Long-term behavior of mortars produced with sulfide-bearing aggregates

ABSTRACT

The variety of sulfides (inorganic compounds formed by combining sulfur with a metal element) present in earth crust is not small, which makes it difficult to avoid the use of sulfide-bearing aggregates in the concrete, especially in places where another source of material is non-existent or economically impracticable, as is the case of dams in Brazil. Within this scenario, this work presents an evaluation of the long-term behavior of mortar produced with aggregates containing pyrite and pyrrhotite. For the study, rock samples from the Irapé Hydroelectric Power Plant area in Minas Gerais (Brazil) were used. This plant was built in a geological site where the rock presented sulfide levels of at least 3%. These rock samples were first ground and then used as aggregates in mortars, which were, during almost 1300 days, subjected to three different exposed conditions: 1-temperature of 23° ± 2°C and relative humidity of 95 to 100%; 2-submerged in calcium hydroxide solution diluted in water almost 850 days and then kept in water up to 1300 days, both in room temperature; 3-submerged in calcium hydroxide solution diluted in water at 50° C during almost 850 days and then kept in water (50° C) up to 1300 days. The presence and amount of pyrrhotite were obtained from a leaching process of the material (aggregate or mortar) in a solution of hydrochloric acid. This procedure allowed also the evaluation of the pyrite content. The results showed that the calcium hydroxide solution speeds up the expansion just like the temperature. It was also observed that the amount of pyrite has remained virtually constant over time in the three exposure situations. This finding indicates that sulfur limits in aggregates should be set according to the type of iron sulfide presented and not only by the total amount of sulfur.

Keywords
sulfide-bearing aggregates; mortar; long-term behavior; dams

1. INTRODUCTION

On the Earth, the lithological variety is huge and, consequently, aggregates for concrete come from many rocks containing sulfides in their mineralogical constitution. These sulfides are pyrite, pyrrhotite, marcasite, chalcopyrite, arsenopyrite, sphalerite, galena, among others. It can be concluded that it is difficult to avoid the use of these aggregates in the concrete. This is even more difficult in places where the availability of another source of material is economically impracticable or non-existent as in the case of concrete dams where large volumes of aggregates are needed [¹,²].

Aggregates containing iron sulfides were used in the Graus and Tavascán dams in Spain in 1992 as well as in buildings in Maresme, also in Spain, in 1970. CHINCHÓN et al. [³], AYORA et al. [⁴], and ARAÚJO et al. [⁵] reported concrete degradation problems in the above-mentioned structures mainly related to severe cracking.

In Brazil, the concern with the use of sulfide-bearing aggregates is relatively recent, and only during the implantation phase of the Irapé Hydroelectric Power Plant (HPP), located in the Jequitinhonha Valley in Minas Gerais, has studies begun to emerge. At this site, rocks with high sulfur contents were found, with sulfide levels of at least 3%, which the predominance of pyrrhotite (Fe(1-x)S) [⁶].

Based on the studies carried out by CEMIG [⁶] and the Leme-Intertechne Consortium [⁷], aggregates from the obligatory excavations of the Irapé rock were not used. This resolution was due to the lack of national standards and information about materials that could effectively contribute to minimize or prevent the deleterious phenomenon of sulfide oxidation present in the aggregates. Thus, materials from the Olhos d’Água and Córrego do Bonito quarries were investigated for the preparation of the concretes. The studies determined the use of aggregates from the Olhos d'Água quarry located six kilometers from the Irapé HPP. These aggregates contained less than 0.5% in sulfide contente [⁷].

Based on this scenario, the objective of this paper is to present an evaluation of the behavior of mortar produced with pyrite and pyrrhotite. The mortar was made with aggregate derived from rocks of the Irapé HPP.

2. MATERIALS AND METHODS

2.1 Materials

The aggregates used in the mortar came from rotary drilling samples of Irapé dam rock formation that was crushed and sieved. This material had an average sulfur content of 2%, which is equivalent to average iron sulfide content (pyrite + pyrrhotite) of 5%, due to the stoichiometry of these compounds.

