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Effect of fresh water leaching on the microstructure of hardened composite binder pastes
Construction and Building Materials 68 (2014) 630–636
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of fresh water leaching on the microstructure of hardened composite binder pastes Fanghui Han a,b, Rengguang Liu b, Peiyu Yan b,⇑ a b
China University of Mining and Technology (Beijing), Beijing, China Key Laboratory of Civil Engineering Safety and Durability of China Education Ministry, Department of Civil Engineering, Tsinghua University, Beijing, China
h i g h l i g h t s Microstructure deteriorated with different degree after leaching for 2–3 years. There were no signs of decomposition of hydration products after long-term leaching. The pastes containing a small amount of fly ash were not damaged severely. Addition of 65% of fly ash deteriorated most severely, Ca(OH)2 was almost exhausted. The pastes containing no more than 70% of slag showed high leaching resistance.
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Article history: Received 1 April 2014 Received in revised form 7 June 2014 Accepted 3 July 2014
Keywords: Cement Slag Fly ash Composite binder pastes Fresh water leaching Porosity Calcium hydroxide Microstructure
a b s t r a c t The microstructure of cement–slag and cement–fly ash pastes leached by fresh water for 2–3 years was investigated by MIP, XRD, TG and SEM. Results indicated that long-term fresh water leaching caused degradation of the pastes in terms of mass loss, increase in porosity and decrease in Ca(OH)2 content and Ca/Si ratio of C–S–H gel, but showed no decomposition of hydration products. The pastes containing a small amount of fly ash were not damaged severely, incorporation of 65% of fly ash deteriorated most severely, whereas the cement–slag pastes (670%) showed high long-term fresh water leaching resistance. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Cementitious materials are widely used in modern civil and hydraulic structures, such as water conveyance tunnel, dam panel and underwater constructions [1]. Pure or deionized water is one of the strong decalcifying agents for cementitious materials [2]. Leaching by fresh water is one of the most significant factors that affect the durability of Portland cement concrete [3,4]. During leaching process, ions in pore solution of cementitious materials, like Calcium ion (Ca2+) and hydroxide (OH), migrate into surrounding environment due to concentration gradients between pore solution and environmental water. This process reduces the alkalinity of pore solution and might cause decomposition of ⇑ Corresponding author. Tel.: +86 13501215836; fax: +86 01062785836. E-mail address: [email protected] (P. Yan). http://dx.doi.org/10.1016/j.conbuildmat.2014.07.019 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
hydration products [5]. Furthermore, it is reported to result in an increased porosity, increased permeability consequently and reduced mechanical properties of concrete [2,6,7]. As partial replacement of Portland cement, mineral admixtures are widely applied in concrete. Owe to low cement content and pozzolanic reaction of mineral admixture, the amount of Ca(OH)2 in composite binder pastes reduced. Meanwhile, Ca(OH)2 will be leached out under high-head flow of fresh water in a long term. Thus, the microstructure of hardened binder pastes could be destabilized. Therefore, it is important to understand the change of microstructure and physical properties of composite binder pastes after long-term fresh water leaching. Many researchers have reported leaching performance of cementitious materials. Cardeet et al. [8] investigated the influence of leaching of Ca(OH)2 and C–S–H gel on the mechanical properties of cement paste. They reported that leaching of Ca(OH)2 reduced
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the strength and increased the porosity of cement paste. The variation of Ca/Si ratio of C–S–H gel had little influence on the strength of pure cement paste. But the strength of the paste containing silica fume leached by fresh water reduced by 30% due to the decrease in Ca/Si ratio of C–S–H gel. Rozière et al. [9] studied the leaching performance of the concrete with different water to binder ratio and with cementitious materials of different chemical compositions. They proposed that the indicators for evaluation of leaching performance were the amount of calcium ion and the depth of leaching, etc. The amount of Ca(OH)2 which could be easily dissolved away under fresh water leaching decreased in composite binder pastes, and thus the extent of leaching damage to hardened concrete reduced. Jain et al. [2] reported the effect of fresh water leaching on the performance of hardened paste blended with fly ash and found that incorporating 10% of fly ash could reduce 20% of leaching depth and that the fly ash modified pastes showed reduced loss of Ca(OH)2 compared to the pure cement paste. The incorporation of a small amount of fly ash in binder pastes was beneficial to the improvement of leaching resistance to fresh water. Due to the complexity of experiment and long experimental period, most researches tend to predict the leaching behavior of hardened binder pastes using mathematical models based on factors such as the amount of calcium ion [2,6], the leaching depth [10–12], the mass loss and strength [8], the porosity [13,14], the diffusion coefficient [15] and other kinetic leaching parameters. However, little information regarding microstructural change of hardened composite binder pastes before and after leaching can be found in the literature. Therefore, in the paper, through measuring the mass loss, pore structure, hydration products, variation of Ca(OH)2 content, Ca/Si ratio of C–S–H gel and observing morphology, the microstructure variation of the hardened composite binder pastes blended with 2 mineral admixtures, fly ash or slag, leached by fresh water was investigated. 2. Experiments 2.1. Materials and specimen preparation
Fig. 1. Particle size distributions of cement and mineral admixtures.
