2. PREPARATION OF TEST SPECIMENS

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1 Leaching of Cement Lining in Newly-Laid Water Mains (Part II) Ong Tuan Chin and Dr. Wong Sook Fun School of Civil and Environmental Engineering, Nanyang Technological University, 5 Nanyang Avenue, Singapore ABSTRACT In the development of public water supplies, the quantity and quality of water are of great importance, with the latter being more significant. Studies show that the corrosion of cement mortar lining may occur in the process of water transmission through the pipes. The leaching of lime consequently increases the ph, calcium content and alkalinity of the water, causing the water quality to deteriorate. In order to meet the water-quality standards established by various specifications and plant operations, research has been conducted to monitor the physical and microstructural properties of the lining as well as to measure selected water quality parameters. Keywords water quality; cement mortar; leaching; ph; microstructure. 1. INTRODUCTION 1.1 Objective The objective of this project is to study the effects of cement matrix on the rate of leaching of calcium hydroxide (Ca(OH) 2 ) due to the corrosion of cement mortar lining in pipes. 1.2 Background In the development of public water supplies, the quantity and quality of water are of great importance, with the latter being more significant. Studies show that the corrosion of cement mortar lining may occur in the process of water transmission through the pipes. The leaching of Ca(OH) 2 consequently increases the ph, calcium content and alkalinity of the water, causing the water quality to deteriorate. In order to meet the water-quality standards established by various specifications and plant operations, research has been carried out to monitor the physical and microstructural properties of the lining as well as to measure selected water quality parameters. 1.3 Scope of Study The scope of work includes: i) Preparation of 5 x 5 x 5mm thin-plate specimens and 1mm concrete cubes. ii) Cement matrix tests based on mass change measurement, microstructure test and thermogravimetric analysis (TGA). iii) Water quality tests based on the measurements of ph and calcium ion concentration of distilled water, which was used as the immersion test solution in this study. iv) Compressive strength test of mortar cubes with different cement types and water-cement ratio (w/c) to evaluate their compressive strengths at different curing ages in water. 1.4 Methodology All information was gathered through books, internet, technical reports, DVGW Standards W343 and W344, BS EN545 (UK National Annex) and experiments. 2. PREPARATION OF TEST SPECIMENS 2.1 Accelerated Leaching Test Thin-plate cement mortar specimens, measuring 5 x 5 x 5mm each, were cast using Perspex formwork of size 35 x 1 x 5mm. After compaction on a vibrating table, the specimens were cut into the required dimensions before the final setting took place. Subsequently, each specimen mold was covered with an impervious plastic sheet, followed by a damp cloth to prevent moisture loss through surface evaporation. The specimens were demolded from the formwork about 24 hours after casting. The demolded specimens were then cured in tap water saturated with Ca(OH) 2 for 3 days. This was to prevent the leaching of Ca(OH) 2 from thin-plate specimens prior to the commencement of the actual testing. The tests would be conducted for two groups of cement mortar mixtures with w/c of.35 and.5. Each group would consist of four different mixtures: i) Ordinary Portland cement consisting of 1% [1] ii) Portland blastfurnace cement PBFC or consisting of 7% and 3% GGBS [2] 1