To have a good concrete workability during batching, different aggregate size compositions were investigated [⁸]. The best result was obtained with 80% of aggregates retained between sieves 4.8 and 0.6 mm and the remaining 20% consisting of material retained between sieves 0.3 and 0.1 mm.

Brazilian Type CP V ARI cement (ASTM Type III) was used in mortar production. The choice of this type of cement was because it did not contain additions in its composition. The concrete mix proportion was 1:2.25 (cement: aggregates) and the water/cement ratio was equal to 0.47. No plasticizer was employed. The values for the mix proportion and the water/cement ratio were according to NBR 15777 [⁹] and ASTM C 1260 [¹⁰].

Cylindrical and prismatic mortar specimens were cast according to the NBR 5738[¹¹] and the NBR 15577 [⁹] standards, respectively. The specimens were stripped from the molds 24 hours after casting and transferred immediately to each one of the exposed conditions. The cylindrical specimens were used in the sulfide content evaluation while the prismatic beams for measuring expansion. According to LNEC E 251 [¹²], expansion strains equal or greater than 3x10^-4 after one to two months indicate that the aggregates are not suitable to be used in concrete or when at the end of six months, these deformations exceed 5x10^-4.

Mortar specimens were exposed for almost 1300 days to three different conditions:

1. Temperature of 23° ± 2°C and relative humidity of 95 to 100%;

2. Submerged in calcium hydroxide solution diluted in water at room temperature for almost 850 days and then kept in water (room temperature) up to 1300 days;

3. Submerged in calcium hydroxide solution diluted in water at 50° C for almost 850 days and then kept in water (50°C) up to 1300 days.

2.2 Experimental procedures

The mortar expansion was measured over time using the equipment shown in Figure 1.

Figure 1
Equipment used to measure the expansion of the prismatic specimens.

It is worth mentioning that in order to read the expansions of the specimens immersed in the solution at 50 °C it was necessary to adjust the values do not take into account the expansion caused by the temperature. The expansion caused by the temperature was calculated according to equation 1.

$$\epsilon =\alpha .\mathrm{\Delta T}$$ (1)

where:

• ε - deformation

• α – concrete thermal expansion coefficient = 10-5

• ΔT – temperature variation = (50°C – 27°C) = 23°C average

In this way, an average expansion of 0.023% was obtained due to the temperature which was subtracted from the result of the expansion reading, so that only the expansion caused by the sulfide reactions was considered.

The total sulfur, pyrite and pyrrhotite contents over time were evaluated as follows. First, a piece of mortar was crushed to a grain size smaller than or equal to 0.15 mm. They were then dried in an oven at 40ºC, for approximately 24 hours for water evaporation. The total sulfur content was then determined through direct combustion (with infrared detection) using the equipment LECO CS 632 (Figure 2).

Figure 2
(a) LECO CS 632 equipment - (b) Sample preparation for sulfur evaluation.

For the evaluation of the content of the different iron sulfides, a leaching process was employed. For the selective dissolution of the pyrrhotite, 1g of the smashed mortar sample was blended with a 100ml hydrochloric acid solution (HCl), in the proportion of (1:1) in an Erlenmeyer flask, and agitated for an hour at 60ºC. Next, the material was put in a funnel containing a filter paper to drain the solution. The material was then washed with distilled water, so all HCl could be removed. Then it was dried in an oven at 40ºC for 24 hours.

A portion of this dried material was subjected to direct combustion testing (LECO) for determination of total residual sulfur content, which corresponds to the pyrite content. The difference between the total initial sulfur content (not leached) and the residual content (pyrite) corresponds to the sulfur content from the pyrrhotite and from cement.

This residue was then subjected to nitric acid leach to destroy all the remaining pyrite. The final sulfur content was measured again to make sure all the pyrite had been dissolved. The sulfur quantification was done up to, approximately, 1300 days for each exposure situation.

The total sulfur content in the cement was 1.07%, which results in a 0.29% sulfur content in the mortar due to the cement dosage (Marcelino [¹³]).

