This experimental study investigates the effects of temperature and curing duration on the stability of slag blended cement systems exposed at 20 °C and 38 °C to combined sodium chloride (30 g/L) – sodium sulphate (3 g/L) solutions. Two slags, designated as slag 1 and 2, having CaO/SiO2 ratios of 1.05 and 0.94, were respectively blended with Portland cement CEM I 52.5R at 30 wt.% replacement level. Mortar prisms and cubes with w/b ratio of 0.5 and binder/aggregate ratio of 1:3 were then prepared for length and mass changes. The samples were cured in lime water for either 7 or 28 days before ponding for a total exposure period of 544 days. Analogous paste samples were also prepared to follow changes in the hydration products using x-ray diffraction (XRD). The results showed that curing at 38°C resulted in less expansion and prolonged curing generally reduced expansion except for slag 1 blend at 20 °C. Also, mass-change was minimal at 38 °C compared to 20 °C, and curing up to 28 days further improved mass stability. There was a positive correlation between mass change and length change for the period of investigation

Basheer, P. A. M.

Black, Leon

Ukpata, Joseph Onah

English

Purdue E-Pubs

105 Sixth International Conference on Durability of Concrete Structures    Paper Number ICC01 18 - 20 July 2018 University of Leeds, Leeds, West Yorkshire, LS2 9JT, United Kingdom  Effects of Temperature and Curing Duration on the Stability of Slag Cements in Combined Chloride-Sulphate Environments   J.O. Ukpata, P.A.M. Basheer and L. Black School of Civil Engineering, University of Leeds, United Kingdom   ABSTRACT   This experimental study investigates the effects of temperature and curing duration on the stability of slag blended cement systems exposed at 20 °C and 38 °C to combined sodium chloride (30 g/L) – sodium sulphate (3 g/L) solutions. Two slags, designated as slag 1 and 2, having CaO/SiO2 ratios of 1.05 and 0.94, were respectively blended with Portland cement CEM I 52.5R at 30 wt.% replacement level. Mortar prisms and cubes with w/b ratio of 0.5 and binder/aggregate ratio of 1:3 were then prepared for length and mass changes. The samples were cured in lime water for either 7 or 28 days before ponding for a total exposure period of 544 days. Analogous paste samples were also prepared to follow changes in the hydration products using x-ray diffraction (XRD). The results showed that curing at 38°C resulted in less expansion and prolonged curing generally reduced expansion except for slag 1 blend at 20 °C. Also, mass-change was minimal at 38 °C compared to 20 °C, and curing up to 28 days further improved mass stability. There was a positive correlation between mass change and length change for the period of investigation.  Keywords: Cement, slag, sulphate, chloride, curing, temperature.   1.0  INTRODUCTION   Excessive volumetric changes in concrete structural elements, whether through shrinkage or expansion, can lead to cracking and spalling of the concrete, and so can pose serious threats to the durability of concrete structures. Slag cements have been associated with early age shrinkage (Darquennes et al., 2008). Hence, there is a need to investigate their stability in service environments. Apart from the cement’s autogenous tendency to shrink or expand in volume during hydration, exposures to certain environments such as sea water can add to the problems of excessive volumetric changes. Sea water contains chlorides and sulphates amongst others. Chlorides are associated with the corrosion of embedded steel reinforcement (Galan and Glasser, 2015), while exposure of cement materials to sulphate solutions has been known to cause expansion and mass loss in the hydrated cements. The sulphate ions react with calcium hydroxide and the hydrated aluminate phase of cement to produce gypsum and ettringite which are associated with expansion  (Whittaker and Black, 2015; Yu et al., 2013). Amongst other causes, the composition of the binder in relation to C3A content, and curing conditions are factors affecting sulphate attack of cements (El-Hachem et al., 2012). Standards give limiting