Effects of low calcium fly ash on sulfate resistance of OPC cement

CEMENT and CONCRETE RESEARCH. Vol. 19, pp. 194-202, 1989. Printed in the USA. 0008-8846/89. $3.00+00. Copyright (c) 1989 Pergamon Press plc.

EFFECTS OF LOW CALCIUM FLY ASH ON SULFATE RESISTANCE OF OPC CEMENT

Fabi6n Irassar* - Oscar Batic** Laboratorio de Entrenamiento M u l t i d i s c i p l i n a r i o para la Investigaci6n y Tecnologia (CIC-LEMIT) Calle 52 entre 121 y 122 1900 - LA PLATA - ARGENTINA (Communicated by D.M. Roy) (Received Feb. 8, 1988) ABSTRACT Low calcium f l y ash is a good pozzolan to improve sulfate resistance of OPC cement. Benefical effects produced by t h i s pozzolan on OPC are studied in the present paper. Results of sulfate resistance evaluated by Koch & Steinegger method are presented for four OPC + f l y ash cements. The changes in mineralogical composition of mortar are analysed by XRD, SEM and EDAX. Fly ash addition delays e t t r i n g i t e formation-cracking phenomenon and decreases the acid attack.

INTRODUCTION The c o n s t r u c t i o n of concrete s t r u c t u r e s in mild environments -water with s u l f a t e s , c h l o r i d e s , low pH, sea water- requests to use a s u i t a b l e concrete mix. In t h i s case, ACI 201 Recommendation, suggests a concrete with low water-cement r a t i o (w/c less than 0.45) and the use of mineral admixtures that increase the s u l f a t e resistance. Many natural pozzolans have demonstrated a satisfactory performance in blended cements exposed to sulfate environment [1]. Low calcium f l y ash, a by-product, has also been showed a good pozzolan for diminishing sulfate attack, especially in high water-cement r a t i o and low unitary cement content concrete [ 2 ] . * Researcher of CONICET - Lecturer of Center National U n i v e r s i t y . **Researcher of ClC - Lecturer of La Plata National U n i v e r s i t y , 194

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195 FLY ASH, LOW CALCIUM, SULFATE RESISTANCE

Concrete corrosion by agressive water with sulfate ions presents three processeS:l) Sulfate ions diffusion in concrete mass. I t is governed by porosity and pore-size distribution in cementitious matrix [3,15]. 2) Chemical reaction between sulfate solution and compounds of portland cement paste with considerable expansion. This reaction involves C3A hydrates and CH to form ettringite (C3A.3SC.31H) and gypsum (CSH2), respectively. 3) Cracking of cementiceuos matrix and increase of concrete damage. The benefical effects that produce pozzolans in concrete to provide resistance to sulfate attack indicated by severals investigators, can be summaryzed as: I) Removal of CH -originated by calcium silicate hydration- by combination with pozzolans [4]. 2) Decrease of porosity by C-S-H ( I I ) formation and protection of unstable cement compounds in sulfate environment. Reduction of CH in paste-aggregate zone [4,5]. 3) Ettringite formation in pore solution without expansion [4]. 4) Relative diminution of C3A content in blended cement [6]. The significance of this investigation is to contribute to clarify the cause whereby low calcium f l y ash improves the sulfate resistance of ordinary portland cements. MATERIALS AND PROCEDURE Ordinary portland cement (OPC-ASTM Type I) was used as reference cement. Its potential composition was: C3S= 37,2%; C S= 32~6%; C A= 7,7%; C.AF= 12,8%; alkalis (in NapO)= 1,22%; and its fineness 3~00 cm /g (~laine). T~is cement presented a po~r sulfate resistance. Fly ash produced in an Argentine power plant by combustion of subbituminous co~l type A (ASTMD-388) was used with two finenesses:2800 (FA1) and 4200 (FA2) cm'/g Blaine. The chemical composition of f l y ash was: SiO~= 59.0%; Al 0 = 24.0%; Fe O = 6.8%; CaD= 4.0% and MgO= 2.0%. This f l y as~ can be classified as ASTM ~l~ss F. Mineralogical analysis by X-Ray Diffraction showed quartz and mullite as principal crystaline compounds. Its glass was aluminosilicate. This composition was indicated by presence of diffuse band between 21 and 25 deg 28 in XRD-pattern [6]. Two blended portland cements (OPC+FA) have been made with each f l y ash; one replacing 20 percent by weight of cement of f l y ash (OPC+2OFA), and the other, 30 percent (OPC+3OFA). The test method adopted to study sulfate resistance of cements was Koch & Steinegger method [7,8]. This test, according to Calleja [9], can be classified as one the best methods to evaluate sulfate resistance of blended portland cements. This test measures the variation of flexural strength, the best index to value the effectiveness of the pozzolans [10]. Briefly, the test involves immersion of small prisms (lOxlOx60 mm) in deionized water during 21 days. After immersion, the specimens are stored in a 4,4% sodium sulfate