prepared with pure cement is 95% when the fluidity of the former mortar is 130– 140 mm. The binder to ISO standard sand ratio for all mortars is 1/3; the water to binder ratio for mortar prepared with pure cement is 0.5. The mix proportions of composite binder pastes are shown in Table 2 and the water to binder ratio (W/B) for all samples is 0.5. 2.2. Test methods The composite binder pastes were prepared according to the mix proportions shown in Table 2. They were cast into polyvinyl chloride (PVC) tubes of 45 mm diameter and 15 mm thickness and cured in an environmental chamber of 90% RH, 20 ± 1 °C. After curing for 90 days, the specimens were moved in a 25 L plastic box filled with deionized water, which was stirred constantly with an electric stirrer to simulate the process of fresh water leaching. The stirring rate was about 50 rpm. Fig. 2 shows a typical sample and the leaching facility. Deionized water was periodically renewed every day in the first week and every week in the following weeks, in order to ensure the specimens placed in a clean fresh water environment. The surface layers (about 5 mm) of the specimens were cut and spit into small pieces and put into acetone to cease further hydration at the prescribed age. 2.3. Mass loss
P.I 42.5 Portland cement, Class I fly ash and S95 ground granulated blast furnace slag conforming to Chinese National Standards (GB175-2007, GB/T1596-2005 and GB/T18046-2008, respectively) were used. P.I 42.5 Portland cement is pure Portland cement whose strength is no less than 42.5 MPa curing under 95% RH, 20 ± 2 °C for 28 days. The classification of fly ash is according to fineness, water requirement ratio and loss on ignition (LOI). The screen residue with 45 lm square hole sieve, the water requirement ratio and LOI of Class I fly ash are no more than 12%, 95% and 5%, respectively. The classification of slag is mainly according to activity index. The ratio of strength of mortar prepared with the binder containing 50% of cement and 50% of slag to that of mortar prepared with pure cement curing under 95% RH, 20 ± 2 °C for 28 days is 95% for S95 ground granulated blast furnace slag. The water to binder ratio and binder to ISO standard sand ratio of mortar is 0.5 and 1/3, respectively. The activity index of fly ash is determined using above method as well, but with a binder containing 30% of fly ash. The chemical compositions of cement and mineral admixtures are given in Table 1. The specific surface areas of the cement and the slag are 350 m2/kg and 442 m2/ kg, respectively. The particle size distributions of cement and mineral admixtures are shown in Fig. 1. The activity index of fly ash and slag is 75% and 95%, respectively. The water requirement ratio of fly ash is 95%, which is determined according to Chinese National Standard GB/T1596-2005. The ratio of water requirement of mortar prepared with the binder containing 70% of cement and 30% of fly ash to that of mortar
The cumulative mass loss rate of the hardened composite binder pastes after leaching (u) can be calculated as follows:
u¼
mn m1 100% m1
ð1Þ
where m1 and mn are the weights of specimen before and after leaching, respectively. Specimens were cured under 90% RH, 20 ± 1 °C for 90 days and then vacuum dried for 4 h, the weight of specimen before leaching was determined (m1). Specimens were removed from the plastic box when they were leached by fresh water to the prescribed age and then the surface water was cleaned, after which they were vacuum dried for 4 h. The weight of specimen after leaching was recorded (mn). Ten specimens were used to obtain the mass loss rate result for each sample. The result of one of specimens would be eliminated if it deviated 15% from the average value of ten specimens’ results, the result of mass loss rate was the average value of the rest of specimens. 2.4. Mercury intrusion porosimetry Porosity and pore size distribution of the hardened composite binder pastes in different leaching duration were measured with AUTOPORE II 9220 mercury intrusion porosimeter with a maximum mercury intrusion pressure of 300 MPa.
Table 1 Chemical compositions of cement and mineral admixtures w (%). Composition
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Na2Oeq
f-CaO
Cl
LOI
Cement Fly ash Slag
20.55 57.60 34.55
4.59 21.90 14.36
3.27 2.70 0.45
62.50 3.87 33.94
2.61 1.68 11.16
2.93 0.41 1.95
0.53 1.05 0.63
0.83 – –
0.010 – –
2.08 7.65 0.70
Na2Oeq = Na2O + 0.658 K2O; w: Mass fraction.
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Table 2 Mix proportions of composite binder pastes. Sample
W/B
Cem FA20 FA40 FA65 SL30 SL70
0.5 0.5 0.5 0.5 0.5 0.5
Mix proportion (by mass) (%) Cement
Fly ash
Slag
100 80 60 35 70 30
0 20 40 65 0 0
0 0 0 0 30 70
2.5. X-ray diffraction The identification of the hydration products of the hardened composite binder pastes in different leaching duration was achieved with a TTR IIIX-ray diffractometer (Cu Ka, 45 kV, 200 mA) over a scanning range of 5–60° in 2h scale. The testing rate applied was 6°/min. Fig. 3. Cumulative mass loss rates of hardened pastes in different leaching duration.
2.6. Thermogravimetry The Ca(OH)2 content of the hardened composite binder pastes in different leaching duration was determined by a TA-Q5000 instrument. Thermogravimetry was carried out from 20 ± 5 °C up to 900 °C, at 10 °C/min heating rate. A nitrogen environment was set up to prevent carbonation of Ca(OH)2. 2.7. Scanning electron microscopy The morphology of hardened binder pastes in different leaching duration was investigated with a FEI Quanta 200 FEG scanning electron microscope. Scanning electron microscopy analysis was carried out on a freshly fractured surface of samples and the Ca/Si ratio of C–S–H gel was analyzed with energy dispersive spectrometer. The sample was coated with carbon. The measuring range of energy dispersive spectrometer is about 1 lm3 and multi-phases can be included. In order to reduce the error, typical C–S–H gel was judged and analyzed, 30 test areas were measured for each sample.
3. Results and discussion 3.1. Mass loss The rates of cumulative mass loss of hardened composite binder pastes in different leaching duration are shown in Fig. 3. It can be seen from Fig. 3 that the hardened composite binder pastes containing fly ash and slag show different mass change at early leaching stage. The mass of pure cement paste and cement–slag pastes increased at early leaching age. With the increased dosage of slag, the rate of mass gain reduced. However, the cement–fly ash pastes showed mass loss. The mass loss rate was greater when the proportion of replacement of cement with fly ash was higher. The results were concerned with the different
reaction mechanism of cement clinker, slag and fly ash. The reaction degree of cement clinker and slag was high compared with fly ash. They produced plenty of hydration products to fill pores, which could make the pastes dense. Half the number of the cement was still unhydrated in pure cement paste after hydrating for 90 days [16]. During fresh water leaching, numerous unhydrated cement particles and mineral admixture particles in hardened pastes continued to hydrate and generated more hydration products, which made the mass of pastes increase. The mass loss of pure cement paste and cement–slag pastes was small in shortterm leaching due to their dense paste structure. The mass increased at the early stage of leaching owe to the less mass loss than the mass gain. The activity of slag was lower than that of cement clinker in cement–slag pastes at early curing age, the effective water to cement ratio of cement clinker in composite binder pastes increased due to the dilution effect of slag. So the hydration degree of cement in the cement–slag pastes was higher than that of pure cement paste, hence less unhydrated cement particles existed before leaching. Thus, with increased dosage of slag, the mass gain of the cement–slag pastes slowed down after short-term leaching. The reaction degree of fly ash was low and few hydration products were produced in cement–fly ash pastes, whose structure was looser than pure cement paste. More dissolvable hydration products could be leached after short-term leaching and its mass loss was higher than the mass increase. So cement–fly ash pastes demonstrated mass loss at early stage of leaching duration.
