MATERIALES DE CONSTRUCCIÓN Vol. 66, Issue 322, April–June 2016, e083 ISSN-L: 0465-2746 http://dx.doi.org/10.3989/mc.2016.01815 Effect of pozzolans with different physical and chemical characteristics on concrete properties a b b a,b H. Paiva *, A. Velosa , P. Cachim , V.M. Ferreira a. Civil Engineering Dept./CICECO, University of Aveiro, (Aveiro, Portugal) b. Civil Engineering Dept./RISCO, University of Aveiro, (Aveiro, Portugal) *[email protected] Received 24 February 2015 Accepted 16 September 2015 Available on line 12 April 2016 ABSTRACT: The durability of concrete structures is an important issue nowadays. Specifically in the case of reinforced concrete bridges or other infrastructures one of the main form of environmental attack is the penetra- tion of chloride ions, which leads to corrosion of concrete steel reinforcement. This study aims to evaluate the effect of the introduction of metakaolin and diatomite, two chemically and physically different pozzolans, on the resistance of concrete to the penetration of chlorides but also the effect on other properties of concrete, namely, its compressive strength and its porosity distribution. The results of this study show that the pozzolans physical and chemical characteristics have a strong influence on the pozzolans behavior and, consequently, on the concrete properties. KEYWORDS: Concrete; Metakaolin; Pozzolans; Durability; Physical properties Citation/Citar como: Pavía, H.; Velosa, A.; Cachim, P.; Ferreira, V.M. (2016) Effect of pozzolans with different physi- cal and chemical characteristics on concrete properties. Mater. Construcc. 66 [322], e083. http://dx.doi.org/10.3989/ mc.2016.01815. RESUMEN: Efecto de puzolanas con características físicas y químicas diferentes sobre las propiedades del hormigón. La durabilidad de las estructuras de hormigón es un tema importante hoy en día. En concreto, en el caso de los puentes de hormigón armado u otras infraestructuras, el principal factor de agresión medioambiental es la pen- etración de iones cloruro, lo que conduce a la corrosion del acero de refuerzo del hormigón. Este estudio tiene como objetivo evaluar el efecto de la introducción de metacaolín y diatomita, dos puzolanas química y físicamente diferentes, sobre la resistencia del hormigón a la penetración de cloruros, y también sobre otras de sus propiedades como la distribución de su porosidad o su resistencia a compresión. Los resultados de este estudio muestran que las características fisicoquímicas de las puzolanas tienen una fuerte influencia en su comportamiento y en consecuencia, en las propiedades del hormigón. PALABRAS CLAVE: Hormigón; Metacaolín; Puzolanas; Durabilidad; Propiedades físicas Copyright: © 2016 CSIC. This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial (by-nc) Spain 3.0 License. 1. INTRODUCTION ions, which leads to corrosion of the concrete steel reinforcement and reduction in strength and weak The concrete structures are often exposed to aesthetics, which usually results in early repairs harsh environments and, for instance, in the case or premature structure replacement. This creates of reinforced concrete bridges, the main form of a problem in the construction materials sector. environmental attack is the penetration of chloride Generally, it is expected that these structures may 2 • H. Paiva et al. last, with little or no maintenance, for long periods better overall performance of concrete, in terms of of time. Production of highly resistant concrete strength, porosity, permeability, chloride ion diffu- structures is required nowadays, which is patent by sivity and freeze-thaw resistance (7–9). the increase of studies concerning concrete durabil- In concrete, chloride ions may be free in the solu- ity. One way to prevent this deterioration is to avoid tion within the pores or may be physically bound the penetration of chlorides in the concrete by mak- with the cement hydration products. The ability of ing the structure more impermeable. Currently, the cementitious products to connect with the chloride use of pozzolanic materials in partial replacement ions reduces the rate of transport of chloride ions of Portland cement in concrete production is widely into the concrete. The rate of reduction of chloride accepted. In addition to the environmental benefit ions depends on their ability to link chemically with related to the decrease of CO2 emissions, the use the hydration products. The partial replacement of of pozzolans have technical advantages, particu- pozzolan in cement, either using flyash or metaka- larly in terms of increasing the durability of con- olin, specifically involves the additional formation crete regarding corrosion, carbonation and attack of hydrated products, thus increasing the binding by sulphates, among other factors that may shorten ability, in comparison with a concrete using cement the service life of structures based on the use of as the sole binder. Thus, the total chloride concen- Portland cement (1). tration (free or bound) in the outer layers of a con- Metakaolin is one of the