IMPROVING SULFATE RESISTANCE OF MORTARS PRODUCED WITH SANDS CONTAMINATED BY NATURAL SULFATE

Transcription

1 International RILEM Conference on Material Science MATSCI, Aachen 2010 Vol. III, AdIPoC 231 IMPROVING SULFATE RESISTANCE OF MORTARS PRODUCED WITH SANDS CONTAMINATED BY NATURAL SULFATE H. N. Atahan, D. Dikme, Istanbul Technical University, Civil Eng. Faculty, Istanbul, Turkey. ABSTRACT: The existence of natural sulfates in many Middle East aggregates would trigger sulfate attack when they are used in concrete mixture, unless proper precautions are taken. In this study, the effect of sulfates on the expansion of mortars mixed with 2 different sands, contaminated by natural sulfate (water soluble sulfate amounts of 1.6% and 2.0%) provided from Bagdad and Kerbela region in Iraq was investigated. A commercially available Type I cement with C 3 A content of 7.6% was used. The effect of mineral admixtures such as ultra fine amorphous colloidal silica (NS), micro silica (MS), fly ash (FA) and ground granulated blast furnace slag (GGBS) on the expansions caused by internal sulfate attack was monitored. Results have shown that depending upon the percentage used, all of the mineral additives, particularly GGBS, have significantly reduced the expansions. Another significant data was obtained from the samples mixed with NS. Although very low replacement ratios such as 4% to 6% are used, the effectiveness of nano silica is very significant. Keywords: Internal sulfate attack, Ettringite, Middle East aggregates, Sulfate contaminated sand, Nano silica, Colloidal silica, Micro silica, Fly ash, GGBS, Expansion. 1 INTRODUCTION Concrete, which is most widely used construction material today, has a complex structure consisting of cement, aggregate, mineral and chemical admixtures, and mixing water. The properties of plain concrete depend on the chemical reactions mainly between cement particles and water, as well as the reactions between other components of concrete. The chemistry of hydration products, pore structure and mechanical properties of hardened concrete is significantly affected by the environmental conditions in which the concrete is exposed during its service life. Sulfate contaminated environment is one of the most important factors that significantly affects the durability of plain and reinforced concrete structures negatively. Lots of research can be found regarding to this aspect [Col03, San02a, San03b]. Sulfate attack can be defined as a series of complex chemical reactions chain which takes place between the sulfate ions and hydration products of cement and, finally results in deterioration of concrete stability. The sulfate ions can diffuse in concrete from the external environment, however, it is also possible that the source of these ions can also be the concrete mixing materials such as cement, aggregate, and chemical and mineral admixtures. Ettringite (3Ca0.Al 2 O 3.3CaSO 4.32H 2 0) formation in hardened concrete is the main reason for concrete deterioration. An example of such reaction is formation of ettringite from monosulfate and gypsum based on the following reaction: C 4 ASH CSH H C 6 AS 3 H 32 (Monosulfate) (Gypsum) (Water) (Ettringite) This reaction known to result in an increase in solid volume of the media and may lead to expansion and deterioartion of concrete [Ska02]. An SEM picture of monosufate hydrate and ettringite can be seen in Figure 1.1.

2 232 ATAHAN, DIKME: Improving Sulfate Resistance of Mortars Produced with Sands Contaminated by Natural Sulfate Fig An SEM picture of typical hexagonal crystals of monosulfate hydrate and needle like crystals of ettringite [Meh06]. ASTM C1038 [Ast04] allows the amount of sulfate in the cement and pozzolan to increase by any amount as long as the expansion after 14 days of immersion in water does not exceed 0.02%. On the other hand, other expansion limits have been proposed for concrete samples subjected to composition induced internal sulfate attack. For moderate resistance, expansion limit of 0.1% after 6 months is generally considered as a safe boundary for determining the maximum percentage of sulfate that can exist in a given mixture without causing any significant deterioration [Ska02, Ast03, Ast05]. Internal sulfate attack, which is caused by the sulfates from aggregates contaminated by natural sulfate, has not been extensively investigated. Gypsum and other sulfates must not be present in the aggregate; however, the existence of sulfate in many Middle East aggregates leads to difficulties. Neville [Nev00] reported that, in a concrete mixture, up to 5% of sulfate by mass of cement (including the sulfate in the cement) is often tolerated. On the other hand, indispensability of the use of sulfate contaminated aggregates in infrastructure and superstructure projects brings potential risks of concrete deterioration due to sulfate attack. Sulfate attack on concrete is mainly due to two principal reactions: (i) the reaction of sulfate ions and Ca(OH) 2 to form gypsum and, (ii) the reaction of the formed gypsum with calcium aluminates hydrates (mono-sulfate hydrate, Afm) to form ettringite. Therefore, it is very important to use mineral additives in order to reduce calcium hydroxide content which is introduced into the mixture by cement hydration. The pozzolanic reaction between calcium hydroxide and the mineral admixtures results in the reduction of calcium hydroxide and improves the sulfate resistance. So, in such cases, understanding which pozzolanic material affects concrete performance best is a crucial knowledge for the engineers in the field. In this experimental study, the effect of sulfates on the expansion of mortar bars mixed with 2 different sands, contaminated by natural sulfate (water soluble sulfate amounts of 1.6% and 2.0% by weight), provided from Bagdad and Kerbela region in Iraq was investigated. The effect of mineral admixtures such as nano-silica (NS), micro silica (MS), fly ash (FA) and ground granulated blast furnace slag (GGBS) on the expansions caused by internal sulfate attack was studied as well. Among these selected admixtures, MS, FA and GGBS are the most commonly used mineral additives in concrete industry.