Fig. 2. The typical sample and leaching facility (a) typical sample and (b) leaching facility.
F. Han et al. / Construction and Building Materials 68 (2014) 630–636
Pure cement paste and cement–slag pastes also showed mass loss after long-term leaching. With the extension of leaching age, the mass loss increased gradually due to less unhydrated particles existing in the hardened pastes, so the mass loss was more than the mass increase. And as the slag content increased, the mass loss rate was lower. This was due to their dense structure and the decreased amount of dissolvable Ca(OH)2. However, the mass loss of cement–fly ash pastes continued increasing with the extension of leaching duration, much more than that of cement–slag pastes. Especially the samples containing high content of fly ash, such as the sample FA65, its mass loss rate was very high. 3.2. Pore structure The porosity variation of hardened composite binder pastes in different leaching duration is shown in Fig. 4. It is evident that the porosity of samples containing fly ash increased with the extension of leaching duration, but the porosity of pure cement paste and cement–slag pastes decreased at early leaching stage and increased slightly after long-term leaching duration. The change rule of the porosity was consistent with that of the mass loss. As shown in Fig. 4, the porosity of pastes containing fly ash was higher than that of pastes containing slag before leaching, indicating that the hardened pastes containing slag were dense. With the leaching prolonged, the dissolvable hydration products could be easily leached from cement–fly ash pastes for their high porosity. Thus, the porosity was gradually increasing. Especially for the sample FA65, the leaching degree (severity) was much more serious and the porosity increased considerably. For pure cement paste and cement–slag pastes, short-term leaching did not cause hydration products to be leached because of their low porosity and dense paste structure. More hydration products generated further filling pores to make decrease of porosity. As the leaching process continued, some hydration products were leached and newly generated hydration products were less and less, so the porosity showed an increasing trend. But the porosity of cement–slag pastes remained stable and showed no trend of a great increase after long-term leaching compared with that before leaching. The density of hardened paste is an important factor in the leaching resistance of concrete. The paste with low porosity and poor pore connectivity made it difficult for water to infiltrate and to dissolve soluble hydration products, so there was only a slight leaching phenomenon in the surface. According to the porosity
Fig. 4. Porosity variation of hardened pastes in different leaching duration.
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variation of the pastes before and after leaching, cement–slag pastes had lower porosity and showed better performance of the leaching resistance to fresh water compared with cement–fly ash pastes. Fig. 5 shows the pore distribution of the hardened composite binder pastes before and after leaching for 2 years. It can be seen that for cement–fly ash pastes, the content of 4.5–50 nm micropores increased a lot after leaching for 2 years, which is indicated that the pastes suffered some degree of leaching and a number of hydration products were eroded off after 2 years leaching. The content of <4.5 nm micropores could not increase in the pastes due to the disconnected micropores where fresh water could not migrate into. The content of 4.5–50 nm micropores accounted for large proportion in the hardened pastes before leaching, what is more, 4.5–50 nm micropores had large specific surface area compared with 50–100 nm and >100 nm pores. Thus, hydration products leached by fresh water had a great influence on the content of 4.5–50 nm micropores. While for cement–slag pastes, the content of 4.5–50 nm micropores did not increase significantly, indicating that the dissolution of hydration products was not serious, which showed high leaching resistance to fresh water. For each sample after long-term leaching, the contents of 50– 100 nm and >100 nm pores did not change significantly. The amount of large pores even became small for samples containing mineral admixture. This phenomenon contributed to continued secondary hydration during fresh water leaching, the obtained result was consistent with literature [2], the increased amount of hydration products filled the large pores, which made the structure dense, thus there was little influence on the strength and permeability of the hardened composite pastes. 3.3. Hydration products The X-ray diffraction spectrums of hardened pastes before and after leaching for 2 years are shown in Figs. 6 and 7, respectively. It is evident that there was no significant change of the crystalline products. The diffraction peaks of unhydrated clinker were weak after long-term leaching, indicating that a little unhydrated clinker existed in the pastes. The diffraction peak of Ca(OH)2 decreased after leaching compared with before leaching, but the Ca(OH)2 diffraction peak of each sample was still strong except sample FA65. For sample FA65, much Ca(OH)2 was consumed for pozzolanic reaction of fly ash and a lot of Ca(OH)2 was dissolved away due to high porosity. In contrast, for sample SL70, the diffraction peak
Fig. 5. Pore distribution of hardened pastes before leaching and after leaching for 2 years.
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Fig. 8. Ca(OH)2 content in hardened pastes in different leaching duration. Fig. 6. XRD spectrum of hardened pastes before leaching.
cement–slag pastes decreased with the increasing leaching age, but the rate of decrease was gentle. The porosity of the pastes containing slag was lower and the pastes were denser than that of the pastes containing fly ash. The refined pores resulted in a difficult leaching of Ca(OH)2. The reaction of slag was not entirely pozzolanic reaction, so only a little Ca(OH)2 was consumed. Thus, the content of Ca(OH)2 was reduced slightly. For sample SL70, the Ca(OH)2 content was still above 7% after leaching for 2 years, which could ensure the high alkalinity of pore solution and stability of C–S–H gel as well as other hydration products. 3.5. Morphology of hardened paste
Fig. 7. XRD spectrum of hardened pastes after leaching for 2 years.
was strong and it is shown that a certain amount of Ca(OH)2 still existed even after 2 years leaching. It is proved that composite binder pastes containing slag were dense, there was no serious leaching of Ca(OH)2.