3. RESULTS AND DISCUSSION

Figure 3 shows the expansion test results evaluated up to 1300 days after casting for all the three exposed conditions. The results indicate that the calcium hydroxide solution kept at higher temperature speeds up the expansion in the first 250 days of exposure. On the other hand, the same level of expansion was reached for the calcium hydroxide solution kept at room temperature only after 850 days. In both cases, the strain values are in the order of 0.052%. The specimens kept at a temperature of 23° ± 2°C and relative humidity of 95 to 100% are still expanding.

Figure 3
Expansion test results.

It is important to mention that the measured strains did not reach the limit of 0.05% at the end of six months established by LNEC E 251 [¹²] for any exposed conditions. For the specimens subjected to the calcium hydroxide solution, both at room temperature and at 50°C, the measured strains exceeded the value of 0.03% after 30 days of exposure. However, no cracking was observed in the bars regardless of the exposure condition after more than 3 years of evaluation (Figure 4).

Figure 4
Concrete specimens after more than 1200 days of exposure in the three different conditions: (a) Specimens exposed to a temperature of 23° ± 2°C and relative humidity of 95 to 100% after 1286 days; (b) Specimens submerged in calcium hydroxide diluted in water at room temperature after 1282 days of exposure; (c) Bars submerged in calcium hydroxide solution diluted in water at 50° C after 1273 days of exposure.

The total sulfur, pyrite and pyrrhotite contents over time for each exposure condition are presented in Tables 1 to 3.

Tabel 1
Results of total sulfur, pyrite and pyrrhotite contents for specimens kept at a temperature of 23° ± 2°C and relative humidity of 95 to 100%.

AGE (DAYS)	TOTAL SULFUR (%)	SULFUR IN THE CEMENT (%)	SULFUR FROM PYRITE (%)	SULFUR FROM PYRRHOTITE (%)
27	1.49	0.29	0.37	0.83
79	1.48	0.29	0.33	0.82
616	1.44	0.29	0.32	0.79
1286	1.40	0.29	0.36	0.75

Tabel 2
Results of total sulfur, pyrite and pyrrhotite contents for specimens submerged in calcium hydroxide solution diluted in water at room temperature for almost 850 days and then kept in water (room temperature) up to 1300 days.

AGE (DAYS)	TOTAL SULFUR (%)	SULFUR IN THE CEMENT (%)	SULFUR FROM PYRITE (%)	SULFUR FROM PYRRHOTITE (%)
27	1.52	0.29	0.43	0.80
90	1.58	0.29	0.43	0.86
609	1.43	0.29	0.44	0.70
1282	1.38	0.29	0.40	0.69

Tabel 3
Results of total sulfur, pyrite and pyrrhotite contents for specimens submerged in calcium hydroxide solution diluted in water at 50°C for almost 850 days and then kept in water (at 50°C) up to 1300 days.

AGE (DAYS)	TOTAL SULFUR (%)	SULFUR IN THE CEMENT (%)	SULFUR FROM PYRITE (%)	SULFUR FROM PYRRHOTITE (%)
27	1.62	0.29	0.37	0.96
63	1.72	0.29	0.43	1.00
602	1.53	0.29	0.42	0.82
1273	1.52	0.29	0.46	0.78

The expected sulfur content from pyrite in the mortar was 0.37% as verified by MARCELINO [¹³]. This result is very close to the values measured shown in Tables 1 to 3 and remains practically constant over time independently of the exposure condition. A reduction of the total sulfur content is observed in all cases over time due to the oxidation process of pyrrhotite.

4. CONCLUSIONS

The results indicate that the calcium hydroxide solution kept at higher temperature speeds up the expansion in the first 250 days of exposure. On the other hand, the same level of expansion was reached for the calcium hydroxide solution kept at room temperature only after 850 days. In both cases, the strain values are in the order of 0.052%. The specimens kept at a temperature of 23° ± 2°C and relative humidity of 95 to 100% are still expanding. However, the studied aggregate did not reach the expansion limits of 0.05% at the end of six months established by LNEC E 2511 for any exposed conditions. No cracking was observed in the specimens regardless of the exposure condition after more than 3 years of evaluation.

