Effect of activator mix on the hydration and strength behaviour of alkali-activated slag cements

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1 Advances in Cement Research, 2003, 15, No. 3, July, Effect of activator mix on the hydration and strength behaviour of alkali-activated slag cements A. Fernández-Jiménez and F. Puertas Instituto de Ciencias de la Construcción Eduardo Torroja (CSIC) This paper examines on the setting time and mechanical strength behaviour of slag cement pastes activated with different alkaline activators. For this purpose three alkaline solutions were used: waterglass solution (27% SiO 2, 18 % Na 2 O and 55% H 2 O), NaOH and Na 2 CO 3, maintaining always a constant concentration of Na 2 O (4% by mass of slag). The solutions were prepared with mixes of 0%, 80%/20% and 20%/80%. The activation process was studied at early ages by conduction calorimetric and fourier transform infrared spectroscopy (FTIR). Results show that the initial ph of the alkaline solution plays an important role in the initial slag dissolution. However the factor playing a decisive role in the acceleration or delay of setting times and in the development of mechanical strengths is the nature of the anion present in the solution. SiO 4 2 ions act as an accelerator of the setting time, but CO 3 2 ions delay the setting time. Introduction Several studies have examined the different factors that influence the process of alkaline activation of granulated blast furnace slag 1 9 (the reactivity of the slag, the nature and concentration of the alkaline activator solution, the curing conditions, etc). Previous studies have shown that the nature of the alkaline activator is the most important factor affecting both setting time and mechanical strength behaviour. 5 The solubility of the soluble silica depends on the ph of the alkali solution. For ph values between 3, ph, 11 the solubility is low, but it increases considerably at very acid (ph, 3) and very basic (ph. 11) ph. However, in an acid medium the hydrates formed are not stable, while in a basic medium the slag is soluble and some insoluble and stable hydrates are formed 3. The essential requirements of a substance to be used in slag alkaline activation are: (a) acceleration of the solubility of slag; (b) promotion of the formation Instituto de Ciencias de la Construcción Eduardo Torroja (CSIC) P.O. Box 19002, Madrid, Spain. (ACR 435) Paper received 7 February 2003; accepted 9 May 2003 of stable hydrates; and (c) formation of a network structure of hydrates. There are few chemical compounds that fit these conditions and that can be used in the alkaline activation process of granulated blast furnace slag. 9 Previous studies 4 8 have shown that the initial setting time in alkali slag pastes (AAS) activated with sodium silicate solution (SiO 2 /Na 2 O ¼ 1 5 by mass) begins after 15 min of reaction. Moreover, these pastes present the highest mechanical strength values, 100 MPa at 28 days in mortar samples. Recent studies 9 have shown that an increase in the Na 2 O content in the waterglass solution results in the induction period in the heat evolution rate decreasing, and when the SiO 2 /Na 2 O ratio increases in the activator solution the induction period increases. When the activator is NaOH, the setting time is longer 8 (ffi 2 3 h), the mechanical strengths are lower 5 (ffi MPa compressive values at 28 days) and the induction period 8,10 is shorter. Na 2 CO 3 as activator increases the setting time, in some cases to longer than 3 days, which gives very low initial mechanical strength. Some papers 1,5,11,12 indicate that the inclusion of alkaline activators has a positive effect on the development of mechanical strengths. However, it is not specified how these mixtures affect setting time and the # 2003 Thomas Telford Ltd

2 A. Fernández-Jiménez and F. Puertas alkali activation process. According to these studies the activation of slag with a mixture of waterglass + Na 2 SO 4 or waterglass + Na 2 CO 3 makes materials with a good balance between mechanical properties and production cost. The slag activated with an optimised mixture of Na 2 CO 3 +Na 2 SO 4 + Ca(OH) 2 gives similar strength values to those obtained with NaOH. 3 Collis 12 found that when slag is activated with a mixture of NaOH + Na 2 CO 3, very good strength was obtained at early ages, similar to those obtained from an ordinary Portland cement (OPC). The objective of this paper was to study the setting time, mechanical strength and heat