pozzolanic materials crete with pozzolans may be equal to or greater than that have been widely studied recently. The inclu- the concentration in a concrete only with cement sion of metakaolin improves the mechanical prop- (10, 11). Simultaneously, due to the products formed erties and durability of concrete due to its high by the pozzolanic reaction inside the capillary pores, pozzolanic activity (2). Metakaolin shows in its in addition to increasing the binding capacity of the constitution amorphous silica but also amorphous cementitious matrix, the pore system also becomes alumina that, in presence of water, quickly reacts thinner and more segmented, hindering the diffu- with calcium hydroxide mainly producing calcium sion of chlorides (10, 11). aluminate hydrates and aluminiumsilicate hydrates. Many others authors tried to study and under- Siddique et al. (3) in a study about the influence stand the action of chloride ions in concrete. Linnhua of metakaolin on mortar and concrete properties Jiang et al. (12) studied the influence of chloride shows several advantages, specifically, an increase salt type on the level of reinforced concrete corro- of mechanical strength and durability, but also a sion and concluded that the chloride salts with the decrease of shrinkage due to the increase of material same valence cation as NaCl and KCl have an iden- density and a better particle packing. tical behavior. However, chloride salts with different Park et al. (4) have evaluated the crack effect on valence cations, as MgCl2 and CaCl2 have a higher chloride diffusion, using accelerated migration test threshold level for the corrosion than NaCl and KCl, in non-stationary condition on concrete specimens when added to the cement. with various crack widths. These authors concluded Zhiguang et al. (13) studied the combined that chloride diffusion coefficient in cracked con- effect of metakaolin and seawater on the proper- crete increases rapidly with crack width and a rela- ties and microstructures of concrete. The compres- tionship was obtained. Yang et al. (5) have shown in sive strength, pore structure, hydration products and their work that the external flexural loading accel- microstructure of concrete were affected by the addi- erate the concrete deterioration and that concrete tion of MK and mixing with seawater. The highest subjected to both sulfate and chloride attack suf- compressive strength was obtained by addition of fers less damage than that which is prone to sulfate 5 wt % of metakaolin and mixing with seawater, attack alone. Chloride ions in concrete can reduce which increased the compressive strength by 52% at and/or delay the deterioration of concrete attacked 28 days. They also observed that concrete chloride by sulfate ions. resistance increased with the amount of metaka- Khatib and Clay (6) have studied the effect of olin. Mixing with seawater was able to improve the replacing cement by metakaolin in contents from chloride resistance. 0 to 20%, keeping constant the water amount, finding Diatomite (DT) is a natural material from a that the presence of metakaolin reduces water pen- sedimentary rock, formed mainly by the deposition etration by capillary action. Metakaolin is obtained of microscopic organisms with a crystalline and by the calcination of kaolinitic clays at tempera- amorphous silica shell (14). Diatomite can be used tures from 650 to 800 °C. Metakaolin (MK) presents as a pozzolan, directly after milling or after a heat a high reactivity with calcium hydroxide, Ca(OH)2, treatment at about 1000 °C followed by grinding. and an ability to accelerate cement hydration due to Subjecting this material to thermal treatment shows its high concentration of silica and alumina. The cal- an increase in the content of amorphous silica and cium to silicon (C/S) ratio of the produced calcium grinding reduces its particle size. The hydraulic silica hydrates (C–S–H gel) is very high, leading to a and pozzolanic activity of silica mineral additions considerably improved concrete microstructure and depends on its crystalline or amorphous state as well Materiales de Construcción 66 (322), April–June 2016, e083. ISSN-L: 0465-2746. doi: http://dx.doi.org/10.3989/mc.2016.01815 Effect of pozzolans with different physical and chemical characteristics on concrete properties • 3 as on its size and particle shape (15). Some authors constitution involves Portland cement (CEM type have shown that the use of diatomite in the produc- I 42.5R) (Cimpor) as a binder, a siliceous natural tion of mortar and concrete does not improve its sand (S) (A. Santos) and two types of crushed lime- mechanical properties (14, 16). stone (B1and B2) (A. Santos and Parapedra) as A disadvantage of the use of diatomite in the aggregate, the particle size distributions of which production of mortar and concrete is the need of are presented in Table 1. a high amount water, due to diatomite high poros- The fundamental formula for this concrete can ity, which promotes a reduction in the mechanical be written as: strength (17, 18).