196

Vol. 19, No. 2 F. Irassar and O. Batic

solution. The rate of attack is increased by high permeability of mortar (cement-sand r a t i o = 1:3 and water-cement r a t i o = 0,6). The c r i t e r i o n selected by these authors to ~ a l u a t e t h e s u l f a t e resistance is the r e l a t i v e transverse strength, which is called "Corrosion Coefficient (R)" This coefficient is calculated with the following expression: R=

Transverse strength of prisms immersed in sulfate solution Transverse strength of prisms cured in water

The Corrosion Coefficient is determined at 21+14, 21+28, 21+56, 21+160, and 21+340 d a y s after test started. Sulfate resistance of cement is satisfactory when the R-Coefficient is higher than 0,70 at 21+56 days. In t h i s study, twelve prisms stored in sulfate solution and twelve prisms cured in water were tested at each age to determine R-Coefficient. In order to distinguish changes in mineralogical composition of mortars cured in water and in sulfate solution, X-Ray power d i f f r a c t i o n technique using Cu-K~ radiation was employed. Scanning Electron Microscope (SEM) equipped with an Energy Dispersive Analysis of X-Ray (EDAX) was used as complement of DRX analisys. XRD analysis was realized on the material resulting from the ~ r i t u r a t i o n of a representative prism, which passed through sieve 75 pm (N°200). SEM analysis was realized on the extreme of the same prism. The mortar porosity was measured using the following procedure: The prism was dried to constant weight at 105°C, then immersed in carbon tetrachloride in a vacuum desiccator. Vacuum was applied u n t i l a i r bubbles ceased to emerge from the prism. Its mass was determinated and the carbon tetrachloride was evaporated at I05°C. F i n a l l y , the mass of dry prism was determinated. The porosity was calculated as follows: Porosity (%) =

(Ws - Wd) 1.593 . Wd

x 100

where: Ws is the weight of satured prism, Wd is the mass of dry prism and 1,593 is the molecular weight of carbon tetrachloride. RESULTS AND DISCUSSION Flexural strength and

corrosion c o e f f i c i e n t

Cured in water: Flexural strength evolution of specimens cured in water is shown in Fig. 1. OPC does not present important increase in flexural strength and develops most of i t s stregth at 21 days. OPC+FA cements present a d i f f e r e n t behaviour. In the f i r s t age, OPC+FA1 show a pronouced loss of strength and i t does not equalize the flexural strength of reference cement. On the other hand, OPC+FA2 cements, i n i t i a l y present a l i t t l e strength loss;

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197

FLY ASH, LOW CALCIUM, SULFATERESISTANCE after 21+28 days, they attain to strength of OPC. At the end of the test, their flexural strength is 20 percent higher than strength of reference cement. This different behaviour of both f l y ashes is attributed to the high pozzolanic a c t i v i t y of FA2 and i t s dispersing effect on cement grains, owing to the very fine particles of this f l y ash. Cured in sulfate solution: Flexural strength variation of mortars cured in sulfate solution can be seen in Fig. 2. OPC presents a quick diminution of flexural strength, once the attack has started. On the other hand, OPC+FAcements present an increasement of flexura] strength during the f i r s t s exposure days (21+14). Later, i t is mantained until 21+28 days, and decreases slowly until 21+56 days. At 21+160 days, the flexural strength is very l i t t l e . I t is important to remark that maximun flexural strength of OPC+2OFA and OPC+3OFA is equal for both f l y ash. Corrosion coefficient (R): Its variation is shown in figure 3. All OPC+FAcements have a major chemical behaviour than OPC. They satisfy the sulfate resistance criterion. At 21+160 days, corrosion coefficient values are similar for all blended cements.

i X

~-'OPC ~--~+20%FAI ,--4+30%FAI o--o+20%FA2 +30%FA2

v 8

8 6

~4

+20%FAI +30%FAI o--o +20%FA2 '--" +30%FA2

O~

x2

+14' +;6 +60 +40

21

4

AGE (days)

FIG 1: Flexural strength of prisms cured in water

+160 +340 AGE (days)

FIG 2: Flexura] strength of prisms cured in sulfate solution.

1.2

,-~OPC ~-~ +20%FAI #-~ +30%FA.L o-o+20%FA2 +30%FA2

Z

0.8 0 ¢..)