The results also show that the pyrite content in the mortars remained virtually constant over time independently of the three exposure situations. This result indicates that the sulfur content limits in aggregates should be set according to the type of iron sulfide presented and not solely by the total amount of sulfur.

BIBLIOGRAPHY

• ¹
  CASANOVA, I., AGULLÓ, L., AGUADO, A., “Aggregate expansivity due to sulfide oxidation - I. Re-action system and rate model”, Cemente and Concrete Research, v. 26, n. 7, pp. 993–998, 1996.
• ²
  HASPARYK, N. P., GOMIDES, M. J., SIGNINI, R., et al, “Metodologias Empregadas na Investigação do Teor de Sulfetos”, In: CITENEL - Anais do III Congresso de Inovação Tecnológica em Energia Elétrica, Florianópolis, 2005.
• ³
  CHINCHÓN, J. S., AYORA, C., AGUADO, A., et al, “Influence of weathering of iron sulfides con-tained in aggregates on concrete durability”, Cement and Concrete Research, v. 25, n. 6, pp. 1264–1272, 1995.
• ⁴
  AYORA C., CHINCHÓN, S., AGUADO, A., et al, “Weathering of Iron Sulfides and Concrete Altera-tion: Thermodynamic Model and Observation in Dams From Central Pyrenees – Spain”, Cement and Concrete Research, v. 28, n. 9, pp. 1223 –1235, 1998.
• ⁵
  ARAÚJO, G. S., CHINCHÓN, J. S., AGUADO, A., “Evaluation of the behavior of concrete gravity dams suffering from internal sulfate attack”, Revista Ibracon de Estruturas e Materiais, v. 1, n. 1, pp. 1–3, 2008..
• ⁶
  CEMIG GERAÇÃO E TRANSMISSÃO S. A., UHE Irapé - P&D 122 - Investigação do comportamento do concreto e da calda de cimento em contato com rocha sulfetada, Belo Horizonte, Brasil (2007).
• ⁷
  CONSÓRCIO LEME-INTERTECHNE. UHE Irapé - Estudos e projetos complementares - Relatório Descritivo da Situação Atual dos Trabalhos (Documentação para a junta de consultores da CEMIG), Abril, 1999.
• ⁸
  MARCELINO, A. P., CALIXTO, J. M., GUMIERI, A. G., et al, “A Methodology for Targeting Sulfur Compound Levels in Concretes for Hydroelectric Dams”, In: Second International Dam World, Conference Lisbon- Portugal, 2015.
• ⁹
  ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS (ABNT), NBR 15577 – Agregados – Reativi-dade álcali-agregado – Parte 4 – Determinação da expansão em barras de argamassa pelo método acelerado, Rio de Janeiro, Brasil (2008).
• ¹⁰
  AMERICAN SOCIETY FOR TESTING MATERIALS (ASTM), C 1260 – Standard test method for potential alkali reactivity of aggregates (mortarbar method), West Conshohocken, United States of America (2007).
• ¹¹
  ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS (ABNT), NBR 5738 – Moldagem e cura de corpos-de-prova de concreto cilíndricos ou prismáticos, Rio de Janeiro, Brasil (1994)
• ¹²
  LABORATÓRIO NACIONAL DE ENGENHARIA CIVIL (LNEC), E 251 – Inertes para Argamassas e Betões – Ensaio de Reactividade com os sulfatos em presença de hidróxido de cálcio, Lisboa, Portugal (1985).
• ¹³
  MARCELINO, A. P., Estudos para o desenvolvimento de uma metodologia de definição de teores dele-térios de enxofre em concretos e argamassas de barragens, Dissertação de M.Sc., Universidade Federal de Minas Gerais, Belo Horizonte, 2015.

Publication Dates

• Publication in this collection
  10 June 2019
• Date of issue
  2019

History

• Received
  14 Aug 2018
• Accepted
  08 May 2019