3.4. Ca(OH)2 content The content of Ca(OH)2 in hardened composite binder pastes in different leaching duration is shown in Fig. 8. There was no reduction in Ca(OH)2 content of pure cement paste but a small increase at the early stage of leaching duration. With the prolonged leaching age, the Ca(OH)2 content had a decreasing trend consistent with the change rule of mass and porosity. For composite binder pastes containing mineral admixture, cement fraction decreased and thus the content of Ca(OH)2 reduced. After long-term leaching, the content of Ca(OH)2 decreased remarkably in cement–fly ash pastes. For sample FA65, the Ca(OH)2 content was only 0.46% and became almost nil after leaching for 3 years. For samples FA20 and FA40 containing relatively low fraction of fly ash, the decreasing trend of Ca(OH)2 became gentle, and it can be expected that leaching will not make Ca(OH)2 consumed seriously or exhausted in a long leaching duration. So a high alkalinity of pore solution in pastes can be maintained and it is impossible that C–S–H gel will suffer from leaching and decomposition. The content of Ca(OH)2 in
Fig. 9 shows morphologies of hardened composite binder pastes before leaching. It can be seen that fly ash was not sculptured on the surface. It was not bonded with the pastes closely and some spherical fly ash particles came off when crushing samples. It is indicated that the reaction degree of fly ash was low after curing for 90 days. The structure of composite binder pastes containing slag was dense and slag particles bonded tightly with the surrounding pastes, indicating a certain reaction degree of slag. Morphologies of hardened composite binder pastes after leaching for 2 years are shown in Fig. 10. After long-term leaching, it can be seen that there were still a lot of sheet Ca(OH)2 crystals in sample Cem as well as the samples FA20 and SL30 containing low content of mineral admixture. For samples FA20 and FA40, fly ash particle was fractured when sampling, indicating that fly ash bonded with the surrounding pastes tightly. The surface of spherical fly ash particles were etched significantly in sample FA40 due to the pozzolanic activity of fly ash. There were many fly ash spheres in sample FA65 and they were bonded mostly closely. Although Ca(OH)2 content was very low, the gel was not damaged resulting in a loose decomposition products. For sample SL70, the slag particles were bonded tightly with the surrounding pastes and it can be judged from gray degree that their reaction degree was high. Thus, it can be said that the structure of hardened composite binder pastes containing mineral admixture was still very dense, there was not loose decomposition products and the paste structure remained stable after long-term fresh water leaching. The Ca/Si ratio of C–S–H gel in hardened pastes in different leaching duration measured by energy dispersive spectrometer is shown in Fig. 11. Ca/Si ratio of C–S–H gel decreased with the increased dosage of mineral admixture in composite binder pastes. It can be seen from the chemical composition of cement and mineral admixture (Table 1) that CaO content of slag is lower than that
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(a) Cem
(b) FA20
(c) FA40
(d) FA65
(e) SL30
(f) SL70
Fig. 9. Morphologies of hardened composite binder pastes before leaching (a) Cem, (b) FA20, (c) FA40, (d) FA65, (e) SL30, and (f) SL70.
(a) Cem
(d) FA65
(b) FA20
(c) FA40
(e) SL30
(f) SL70
Fig. 10. Morphologies of the hardened pastes after leaching (a–d) for 3 years; (e) and (f) for 2 years (a) Cem, (b) FA20, (c) FA40, (d) FA65, (e) SL30, and (f) SL70.
of cement, which is the lowest for fly ash, so Ca/Si ratio of C–S–H gel produced by reaction of mineral admixture was lower than that produced by cement reaction. With the extent of leaching duration, Ca/Si ratio of C–S–H gel in pure cement paste decreased slightly and remained stable. It is proved that the degree of leaching was low and there were not a lot of calcium ions separated out. However, the Ca/Si ratio of C–S–H gel in composite binder pastes decreased; there was an obvious downward trend for composite binder containing 40%, 65% of fly ash and 70% of slag. The reaction rate of mineral admixture was slow and its reaction degree was
low at early age, but the reaction degree increased gradually at late age. In long-term duration, some mineral admixture particles continued to react, which produced more C–S–H gel with low Ca/Si ratio, which made a decrease in the Ca/Si ratio of the composite binder pastes. If decomposition of C–S–H gel in composite binder pastes occurred after 2–3 years’ leaching, Calcium ions would be leach out from C–S–H gel and the remaining substance was silicon dioxide hydrate, which had no characteristics of gel and could be easily dissolved away. Thus, the mass loss and porosity would increase
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micropores in the cement–fly ash pastes increased due to hydration products leached by fresh water. The continued secondary hydration during fresh water leaching could lead to decrease in large pores content. (4) Composite binder pastes blended with slag had denser paste microstructure and lower porosity. The pastes containing no more than 70% of slag had high long-term fresh water leaching resistance and Ca(OH)2 content was plentiful in the pastes after long-term leaching.
Acknowledgment Authors would like to acknowledge the National Natural Science Foundation of China (Nos. U1134008 and 51278277). References Fig. 11. Variation of Ca/Si ratio of C–S–H gel in hardened pastes in different leaching duration.
significantly, paste morphologies would be loose, Ca(OH)2 content and Ca/Si ratio of C–S–H gel would decrease obviously. But the results (Figs. 3–10) were not consistence with above mentioned. Then it could be concluded herein that composite binder pastes did not suffer from serious corrosion after long-term leaching and the decrease of Ca/Si ratio was not mainly due to the fresh water leaching. For pure cement paste, composite binder pastes blended with a small amount of fly ash or the pastes containing no more than 70% of slag, the trend of reduction in Ca/Si ratio of C–S–H gel was not evident. But it is noted that Ca/Si ratio of C–S–H gel in the paste containing 65% of fly ash decreased obviously after leaching for 2–3 years, which could be the starting point for the decomposition of C–S–H gel. 4. Conclusions (1) After long-term leaching by fresh water, the pastes were degraded in terms of mass loss, increase in porosity, decrease in Ca(OH)2 content and Ca/Si ratio of C–S–H gel, but showed no decomposition of hydration products. (2) Composite binder pastes containing a small amount of fly ash were not damaged severely. Incorporation of 65% of fly ash deteriorated severely, the rate of mass loss and porosity increased significantly after leaching for 2–3 years, Ca(OH)2 was almost exhausted due to the pozzolanic reaction and fresh water leaching, Ca/Si ratio of C–S–H gel decreased obviously. (3) The rate of mass loss and the porosity of cement–fly ash pastes were higher than those of cement–slag pastes after long-term fresh water leaching. The content of 4.5–50 nm
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Effect of fresh water leaching on the microstructure of hardened composite binder pastes Fanghui Han a,b, Rengguang Liu b, Peiyu Yan b,⇑ a b
China University of Mining and Technology (Beijing), Beijing, China Key Laboratory of Civil Engineering Safety and Durability of China Education Ministry, Department of Civil Engineering, Tsinghua University, Beijing, China
h i g h l i g h t s Microstructure deteriorated with different degree after leaching for 2–3 years. There were no signs of decomposition of hydration products after long-term leaching. The pastes containing a small amount of fly ash were not damaged severely. Addition of 65% of fly ash deteriorated most severely, Ca(OH)2 was almost exhausted. The pastes containing no more than 70% of slag showed high leaching resistance.
