MICROSTRUCTURAL ANALYSIS AND GLOBAL PERFORMANCE OF MORTAR WITH TAILORED NANO AGGREGATES

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1 MICROSTRUCTURAL ANALYSIS AND GLOBAL PERFORMANCE OF MORTAR WITH TAILORED NANO AGGREGATES Jie Hu (1), D.A. Koleva (1) and K. van Breugel (1) (1) Faculty of Civil Engineering and Geosciences, Department Materials and Environment, Delft University of Technology, Stevinweg 1, 2628 CN, Delft, the Netherlands Abstract: Polymeric nano-aggregates (Poly (ethylene oxide)-block-polystyrene (PEO 113 -b-ps 70 ) micelles, hydrodynamic radius of approx. 50 nm) were admixed in mortar (i.e. solution of the stabilized in water micelles (in concentration of 0.5 g/l) was added to the mortar mixture as the mixing water). The microstructural properties (porosity, pore size distribution) and more general (global) parameters as water permeability and compressive strength were investigated. A comparison of these latter properties was made between micelle-containing (0.006 wt. % per mortar weight) and micelle-free mortar specimens. The results indicate that the compressive strength of the micelle-containing mortar was slightly increased. The coefficient of water permeability was 3 orders of magnitude lower for the micelle-containing specimen, compared to the reference (micelle-free) specimen. Additionally, the presence of very low concentration of micelles in the mortar mixture exerted influence on microstructural properties e.g. significantly reduced porosity, compared to micelle-free matrix. The most plausible mechanism seems to be altered cement hydration and re-distribution of hydration products in the presence of micelles in the cementitious matrix. 1. INTRODUCTION In recent years, the use of nano-particles has received particular attention in many fields of applications to fabricate materials with new functionalities. Due to ultrafine size, when nanoparticles are incorporated in cement paste, mortar or concrete, materials with different characteristics, compared to the conventional ones, can be obtained [1, 2]. There are many researches investigating the influence of incorporated nano-particles on material properties, such as mechanical performance, permeability and microstructure of the cement-based materials [3-5]. However, the research on corrosion prevention or protection in reinforced concrete, by using tailored nano-particles is very limited. This study is part of a two-year research project on the application of nano-materials with tailored properties for self-healing of corrosion damages in reinforced concrete. The motivation for applying nano-particles in this project is to establish an innovative technology 791

2 for corrosion control of steel reinforcement. On one hand, the global performance and microstructure of the cement matrix will be improved by the incorporated tailored nanoparticles; on the other hand, the reinforcing steel can be protected by using autonomous selfhealing mechanisms, triggered via the release of a healing agent from the particles core. Since previous preliminary investigation on the above mentioned properties of cement-based materials in the presence of core shell micelles, containing a longer PS chain (PEO 113 -b-ps 218 ) have shown very promising results [6, 7], this study is an extensive prolongation of the initial tests, but using a more stable formation (PS 70 ) and following a systematic experimental schedule for deriving parameters at different hydration ages in plain (not reinforced) mortar. The paper will present the results on microstructral properties of mortar specimens with and without admixed micelles for the curing ages of: 1 day, 7 days, 14 days and 28 days. Further, one of the objectives of this investigation includes also the correlation between the microstructure properties of the bulk matrix (e.g. porosity, pore size distribution) and more global parameters as compressive strength and water permeability. 2. EXPERIMENTAL 2.1 Materials Mortar specimens The specimens used in this investigation were plain mortar cubes. Casting was according standard procedures, using OPC CEM I 42.5, cement-to-sand (c/s) ratio of 1:3 and water-tocement (w/c) ratio of 0.5. Two main groups of specimens were investigated: Cube-Ref as a control, micelle-free group and Cube-Nano, specimens cast with micelles (0.5 g/l PEO 113 -b- PS 70 in the mixing water). All specimens were cured in a fog room (20ºC, 98%RH) before all tests. Additionally, separate mortar specimens (same mixture and handling) were prepared for the permeability tests, performed at the hydration age of 7 days (details further below) Polymeric nano-aggregates The micelles employed in this study were produced from polyethylene oxide (PEO) block polystyrene (PS) di-block copolymer (PEO 113 -b-ps 70 ), synthesized by atom transfer radical polymerization (ATRP), employing the macroinitiator technique [8]. (The micelles were received as a solution with micelle concentration of 0.5 g/l). For the micelle-containing specimens, Nano-Cube, (further in the text, for simplicity, referred to as nano specimens) the as received solution was used as the mixing water instead of the tap water. The micelles concentration in the mortar matrix was thus wt% per mortar weight. The hydrodynamic radius of the micelles was approximately 50 nm, as evident by DLS tests (Figure1) of the as received solution. 2.2 Methods Standard compressive strength The standard compressive strength test was conducted on mm 3 mortar cubes, and there were at least 3 replicates per each curing age Microstructure and image analysis (for porosity and pore size distribution) 792

