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, 50 Nanyang Avenue, Singapore 639798
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 50 x 50 x 5mm thin-plate specimens and 100mm 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 50 x 50 x 5mm each, were cast using Perspex formwork of size 350 x 100 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 0.35 and 0.50. Each group would consist of four different mixtures:
iii) High slag blastfurnace cement – HSBFC or P4246 consisting of 30% OPC and 70% GGBS [3] iv) Calcium aluminate cement CAC “Ciment Fondu” – FONDU consisting of 100% 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 20 N/mm2 must be achieved in order to minimize such damages that could affect the pipe performance [5].
Cement mortar cubes (100mm) 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 20 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 20 ± 2oC. 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 0, 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 300mm diameter pipe, the volume of distilled water required in the tank was based on the number of specimens immersed multiplied by 450cm3 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 pre-scheduled periods just before the next flushing (e.g. pH reading for the 1st flushing was taken at the end of the 7th 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. 10ml of filtrate samples, containing two drops of nitric acid each, were prepared and stored at a temperature below 10oC. 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 (0, 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.
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 20 N/mm2 after 7 days of curing. For mortar cubes with w/c of 0.50 and
0.35, OPC mixtures reflected the highest strength at 7 days among all the mixtures. Meanwhile, mortar cubes comprising 100% FONDU 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.
Mortar Cube (W/C:0.50) Compressive Strength
0 10 20 30 40 50 60 0 5 10 15 20 25 30
Curing Period (days) Fig.1(a) Compressive Strengt h (N/mm 2 ) OPC P146 P4246 FONDU
Mortar Cube (W/C:0.35) Compressive Strength
0 10 20 30 40 50 60 70 0 5 10 15 20 25 30
Curing Period (days) Fig.1(b) Compressive Strength (N/mm 2 ) OPC P4246 FONDU 4.2 Measured pH
Fig.2(a) and 2(b) show there was fluctuation between the 2nd and 3rd 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
in the results obtained by SsangYong Central Laboratory [7]. Hence, based on the pH measured at the 4th flushing only, thin-plate specimens of P4246 mixtures reflected the lowest pH (e.g. 10.99 for P4246 of w/c 0.35) compared to the other cement mixtures. On the other hand, leaching of Ca(OH)2 from OPC specimens was the highest, followed by P146 and FONDU 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 (P4246 or P146) rather than 100% OPC 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 (W/C:0.35) vs. no. of flushing 10.5 11 11.5 12 12.5 1 2 3 4 no. of flushing Fig.2(a) pH OPC P146 P4246 FONDU pH (W/C:0.50) vs. no. of flushing 10.5 11 11.5 12 1 2 3 4 no. of flushing Fig.2(b) pH OPC P146 P4246 FONDU
4.3 Calcium Concentration
es shown in Fig.3(a) and 3(b) indicated that the amount of calcium ions that lea
s found to have the highest
t should be noted that due to the duration of the course (6 weeks), the results reported are based on
. CONCLUSIONS AND RECOMMENDATIONS
ibution pipes against electrochemical corrosion may
low-flowing water conditions, especially stagnant water that remains relatively long in newl
The mixtures investigated were able to meet the minimum compressive strength requirement for cement
ii) e more suitable for use as cement lining than those of
iii) cement mortar mixtures with GGBS could be considered for use as cement lining, since they Ca concentration (W/C:0.35) vs. no. of flushing
0 50 100 150 200 1 2 3 4 no. of flushing Fig.3(a) Ca concentration (Mg/L ) OPC P146 P4246 FONDU
Ca concentration (W/C:0.50) vs. no. of flushing
0 50 100 150 200 250 1 2 3 4 no. of flushing Fig.3(b) Ca concentration (Mg/L) OPC P146 P4246 FONDU
The declining trend of the curv
ched 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 4th flushing, P4246 mixture with a w/c of 0.35 wa
resistance against leaching while OPC mixtures reflected the lowest, with P146 and FONDU 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 0.35. This suggests that a decrease in w/c would generally lead to a lower rate of leaching.
I
those collected up to this point of time. Ongoing tests will be carried out up to an exposure period of 56 days.
5
Cement mortar lining which protects metal water distr
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
y-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)
mortar lining of 20 N/mm2 after 7 days of curing. Cement mortar mixtures with lower w/c would b
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
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: 2000, Ordinary Portland cement, Singapore Standards, 2000. 2. SS 477: 2000, Portland blastfurnace cement, Singapore Standards, 2000. 3. SS 476: 2000, High slag blastfurnace cement, Singapore Standards, 2000.
4. NF P 15-315, Hydraulic binder; Melted aluminous cement, French norm, April 1991.
5. H. Holtschulte and M. R. Schock, Internal corrosion of water distribution systems, AWWA Research Foundation and DVGW-Forschungsstelle, Denver: AWWARF, 1985.
6. SS 78: Part A16: 1987, Methods for determination of compressive strength, Singapore Standards, 1987. 7. SsangYong Central Laboratory, Influence of the type of cement on the alkali leaching behavior of
concrete pipe lining, SsangYong Cement Singapore Ltd., 2000.