a r t i c l e
i n f o
Article history: Received 1 April 2014 Received in revised form 7 June 2014 Accepted 3 July 2014
Keywords: Cement Slag Fly ash Composite binder pastes Fresh water leaching Porosity Calcium hydroxide Microstructure
a b s t r a c t The microstructure of cement–slag and cement–fly ash pastes leached by fresh water for 2–3 years was investigated by MIP, XRD, TG and SEM. Results indicated that long-term fresh water leaching caused degradation of the pastes in terms of mass loss, increase in porosity and decrease in Ca(OH)2 content and Ca/Si ratio of C–S–H gel, but showed no decomposition of hydration products. The pastes containing a small amount of fly ash were not damaged severely, incorporation of 65% of fly ash deteriorated most severely, whereas the cement–slag pastes (670%) showed high long-term fresh water leaching resistance. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Cementitious materials are widely used in modern civil and hydraulic structures, such as water conveyance tunnel, dam panel and underwater constructions [1]. Pure or deionized water is one of the strong decalcifying agents for cementitious materials [2]. Leaching by fresh water is one of the most significant factors that affect the durability of Portland cement concrete [3,4]. During leaching process, ions in pore solution of cementitious materials, like Calcium ion (Ca2+) and hydroxide (OH), migrate into surrounding environment due to concentration gradients between pore solution and environmental water. This process reduces the alkalinity of pore solution and might cause decomposition of ⇑ Corresponding author. Tel.: +86 13501215836; fax: +86 01062785836. E-mail address: [email protected] (P. Yan). http://dx.doi.org/10.1016/j.conbuildmat.2014.07.019 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
hydration products [5]. Furthermore, it is reported to result in an increased porosity, increased permeability consequently and reduced mechanical properties of concrete [2,6,7]. As partial replacement of Portland cement, mineral admixtures are widely applied in concrete. Owe to low cement content and pozzolanic reaction of mineral admixture, the amount of Ca(OH)2 in composite binder pastes reduced. Meanwhile, Ca(OH)2 will be leached out under high-head flow of fresh water in a long term. Thus, the microstructure of hardened binder pastes could be destabilized. Therefore, it is important to understand the change of microstructure and physical properties of composite binder pastes after long-term fresh water leaching. Many researchers have reported leaching performance of cementitious materials. Cardeet et al. [8] investigated the influence of leaching of Ca(OH)2 and C–S–H gel on the mechanical properties of cement paste. They reported that leaching of Ca(OH)2 reduced
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the strength and increased the porosity of cement paste. The variation of Ca/Si ratio of C–S–H gel had little influence on the strength of pure cement paste. But the strength of the paste containing silica fume leached by fresh water reduced by 30% due to the decrease in Ca/Si ratio of C–S–H gel. Rozière et al. [9] studied the leaching performance of the concrete with different water to binder ratio and with cementitious materials of different chemical compositions. They proposed that the indicators for evaluation of leaching performance were the amount of calcium ion and the depth of leaching, etc. The amount of Ca(OH)2 which could be easily dissolved away under fresh water leaching decreased in composite binder pastes, and thus the extent of leaching damage to hardened concrete reduced. Jain et al. [2] reported the effect of fresh water leaching on the performance of hardened paste blended with fly ash and found that incorporating 10% of fly ash could reduce 20% of leaching depth and that the fly ash modified pastes showed reduced loss of Ca(OH)2 compared to the pure cement paste. The incorporation of a small amount of fly ash in binder pastes was beneficial to the improvement of leaching resistance to fresh water. Due to the complexity of experiment and long experimental period, most researches tend to predict the leaching behavior of hardened binder pastes using mathematical models based on factors such as the amount of calcium ion [2,6], the leaching depth [10–12], the mass loss and strength [8], the porosity [13,14], the diffusion coefficient [15] and other kinetic leaching parameters. However, little information regarding microstructural change of hardened composite binder pastes before and after leaching can be found in the literature. Therefore, in the paper, through measuring the mass loss, pore structure, hydration products, variation of Ca(OH)2 content, Ca/Si ratio of C–S–H gel and observing morphology, the microstructure variation of the hardened composite binder pastes blended with 2 mineral admixtures, fly ash or slag, leached by fresh water was investigated. 2. Experiments 2.1. Materials and specimen preparation
Fig. 1. Particle size distributions of cement and mineral admixtures.