3 Environmental SEM (ESEM Philips XL30) was used for visualization, morphological and microstructure investigation; OPTIMAS software was used for image analysis. A set of ESEM images of the cement matrix was obtained in backscattered electrons (BSE) mode with the magnification of 500. The results are an average of 35 locations per sample (details about the sample preparation and procedures as reported in [9-11]). 12 Intensity (%) Figure1: Dynamic light scattering (DLS) results Permeability measurements (water and 3% NaCl solution) Water and NaCl permeability were monitored. The motivation for the additional test with NaCl solution was to investigate the permeability of the mortar when chlorides are involved (as in an environment of chloride-induced corrosion). The specimens were cured for 7 days in sealed conditions; water saturated before the test and placed in the permeability cells, and subjected to a constant hydrostatic pressure of about 7 bars. The coefficient of permeability K [m/s] was calculated using Darcy s Law: k w =LQ/A sec Δh (1) where Q = the volume of fluid passed in unit time, m 3 /s, A sec = cross-sectional area of the sample in m 2, Δh = drop in hydraulic head through the sample, measured in m, L = thickness of the sample in m For details on the experimental set-up and procedures, please see [9]. 3. RESULTS AND DISCUSSION Size (d.nm) 3.1 Compressive strength Compressive strengths of both control and nano mortar cubes, for different curing ages are shown in Figure 2. It can be seen that, except for the samples cured during 1 day, the compressive strength of the nano sample was slightly higher than that of the control sample. For example, at 28 days curing age, the average compressive strength of the nano samples was Mpa, compared to the Mpa obtained for the control samples. The results indicate that the compressive strength of the mortar is slightly improved when the micelles were used as the mixing water instead of tap water. 793

4 Figure 2: Compressive strength of mortar cubes at different hydration ages 3.2 Water permeability Permeability is considered to be one of the most important properties affecting concrete durability. Concrete with higher permeability allows faster penetration of gases, liquids and other aggressive aggregates such as chlorides, resulting in rapid corrosion of the reinforcing steel. The permeability of concrete is related to its microstructure, especially the volume, size distribution, connectivity and shape of the pores [12]. The permeability tests in this study were conducted as follows. First, water permeability was tested. After the water permeability reached a stable value, the testing solution was changed to 3% NaCl solution (in order to determine the NaCl permeability). Finally, the NaCl solution was changed back to water. The recorded permeability results are shown in Figure3. As seen from the plot, the coefficient of water permeability K for the Nano samples is about 3 orders of magnitude lower than the control (Ref) ones ( m/s for nano sample, compared to m/s for control sample). Obviously, a very low concentration of PEO 113 -b-ps 70 micelles is able to significantly reduce the water permeability of the mortar matrix. It can be also observed that for the control (micelle-free) group, the permeability continued decreasing after changing the water to NaCl solution. The reason is the reduction of porosity and pore connectivity as a result from continuous cement hydration (especially in the presence of NaCl [13, 14]). However, the result for the nano samples was different: when the test solution was changed to 3%NaCl, the permeability slightly increased (rather than decrease as in the control group ). Changing back to water resulted in subsequent decrease of permeability (Figure3 bottom right). When NaCl was involved, the permeability of the nano samples was mainly influenced by two factors: the first factor is the cement hydration which will result in a reduction of porosity and permeability; the second factor is the specific properties of the micelles in the presence of chlorides (i.e. shrinkage of the PEO shell [15]), which apparently results in an increase of permeability. The hereby presented experimental results indicate that the latter one is the main factor affecting the permeability of the nano sample. However, the PS core of the micelles still remains incorporated in the cement matrix. Therefore, the NaCl permeability of the nano sample is still lower than the control sample. 794