3 International RILEM Conference on Material Science MATSCI, Aachen 2010 Vol. III, AdIPoC EXPERIMENTAL STUDY In this experimental study, the effect of sulfates on the expansions mortar bars mixed with 2 different naturally sulfate contaminated sands (water soluble sulfate amounts of 1.6% and 2.0% by weight) provided from Bagdad and Kerbela region in Iraq was investigated. Some of the physical and chemical characteristics of these sands are given in Table 2.1. A commercially available Type I cement with C 3 A content of 7.6% was used. The chemical and physical properties of the cement are given in Table 2.2. For all mixtures, the mixture proportions of the mortars were kept constant as following: water/sand/cementitious material = 0.5/2.75/1. Workability (flow) of the mixtures was kept constant at 15±1cm when tested according to ASTM C1437 [Ast07]. A commercially available high range water reducer was also used when necessary. Table 2.1. Physical and chemical properties of the sand samples. Sample # Sand1_BAGDAD Sand2_KERBELA Density, kg/m Water absorption, % by weight Water soluble sulfate, % by weight Chloride, % by weight Dissolved silica, m.mol/l 33 9 Reduction in alkalinity, m.mol/l Table 2.2. Chemical and physical properties of cement. Chemical Analysis (%) Mineralogical Composition, % Physical and Mechanical Properties SiO C3S Density 3150 kg/m 3 Al 2 O C2S CaO C3A 7.59 Specific MgO 1.28 C4AF Surface: 373 m 2 /kg SO Cl Standard Na 2 O 0.21 Strength: 51.3 MPa K 2 O 0.92 Free lime 1.15 Insoluble residue 0.49 The fresh mixes of mortar were placed in 40x40x160 mm 3 molds and compacted on a vibration table. After that, the samples were sealed and kept in 22±2 o C and 60% RH laboratory environment for 24 hours. After this period, samples were removed from the molds and the first length-comparator readings were done as a reference reading with a precision of 0.01mm. The samples were then placed in 22±2 o C water saturated with calcium hydroxide and consecutive length-comparator readings were done once a week. The effect of mineral admixtures such as nano-silica (NS), micro silica (MS), fly ash (FA) and ground granulated blast furnace slag (GGBS) on the expansions caused by internal sulfate attack was studied. Some of the physical and chemical properties of the mineral additives used in this study are given in Table 2.3. As well as the reference mixtures which