prepared with pure cement is 95% when the fluidity of the former mortar is 130– 140 mm. The binder to ISO standard sand ratio for all mortars is 1/3; the water to binder ratio for mortar prepared with pure cement is 0.5. The mix proportions of composite binder pastes are shown in Table 2 and the water to binder ratio (W/B) for all samples is 0.5. 2.2. Test methods The composite binder pastes were prepared according to the mix proportions shown in Table 2. They were cast into polyvinyl chloride (PVC) tubes of 45 mm diameter and 15 mm thickness and cured in an environmental chamber of 90% RH, 20 ± 1 °C. After curing for 90 days, the specimens were moved in a 25 L plastic box filled with deionized water, which was stirred constantly with an electric stirrer to simulate the process of fresh water leaching. The stirring rate was about 50 rpm. Fig. 2 shows a typical sample and the leaching facility. Deionized water was periodically renewed every day in the first week and every week in the following weeks, in order to ensure the specimens placed in a clean fresh water environment. The surface layers (about 5 mm) of the specimens were cut and spit into small pieces and put into acetone to cease further hydration at the prescribed age. 2.3. Mass loss
P.I 42.5 Portland cement, Class I fly ash and S95 ground granulated blast furnace slag conforming to Chinese National Standards (GB175-2007, GB/T1596-2005 and GB/T18046-2008, respectively) were used. P.I 42.5 Portland cement is pure Portland cement whose strength is no less than 42.5 MPa curing under 95% RH, 20 ± 2 °C for 28 days. The classification of fly ash is according to fineness, water requirement ratio and loss on ignition (LOI). The screen residue with 45 lm square hole sieve, the water requirement ratio and LOI of Class I fly ash are no more than 12%, 95% and 5%, respectively. The classification of slag is mainly according to activity index. The ratio of strength of mortar prepared with the binder containing 50% of cement and 50% of slag to that of mortar prepared with pure cement curing under 95% RH, 20 ± 2 °C for 28 days is 95% for S95 ground granulated blast furnace slag. The water to binder ratio and binder to ISO standard sand ratio of mortar is 0.5 and 1/3, respectively. The activity index of fly ash is determined using above method as well, but with a binder containing 30% of fly ash. The chemical compositions of cement and mineral admixtures are given in Table 1. The specific surface areas of the cement and the slag are 350 m2/kg and 442 m2/ kg, respectively. The particle size distributions of cement and mineral admixtures are shown in Fig. 1. The activity index of fly ash and slag is 75% and 95%, respectively. The water requirement ratio of fly ash is 95%, which is determined according to Chinese National Standard GB/T1596-2005. The ratio of water requirement of mortar prepared with the binder containing 70% of cement and 30% of fly ash to that of mortar
The cumulative mass loss rate of the hardened composite binder pastes after leaching (u) can be calculated as follows:
u¼
mn m1 100% m1
ð1Þ
where m1 and mn are the weights of specimen before and after leaching, respectively. Specimens were cured under 90% RH, 20 ± 1 °C for 90 days and then vacuum dried for 4 h, the weight of specimen before leaching was determined (m1). Specimens were removed from the plastic box when they were leached by fresh water to the prescribed age and then the surface water was cleaned, after which they were vacuum dried for 4 h. The weight of specimen after leaching was recorded (mn). Ten specimens were used to obtain the mass loss rate result for each sample. The result of one of specimens would be eliminated if it deviated 15% from the average value of ten specimens’ results, the result of mass loss rate was the average value of the rest of specimens. 2.4. Mercury intrusion porosimetry Porosity and pore size distribution of the hardened composite binder pastes in different leaching duration were measured with AUTOPORE II 9220 mercury intrusion porosimeter with a maximum mercury intrusion pressure of 300 MPa.
Table 1 Chemical compositions of cement and mineral admixtures w (%). Composition
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Na2Oeq
f-CaO
Cl
LOI
Cement Fly ash Slag
20.55 57.60 34.55
4.59 21.90 14.36
3.27 2.70 0.45
62.50 3.87 33.94
2.61 1.68 11.16
2.93 0.41 1.95
0.53 1.05 0.63
0.83 – –
0.010 – –
2.08 7.65 0.70
Na2Oeq = Na2O + 0.658 K2O; w: Mass fraction.
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Table 2 Mix proportions of composite binder pastes. Sample
W/B
Cem FA20 FA40 FA65 SL30 SL70
0.5 0.5 0.5 0.5 0.5 0.5
Mix proportion (by mass) (%) Cement
Fly ash
Slag
100 80 60 35 70 30
0 20 40 65 0 0
0 0 0 0 30 70
2.5. X-ray diffraction The identification of the hydration products of the hardened composite binder pastes in different leaching duration was achieved with a TTR IIIX-ray diffractometer (Cu Ka, 45 kV, 200 mA) over a scanning range of 5–60° in 2h scale. The testing rate applied was 6°/min. Fig. 3. Cumulative mass loss rates of hardened pastes in different leaching duration.
2.6. Thermogravimetry The Ca(OH)2 content of the hardened composite binder pastes in different leaching duration was determined by a TA-Q5000 instrument. Thermogravimetry was carried out from 20 ± 5 °C up to 900 °C, at 10 °C/min heating rate. A nitrogen environment was set up to prevent carbonation of Ca(OH)2. 2.7. Scanning electron microscopy The morphology of hardened binder pastes in different leaching duration was investigated with a FEI Quanta 200 FEG scanning electron microscope. Scanning electron microscopy analysis was carried out on a freshly fractured surface of samples and the Ca/Si ratio of C–S–H gel was analyzed with energy dispersive spectrometer. The sample was coated with carbon. The measuring range of energy dispersive spectrometer is about 1 lm3 and multi-phases can be included. In order to reduce the error, typical C–S–H gel was judged and analyzed, 30 test areas were measured for each sample.
3. Results and discussion 3.1. Mass loss The rates of cumulative mass loss of hardened composite binder pastes in different leaching duration are shown in Fig. 3. It can be seen from Fig. 3 that the hardened composite binder pastes containing fly ash and slag show different mass change at early leaching stage. The mass of pure cement paste and cement–slag pastes increased at early leaching age. With the increased dosage of slag, the rate of mass gain reduced. However, the cement–fly ash pastes showed mass loss. The mass loss rate was greater when the proportion of replacement of cement with fly ash was higher. The results were concerned with the different
reaction mechanism of cement clinker, slag and fly ash. The reaction degree of cement clinker and slag was high compared with fly ash. They produced plenty of hydration products to fill pores, which could make the pastes dense. Half the number of the cement was still unhydrated in pure cement paste after hydrating for 90 days [16]. During fresh water leaching, numerous unhydrated cement particles and mineral admixture particles in hardened pastes continued to hydrate and generated more hydration products, which made the mass of pastes increase. The mass loss of pure cement paste and cement–slag pastes was small in shortterm leaching due to their dense paste structure. The mass increased at the early stage of leaching owe to the less mass loss than the mass gain. The activity of slag was lower than that of cement clinker in cement–slag pastes at early curing age, the effective water to cement ratio of cement clinker in composite binder pastes increased due to the dilution effect of slag. So the hydration degree of cement in the cement–slag pastes was higher than that of pure cement paste, hence less unhydrated cement particles existed before leaching. Thus, with increased dosage of slag, the mass gain of the cement–slag pastes slowed down after short-term leaching. The reaction degree of fly ash was low and few hydration products were produced in cement–fly ash pastes, whose structure was looser than pure cement paste. More dissolvable hydration products could be leached after short-term leaching and its mass loss was higher than the mass increase. So cement–fly ash pastes demonstrated mass loss at early stage of leaching duration.