2 iii) High slag blastfurnace cement HSBFC or consisting of 3% and 7% GGBS [3] iv) Calcium aluminate cement CAC Ciment Fondu consisting of 1% CAC [4] GGBS: ground granulated blastfurnace slag The mixture proportions of test specimens and cement matrix tests (which include measurement of mass change, microstructure test and thermogravimetric analysis) have been detailed in the report by project partner Leow Boon Huat. 2.2 Compressive Strength Test As pipes are subjected to stresses and damages during handling process as well as transportation to site, it is much more meaningful to monitor the early strength of cement mortar cube than its ultimate strength. A minimum early strength of 2 N/mm 2 must be achieved in order to minimize such damages that could affect the pipe performance [5]. Cement mortar cubes (1mm) of various mixtures stated in Section 2.1 were cast using steel molds. The procedure of preparing these cubes, excluding the stage of cutting, was the same as that stated for thin-plate specimens. These cubes were then used to study the compressive strength development of selected mixtures. At curing ages of 3, 7 and 28 days, the cubes were removed from the curing tank for compressive strength tests in accordance with SS78 Part A16: 1987 [6] using an Avery-Denison compression machine. A loading rate of 2 kn/min was employed during the test. 3. WATER QUALITY TESTS After 3 days of curing, the specimens were transferred into an immersion tank containing distilled water at ambient temperature of 2 ± 2 o C. Distilled water was used as the immersion solution, since it acted more aggressively on the lining. The specimens would be exposed to the distilled water for, 3, 7, 14 and 28 days before they were taken out for the following tests: i) Measurement of ph ii) Inductively coupled plasma (ICP) In accordance to a 3mm diameter pipe, the volume of distilled water required in the tank was based on the number of specimens immersed multiplied by 45cm 3 of water for one thin-plate specimen. To simulate the constant pipe flow condition of weekly flushing, the distilled water was renewed on a weekly basis. 3.1 Measurement of ph The ph of distilled water, in which thin-plate mortar specimens were immersed, was recorded at various prescheduled periods just before the next flushing (e.g. ph reading for the 1 st flushing was taken at the end of the 7 th day exposure). A significant increase in the ph value compared to the preceeding value would be due to the leaching of Ca(OH) 2 from the specimens. Since ph also varies with temperature (ph increases with temperature), the measurement of ph was carried out in the constant temperature room where the test specimens were stored. 3.2 Inductively Coupled Plasma (ICP) Inductively coupled plasma (ICP) tests were conducted to measure the calcium (Ca) concentration in the distilled water which had leached out from the cement mortar specimens. 1ml of filtrate samples, containing two drops of nitric acid each, were prepared and stored at a temperature below 1 o C. To prevent variation in the results, ICP tests were only performed after all the filtrate samples for the specified periods had been collected. Based on the ICP results, one could verify whether the ph measured at a certain period (, 7, 14, 21 or 28 days) was correct, i.e. the leaching of Ca(OH) 2 would increase the alkalinity of distilled water thus causing the ph value to rise. 2

3 4. RESULTS AND DISCUSSION 4.1 Compressive Strength From the compressive strength results in Fig.1(a) and 1(b), all four types of cement mortar mixtures were able to meet the minimum strength of 2 N/mm 2 after 7 days of curing. For mortar cubes with w/c of.5 and Mortar Cube (W/C:.5) Compressive Strength Mortar Cube (W/C:.35) Compressive Strength Compressive Strength (N/mm 2 ) Curing Period (days) Fig.1(a) Compressive Strength.35, mixtures reflected the highest strength at 7 days among all the mixtures. Meanwhile, mortar cubes comprising 1% achieved their ultimate strength at approximately 21 days. However, in terms of long-term strength (i.e. at 28 days), blended cement mortar mixtures containing GGBS were the highest. In addition, the results showed that a higher compressive strength was achieved with a lower w/c. This could be explained by the formation of a denser microstructure at a lower w/c. 4.2 Measured ph Fig.2(a) and 2(b) show there was fluctuation between the 2 nd and 3 rd flushing. Such fluctuation might be due to an increase in temperature in the room where the test specimens were stored. In actual case, the leaching of Ca(OH) 2 from the specimens immersed in the distilled water would cause the amount of calcium ions to deplete continuously. The ph of the distilled water should then decrease with subsequent flushing, as reflected (N/mm 2 ) Curing Period (days) Fig.1(b) ph (W/C:.35) vs. ph (W/C:.5) vs ph Fig.2(a) in the results obtained by SsangYong Central Laboratory [7]. Hence, based on the ph measured at the 4 th flushing only, thin-plate specimens of mixtures reflected the lowest ph (e.g for of w/c.35) compared to the other cement mixtures. On the other hand, leaching of Ca(OH) 2 from specimens was the highest, followed by and mixtures. Nevertheless, the lowest ph value observed in Fig.2(a) was still higher than the acceptable limit of 9.5 [8]. Despite that higher resistance to leaching could be achieved using double blended mixtures ( or ) rather than 1% mixture, future research need to be conducted to establish methods, e.g. application of protective coating on the lining, which would improve the water quality. ph Fig.2(b) 3

