The Influence of Slag and Fly Ash on the Carbonation of Concretes. By M. Collepardi, S. Collepardi, J.J. Ogoumah Olagot and F.

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1 The Influence of Slag and Fly Ash on the Carbonation of Concretes By M. Collepardi, S. Collepardi, J.J. Ogoumah Olagot and F. Simonelli Synopsis: The paper shows the influence of mineral additions (in form of fly ash, slag and ground limestone) replacing portland cement on the CO 2 penetration rate of concretes manufactured at a given water-cementitious material ratio (w/cm). The results indicate that at a given w/cm there is an increase in the carbonation rate in concretes with mineral additions, except when the amount of portland cement replacement is relatively low (15%). On the other hand, when the comparison of the carbonation rate is made on concretes at the same strength level, there is no significant difference between concretes with portland cement and those with replacement by mineral addition up to 50%. Keywords: Carbonation, Compressive Strength, Fly Ash, Ground Limestone, Mineral Addition

2 Mario Collepardi is Professor at the Civil Engineering Faculty Leonardo da Vinci, Politechnic of Milan, Italy. He is author or co-author of numerous papers on concrete technology and cement chemistry. He is also the recipient of several awards for his contributions to the fundamental knowledge of superplasticizers and their use in concrete. Silvia Collepardi is a research civil engineer and director of the Enco Laboratory, Spresiano, Italy. She is working in the field of concrete durability and superplasticized concrete mixtures and has published several papers in this area. Jean Jacob Ogoumah Olagot is a civil engineer working as researcher in the field of concrete at Enco, Spresiano (TV), Italy. Francesca Simonelli is a civil engineer working as researcher in the field of concrete at Enco, Spresiano (TV), Italy. INTRODUCTION The replacement of portland cement by fly ash and granulated ground blast furnace slag produce a cementitious matrix which is more resistant to many aggressive chemical conditions including alkali-aggregate reaction, sulphate attack and corrosion of steel promoted by chloride penetration. Concretes with slag and/or fly ash are considered to be more durable than the corresponding concretes without any replacement of portland cement [1-4]. On the other hand, the carbonation process (1) can cause depassivation and promote corrosion of steel reinforcements in structures exposed to air through the reaction Ca(OH) 2 +CO 2 CaCO 3 +H 2 O (1) and there are some doubts about a durable behavior of concretes with slag or fly ash replacing part of portland cement, due to the lower amount of Ca(OH) 2 which is available to be neutralized by CO 2. There are two mechanisms, both related to the presence of slag and/or fly ash, which can cause a significant reduction of Ca(OH) 2 in the cement matrix: - firstly, a lower amount of portland cement available, through its hydration, for the Ca(OH) 2 formation according to the equation (2);

3 portland cement + H 2 O C-S-H + Ca(OH) 2 + C-A-H (2) fly ash & slag H 2 O C-S-H* (3) - secondly, the combination of Ca(OH) 2 with fly ash or slag to produce additional calcium silicate hydrate (C-S-H*) according to the equation (3). It is difficult to say whether the additional C-S-H*, produced according to equation (3) can furtherly decrease the capillary porosity and then compensate for the lower amount of Ca(OH) 2 in reducing the carbonation process. The purpose of this work was to study the influence of replacement of portland cement by fly ash or slag on the carbonation rate in order to check whether or not this replacement can really increase the carbonation rate and therefore the risk of steel corrosion promoted by carbonation. EXPERIMENTAL: MATERIALS AND METHODS Several concrete types were manufactured by replacing portland cement with slag (15-50%) or fly ash (25%). Ground limestone (15-25%) was also used in order to compare the influence of a mineral addition without any pozzolanic activity, such as CaCO 3, with that of active cementitious materials as fly ash and slag. Table 1 shows the characteristics of portland cement and that of the mineral addition. Different water-cementitious material ratios (w/cm) were adopted ( ) for each concrete type where the cementitious material (cm) is portland cement or its combinations with mineral additions for a total of six mixtures (Table 2). For each of the six concrete mixtures shown in Table 2 related to a specific cementitious material - there are four mixture codes ( ; ; ; ), where the first figure (400, 350, 300 or 250) indicates the content of cementitious materials, in kg/m 3, and the second one (0.40, 0.50, 0.60 or 0.70) is related to the nominal water-cementitious material ratio (w/cm). The water-cement ratio (w/c) is shown in the sixth column of Table 2. The amount of total aggregate in kg/m 3 is shown in the seventh column. It was obtained combining sand (0-4 mm), natural aggregate (5-12 mm), and gravel (4-19 mm) according to the Fuller s curve grading. In order to keep about the same workability level (slump = mm), as shown in the ninth comumn of Table 2 for all the concrete mixtures an adequate dosage of an acrylic superplasticizer, was used, specially in concrete mixtures with the lowest w/cm (0.40).