Fig. 2. The typical sample and leaching facility (a) typical sample and (b) leaching facility.
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Pure cement paste and cement–slag pastes also showed mass loss after long-term leaching. With the extension of leaching age, the mass loss increased gradually due to less unhydrated particles existing in the hardened pastes, so the mass loss was more than the mass increase. And as the slag content increased, the mass loss rate was lower. This was due to their dense structure and the decreased amount of dissolvable Ca(OH)2. However, the mass loss of cement–fly ash pastes continued increasing with the extension of leaching duration, much more than that of cement–slag pastes. Especially the samples containing high content of fly ash, such as the sample FA65, its mass loss rate was very high. 3.2. Pore structure The porosity variation of hardened composite binder pastes in different leaching duration is shown in Fig. 4. It is evident that the porosity of samples containing fly ash increased with the extension of leaching duration, but the porosity of pure cement paste and cement–slag pastes decreased at early leaching stage and increased slightly after long-term leaching duration. The change rule of the porosity was consistent with that of the mass loss. As shown in Fig. 4, the porosity of pastes containing fly ash was higher than that of pastes containing slag before leaching, indicating that the hardened pastes containing slag were dense. With the leaching prolonged, the dissolvable hydration products could be easily leached from cement–fly ash pastes for their high porosity. Thus, the porosity was gradually increasing. Especially for the sample FA65, the leaching degree (severity) was much more serious and the porosity increased considerably. For pure cement paste and cement–slag pastes, short-term leaching did not cause hydration products to be leached because of their low porosity and dense paste structure. More hydration products generated further filling pores to make decrease of porosity. As the leaching process continued, some hydration products were leached and newly generated hydration products were less and less, so the porosity showed an increasing trend. But the porosity of cement–slag pastes remained stable and showed no trend of a great increase after long-term leaching compared with that before leaching. The density of hardened paste is an important factor in the leaching resistance of concrete. The paste with low porosity and poor pore connectivity made it difficult for water to infiltrate and to dissolve soluble hydration products, so there was only a slight leaching phenomenon in the surface. According to the porosity
Fig. 4. Porosity variation of hardened pastes in different leaching duration.
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variation of the pastes before and after leaching, cement–slag pastes had lower porosity and showed better performance of the leaching resistance to fresh water compared with cement–fly ash pastes. Fig. 5 shows the pore distribution of the hardened composite binder pastes before and after leaching for 2 years. It can be seen that for cement–fly ash pastes, the content of 4.5–50 nm micropores increased a lot after leaching for 2 years, which is indicated that the pastes suffered some degree of leaching and a number of hydration products were eroded off after 2 years leaching. The content of <4.5 nm micropores could not increase in the pastes due to the disconnected micropores where fresh water could not migrate into. The content of 4.5–50 nm micropores accounted for large proportion in the hardened pastes before leaching, what is more, 4.5–50 nm micropores had large specific surface area compared with 50–100 nm and >100 nm pores. Thus, hydration products leached by fresh water had a great influence on the content of 4.5–50 nm micropores. While for cement–slag pastes, the content of 4.5–50 nm micropores did not increase significantly, indicating that the dissolution of hydration products was not serious, which showed high leaching resistance to fresh water. For each sample after long-term leaching, the contents of 50– 100 nm and >100 nm pores did not change significantly. The amount of large pores even became small for samples containing mineral admixture. This phenomenon contributed to continued secondary hydration during fresh water leaching, the obtained result was consistent with literature [2], the increased amount of hydration products filled the large pores, which made the structure dense, thus there was little influence on the strength and permeability of the hardened composite pastes. 3.3. Hydration products The X-ray diffraction spectrums of hardened pastes before and after leaching for 2 years are shown in Figs. 6 and 7, respectively. It is evident that there was no significant change of the crystalline products. The diffraction peaks of unhydrated clinker were weak after long-term leaching, indicating that a little unhydrated clinker existed in the pastes. The diffraction peak of Ca(OH)2 decreased after leaching compared with before leaching, but the Ca(OH)2 diffraction peak of each sample was still strong except sample FA65. For sample FA65, much Ca(OH)2 was consumed for pozzolanic reaction of fly ash and a lot of Ca(OH)2 was dissolved away due to high porosity. In contrast, for sample SL70, the diffraction peak
Fig. 5. Pore distribution of hardened pastes before leaching and after leaching for 2 years.
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Fig. 8. Ca(OH)2 content in hardened pastes in different leaching duration. Fig. 6. XRD spectrum of hardened pastes before leaching.
cement–slag pastes decreased with the increasing leaching age, but the rate of decrease was gentle. The porosity of the pastes containing slag was lower and the pastes were denser than that of the pastes containing fly ash. The refined pores resulted in a difficult leaching of Ca(OH)2. The reaction of slag was not entirely pozzolanic reaction, so only a little Ca(OH)2 was consumed. Thus, the content of Ca(OH)2 was reduced slightly. For sample SL70, the Ca(OH)2 content was still above 7% after leaching for 2 years, which could ensure the high alkalinity of pore solution and stability of C–S–H gel as well as other hydration products. 3.5. Morphology of hardened paste
Fig. 7. XRD spectrum of hardened pastes after leaching for 2 years.
was strong and it is shown that a certain amount of Ca(OH)2 still existed even after 2 years leaching. It is proved that composite binder pastes containing slag were dense, there was no serious leaching of Ca(OH)2.