values of sulphate concentrations for severe exposure conditions ranging from 3000 to 6000 mg/l (BS EN 206, 2013). However, expansive effects of sulphate attack in plain and slag blended cements have been observed at concentrations as low as 3000mg/l, associated with sea water (El-Hachem et al., 2012; Whittaker et al., 2016). Yet, it is still not clear how the changes occur when there is a simultaneous presence of chlorides and sulphates. Previous studies used high concentrations which do not reflect actual service conditions. Conflicting views had been expressed previously about the influence of chlorides on sulphate attack (Al-Amoudi et al., 1995; Sotiriadis et al., 2013). According to some, the presence of chlorides reduced deterioration of concrete due to sulphate attack (Santhanam et al., 2006; Zuquan et al., 2007). This view seems more acceptable than the contrary, although the effects of changes in environmental conditions and pre-conditioning of slag-blended systems need to be explored further along with their hydration characteristics.  This study investigates the influence of temperature and curing duration on length and mass changes of slag blended cement mortars in relation to their previously published water absorption and hydration characteristics (Ukpata et al., 2018)  2.0  MATERIALS AND METHODS  This study has used two slags (1 & 2), with CaO/SiO2 ratios of 1.05 and 0.94 respectively. These were each blended with CEM I 52.5R at 30 wt.% replacement. Blending was carried out for 2 hours using a roller ball mill incorporating plastic charges. The particle size distributions of the 2 slags are shown in Fig. 1, while the binders’ physical properties and chemical compositions are presented in Tables 1 and 2 respectively. ICDCS 2018: ICC01 106   Fig. 1. Particles size distributions of slags 1 and 2.  Table 1. Physical properties of binders  Property  Unit  CEM I  52.5R  Slag 1  (S1)  Slag 2  (S2)  Blaine  cm2/g  7357  5995  5540  Density  g/cm3  3.16  2.93  2.91  D10 µm 2.94 2.27 2.87 D50 µm 9.43 11.56 12.91  Table 2. Chemical compositions of binders  Property  Unit  CEM I 52.5R  Slag 1  (S1)  Slag 2  (S2)  SiO2  %  20.50 36.58  40.14  Al2O3  %  5.43 12.23  7.77  TiO2  %  0.29 0.83  0.30  MnO  %  0.05 0.64  0.64  Fe2O3  %  2.51 0.48  0.78  CaO  %  63.43 38.24  37.90  MgO  %  1.51 8.55  9.51  K2O  %  0.79 0.65  0.55  Na2O  %  0.17 0.27  0.36  SO3  %  3.43 1.00  1.47  P2O5  %  0.14 0.06  0.02  LOI 950°C  %  1.37 1.66 0.40 Total  %  99.62 99.88  99.43  Glass content  %  na  99.3  97.1   2.1  Sample Preparation, Curing and Exposure  Mortar samples (prisms and cubes) were cast in steel moulds with w/b ratio of 0.5, and aggregate to binder ratio of 3.0. The samples were demoulded after 24 hours before curing. Meanwhile, paste samples were cast in 14 mm ɸ x 50 mm plastic vials with w/b of 0.5. The fresh samples were sealed and rotated in a sample rotator at 20 rpm for 24 hours to prevent bleeding. At specified test ages, hydration was stopped using isopropanol solvent replacement method. Samples could then be stored for subsequent analysis.  The curing and exposure conditions are detailed in Table 3. A salt solution composed of 30 g/l sodium chloride and 3 g/l sodium sulphate was prepared from 99% analytical reagents. Mortars samples were exposed to the solution at either 20°C or 38°C, after initial curing for either 7 or 28 days in saturated lime water to prevent leaching of calcium hydroxide. Pastes samples were kept sealed in water baths and demoulded only for either exposure to the salt solution or for hydration stopping. Parallel reference mortar samples were left in limewater throughout the test period (X1).  Table 3. Exposure conditions  Code Initial curing Exposure condition Exposure temperature X1 7 or 28 days in lime water Ponding in lime water 20°C X2 7 or 28 days in lime water Ponding in combined chloride-sulphate solution 20°C or 38°C  2.2  Test Methods  Length changes were measured in triplicate using  25 x 25 x 200 mm mortar prisms, in line with ASTM C490 (2011). Readings were taken from each sample face and the average of 12 measurements from the 3 samples were used as in Equation 1.  