FIG 3: R-Coefficient Corrosion variation

"o.6 h X

0.4 I

21

I

I

I

+14 +28 +56

+160

Age(da s)

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Vol. 19, No. 2 F. I r a s s a r and O. Batic

P o r o s i t y , XRD and SEM This a n a l y s i s has been realized to i n t e r p r e t the mechanical behavior described p r e v i o u s l y . In figures 4 and 5 i t can be seen XRD patterns of OPC ana OPC+3OFA2 cements for each test age of the prisms cured in water and the prisms cured in s u l f a t e s o l u t i o n . Table I shows p o r o s i t y e v o l u t i o n . The other cements present s i m i l a r behaviour. TABLE [: Porosity of mortar prisms in percent. CEMENT OPC AGE (days) water s u l f a t e 21+0 21+14 21+56

6.5 6.5 6.8

OPC+3OFAI Cured in water s u l f a t e

9.2 9.3

9.5 9.2 9.3

8.5 9.7

OPC+3OFA2 water

sulfate

7.6 7.7 8.0

7.~ 8.1

Before the s u l f a t e attack (21 days), i t is impossible detect monosulfoaluminate (C3A.~.I2H) or hydrogarnets (C3AH,4) by XRD a n a l y s i s . SEM zecnnique permits t h i s detection (see f i g u r e 6}. ~n OPC+FA cements i t is ooserved a low reaction and a l i t t l e adherence between f l y ash spheres and the m a t r i x . This observation j u s t i f i e s lower strength of mortar and higher p o r o s i t y at f i r s t ages. At a l l ages, specimens stored in water, present an important advance of surface carbonation. I t is caused by the reaction between CH - i n paste- and C,G

O,

G.(:

a) r

f. ,~

m

£

21 - g 6 d

m

b)

zl 1~ d

c

Q

~I - ~ o

Q

T

P

d

c

2G

FIG 4: XRD-patterns of OPC mortar: (a) Cured in water. (b) Cured in sodium s u l f a t e s o l u t i o n . / / Q: Quartz. C= C a l c i t e . P: P o r t l a n d i t e . E: E t t r i n g i t e . G= Gypsum

Vol. 19, No. 2

199 FLY ASH, LOW CALCIUM, SULFATE RESISTANCE

CG

¢

"

,'

2o

~

(:

"

,o

P

I15

,~

1

,~

,

I@

FIG 5: XRD-patterns of OPC+3OFA2 mortar: (a) cured in water. (b) Cured in sodium sulfate solution. / / Q= Quartz. C= Calcite. P= Portlandite. E= Ettringite. G= Gypsum.

CC^H -in water-. Carbonation takes place whenthe water-oH i s 7 t o l O [12]. The pH measured in water storage was 7-7.5 at 21 days and 10-11 at 21+340 days. Progress of this reaction can be seen in XRD patterns (28=29.4° Fig. 4 and 5). Crystals of calcite with well developed face cover all prisms surface of OPC and OPC+FA cements at 21+56 days (Fig. 7). An increase of sulfate resistance in OPC+FAcements has been measured using the same test method, when previous cu~ngwas extended to 60 days [13]. This behaviour can be undertood by the progress of pozzolanic reactions that reduce the porosity [14] and by the formation of layer of CC which protects the mortar. XRD patterns of OPC+FA cement cured in water show a decrease of CH (28=34,06). Previous research corroborates this observation [14]. OPC+FA cements exposed at agressive solution show the growing of ettringite crystals (28=9,08), and a decrease of mortar porosity at 21+14 days. SEM observation permits to affirm that ettringite crystals develop in mortar pores (see figure 8). For this reason, flexural strength is increased. OPC does not show this increase in strength because the cracking took place before the f i r s t measure. Ettringite crystals continue growing. New contributions of sulfate ions lead to prisms cracking and flexural strength decreases. This phenomenon takes place between 21+14 and 21+28 days in OPC+FA cements and between 21 and 21+14

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Vol. 19, No. 2 F. I r a s s a r and O. Batic

i¸

i ~

i I I I i i ~

FIG 6: Monosulfoaluminate in OPC at 21 days of curingin water. EDAX: C= 58.0. A= 24.0. S= 18.0.

FIG 7: Calcium carbonate c r y s t a l s in OPC at 21+56 days of curing in wa te r.

FIG 8: E t t r i n g i t e f i b e r s into OPC+ 30FA2 mortar pore at 21+56 days. EDAX: C= 55.0%. A= 12.0~. S= 33.0%

FIG 9: Gypsum c r y s t a l i n t o OPC+3OFA2 mortar pore at 21+56 days. EDAX: S= 60.0%. C= 40.0%

FIG I0: Gypsum c r y s t a l in pasteaggregate zone in OPC at 21+56 days.

