------A 2

j //~ aJ

block masonry ..~------"

/

J

1 day mortar 1 °

5'

30'

lh

2h

3h

I

I

I

l

5'

30'

lh

2h

I~

3h

FIG. 5 Evolution of free shrinkage in contact with d i f f e r e n t porous substrates Role of c h a r a c t e r i s t i c

parameters of mortar

The expression of v. i n d i c a t e s t h a t the increase of binder q u a n t i t y or i t s fineness f o l l o w s t h ~ d i r e c t i o n of decrease of average radius r M of mortar and hence reduces the d e s i c c a t i o n ( f i g u r e 6). The increase of bidder r e a c t i v i t y tends in the same d i r e c t i o n ( f i g u r e 7), a f f e c t i n g the k i n e t i c s of average radius e v o l u t i o n and also the v e l o c i t y of water engagement in the hydrates.

c/s =1/5 60

40 .= t f .Jr'~

pure paste o

~

o

20

J° I

I

I

I

15"

30'

lh

3h

FIG. 6 Desiccation by capillary absorption Influence of the ratio cement/sand

I > 6h

For a range of mix tures giving a constancy in behaviour with respect to the water absorption, it is necessary to examine more p a r t i cularly the regul a r i t y of the f o l lowing factors : gradation of sand; - mortar cement/ sand ratio ; - quality of the binder (fineness, composition and constancy of proportioning in the case of a mixture of b i n d e r s ) . -

Vol. 16, No. 3

435 DEFORMATIONS, MULTILAYERS, MORTARS, RENDERINGS, HYDRATION

A11 variations of these parameters will be expressed by a modification of the desiccation conditions and therefore of behaviour as shown for example in figures 7 and 8.

I To kPa. I

/

10~

a CPA 55R E/C=0,65

0

.o CPA 55R.XHN 60 /

/o

/

°,,

.~ XHN 60

/

/

E/c ;076

"

102 30'

lh

2h

3h

FIG. 7 Evolution of the consistency ; influence of binder nature

pure paste "O

°~

°

~. + ..----~'---~

+

+fl I

mortar

1/2

"-O--

~

+

C/S=1/3 ,4

~

•

0

~

mortar 0

0

C/S=1/5

O

'l

•

--0--

I

I

1h

3h

I

-

I

FIG. 8 Evolution of free shrinkage ; influence of dosage in binder

436

Vol. 16, No. 3 C.H. D~trich~ and J.C. Maso

Role of characteristic parameters of placing conditions a) Thickness and thickness ratio. The volume of water susceptible of absorption by a porous substrate depends on the volume of its empty capillary tubes (i.e. per unit area of its thickness), while the mixing water is proportional to the mortar thickness e M. The desiccation increases therefore with the ratio eps/e M (figure 9). The d e s i c c a t i o n p r o cess will be t h e r e f o r e a f u n c t i o n of l o c a l v a r i a t i o n s of t h i c k n e s s r a t i o acc o r d i n g to : - v a r i a t i o n s of the t h i c k n e s s of mortar layer ; - c o m p o s i t i o n of h o l low b l o c k w a l l s ; - presence of j o i n t s between the b l o c k s .

50

I t l e a d s , i n a l l cases, to the j u x t a p o sition of m a t e r i a l s placed i n d i f f e r e n t c u r i n g c o n d i t i o n s and rapidly developing different characteristics. This is seen, for example, i n the results of f i g u r e 10.

10

15"

lh

3h

FIG, 9 Desiccation by capillary absorption Influence of the thickness ratio eps/e M

b) Substrate impregnation The sweeping velocity of substrate capillary tubes is given by the following formula

: dl(t)~[

= K1/~V2 . r p S ( l _ r p s / r M ) . l / l ( [

tube l e n g t h which sweeps t h r o u g h at an i n s t a n t

) where l ( t )

i s the c a p i l l a r y

t.

For a dry s u b s t r a t e l ( o ) = o. On the o t h e r hand f o r a porous s u b s t r a t e i n i tially impregnated w i t h w a t e r , the c a p i l l a r y tubes at the moment of c o n t a c t are a l r e a d y f i l l e d a l o n g a l e n g t h 1(o) = l o . C o n s e q u e n t l y , the v e l o c i t y of sweeping i s slower w i t h more s i g n i f i c a n t initial i m p r e g n a t i o n . The e f f e c t of i m p r e g n a t i o n i s t h e r e f o r e to reduce the volume of water a v a i l a b l e f o r a b s o r p t i o n and a l s o to d e l a y the e f f e c t s of the s u c t i o n . In p r a c t i c e w a l t i m p r e g n a t i o n by a s p e r s i o n i s not u n i f o r m ; f u r t h e r m o r e , the d e l a y between the p r e p a r a t i o n of s u b s t r a t e and the p l a c i n g of r e n d e r i n g i s variable. The r e s u l t i s the j u x t a p o s i t i o n of m a t e r i a l s which w i l l rapidly acquire different characteristics as shown i s f i g u r e 11. ( I : the r a t i o of absorbed water volume to the t o t a l at s a t u r a t i o n ) .