3.4. Ca(OH)2 content The content of Ca(OH)2 in hardened composite binder pastes in different leaching duration is shown in Fig. 8. There was no reduction in Ca(OH)2 content of pure cement paste but a small increase at the early stage of leaching duration. With the prolonged leaching age, the Ca(OH)2 content had a decreasing trend consistent with the change rule of mass and porosity. For composite binder pastes containing mineral admixture, cement fraction decreased and thus the content of Ca(OH)2 reduced. After long-term leaching, the content of Ca(OH)2 decreased remarkably in cement–fly ash pastes. For sample FA65, the Ca(OH)2 content was only 0.46% and became almost nil after leaching for 3 years. For samples FA20 and FA40 containing relatively low fraction of fly ash, the decreasing trend of Ca(OH)2 became gentle, and it can be expected that leaching will not make Ca(OH)2 consumed seriously or exhausted in a long leaching duration. So a high alkalinity of pore solution in pastes can be maintained and it is impossible that C–S–H gel will suffer from leaching and decomposition. The content of Ca(OH)2 in
Fig. 9 shows morphologies of hardened composite binder pastes before leaching. It can be seen that fly ash was not sculptured on the surface. It was not bonded with the pastes closely and some spherical fly ash particles came off when crushing samples. It is indicated that the reaction degree of fly ash was low after curing for 90 days. The structure of composite binder pastes containing slag was dense and slag particles bonded tightly with the surrounding pastes, indicating a certain reaction degree of slag. Morphologies of hardened composite binder pastes after leaching for 2 years are shown in Fig. 10. After long-term leaching, it can be seen that there were still a lot of sheet Ca(OH)2 crystals in sample Cem as well as the samples FA20 and SL30 containing low content of mineral admixture. For samples FA20 and FA40, fly ash particle was fractured when sampling, indicating that fly ash bonded with the surrounding pastes tightly. The surface of spherical fly ash particles were etched significantly in sample FA40 due to the pozzolanic activity of fly ash. There were many fly ash spheres in sample FA65 and they were bonded mostly closely. Although Ca(OH)2 content was very low, the gel was not damaged resulting in a loose decomposition products. For sample SL70, the slag particles were bonded tightly with the surrounding pastes and it can be judged from gray degree that their reaction degree was high. Thus, it can be said that the structure of hardened composite binder pastes containing mineral admixture was still very dense, there was not loose decomposition products and the paste structure remained stable after long-term fresh water leaching. The Ca/Si ratio of C–S–H gel in hardened pastes in different leaching duration measured by energy dispersive spectrometer is shown in Fig. 11. Ca/Si ratio of C–S–H gel decreased with the increased dosage of mineral admixture in composite binder pastes. It can be seen from the chemical composition of cement and mineral admixture (Table 1) that CaO content of slag is lower than that
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(a) Cem
(b) FA20
(c) FA40
(d) FA65
(e) SL30
(f) SL70
Fig. 9. Morphologies of hardened composite binder pastes before leaching (a) Cem, (b) FA20, (c) FA40, (d) FA65, (e) SL30, and (f) SL70.
(a) Cem
(d) FA65
(b) FA20
(c) FA40
(e) SL30
(f) SL70
Fig. 10. Morphologies of the hardened pastes after leaching (a–d) for 3 years; (e) and (f) for 2 years (a) Cem, (b) FA20, (c) FA40, (d) FA65, (e) SL30, and (f) SL70.
of cement, which is the lowest for fly ash, so Ca/Si ratio of C–S–H gel produced by reaction of mineral admixture was lower than that produced by cement reaction. With the extent of leaching duration, Ca/Si ratio of C–S–H gel in pure cement paste decreased slightly and remained stable. It is proved that the degree of leaching was low and there were not a lot of calcium ions separated out. However, the Ca/Si ratio of C–S–H gel in composite binder pastes decreased; there was an obvious downward trend for composite binder containing 40%, 65% of fly ash and 70% of slag. The reaction rate of mineral admixture was slow and its reaction degree was
low at early age, but the reaction degree increased gradually at late age. In long-term duration, some mineral admixture particles continued to react, which produced more C–S–H gel with low Ca/Si ratio, which made a decrease in the Ca/Si ratio of the composite binder pastes. If decomposition of C–S–H gel in composite binder pastes occurred after 2–3 years’ leaching, Calcium ions would be leach out from C–S–H gel and the remaining substance was silicon dioxide hydrate, which had no characteristics of gel and could be easily dissolved away. Thus, the mass loss and porosity would increase
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micropores in the cement–fly ash pastes increased due to hydration products leached by fresh water. The continued secondary hydration during fresh water leaching could lead to decrease in large pores content. (4) Composite binder pastes blended with slag had denser paste microstructure and lower porosity. The pastes containing no more than 70% of slag had high long-term fresh water leaching resistance and Ca(OH)2 content was plentiful in the pastes after long-term leaching.
Acknowledgment Authors would like to acknowledge the National Natural Science Foundation of China (Nos. U1134008 and 51278277). References Fig. 11. Variation of Ca/Si ratio of C–S–H gel in hardened pastes in different leaching duration.
significantly, paste morphologies would be loose, Ca(OH)2 content and Ca/Si ratio of C–S–H gel would decrease obviously. But the results (Figs. 3–10) were not consistence with above mentioned. Then it could be concluded herein that composite binder pastes did not suffer from serious corrosion after long-term leaching and the decrease of Ca/Si ratio was not mainly due to the fresh water leaching. For pure cement paste, composite binder pastes blended with a small amount of fly ash or the pastes containing no more than 70% of slag, the trend of reduction in Ca/Si ratio of C–S–H gel was not evident. But it is noted that Ca/Si ratio of C–S–H gel in the paste containing 65% of fly ash decreased obviously after leaching for 2–3 years, which could be the starting point for the decomposition of C–S–H gel. 4. Conclusions (1) After long-term leaching by fresh water, the pastes were degraded in terms of mass loss, increase in porosity, decrease in Ca(OH)2 content and Ca/Si ratio of C–S–H gel, but showed no decomposition of hydration products. (2) Composite binder pastes containing a small amount of fly ash were not damaged severely. Incorporation of 65% of fly ash deteriorated severely, the rate of mass loss and porosity increased significantly after leaching for 2–3 years, Ca(OH)2 was almost exhausted due to the pozzolanic reaction and fresh water leaching, Ca/Si ratio of C–S–H gel decreased obviously. (3) The rate of mass loss and the porosity of cement–fly ash pastes were higher than those of cement–slag pastes after long-term fresh water leaching. The content of 4.5–50 nm
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