𝛿௅ =  (௅ೣି ௅೔)௅೔ 𝑥 100%                        (1)   Where: δL = length change (%), Lx = Measured length at a given age, Li = initial baseline length for the same sample.  Mass changes were measured in triplicate on 50 mm3 mortar samples to 0.01 g sensitivity. Mass change was calculated according to Equation 2.  𝛿ெ =  (ெೣି ெ೔)ெ೔ 𝑥 100%                       (2)    Where: δM = mass change (%), Mx = Measured mass at a given age, Mi = initial baseline mass for the same sample.  XRD analysis was performed using a Bruker D2 phaser diffractometer on finely ground powder samples passing 63 µm, obtained from hydration stopped samples. Scanning was performed from 5°-70° 2θ with a 0.034° step size and a 2s dwell time.   3.0  RESULTS AND DISCUSSION  3.1  Length Change  The length change results are presented in Figs. 2 to 5.  Figure 2a shows length change for the more reactive, slag 1 blend exposed to the salt solution following 7 days of curing, while Fig. 2b shows the Ukapa et al. 107 results for samples cured for 28 days before exposure. The samples cured for 7 days before exposure showed slight shrinkage, but this disappeared by 28 days. Generally, the reference samples (X1) remained stable, showing minimal length change throughout the test period. Exposure to the salt solution at 38°C led to slight expansion beyond 6 months. However, samples exposed at 20°C showed significant expansion from about 15 weeks’ exposure. The samples cured for 28 days before exposure behaved differently. The samples cured at 38°C suffered slight initial shrinkage prior to exposure, but remained stable throughout the test period. Meanwhile, the samples cured and exposed at 20°C from about 15 weeks onwards, with expansion ultimately being greater than for the samples cured and exposed at lower temperature.    Fig. 2a. Length change for slag 1 blends cured at 7 days prior to exposure    Fig. 2b. Length change for slag 1 blends cured at 28 days prior to exposure  Figures 3 (a & b) show the length change for the less reactive, slag 2 blends. The reference samples showed no expansion, as did the samples cured and exposed at 38°C. However samples cured and exposed at 20°C showed significant length change from about 16 weeks onwards. In this case, expansion was greater for samples cured for 7 days than 28 days.   The effects of curing duration at 20 and 38°C are exemplified in Figs. 4 and 5. In almost all cases, the importance of prolonged curing was evident in the 2 slag blends and at both temperatures. More noticeable was the impact of curing temperature, where increased temperatures greatly improved performance. The length changes at 38°C agreed well with water sorptivity data for the same samples, where sorptivity reduced with increased temperature (Table 4). No previous data on the effect of temperature on the expansion of slag cements could be found in the literature. However, the results of expansion at 20°C are consistent with earlier research (Maes and De Belie, 2014).   Table 4. 90d sorptivity (g/mm2/min0.5) x10-3  Mix Exposure 20°C Std Dev. 38°C Std Dev. C2S1 90d (X1) 0.024 0.0017 0.022 0.0034   90d (X2) 0.044 0.0012 0.048 0.0075             C2S2 90 (X1) 0.037 0.0020 0.028 0.0025   90 (X2) 0.060 0.0026 0.051 0.0108    Fig. 3a. Length change for slag 2 blends cured at 7 days prior to exposure    Fig. 3b. Length change for slag 2 blends cured at 28 days prior to exposure  3.2  Mass Change  Mass change results are presented from Figs. 6 to 9. Figs. 6 (a & b) show respectively the mass changes for the slag 1 blend initially cured for 7 and 28 days before exposure, while Figs. 7 (a & b) show similar results for the slag 2 blends. For both systems, the reference samples (X1) show only slight increases in mass, while the samples exposed to the solution ICDCS 2018: ICC01 108 showed mass increases. When cured at 20°C, the increases were considerable from about 24 weeks’ exposure. However, the samples exposed at 38°C showed much more modest increases. No spalling of surface material was observed, which possibly explains why no mass losses were yet observed.    