evolution rate in slag cement pastes activated with a mixture of alkaline activators. These mixtures have different types of anions (SiO 4 2,CO 3 2 and OH ). Experimental A Spanish blast furnace slag was used. The chemical composition of this slag was: 40 3% CaO, 34 47% SiO 2 and 11 80% MgO. This slag had a vitreous phase content of 95% and a specific surface area of 460 m 2 / kg. The pastes were prepared with an alkaline solution/ slag ratio of 0 4. Alkaline solutions used were: waterglass solution (27% SiO 2 ;18%Na 2 O and 55% H 2 O), NaOH and Na 2 CO 3. The Na 2 O concentration was constant, equal to 4% Na 2 O by mass to the slag. Table 1 displays the chemical composition of the different alkaline solutions used and their ph values. The setting time of the pastes was determined according to standard UNE EN The mechanical strength of the slag pastes was determined from prismatic specimens of mm at 3, 7 and 28 days. These pastes were also studied by isothermal conduction calorimetry, with an IBM JAF Calorimeter, AWCAL4 Program, Wexham Developments. With this technique the reaction heat and the heat evolution rate were determined as a function of time. Depending on the calorimetric results some AAS pastes were fixed with acetone and ethanol and analysed through FTIR. The specimens were prepared by mixing 1 mg of sample in 300 mg of KBr. The spectral analysis was performed in the range cm 1 with a spectral resolution of 1 cm 1 and the equipment used was a ATIMATTSON apparatus, FTIR- TM series. Results Flexural and compressive strength values at 3, 7 and 28 days for the AAS slag pastes are shown in Table 2. Samples 1, 2 and 3, when the main alkaline activator is Table 1. ph and chemical composition of solution by mass to the slag for alkaline solution to slag ratio ¼ 0 4. N8 AAS Activator ph [Na 2 O]: % [SiO 2 ]: % [CO 3 ] 2- SiO 2 /Na 2 O 1 Waterglass %Wat/20%NaOH %Wat/20%Na 2 CO NaOH %NaOH/20%Wat %NaOH/20%Na 2 CO Na 2 CO %Na 2 CO 3 /20%Wat %Na 2 CO 3 /20%NaOH Table 2. Mechanical strengths of alkali activated slag pastes N8 Activator Flexural strength: MPa Compressive strength: MPa 3 days 7 days 28 days 3 days 7 days 28 days 1 Waterglass %Wat/20%NaOH %Wat/20%Na 2 CO NaOH %NaOH/20%Wat %NaOH/20%Na 2 CO Na 2 CO %Na 2 CO 3 /20%Wat %Na 2 CO 3 /20%NaOH Advances in Cement Research, 2003, 15, No. 3

3 Activator mix and the hydration and strength behaviour of alkali-activated slag cements waterglass, show the highest mechanical strength at all ages, (about MPa compressive strength at 28 days). When the waterglass dose decreases, the mechanical strength decreases. The mechanical strength of pastes 5 (80%N/20%W) and 8 (80%C/20%W) are higher than those of 0% waterglass (samples 4 and 7 respectively). When the main alkaline activator is Na 2 CO 3 (samples 7, 8 and 9) the mechanical strengths at early age were the lowest, which is why in these three cases the test specimens were taken off the mould after 75 h curing. However, these mechanical strengths improved when the reaction time increased. The cause of this phenomenon will be discussed later. In Table 3, the experimental results obtained for initial, t I, and final setting times, t F, are shown, and also the time this process lasted ( t ¼ t F t i ). These results indicate that paste activated with 100% waterglass, 80%W/20%N and 80%W/20%C (samples 1, 2 and 3) have the shortest initial setting times (t I, 1h 30 min), along with as the slag activated with 80%N/ 20%W (sample 5). The slag samples 4 and 6 activated with 100%NaOH and 80%N/20%C undergo a small increase of initial setting time (2 h 45 min and 3 h 22 min, respectively). However the slag paste activated with 100% Na 2 CO 3 solution (sample 7) shows the longest initial setting time (t I, 3 days); in this paste a change of plasticity at 25 h was observed. The heat evolution rate of AAS pastes versus time for the different alkaline activators is shown in Fig. 1. This study was performed in order to determine which areas of the calorimetric curves correspond to initial and final setting times in AAS pastes. The heat evolution curves of AAS pastes can be classified into five periods, the same as those of the Portland cement: 6,8 (a) initial (pre-induction) period; (b) induction period; (c) acceleration (post-induction) period; (d) deceleration period; and (e) diffusion period. In samples 1, 2, 3, 7, 8 and 9 in which the main activators were waterglass and Na 2 CO 3, two heat evolution peaks were observed. However, in samples 4, 5 and 6 in which the main activator