5 When the medium changes back to water, the PEO shell would swell again and cause a decrease of water permeability. Figure 3: Coefficient of water and NaCl permeability of both groups 3.3 Microstructural observation and image analysis results Permeability results of the mortar are well supported by the microstructure properties. Figure 4 shows micrographs of the bulk matrix at the hydration age of 24h in both the control (Figure4a) and nano samples (Figure4b), depicting a distinguishable difference in porosity. The porosity and pore size distribution of both the control and nano specimens are shown in Figure5. The results were calculated from the image analysis of 35 locations per sample. As seen from the plots, the incorporated micelles have a significant influence on the microstructure of the cement matrix: the porosity of nano sample is lower than the control sample at each curing age, especially at early age. For example, for 1d samples, the porosity of nano sample is 8.97, compared to 15.12% for control sample. Apparently, the micelles (even in the very low concentration of wt%) resulted in a significant densification of the bulk matrix which is well in line with the lower water permeability results for the nano specimen. The reduction of critical pore size is not so evident. The effect of the micelles on the bulk matrix properties (including microstructural properties) is due to the role of the micelles, acting as nucleation sites for the formation of new hydration products, thus resulting in a more uniform matrix, which was also previously observed for PEO 113 -b- PS PS 218 [6]. 795

6 (a) Control sample, (b) Nano sample at 24h age Figure 4: Morphology of mortar matrix at 500x magnification, (a) 1d (b) 7d (c) 14d (d) 28d Figure 5: Porosity and pore size distribution of mortar calculated from the ESEM images 4. CONCLUSIONS Based on the experimental results, it can be concluded that a very low concentration (0.006 wt.% per mortar weight) of PEO 113 -b-ps 70 micelles can affect the global performance and microstructure of the mortar matrix. The porosity of the nano samples was significantly reduced at early age. Further, the nano samples present still lower porosity at later stages, along with slightly higher compressive strength. Most importantly, the micelles-containing 796

7 specimens exhibit significantly reduced water and NaCl permeability, the responsible mechanism being related to the specific amphiphilic properties of the micelles on one hand and a more uniform distribution of hydration products on the other. Further research is necessary for elucidation of the exact chemical mechanism, relevant to the observed behavior. REFERENCES: [1] Older I. Lea s chemistry of cement and concrete. 4th ed. London: Arnold; [2] Neville AM. Properties of concrete. 4th ed. England: ELBS with Addison Wesley Longman; [3] Collepardi S, Borsoi A, Ogoumah Olagot JJ, Troli R, Collepardi M, Cursio AQ. Influence of nano-sized mineral additions on performance of SCC. In: Proceedings of the 6th international congress, global construction, ultimate concrete opportunities, Dundee, UK; 5 7 July [4] Zhang MH, Li H (2006) Chloride permeability of concrete containing nano-particles for pavement. Structural Health Monitoring and Intelligent Infrastructure, Vols 1 and 2, Proceedings and Monographs in Engineering, Water and Earth Sciences [5] Hui L, Xiao H, Yuan J and Ou J 2004 Microstructure of cement mortar with nano-particles Composites B [6] D.A.Koleva, K. van Breugel, G.Ye, J. Zhou, G. Chamululu and E.Koenders, Porosity and Permeability of Mortar Specimens Incorporating PEO113 b PS218 Micelles, Special issue of ACI Materials Journal, SP267, (2009). [7] D.A. Koleva, G. Ye, J. Zhou, P. Petrov, K.van Breugel, Material properties of mortar specimens at early stage of hydration in the presence of polymeric nano-aggregates, International Conference on "Microstructure related Durability of Cementitious Composites" Nanjing, China, 13th-15th October, 2008, RILEM Publications SARL 2008, pp [8] Petrov P., Bozukov M. et al (2005) J. Mater. Chem., 15: [9] Ye, G., Experimental study and numerical simulation of the development of the microstructure & permeability of cementitious materials, PhD thesis, Delft University of Technology, Delft (2003). [10] Hu, J., Porosity of Concrete, Morphological Study of Model Concrete, PhD thesis, Delft University of Technology, Delft (2004). [11] Koleva, D.A., Corrosion and Protection in Reinforced Concrete, PhD Thesis, Delft University of Technology, Delft (2007). [12] Hughes, D.C. (1985) Pore structure and permeability of hardened cement paste, Magazine of concrete research, vol. 37 (133) pp [13] Suryavanshi A.K., Scantlebury J.D., Lyon S.B., Cem. Concr. Res., 25, (1995). [14] Díaz B., Nóvoa X.R., Pérez M.C., Cem. Concr. Comp., 28, (2006). [15] Patel K. et al (2007), European Polymer J. 43: