INVESTIGATION OF HYDRATION PRODUCTS FORMATION AT EARLY AGES

Transcription

1 INVESTIGATION OF HYDRATION PRODUCTS FORMATION AT EARLY AGES A. U. Ozturk (1), B. Baradan (1) (1) Civil Engineering Department, Dokuz Eylul University, TURKEY Abstract In this study, hydration development of cement mortars has been investigated to simulate the development of hydration products formation at early ages. Cement mortars were prepared by adding naphthalene sulphonate based (NSB) admixture having a retardation effects on cement hydration by different ratios. Microstructural investigation has been made on each cement mortar to characterize the microstructural development of hydration process. Micrographs have been taken under scanning electron microscope (SEM) at early ages such as 1, 2 and 7 days in which naphthalene sulphonate based admixture has the retardation effect more effectively. The area ratio values of each hydration product phase such as undifferentiated hydrates, unhydrated parts, calcium hydroxide ( Ca(OH) 2 ) and pore structure in cement mortar prepared with ordinary Portland cement (OPC) and naphthalene sulphonate based admixture were determined by image analysis. These area ratio values developments with time were plotted for each phase. The study indicates that admixture retards formation of undifferentiated hydrates and, results as an increase in pore area ratio values at early ages. Therefore, the area ratio values of calcium hydroxide part increases as the area ratio values of unhydrated part decrease with time. 1. INTRODUCTION Engineers and materials scientists have struggled with the characterization of existing construction materials. Microstructural characterization of cementitious materials is needed to simulate the micro properties. Microstructure property relationship has become more important. Within the past two decades, technological advantages in the understanding of materials has led material scientists have more appropriate solutions to improve macro properties. Microstructure property relationship is the most important criteria which can lead to improve mechanical and durability properties of cementitious materials by investigating microstructural characteristics. Currently, new experimental technologies according to development in computer analysis are performed to define microstructural 233

2 properties of a material. Determining the microstructural characteristics of materials such as cementitious materials having a heterogeneous microstructure, and improve the microstructure lead engineers develop new and present mechanical and durability properties of cementitious materials. Hydration products have a great effect on these macro properties of cementitious materials. Investigation of hydration products formation leads to have a better view on predicting the properties of cementitious materials at later ages. To determine developments of phases in cement matrix need to use the current experimental technologies and to analyze their results. During last 2 years, to investigate polished sections of specimens by scanning electron microscope (SEM) with backscattered electron (BE) mode has become more important [1]. There are several advantages of this technique. Although cement paste has a reasonably heterogeneous structure, this technique has a great capacity on determining the relationship between microstructure-property by quantitive measures. SEM applications improve the ability to characterize the microstructure of cement mortar and concrete, and helps in investigating effects of admixtures, determining durability problems and service life [2]. Since the debut in the early of 198, SEM micrographs under BE mode have shown a great potential to investigate cementitious materials [3,4]. Basically, secondary electron (SE) mode gives the morphological structure of a specimen. Generally, fractured specimens were used for SE mode. Minimum requirements to prepare specimen and to identify the topographical images easily make SE mode with fractured specimens more attractive. However, SE mode imaging of fractured specimens must be investigated more carefully. Because, during preparing specimens, cracks in cementitious material may propagate through the weakest zone and a fractured specimen may not represent the whole specimen [5]. The roughness of fractured specimen affects the image processing inversely [6,7,8]. To get the benefits of SEM (BE mode imaging / image processing) needs flat specimen. In this study, polished sections of mixes were prepared in different ages to investigate the improvement of microstructure. The development of each phase such as undifferentiated hydrates, unhydrated parts, pore structure and calcium hydroxide (Ca(OH) 2 ) were plotted to simulate hydration process with time. 2. EXPERIMENTAL 2.1. Materials Naphthalene sulphonate based admixture, which is type G admixture according to ASTM- C 494, was used. The properties and usage conditions are given in Table 1. Ordinary Portland cement (OPC) is CEM I 42,5 R. The properties of cement are given in Table Method Specimen Preparation Cement mortars were prepared for SEM analysis and compressive strength investigation. Admixture was added %1 and %2 by the ratio of cement weight. Mix designs of cement mortars are given below in Table 3. Table 1: Properties of retarder Type Base Density Solid phase ASTM-C 494 Type G Naphthalene TS EN Part Sulphonate kg/l 3-4% 234

3 Table 2: Properties of OPC Cement Type CEM I 42,5 R Results of Chemical Analysis Property Unite Test Method Standard Criteria (TS EN 197-1) Value (Cl - ) Amount % TS EN %,1,76 (SO 3 ) Amount % TS EN %4, 3,4 LOI % TS EN %5, 3,76 Results of Physical Analysis Property Unite Test Method Standard Criteria (TS EN 197-1) Value Initial Setting Time min. TS EN Final Setting Time min. TS EN Spesific Surface (Blaine) cm 2 /gr TS EN Volume Expansion in.(mm) TS EN (1).39 (1) Results of Compressive Strength Test Property Unite Test Method Standard Criteria (TS EN 197-1) Value (2 day) Strength N/mm 2 TS EN , - 24,1 (28 day) Strength N/mm 2 TS EN ,5 %62,5 5 Table 3: Mix Design of Cement Mortars Mix Name Amount by Weight in Mix (gr) Aggregate Cement Water Admixture Control Rheobuild % ,43 4,5 Rheobuild % ,85 9, To avoid the effects of cutting specimens, during casting some amount of mixtures were cast into plastic caps. After 1, 2, and 7 days, hydrations of mixes were stopped by putting specimens into alcohol ispropylique during 5 days. The specimens were covered by polyester, and, then each specimen was examined by polishing process. Each specimen was sanded by 6 and 12 sandpapers. After sanding, each specimen was polished by.25, 1, 3 and 9 µm diamond paste for 12 seconds [9]. Therefore, the specimens were covered by gold (Au) for SEM analysis TESTS PROCEDURES ON CEMENT MORTARS Compressive strength testing was conducted on the 5 mm cement mortar cubes at the ages of 1, 2, 7 and 28 days. Microstructure studies were conducted on cement mortars which were taken during preparation of cement mortars into the plastic tubes for image processing. Segmentation analysis was conducted to determine the area ratio values of phases. 235

4 Compressive Strength Development 6 5 Compressive Strength Values ( Mpa ) % 1 NSB % 2 NSB Time ( day ) Fig. 1 - Compressive Strength Development of Cement Mortars 3. RESULTS Axial compressive strength test were performed for 1, 2, 7 and 28-day old specimens. Compressive strength developments of specimens are given in Figure1. Also, flow values were determined for each mix. The flow values are shown in Figure 2. To investigate early age characteristics of cement mortars, SEM analysis and image processing were performed for each 1,2 and 7-day old specimens. Micrographs of prepared specimens were taken from 3 different areas of polished section to provide homogeneity of analysis. Micrographs taken with 1X magnification in backscattered mode for each mix at 1, 2 and 7 days are given below. Flow Values ,33 17, ,67 12 Flow (mm.) 1 8 Flow Values (mm) No Admixture Fig. 2 - Flow Values of Cement Mortars 236

5 (c) Fig. 3 - Micrographs of 1 day Cement mortars (1 X) Control; ; (c) % 2 NSB (c) Fig. - 4 Micrographs of 2 day Cement Mortars (1 X) Control; ; (c) % 2 NSB 237

6 (c) Fig. 5 - Micrographs of 7 day Cement Mortars (1 X) Control; ; (c) % 2 NSB To define the amounts of area ratio of each phase, segmentation analysis was conducted for each micrograph. Phases were separated into five groups, these are pores, undifferentiated hydrates, unhydrated phase, calcium hydroxide (Ca(OH) 2 ) and aggregate phase. The development of phase area ratio values of hydration products obtained by image processing of micrographs with 1X magnification is given below. Pore Area Ratio Development 4 35 Pore Area Ratio Development (%)

7 Undifferentiated Hydrates Part Area Ratio Development 45 Undifferentiated Hydrates Part Area Ratio Development (%) Time (day ) Ca(OH)2 Area Ratio Development 4 35 Ca(OH)2 Area Ratio Development (%) (c) Unhydrated Part Area Ratio Development 35 Unhydrated Part Area Ratio Development (%) % 2 NSB 5 (d) Figure 6 Development of phase area ratio values; Pore area ratio, Undifferentiated hydrates part area ratio, (c) Ca(OH) 2 part area ratio, (d) Unhydrated part area ratio 239

8 Due to hydration development with time, pore area ratio and unhydrated part area ratio values decrease for each specimen. However, area ratio values of these parts are always higher for specimens with retarders at early ages. The values of area ratio values of undifferentiated hydrates and Ca(OH) 2 phases for mixes with admixtures are lower than the values for control mixes. Increase in undifferentiated hydrates part area ratio values for control mixes has got the same trend with decrease in unhydrated part area ratio values for the same mixes. The same trend can be seen in pore area ratio values. Pore Area Ratio / Undifferentiated Hydrates Part Area Ratio Development 2, Pore Area Ratio / Undifferentiated Hydrates Part Area Ratio 1,8 1,6 1,4 1,2 1,,8,6,4,2 - Unhydrated Part Area Ratio / Undifferentiated Hydrates Part Area Ratio Development Unhydrated Part Area Ratio / Undifferentiated Hydrates Part Area Ratio 2 1,8 1,6 1,4 1,2 1,8,6,4,2 Figure. 7: Developments of related values of phases Pore area ratio/undifferentiated hydrates part area ratio, Unhydrated part area ratio/ Undifferentiated hydrates part area ratio 4. CONCLUSIONS - Hydration products formation has been investigated by image analysis results. Development of area ratio value for each phase were plotted to simulate the hydration process at early ages. Naphthalene based admixture were used by different ratio to determine the effects of retarder on the early age hydration. - The delay in compressive strength development of specimens prepared with admixture can be seen clearly. Therefore, 2 % addition shows an overdosage effect and, the 24

9 compressive strength values of these specimens are much lower than other specimens. While mixes with %1 addition have 28-day old compressive strengths as much as compressive strengths of control specimens, specimens with 2 % have the lowest compressive strength values due to its more porous structure according to overdosage effect. Pore area ratio and unhydrated part area ratio decrease with time. However, the maximum area ratio values of these phases are obtained at 2 % addition which has overdosage effect. Undifferentiated hydrates part and Ca(OH) 2 area ratio values increase with time. Control specimens have the maximum values for these phases. Undifferentiated hydrates part and Ca(OH) 2 area ratio graphs of specimens prepared by using naphthalene sulphonate based admixture with different ratios pretend to behave the same. Investigating these graphs, undifferentiated hydrates part area ratio values increase with a decrease from 2 days to 7 days. In the same period, there is a slope with an increase for Ca(OH) 2 area ratio. - Area ratio values of pore and undifferentiated hydrates part which are the most effective phases on strength are related to each other by dividing pore area ratios with undifferentiated hydrates part area ratios. The developments of these values with time are given. Decrease in Pore area ratio / Undifferentiated hydrates part area ratio values indicate that an increase in undifferentiated hydrates part according to hydration process fills pore structure and, therefore, a decrease in pore area ratio becomes as a result of this formation. REFERENCES [1 Scrivener K. L.. Backscattered Electron Imaging of Cementitious Microstructures: Understanding and Quantification. Cem Con Comp., 24, 26 (8): [2] Skalny J, Gebauer J, Odler I. Scanning Electron Microscopy in Concrete Petrography. Materials Science of Concrete Special Volume :Calcium Hydroxide in Concrete, American Ceramic Society. Anna Maria Island, Florida, November 1-3, 2, [3] Scrivener KL, Pratt PL. Characterization of Portland Cement Hydration by Electron Optical Techniques. In: Electron microscopy of materials. Proc Mat Res Soc Symp, 1983, [4] Scrivener KL, Pratt PL. Backscattered Electron Images of Polished Cement Sections in Scanning Electron Microscope. In: Proc Sixth Int Conf Cement Microscopy, Albuquerque, 1984, [5] Detwiller R.J.,Powers L.J., Jakobsen U.H., Ahmed W.U., Scrivener K.L., Kjellsen K.O. Preparing Specimens for Microscopy. Concr Int., 21, [6] Goldstein J.L., Newbury D.E., Echlin P., Joy D.C., Fiori C., Lifshin E.(1981), Scanning Electron Microscopy and X-ray Microanalysis, Plenum, New York, [7 Stutzman P.E., Clifton J.R. "Specimen Preparation for Scanning Electron Microscopy, Proceedings of the 21st International Conference on Cement Microscopy, Las Vegas, 1999,1-22. [8 Crumbie A.K., SEM Microstructural Studies of Cementitious Materials: Sample Preparation of Polished Sections and Microstructural Observations with Backscattered Images-Artifacts and Practical Considerations, Proceedings of the 23rd International Conference on Cement Microscopy, Albuquerque, 21. [9] Stutzman P. Scanning Electron Microscopy Imaging of Hydraulic Cement Microstructure. Cem Con Comp, 24, 26 (8):