4 4.3 Calcium Concentration Ca concentration (W/C:.35) vs. Ca concentration (W/C:.5) vs. Ca concentration (Mg/L) Fig.3(a) Ca concentration (Mg/L) Fig.3(b) The declining trend of the curves shown in Fig.3(a) and 3(b) indicated that the amount of calcium ions that leached out from the specimens was decreasing with subsequent flushing of distilled water. This was because the calcium ions present in the specimens were depleting continuously, hence lesser ions would be leached into the distilled water of each subsequent flushing. The declining trend observed thus verifies the downward sloping ph curves shown in the report by SsangYong Central Laboratory [7]. In other words, the ph of the distilled water was directly proportional to the amount of calcium ions leached. Based on the results at the 4 th flushing, mixture with a w/c of.35 was found to have the highest resistance against leaching while mixtures reflected the lowest, with and mixtures having the moderate performance against leaching. In addition, it was also observed that the leaching of Ca(OH) 2 was lesser from specimens having a w/c of.35. This suggests that a decrease in w/c would generally lead to a lower rate of leaching. It should be noted that due to the duration of the course (6 weeks), the results reported are based on those collected up to this point of time. Ongoing tests will be carried out up to an exposure period of 56 days. 5. CONCLUSIONS AND RECOMMENDATIONS Cement mortar lining which protects metal water distribution pipes against electrochemical corrosion may result in the leaching in Ca(OH) 2 from the mortar itself. Besides leading to structural deterioration of the lining, the calcium loss due to leaching also impairs the water quality by increasing its ph, alkalinity and calcium concentration. Under extended low-flowing water conditions, especially stagnant water that remains relatively long in newly-lined pipe sections, causes the leaching rate to increase significantly. In addition, waters of low ph, alkalinity and calcium cause further acceleration of leaching, as they appear to be most aggressive to the linings. The corrosion rate of lining is also influenced by the different cement mortar compositions including w/c, cement type and the incorporation of mineral admixture. Hence, based on the experiment and research, the following conclusions and recommendations are made: i) ii) iii) iv) The mixtures investigated were able to meet the minimum compressive strength requirement for cement mortar lining of 2 N/mm 2 after 7 days of curing. Cement mortar mixtures with lower w/c would be more suitable for use as cement lining than those of higher w/c, since the ph and ICP results show that there was a decrease in ph and calcium concentration of the distilled water. Consequently, greater resistance to leaching can be achieved. In addition, with a lower w/c, a higher compressive strength can be obtained which will aid in the structural strength of lining. Blended cement mortar mixtures with GGBS could be considered for use as cement lining, since they contain less free Ca(OH) 2 than 1% mixtures. Also, the resistance to leaching generally improved with an increase in slag content of the cement mortar mixture. Based on the ICP results, the calcium concentration of distilled water decreased with increasing number of flushing. Hence, the pipe sections lined with cement mortar should be flushed regularly to prevent lowflowing water conditions. 4

5 v) Dead ends in water distribution systems should be avoided to eliminate stagnant water conditions. Otherwise, drains should be provided to facilitate flushing at dead ends. REFERENCES 1. SS 26: 2, Ordinary Portland cement, Singapore Standards, SS 477: 2, Portland blastfurnace cement, Singapore Standards, SS 476: 2, High slag blastfurnace cement, Singapore Standards, NF P , Hydraulic binder; Melted aluminous cement, French norm, April H. Holtschulte and M. R. Schock, Internal corrosion of water distribution systems, AWWA Research Foundation and DVGW-Forschungsstelle, Denver: AWWARF, SS 78: Part A16: 1987, Methods for determination of compressive strength, Singapore Standards, SsangYong Central Laboratory, Influence of the type of cement on the alkali leaching behavior of concrete pipe lining, SsangYong Cement Singapore Ltd., M. G. Alexander, Soft-water attack, J. Mater. Civ. Engrg., 5(4), , November