Fig. 4. Effect of curing duration at 20°C on length change    Fig. 5. Effect of curing duration at 38°C on length change  Figures 8 and 9 show the effects of curing duration on mass change at 20 and 38°C respectively. At both temperatures, prolonged curing before exposure to the salt solution led to lower mass increase compared to exposure following curing for 7 days.  Another important difference was that, despite showing a lower sorptivity (Table 4), the more reactive slag (slag 1) showed a greater mass increase than the less reactive slag 2. This is possibly due to the higher aluminium content of the slag, increasing the potential for AFm, AFt and Friedel’s salt formation.  3.3  Relationship between Length and Mass Changes  Figure 10 shows the relationship between mass change and length change for the slag 1 blended mortars. Irrespective of curing duration or temperature, a positive relation was established between mass change and length change. The positive correlation  was  stronger  for  samples    Fig. 6a. Mass change for slag 1 blends cured at 7 days prior to exposure    Fig. 6b. Mass change for slag 1 blends cured at 28 days prior to exposure    Fig. 7a. Mass change for slag 2 blends cured at 7 days prior to exposure    Fig. 7b. Mass change for slag 2 blends cured at 28 days prior to exposure Ukapa et al. 109 exposed at 20 °C than 38 °C, with r2 values greater than 0.95. Similar mass gain and length increase in 50 wt.% slag blended mortar samples exposed to 50 g/L Na2SO4 + 50 g/L NaCl solution, up to about 600 and 300 days respectively had been reported in a previous study (Maes and De Belie, 2014).    Fig. 8. Effect of curing duration on mass change at 20°C    Fig. 9. Effect of curing duration on mass change at 38°C    Fig. 10. Relationship between mass change and length change for slag 1 systems  3.4  X-Ray Diffraction (XRD) Analysis  XRD data from paste samples exposed to the combined salt solution are presented in Figs. 11 and 12, with Fig. 11 showing the data obtained after exposure for 28 days and Fig. 12 showing that obtained after exposure for 180 days. The 2 periods represent periods before and after significant mass change or expansion. The ettringite content only increased slightly upon exposure, and showed no discernible increase between 28 and 180 days’ immersion in the solution. This suggests that the expansion observed in the mortar samples may not be due to ettringite precipitation usually associated with expansion. Monosulphate was converted to Friedel’s salt and Kuzel’s salt upon exposure to the solution. For samples exposed at 20°C, there was a significant increase between 28 and 180 days. There were no significant changes for those samples exposed at 38 °C, consistent with the mass and length change behaviours. Portlandite peaks decreased with time at both temperatures reflecting increased slag hydration.    Fig. 11. XRD patterns at 28 days (P: portlandite; B: brucite; G: gypsum; E: ettringite; FS: Friedel’s salt; KS: Kuzel’s salt; Ms: monosulphate; Hc: Hemicarboaluminate)    Fig. 12. XRD patterns at 180 days (P: portlandite; B: brucite; G: gypsum; E: ettringite; FS: Friedel’s salt; KS: Kuzel’s salt; Ms: monosulphate; Hc: Hemicarboaluminate)   4.0  CONCLUSIONS   The effects of a change in temperature from 20°C to 38°C, and of the duration of initial curing before exposure to an aggressive, combined chloride-sulphate solution have been investigated in 2 slag 10 15 20 25 30 35 402Theta (o)S1-20S1-38S2-20S2-38WaterSalt solutionS1-20S1-38S2-20S2-38MsEHc FSGEPP GPBFSKSKS10 15 20 25 30 35 40PFSPPS1-20S1-38S2-20S2-38S1-20S1-38S2-20S2-382Theta (o)WaterSalt solutionEMs HcGFSGFSEKSICDCS 2018: ICC01 110 blended systems. In terms of mass and length changes, curing and exposure at higher temperature generally improved the mortars’ resistance to these changes. This may be due a more compact microstructure following improved hydration at higher temperatures, as confirmed in other studies. Except for the length change of slag 1 at 20°C, longer curing, from 7 to 28 days, generally had a positive impact. This is also linked with the slow hydration of slags and the poorly formed microstructure at early age, which allows more ingress of the exposure solution into the mortar matrix. This underscores the need for longer curing periods when mortars and concretes incorporate slags as part of their binders. The need for prolonged curing is more significant at 20°C than 38°C.  