4 234 ATAHAN, DIKME: Improving Sulfate Resistance of Mortars Produced with Sands Contaminated by Natural Sulfate does not contain mineral additives, three different replacement ratios by weight of cement was selected for each mineral additive. These replacement ratios were 2%, 4% and 6% for NS, 6%, 9% and 12% for MS, 15%, 30% and 45% for FA and, 20%, 40% and 60% for GGBS. It should be explained that the nano silica sol (NS) which was used in the mixtures has a solid content of 50% (50% SiO 2 and 50% water by weight), and the given replacement ratios for this material are for pure NS amounts. A summary of mortar mixtures used are shown in Table 2.4. Table 2.3. Chemical and physical properties of mineral admixtures. Composition, % Nano Silica (NS) (Ultra Fine Amorphous Colloidal Silica) Micro Silica (MS) Fly Ash (FA) Ground Granulated Blast Furnace Slag (GGBS) SiO 2 >99 > Al 2 O Fe 2 O CaO - < MgO Na 2 O K 2 O SO 3 - < Specific Surface, m 2 /kg > > Density, kg/m Mean size, µm [Ott06] Table 2.4. Mixture properties of the mortars. Replacement ratio (% of cement) Nano Silica (NS) Micro Silica (MS) Fly Ash (FA) Blast Furnace Slag (GGBS) Sand1-BAGDAD (1.63% Sulfate) 0% 2% 4% 6% 0% 6% 9% 12% 0% 15% 30% 45% 0% 20% 40% 60% Sand2-KERBELA (1.96% Sulfate) 0% 2% 4% 6% 0% 6% 9% 12% 0% 15% 30% 45% 0% 20% 40% 60% (For all mixtures: Water / Sand / Cementitious material = 0.5 / 2.75 / 1 ) (Curing: In 22±2 o C lime saturated water) 3 RESULTS AND DISCUSSION Comparator length readings of the 40x40x160 mm mortar prisms produced with Sand-1 (Bagdad) and Sand-2 (Kerbela) samples are shown in Figure 3.1 through Figure 3.4. As stated before, an expansion limit of 0.1% after 6 months is generally considered as a safe boundary for determining the maximum percentage of sulfate that can exist in a given mixture without causing any significant deterioration. When the time stabilization of expansion curves which can be observed in Figures 3.1 through 3.4 is taken into consideration, it can be concluded that the maximum expansion of 0.1% at 6 months would also be considered as an acceptable

5 International RILEM Conference on Material Science MATSCI, Aachen 2010 Vol. III, AdIPoC 235 limit for determining the sulfate resistance of the mixtures prepared in this work. However, it should be denoted that the sulfate attack investigated in this study was internal sulfate attack which means that the sulfate ions were readily available in the mixture. Therefore, when the external sulfate attack is considered, since it takes time for sulfate ions to diffuse in hardened concrete (depending on the concrete permeability), the maximum expansion limit of 0.1% might be expected at a later age or the maximum expansion limit at 6 months might be lowered. The mortar samples produced with 2 different naturally sulfate contaminated sands without any mineral additives (reference samples) has shown very significant expansions due to internal sulfate attack. The expansions of reference samples at the end of 6 months (26 weeks) are 0.34% and 0.27% for Sand-1 (Bagdad) and Sand-2 (Kerbela), respectively. Moreover, 34 week expansions are 0.36% and 0.34% for Sand-1 (Bagdad) and Sand-2 (Kerbela), respectively. For a visual comparison of the expansions caused by internal sulfate attack, Figures 3.5 and 3.6 can be seen. Fig Expansion of mortars produced with and without Micro Silica (MS). Figure 3.1 shows the length changes of mortar samples with and without micro silica (MS). As seen in this figure, addition of micro silica significantly reduced the expansions for both sands. If the expansion limit of 0.1% is a safe limit, at least 9% of micro silica is needed for a safe margin. When Sand-1 is considered, 6 month expansions of MS mortar samples are 0.11%, 0.09% and 0.04% for 6%, 9, and 12% micro silica replacement ratios, respectively. 6 month expansions for Sand-2 mortar samples are 0.14%, 0.08% and 0.06%, respectively for 6%, 9, and 12% micro silica replacement ratios. Figure 3.6 shows the comparison of the expansions of different mortars.

6 236 ATAHAN, DIKME: Improving Sulfate Resistance of Mortars Produced with Sands Contaminated by Natural Sulfate Fig Expansion of mortars produced with and without nano silica (NS) (Ultra Fine Amorphous Colloidal Silica). Fig Expansion of mortars produced with and without Fly Ash (FA).

7 International RILEM Conference on Material Science MATSCI, Aachen 2010 Vol. III, AdIPoC 237 Fig Expansion of mortars produced with and without slag (GGBS). Figure 3.2 shows the length changes of mortar samples with and without ultra fine amorphous colloidal silica (NS). Colloidal silica can be defined as stable dispersions or sols of amorphous silica in water (aquasols) or in an organic solvent (organosols) [Ott06]. Similar to the results obtained from micro silica replacement, addition of nano silica significantly reduced the expansions for both sands as well. When Sand-1 is considered, 6 month expansions of NS mortar samples are 0.15%, 0.07% and 0.04% for 2%, 4%, and 6% nano silica replacement ratios, respectively. 6 month expansions for Sand-2 mortar samples are 0.15%, 0.07% and 0.06%, respectively for 2%, 4%, and 6% nano silica replacement ratios. If the expansion limit of 0.1% is considered as a safe limit, at least 4% of ultra fine amorphous colloidal silica, which is, by weight, less than half of the micro silica needed for limiting the expansions, is enough for a safe margin. It was reported in the literature that the addition of colloidal silica to concrete mixture would substantially reduce chloride ion permeability and enhance sulfate resistance [Ott06, Ska01]. In addition to this information, Greenwood [Ott06, Gre02] has shown that the smaller particles in the sol provided most of the sulfate resistance whereas the larger particles provided chloride resistance of concrete. Current study has also shown that in spite of the fact that very low replacement ratios such as 4% to 6% were used; the effectiveness of nano silica is very significant. This result can be attributed to very high surface area (>80000m 2 /kg) and high purity (>99% SiO 2 ) of this nano technological material. The major component of this material is the amorphous (glassy) silica which reacts with calcium hydroxide to form calcium silicate hydrates (C-S-H). The rate of the pozzolanic reaction is proportional to the amount of surface area available for reaction [Byu07]. Figure 3.3 shows the length changes of mortar samples with and without fly ash (FA). The fly ash used in this experimental work can be classified a Class F fly ash. As seen in this figure, addition of fly ash significantly reduced the expansions for both sands as well. Except for 15% replacement ratio of mortars mixed with Sand-2 (Kerbela), all the samples remained below 0.1% expansion after 6 months (Figure 3.3 and Figure 3.6). Especially for the mortars

8 238 ATAHAN, DIKME: Improving Sulfate Resistance of Mortars Produced with Sands Contaminated by Natural Sulfate mixed with Sand-1 (Bagdad), the expansions did not change from 3 to 6 months for all replacement ratios and remained below 0.1%. On the other hand, fly ash replacement made different effects on the expansion behaviour of two different mortars produced with different sands. In other words, the performance of fly ash in reducing the expansions due to internal sulfate attack was more favourable for Bagdad sand. It should be noted here that the Sand-1 sample has better physical characteristics compared to Sand-2 sample. Table 2.1 can be seen for the comparison of some of the physical properties of these two sands. For a general comparison, when Sand-1 is considered, 6 month expansions of FA mortar samples are 0.05%, 0.05% and 0.06% for 15%, 30%, and 45% fly ash replacement ratios, respectively. 6 month expansions for Sand-2 mortar samples are 0.16%, 0.10% and 0.08%, respectively for 15%, 30%, and 45% fly ash replacement ratios. If the expansion limit of 0.1% is considered as a safe limit, 30% of fly ash can be considered as a safe margin. Relative Expansion (percent of referance sample) Sand Sample 1 (after 30 weeks) (Bagdad 1.63% Water Soluble Sulfate) Relative Expansion (percent of referance sample) Sand Sample 2 (after 30 weeks) (Kerbela 1.96% Water Soluble Sulfate) Sand1_Ref 6%MS 9%MS 12%MS 2%NS 4%NS 6%NS 15%FA 30%FA 45%FA 20%GGBS 40%GGBS 60%GGBS Sand2_Ref 6%MS 9%MS 12%MS 2%NS 4%NS 6%NS 15%FA 30%FA 45%FA 20%GGBS 40%GGBS 60%GGBS Fig (a) Relative expansions of mortars as percent of reference samples (a) Mortars produced with Sand-1 (Bagdad) and, (b) Mortars produced with Sand-2 (Kerbela). (b) Figure 3.4 shows the length changes of mortar samples with and without ground granulated blast furnace slag (GGBS). Similar to other mineral additives, addition of slag significantly reduced the expansions for both sands. Similar to fly ash mortars, except for 20% replacement ratio of mortars mixed with Sand-2 (Kerbela), all the samples remained below 0.1% expansion after 6 months (Figure 3.4 and Figure 3.6). Especially for the mortars mixed with Sand-1 (Bagdad), the expansions did not change significantly from 3 to 6 months for all replacement ratios and remained below 0.1%. Particularly 40% and higher replacement ratios, expansion of GGBS mortars were below 0.05% after 6 months. Similar to fly ash mortars, the performance of GGBS in reducing the expansions due to internal sulfate attack was more favourable for Bagdad sand. For a general comparison, when Sand-1 is considered, 6 month expansions of GGBS mortar samples are 0.05%, 0.03% and 0.02% for 20%, 40%, and 60% GGBS replacement ratios, respectively. 6 month expansions for Sand-2 mortar samples are 0.14%, 0.08% and 0.07%, respectively.