4 The cube 28-day compressive strength shown in the last column of Table 2, characterizes the concrete from a mechanical point of view. Concretes were exposed to air (20 C; R.H. of 60%) after a curing of 28 days at 20 C at R.H. of 95%. The penetration of CO 2 was measured through the phenolphtalein test as a function of the exposure time up to 1 year. RESULTS Figures 1-6 show, for each of the 24 concrete mixtures shown in Table 2, the carbonation depth as a function of the exposure time to the air. Since the carbonation rate is usually represented through the following equation (4) x = k t (4) where x is the depth of concrete penetrated by CO 2 and t is the exposure time to the air, the plot x vs. t is approximately linear and the slope is related to the k value. For each cementitious material the k value is shown on the four curves related to the corresponding w/cm. When portland cement is used (Fig. 1), the higher the w/c the higher the penetration rate: for instance k is 1 mm year -½ with a w/c of 0.40 and becomes 8 mm year -½ with w/c of Similar results are obtained when portland cement is replaced by mineral additions (Fig. 2-6): for a given cementitious material the higher the w/cm the higher the k value. At a given w/cm, the concrete manufactured with portland cement appears to be more resistant to the CO 2 penetration than all the other mixes where portland cement was replaced by mineral addition. However, there is no difference in the CO 2 penetration rate when the concrete with portland cement (Fig. 1) is compared with that containing 15% of slag substituting for portland cement (Fig. 5): in such a case the k value, at a given w/cm, is approximately the same, although the w/c of the concrete with slag is about 18% higher than that of the corresponding without slag (Table 2). On the other hand, when the percentage of slag replacing portland cement is as high as 50%, the difference in the w/c, at a given w/cm, between the two concretes is very high (100% higher in the slag-portland cement concrete ad shown in Table 2). Consequently, the carbonation rate, in terms of k, is much higher in the concrete containing 50% of slag in the cementitious material (Fig. 6) with respect to the concrete without slag (Fig. 1) at the same w/cm. In concretes with moderate levels of replacement (15%) by ground limestone the difference in the CO 2 penetration rate with respect to the concrete with portland cement is negligible (Fig. 1-2). In concretes with higher levels of replacement (25%) by fly ash (Fig. 4) and specially by ground limestone (Fig. 3) there is a significant increase in the

5 CO 2 penetration rate compared with the concrete with portland cement (Fig. 1). When the comparison is made at the same strength level, the replacement of portland cement by mineral additions does not sustantially change the carbonation rate. An example of this comparison is shown in Table 3: in concretes, all with a 28-day compressive strength in the range of N/mm 2, the carbonation depth at different times of exposure to air is aproximately the same. Figure 7 shows the carbonation depth as a function of t for all the concrete mixtures shown in Table 3: there is only one curve needed to describe the carbonation rate which is independent of the concrete composition. These results indicate that compressive strength, rather than w/c or w/cm, is an important parameter affecting the CO 2 penetration into the concrete. This does not seem to be surprising since strength and carbonation are both related to the porosity of the cement matrix. CONCLUSIONS The replacement of portland cement by mineral additions at a given watercementitious material ratio increases the carbonation rate, with negligible difference at lower levels (15%) of replacement. The replacement of portland cement by mineral additions for concretes of a given strength does not cause any change in the carbonation rate. This result is very important for the durability of concretes containing mineral additions exposed to the risk of corrosion of metallic reinforcements promoted by CO 2 penetration. In addition to the excellent performance [1-4] for the chemical attack caused by alkali aggregate reaction, sulfate attack, chloride penetration, there is no risk of corrosion promoted by carbonation in concretes containing fly ash and slag provided that the strength level is not reduced. REFERENCES [1] Idorn, G.M., The effect of slag cement in concrete, NRMCA Publication No. 167, 10 pp., Silver Spring, Maryland, April 1983 [2] Bilodeau, A., Sivasundaram, V., Painter, K.E., Malhotra, V.M., «Durability of concrete incorporating high volumes of fly ash from sources in the U.S.A.», ACI Materials Journal, 91, N 11, pp 3-12, 1994 [3] Tikalsky, P.J., Carrasquillo, P.M., and Carrasquillo, R.,L., Strength and durability considerations affecting mix proportioning of concrete containing fly ash, ACI Materials Journal, 85, No 6, pp , 1988

6 [4] Currie, R.J. Carbonation depths in structural quality concrete, Building Research Establishment Report, 19 pp, Watford, U.K., 1986

7 Table 1 Chemical composition and Blaine fineness of the cementitious materials Composition & Fineness* Portland Cement Limestone Fly Ash Slag l.o.i SiO Al 2 O Fe 2 O CaO MgO K 2 O Na 2 O SO CO *Blaine (m 2 /kg)

8 Table 2.Composition and properties of concrete mixtures Mixture Code (cm w/cm) Portland Cement Cementitious Material (cm) Limestone* Fly Ash Slag w/c Aggregate (kg/m 3 ) Superplasticizer (% by cm) Slump (mm) f c28 (N/mm 2 ) *Ground limestone is really a powder rather than a cementitious material

9 Table 3 Carbonation rate of concrete with and without mineral additions at a given 28-day compressive strength (f c28 ) in the range of MPa Composition of cementitious materials (cm) Portland cement Limestone Fly ash Slag cm (kg/m 3 ) w/cm f c28 (N/mm 2 ) 30 days Carbonation depth (mm) at: 45 days 60 days 90 days 180 days days Average Fig. 1 Carbonation rate as a function of w/cm for concretes with portland cement (CEM I 52.5R).

10 Fig. 2 Carbonation rate as a function of w/cm for concretes with limestone (15%)-portland blended cement. Fig. 3 Carbonation rate as a function of w/cm for concretes with limestone(25%)-portland blended cement.

11 Fig. 4 Carbonation rate as a function of w/cm for concretes with fly ash (25%)-portland blended cement. Fig. 5 Carbonation rate as a function of w/cm for concretes with slag (15%)-portland blended cement.

12 Fig. 6 Carbonation rate as a function of w/cm for concretes with slag (50%)-portland blended cement. Fig. 7 Carbonation rate for all the concretes shown in Table 3 characterized by the same strength level (40-45 N/mm 2 ).