volume of absorbed water

Long term e f f e c t s I f the mass of t o t a l water ( i n t e r s t i t i a l or i n v o l v e d i n the f o r m a t i o n of h y d r a t e d p r o d u c t s ) i s d e s i g n a t e d by E. and the mass of water d e r i v e d s o l e l y from t h e r m a l d e c o m p o s i t i o n of the h y d r a t e d p r o d u c t s by Ht, i t i s observed

437

Vol. 16, No. 3 DEFORMATIONS, MULTILAYERS, MORTARS, RENDERINGS, HYDRATION

X

10~

B.C.1

M

/

o

/

o/ x/ 1o:

/ / /

x/'aJo/°/°

.__~o_~ . . . . . ,~'

30'

+/./ +

+

/

•

/

9

.~_~/c__t~_.__-~_____

~h (a)

2h

5 kPa

~-_3h

/°

i ;sio.i I

I

I

0,5

1

2

FIG. 10 Influence of the thickness r a t i o (a) - evolution of the consistency (b) - evolution of free shrinkage

-'

that the r a t i o H~/C t'representative ~of the evolution of binder hydration) follows in genera} that of ratio Et/C. Studying a great number of experimental results, allows us to deduce a relationship of the type d(Hl/c)/dL~ t=

a(ct/c-K~ H~/c) where ku is defined as the"ratio L/H~ at the end of Eth~ evaporation and K is a parameter depending upon the type of binder and the ratio E /c of mixing (5).° This relation shows that the hydration rate of the binder at a given age depends upon all stages of water movement and shows also that the rapid elimination of a considerable quantity of mixing water in the mortar may cause a slowing down of the hydration. It clearly results in a modification of the evo lution process of the mechanical characteristics which have been previously analysed (5). Thus two volumes of material juxtaposed, but subjected to different desiccation conditions during the first hours which follow the application must be considered as distinct materials having different mechanical responses.

438

Vol. 16, No. 3 C.H. D4trich4 and J.C. Maso

X

103

B.C.1 -

M

-

0

Y /i.

/

102

15'

+~+~....o~

30'

lh

(a)

/ i 0.5

+~+~+

o~~*/ 2h

. ~ , 2h 3h

0

:

o,o

~

+

+ 30

.~o •

,i_ lh

o

:

/i/,___,__,~

///

3h

,

,

Z~h

5h

.0

(b)

100

,[~-~ 6h

FIG. 11 Influence of i n i t i a l impregnation of substrate (a) - e v o l u t i o n of the consistency (b) - e v o l u t i o n of f r e e shrinkage

Vol. 16, No. 3

439 DEFORMATIONS, MULTILAYERS, MORTARS, RENDERINGS, HYDRATION

Conclusion Rendering behaviour i s governed by a g r e a t number of parameters which are subjected to unavoidable and perhaps u n c o n t r o l a b l e f l u c t u a t i o n s . I t i s the d e s i c c a t i o n process of mortars in place t h a t determines the behav i o u r as much f o r s h o r t age as f o r the end. This process i s a f u n c t i o n of parameters e s s e n t i a l l y l i n k e d to m o r t a r , s u b s t r a t e , and to the p l a c i n g c o n d i t i o n s . The r o l e of c o n s e r v a t i o n c o n d i t i o n s , being t o t a l l y u n c o n t r o l a b l e i n p r a c t i c e , are not here taken i n t o c o n s i d e r a t i o n . The f l u c t u a t i o n s of these parameters, some of which are attached to the unknown f a c t o r s of f a b r i c a t i o n ( r e g u l a r i t y of composition and s t o r i n g f o r examples) and o t h e r s which are directly i n h e r e n t i n the nature of the s u b s t r a t e r e n d e r i n g s (non-planeness, e x i s t e n c e of j o i n t s . . . ) , lead to the j u x t a p o s i t i o n of volume of m a t e r i a l which a c q u i r e r a p i d l y d i f f e r e n t b e h a v i o u r s . The d i f f e r e n t i a l h y d r a t i o n of b i n d e r s , t h e r e f o r e , c r e a t e s i n the f r o n t i e r zones between volumes of d i f f e r e n t c h a r a c t e r shear d e f o r m a t i o n s which can lead to immediate d i s o r d e r s ( c r a c k s ) o r , a t l e a s t , t o l o c a l weakenings w i t h i n the s t r u c t u r i n g of m o r t a r . These can be r e v e a l e d a f t e r w a r d s by the development of s o l l i c i t a t i o n s related to the o b s t a c l e of p o t e n t i a l s h r i n k a g e . References I.

S. Rakotonirana - Th~se D . I .

2. ] .

U n i v e r s i t ~ de P a r i s Vl - 1980.

Baron - Rapport des recherche

au L.C.P.C. - mai 1971.

3. Ch. H. D ~ t r i c h ~ , ] .

Grandet, ] . C .

4. Ch. H. D ~ t r i c h ~ ,

Grandet - RILEM 80, 91 - 102, 1981.

5.

].

Ch . H. D ~ t r i c h ~ , 302, 1984.

].L.

Gallias,

Maso - RILEM 94, 251 - 258, 1983.

3.

Grandet,

].C.

Maso - RILEM I00,

297