Reference samples, which were kept in lime water to prevent leaching of portlandite, did not show any length change but did show a slight increase in mass. This confirms that the observed change in length was due to the ingress of the salt solution. This may be attributed to the formation of Friedel’s salt, Kuzel’s salt and perhaps, additional ettringite due to the reaction of the aluminate phase with portlandite and the ingressing sulphate from the exposure solution. The slight increase in mass of the reference sample is likely due to water imbibition as explained in literature.  Generally, there was positive correlation between mass change and length change within the period of this investigation.  Acknowledgement  The scholarship from Niger Delta Development Commission (NDDC), Nigeria is gratefully acknowledged.   References  Al-Amoudi, O.S.B., Maslehuddin, M., Abdul-Al, Y.A.B., 1995. Role of chloride ions on expansion and strength reduction in plain and blended cements in sulfate environments. Constr. Build. Mater., 9(1): 25-33. ASTM C490, 2011. Annual Book of the ASTM standards, Standard practice for use of apparatus for the determination of length change of hardened cement paste, mortar, and concrete. ASTM International, USA, pp. 331-335. BS EN 206, 2013. Concrete — Specification, performance, production and conformity. CEN, United Kingdom. Darquennes, A., Staquet, S., Belie, N.D., Robeyst, N., Espion, B., 2008. Autogenous shrinkage development and setting monitoring of slag cement concrete, Creep, Shrinkage and Durability Mechanics of Concrete and Concrete Structures, Two Volume Set. Taylor & Francis, pp. 315-319. El-Hachem, R., Rozière, E., Grondin, F., Loukili, A., 2012. Multi-criteria analysis of the mechanism of degradation of Portland cement based mortars exposed to external sulphate attack. Cem. Concr. Res., 42(10): 1327-1335. Galan, I., Glasser, F.P., 2015. Chloride in cement. Adv. Cem. Res., 27(2): 63-97. Haha, M.B., Lothenbach, B., Le Saout, G., Winnefeld, F., 2011. Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag — Part I: Effect of MgO. Cem. Concr. Res., 41(9): 955-963. Maes, M., De Belie, N., 2014. Resistance of concrete and mortar against combined attack of chloride and sodium sulphate. Cem. Concr. Compos., 53: 59-72. Santhanam, M., Cohen, M., Olek, J., 2006. Differentiating seawater and groundwater sulfate attack in Portland cement mortars. cement & Concrete Research, 36(2006): 2132-2137. Sotiriadis, K. et al., 2013. The effect of chlorides on the thaumasite form of sulfate attack of limestone cement concrete containing mineral admixtures at low temperature. Constr. Build. Mater., 43(0): 156-164. Ukpata, J.O., Basheer, P.A.M., Black, L., 2018. Performance of plain and slag-blended cements and mortars exposed to combined chloride–sulfate solution. Adv. Cem. Res., 0(0): 1-16. Whittaker, M., Black, L., 2015. Current knowledge of external sulfate attack. Adv. Cem. Res., 27(9): 532-545. Whittaker, M., Zajac, M., Ben Haha, M., Black, L., 2016. The impact of alumina availability on sulfate resistance of slag composite cements. Constr. Build. Mater., 119: 356-369. Yu, C., Sun, W., Scrivener, K., 2013. Mechanism of expansion of mortars immersed in sodium sulfate solutions. Cem. Concr. Res., 43: 105-111. Zuquan, J., Wei, S., Yunsheng, Z., Jinyang, J., Jianzhong, L., 2007. Interaction between sulfate and chloride solution attack of concretes with and without fly ash. Cem. Concr. Res., 37: 1223-1232.  

Effects of Temperature and Curing Duration on the Stability of Slag Cements in Combined Chloride-Sulphate Environments

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Effects of Temperature and Curing Duration on the Stability of Slag Cements in Combined Chloride-Sulphate Environments

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