was NaOH only one peak was observed, (see Fig. 1, Table 4). V 1 is the heat evolution rate associated with the pre-induction peak and V 2 is the heat evolution rate associated with the acceleration-deceleration peak when the main reaction product, a calcium silicate hydrate, 6,8 is formed. The numerical values obtained for the first and second peak rates and the times at which these signals appear are shown in Table 4. In this table the duration time of the second reaction peak, t, associated with the precipitation of the main reaction products is also shown, as well as the heat released, Q. FTIR spectra of samples 1, 3, 4 and 7 are shown in Fig. 2. All samples produced a complex group of bands in the range of cm 1 corresponding to asymmetric and symmetric stretching vibrations of Si- O bands, and a group of bands in the range of cm 1 due to deformation of SiO 4 tetrahedra. Silicate vibration bands in FTIR spectra of all AAS samples, at different ages, are presented in Table 5. Bands associated with vibrations of carbonate groups are also present in all spectra of AAS pastes. Also in sample 7, activated with Na 2 CO 3 band associated with the formation of sodium calcium silicate was detected (1450 cm 1 with a shoulder at 1410, 876, 708 and 685 cm 1 ). Discussion The alkali activated slag pastes with 100% (sample 1), and 80% (sample 2 and 3) of waterglass solution gave high mechanical strength values in flexure and compression, even at initial ages (3 days, see Table 2). In these alkali solutions the ph values are very similar ( ) to those used in samples 4, 5 and 6, where the mechanical strengths were lower. This indicates that the presence of silicate ions in the previously mentioned solutions plays a decisive role in the development of high mechanical strengths. On the other hand, the presence of carbonate ions reduces the ph values in the solution (samples 7, 8 and 9) and for these the initial mechanical strength decreases (this behaviour will be discussed later in more detail). A ternary diagram is presented in Fig. 3, where a summary of the results obtained is represented as a Table 3. Setting time for alkali activated slag cements pastes Sample Activators Initial time: t i Final time: t f t ¼ t i - t f 1 Waterglass 1 h 16 min 1 h 46 min 20 min 2 80%Wat/20%NaOH 1 h 15 min 1 h 55 min 25 min 3 80%Wat/20%Na 2 CO 3 1 h 20 min 1 h 55 min 30 min 4 NaOH 2 h 45 min 3 h 50 min 30 min 5 80%NaOH/20%Wat. 1 h 10 min 1 h 40 min 20 min 6 80%NaOH/20%Na 2 CO 3 3 h 22 min 5 h 12 min 2 h 20 min 7 Na 2 CO 3.3day.3 day 8 a 80%Na 2 CO 3 /20%Wat. 2 h 42 min 5 h 42 min 3 h 9 a 80%Na 2 CO 3 /20%NaOH 5 h 47 min 9 h 47 min 4 h a false setting. Advances in Cement Research, 2003, 15, No

4 A. Fernández-Jiménez and F. Puertas 5 5 Rate heat evolution: kj/kg/h Waterglass W/20N 3. 80W/20C Rate heat evolution: kj/kg/h NaOH 5. 80N/20W 6. 80N/20C Time: h Time: h Rate heat evolution: kj/kg/h Na 2 CO C/20W 9. 80C/20N Time: h Fig. 1. Heat evolution rates of alkali activated slag pastes Table 4. Numerical value of heat evolution rate and heat curves of alkali activated slag pastes N8 Sample Activator First signal Second signal t 1 :h V 1 : kj/kgh t 2 :h V 2 : kj/kgh t Q 1 Waterglass %Wat/20%NaOH %Wat/20%Na 2 CO NaOH %NaOH/20%Wat %NaOH/20%Na 2 CO Na 2 CO %Na 2 CO 3 /20%Wat %Na 2 CO 3 /20%NaOH function of the composition of the alkaline solution used. A discussion of the results obtained is presented as follows. Waterglass! NaOH (samples 1, 2, 5 and 4) When the amount of silicate ions decreases in the alkaline activation solution, the setting time of AAS pastes is scarcely modified. Mechanical strength values decrease (see Table 2), and a reduction of the induction period in the heat evolution rate (see Fig. 1) is observed as well. The causes of this behaviour are related to the nature and structure of the reaction products formed. In sample 1 (100% waterglass, with a high ph ¼ 13 2 and high content of silicate ions (SiO 2 /Na 2 O ¼ 1 5 ratio)), the silicate ions from the alkaline solution initially react with the Ca 2þ ions from the slag to form a C S H responsible for the fast setting time of this paste. The setting time and hardening of the paste takes 132 Advances in Cement Research, 2003, 15, No. 3