FIG I i : Ettringite in OPC+3OFA2 mortar at 21+340 days. EDAX: C= 48.0%. S= 40.0%. A= 12.0%

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201 FLY ASH, LOW CALCIUM, SULFATE RESISTANCE

days in OPC (see figure 2). Mortar cracking originates a large exposed surface to~gressive solution. Therefore, corrosion rate is greater. In this moment, the gypsum corrosion -acid attack- acquires a great importance (28=11,7°). Gypsum is formed by combination of HC in cement paste with sulfate ions in solution. Gypsum crystallizes with expansion. In OPC+FA cements, gypsum formation is delayed. At 21+56 days, gypsum in OPC is greater than in OPC+FA cements (see figures 4 and 5), on account of CH removal by pozzolanic reaction. All mixes present an increase of mortar porosity. Gypsum formation occurs in p o r e s and fissures. I t can be seen in the transition zone paste-aggregate, too (see figures 9 and 10). After one year, the prisms have been destroyed, CH can't be detected by XRD. I t has been transformed to gypsum. Gypsum crystals are in block of 30 to 150 um and are localized in cracks (figure 9). Size of e t t r i n g i t e crystals in OPC+FA cements is 15 to 25 pm by 2 to 3 pm (see figure 11). The pris~,s casted with OPC+2OFA present a greater relative degradation than OPC+3OFA cements. I t should De attributed to a greater formation of gypsum. Figure 12 shows XR~patterns of the four OPC+FA cements after 21+160 days cured in sulfate solution. Surface carbonation phenomenon takes place in prisms cured in sulfate Solution too. CONCLUSIONS For the materials and test methods used in this research, the following conclusions can be enumerated: 1) Sulfate attack to l i t t l e mortar prisms presents three well defined stages: a)A~ressive ions diffusion in mortar mass and chemical reaction with CRA hydrates to form e t t r i n g i t e . In this stage exists an increase in fTexural strength. Ettringite crystallizes into mortar pores and decreases the mortar porosity. QG

G

20%FA 2

L

40

!

15

I

30

I

zS

20

a

15

I

10

20% FA!

I

S

2e

FIG 12: XRD-patterns of OPC+FA mortars at 21+160 days of cured in sodium sulfate solution

202

Vol. 19, No. 2 F. Irassar and O. Batic

b) Prisms fissuration due to the ettringite crystals growth; in this stage, starts the flexural strength loss. c) Acid attack gypsum formation due to the greater exposed surface and the high sulfate concentration in solution. At the end of this stage the specimens are destroyed. 2) The improvements in sulfate resistance of OPC cement by addition of low calcium f l y ash have two principal causes: at f i r s t ages, ettringite formation and cracking is delayed. Then pozzolanic reaction reduces CH in mortars and consequently gypsum formation. 3) Fly ash fineness doesn't have influence on sulfate resistance; on the other hand, i t influences flexural strength development.

REFERENCES L1] Kalousek, G.L.-Porter, L.C.-Beton, E.J.. Cem. Concr. Res. 2 (1), 79-89 (1972) [2] Stark, D.. ACI Publication. SP 77-2, 21-40 (1980) [3] Bernaudat, F.-Revertegat, E.. Vll Int. Cong. Chem. Cem. V, 41-46 (1986) [4] Massazza, F.. II Cim. 1/76, 3-38 (1979) [5] Mehta, P.K.. "Concrete: Structure and M~terials", p.269.Prentice Hall Inc. N.Y. (1986) [6] Mehta, P.K.. J. Amer. Concr. Inst. 83, 994-1000 (1986) [7] Koch, A.-Steinegger, H.. Zem. Kalk Gips 7, 317-324 (1960) [8] jasper, M.J.. Mat. et Const. 704, 1/77, 51-58 (1977) [9] Calleja, J.. Vll Int. Cong. Chem. Cem. VII-2, 1-49 (1980) [I0] Samanta, S.-Chatterjee, M.K.. Cem. Concr. Res. I.__22(6), 726-734 (1982) [ I I ] Wong, G.-Poole, T.. ACI Publication. SP-IO0-109, 2121-2134 (1986). [12] Koelliker, E.. VIII Int. Cong. Chem. Cem. V, 159-164 (1986) [13] Irassar, E.F.. Data to be published [14] Batic, O.-Sota, J . - V i s i n t i n , L.. Anuario Asoc. Quim. Arg. 73 (6), 539-552 (1985) [15] Huges, D.C.. Cem. Concr. Res. I_55(6), 1003-1012 (1985)