9 International RILEM Conference on Material Science MATSCI, Aachen 2010 Vol. III, AdIPoC 239 Expansion, % Sand Sample 1 (Bagdad 1.63% Water Soluble Sulfate) MS NS FA 3 Month 6 Month GGBS 0.00 Sand1_Ref 6%MS 9%MS 12%MS 2%NS 4%NS 6%NS 15%FA 30%FA 45%FA 20%GGBS 40%GGBS 60%GGBS (a) Expansion, % Sand Sample 2 (Kerbela 1.96% Water Soluble Sulfate) 3 Month 6 Month MS NS FA GGBS 0.00 Fig Sand1_Ref 6%MS 9%MS 12%MS 2%NS 4%NS 6%NS Expansions of mortars after 3 and 6 months of water cure (a): Mortars produced with Sand-1 (Bagdad) and, (b) Mortars produced with Sand-2 (Kerbela) 15%FA 30%FA 45%FA 20%GGBS 40%GGBS 60%GGBS (b) 4 CONCLUSIONS Sulfate ions can diffuse in concrete from the external environment; however, as investigated in this paper, it is also possible that the source of these ions can also be the concrete mixing materials. In some cases, the sulfate ions are unavoidably introduced in the mixture and additional precautions must be taken while preparing the concrete mixture design. This experimental work has studied the effect of different mineral admixtures on the expansion of mortar bars caused by internal sulfate attack and the following concluding remarks were drawn: Within the limits of this work, depending upon the percentage used, all of the mineral additives, particularly GGBS, have significantly reduced the expansions caused by internal sulphate attack. A blend of Portland cement and GGBS contains more silica and less lime compared to Portland cement alone. As a consequence, because of higher replacement ratios and low calcium hydroxide content of the GGBS mortar, the resistance of sulfate attack is improved best.

10 240 ATAHAN, DIKME: Improving Sulfate Resistance of Mortars Produced with Sands Contaminated by Natural Sulfate Another significant data was obtained from the mortar samples mixed with ultra fine amorphous colloidal silica (NS). Even if very low replacement ratios such as 4% to 6% are used, the effectiveness of nano silica is very significant. This result can be attributed to very high surface area (>80000 m 2 /kg) and high purity (>99% SiO 2 ) of this nano technological material. The rate of the pozzolanic reaction is proportional to the amount of surface area available for the reaction. When the time stabilization of expansion curves drawn in this paper is taken into consideration, it can be concluded that the maximum expansion of 0.1% at 6 months would be considered as an acceptable limit for determining the sulfate resistance of the mixtures. It is very important to use mineral additives in order to reduce Ca(OH) 2 content which is introduced into the mixture by cement hydration. The pozzolanic reaction between calcium hydroxide and the mineral admixtures results in the reduction of calcium hydroxide and improves the sulfate resistance. REFERENCES [Ast03] ASTM C , Standard Performance Specification for Hydraulic Cement, ASTM International, [Ast04] ASTM C , Standard Test Method for Expansion of Hydraulic Cement Mortar Bars Stored in Water, ASTM International, [Ast05] ASTM C , Standard Specification for Silica Fume Used in Cementitious Mixtures, ASTM International, [Ast07] ASTM , Standard Test Method for Flow of Hydraulic Cement Mortar, ASTM International, [Byu07] Byung, W.J, Kim, C.H, et.al., Characteristics of Cement Mortar with Nano-SiO 2 Particles, Construction and Building Materials, Vol.21, 2007, pp [Col03] Collepardi, M., A state-of-the-art Review on Delayed Ettringite Attack on Concrete, Cement and Concrete Composites, Vol.25, 2003, pp [Gre02] Greenwood, P., Bergqvist, H., Skarp, U., U.S. Patent 0, 387,173, [Meh06] Mehta, K.M. and Monteiro, P.J.M, Concrete: Microstructure, Properties and Materials, McGraww Hill, Third Edition, 2006, 660p. [Nev00] Neville, A.M., Properties of Concrete, Prentice Hall, Fourth Edition, 2000, 844p. [Ott06] Otterstedt, J., Greenwood, P., Some Important, Fairly New Uses of Collidal Silica/Silica Sol, in Colloidal Silica, Fundamentals and Applications, Editors: Bergna H.E and Roberts W.O., 2006, p [San02a] Santhanam, M, Cohen, M.D, and Olek, J., Mechanism of Sulfate Attack: A Fresh Look Part 1. Summary of Experimental Results, Cement and Concrete Research, Vol.32, 2002, pp [San03b] Santhanam, M, Cohen, M.D, and Olek, J., Mechanism of Sulfate Attack: A Fresh Look Part 2. Proposed Mechanisms, Cement and Concrete Research, Vol.33, 2003, pp [Ska01] Skarp, U., Sarkar, S., Proceedings from the World of Concrete, Las Vegas, [Ska02] Skalny, J., Marchand, J and Odler, I., Sulfate Attack on Concrete, Modern Concrete Technology Series, 2002, 217p.