5 Activator mix and the hydration and strength behaviour of alkali-activated slag cements 14h. No 1, 100% Waterglass No 3, 80% Wat./20% Na 2 CO 3 2h. 2h. 14h. 1h. 1h. Slag Slag cm cm 1 No 4, 100% NaOH No 7, 100% Na 2 CO 3 4h. 14h. 200h. 2h. 38h. 20h. Slag Slag cm 1 cm 1 Fig. 2. FTIR spectra for samples 1, 3, 4 and 7 place before the maximum precipitation of the main reaction products occur. It results in a pre-induction peak in the curves of the heat evolution rate. When the amount of SiO 2 ions in the alkaline solution decreases (from 6% in sample 1 to 4 8% in sample 2 and 1 2% in sample 5), the intensity of this pre-induction peak decreases too (from V 1 ¼ 4 22 to V 1 ¼ 2 44 and 1 27 for samples 2 and 5 respectively, see Table 4). This is due to the lower amount of this initial calcium silicate hydrate formed. This is confirmed by the fact that in the absence of silicate ions in the alkaline solution (sample 4) this pre-induction signal is not detected. In FTIR spectra, the Si-O silicate vibration regions of C S H generally give a band centred at 970 cm 1, assigned to Si-O stretching vibrations. 8,13 This band shifts towards higher frequencies as the C/S ratio decreases. In Fig. 2(a) and in Table 5 it is clearly observed how at 2 h (time associated with the initial setting time), the band assigned to stretching vibrations in samples 1, 2 and 5 shifts toward higher frequencies than in the unreacted slag spectrum (958 cm 1 shifts to 985 3, and 968 cm 1, respectively). Over long time periods (about 8 11 h) a calcium silicate hydrate response to the acceleration deceleration peak in the curves of heat evolution rate is formed. At this Advances in Cement Research, 2003, 15, No

6 A. Fernández-Jiménez and F. Puertas Table 5. Bands of asymmetric and symmetric stretching vibrations of Si-O bands, and bands of bending vibrations of Si-O-Si bands (cm 1 ) Frequency of bands of í 3 (Si-O) and í 4 (Si-O-Si) in cm 1 Slag N81 (1 h) (2 h) (14 h) N82 (1 h) (2 h) (14 h) N83 (1 h) (2 h) (14 h) N84 (2 h) (4 h) (14 h) N85 (1 h) (2 h) (14 h) N86 (2 h) (6 h) (14 h) N87 (20 h) (38 h) (200 h) N88 (2 h) (6 h) (60 h) (105 h) N89 (5 h) (14 h) (60 h) ***t 2 *MS **t i same NaOH 4 7 Na 2 CO 3 **t i Waterglass *MS same ***t 2 *MS **t i ***t 2 Fig. 3. Mixes of alkaline activator. MS ¼ mechanical strength; t i ¼ initial setting time; t 2 ¼ time that the peak associated to the main reaction products in the curve of heat evolution rate appear. time the í 3 (Si-O) bands shift to lower frequencies than at 2 h (979 6, and 968 cm 1 at 14 h for samples 1, 2 and 5 respectively), indicating that this calcium silicate has a higher C/S ratio than the initial one. In previous studies, 8,14,15 it has been demonstrated that when a waterglass solution is used as an alkali activator the main reaction product formed is a semicrystalline C S H gel with a dreierketten-type structure. These structures have a C/S ratio of and also some tetrahedral aluminium (Al T ) in bridging positions. On the other hand, the presence of Q 3 entities associated with possible formation of cross-linked structures was also detected. That structure could be responsible for the higher mechanical strengths obtained in these AAS pastes. When the silicate anions in the alkaline solution decrease, the C/S ratio in the main C S H increases and the content of Q 3 units decreases or does not appear, the initial setting time increases and mechanical strength decreases. NaOH! Na 2 CO 3 (samples 4, 6, 9 and 7) In the presence of carbonate ions in the alkaline solutions, the initial ph decreases, the setting time increases and the early mechanical strengths decrease. However, in the absence of carbonate and silicate ions (sample 4, ph ¼ 13 5) the activation process is very fast. (see Fig. 1). Thus, when the alkali activator is just an NaOH solution, the initial peak associated with the slag dissolution, or the pre-induction peak associated with the formation of initial reactions, are not detected in the heat evolution rate curves. In this sample (AAS activated with NaOH solution) the reaction process is similar to that of Portland cement hydration; 6,8 the initial and final setting time are directly affected by the acceleration peak in the heat evolution curve, when a C S H gel, responsible for the setting time of the pastes, is formed. This explains why the heat associated with the acceleration deceleration peak is more intense than in the other samples studied. As previously indicated, the presence of carbonate ions in the alkali solution increases the setting time, it also increases the induction period in the heat evolution curve. With 20% of Na 2 CO 3 this effect is not as 134 Advances in Cement Research, 2003, 15, No. 3

7 Activator mix and the hydration and strength behaviour of alkali-activated slag cements significant as with 80% (sample 8) where the presence of a pre-induction peak (V 1 ¼ 0 35 at 4 33 h) was detected. This peak can be associated with the formation of a sodium calcium carbonate responsible for both the false setting time in these pastes and for the decrease in the mechanical strength at early ages. When the alkaline activator is NaOH, Si-O stretching vibration bands (see Table 5, sample 4) shift toward higher frequencies than those of the unreacted slag. However they always present at lower frequencies than in AAS paste with silicate ions in the alkaline solution (samples 1, 2 and 5). This indicates that a C S H with a higher C/S ratio is being formed. It has been proved in previous studies 8,13,14 that when the alkaline activator is an NaOH solution, a semicrystalline C S H with dreierketten-type structure is formed, with a high amount of Al T in bridging positions and a C/S ratio between , but the presence of Q 3 entities went undetected. 14 When carbonate ions are present in the activated solution the initial ph is lower; as the kinetic process is delayed. At early ages, the low ph solution slows the kinetic process, therefore more time is needed for the alkali reaction. Later, a C S H is formed (Si-O stretching vibrations bands in the FTIR spectra shift toward higher frequencies). This proves that the presence of carbonate ions, in the alkaline solution, reduces mechanical strength, but only at early ages. Na 2 CO 3! waterglass (samples 7, 8, 3 and 1) As previously observed, the presence of carbonate ions increases the setting time, while the presence of silicate ions reduces it. Sample 7 (ph ¼ 11 6, Na 2 CO 3 solution) has the longest initial setting time (. 3 days). It is due, partly, to the small ph of the alkaline activator that slows the rate of slag dissolution, and also, to the formation of a sodium calcium carbonate. Consequently, a initial signal, between 8 and 20 h, was observed in the heat evolution curve in response to a pre-induction peak. Also, in the FTIR spectrum of this sample, a band of great intensity was detected at 1450 cm 1 with a shoulder at 1410 cm 1 together with the bands that appear at 876, 708 and 685 cm 1 associated with formation of sodium calcium carbonate. This sodium calcium carbonate is responsible for the lost of paste plasticity, which neither hardens nor sets. Later, at increased intervals, the C S H gel is formed. In samples 3 and 8, the ph of the solution and the content of silicate ions increases while the content of carbonate ions decreases. It results in an increase in kinetics; therefore the setting time decreases and the mechanical strength increases, both at early and at late ages. The results obtained in this work show that mix 3 is the most appropriate for improving workability without diminishing mechanical strength. The high ph favours the initial slag dissolution, the silicate ions increase the mechanical strength and carbonate ions act as regulators of the setting time. A more detailed study of the three mixture activators will be carried out in the future to determine which is the most appropriate amount of silicate and carbonate ions. Conclusion The main conclusions drawn from this work are (a) The ph of the activated solution plays a very important role in the initial slag dissolution, favouring Ca 2þ ions from the slag to the solution. This process is highly favoured by ph values > 12. Lower values delay the activation process, although it still occurrs. (b) For ph values higher than 12, the main factor that controls the setting time as well as the development of mechanical strengths, is the anion type of alkaline activator solution. This anion can react with the ions dissolved from the slag, mainly with the Ca 2þ ions, generating stable hydration products. In the presence of silicate ions, an initial calcium silicate hydrate that accelerates the setting times and increases the mechanical strengths is formed. Carbonate ions have an opposite effect, extending the setting time and decreasing the mechanical strengths at early ages. This is due partly to lower ph in the solution and to the initial formation of sodium calcium carbonate. Acknowledgement The authors of the present work wish to thank the CICYT for funding this research through the project MAT Thanks go to the Regional Government of Madrid for awarding a post-doctoral grant. Also, thanks to J. L. García and A. Gil for the preparation of AAS cement. References 1. 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