CEMENT and CONCRETERESEARCH. Vol. 15, pp. 89-92, 1985. Printed in the USA. 0008-8846/85 $3.00+00. Copyright (c) 1985 Pergamon Press, Ltd.
PASTES OF TRICAIEIUM SILICATE WITH RICE HUSK A ~
M.H. Khan*, K. M~han and H.F.W. Taylor Department of Chemistry, University of Aberdeen Meston Walk, Old Aberdeen AB9 2UE, Scotland, UK
(Communicated by D.M. Roy) (Received May 3, 1984) ABSTRACT Pastes of tricalcium silicate with rice husk ash (RHA) were cured at 25CC for 1 - 245 days and examined by XRD, analytical electron microscopy, TG, acid extractions to determine unreacted rice husk ash, and trimethylsilylation. The rice husk ash was highly reactive, and contents of calci~nhydroxide, referred to the ignited weight, never exceeded 3 %. Initially, a product having a Ca/Si atom ratio of 0.i - 0.2 was formed, but this was later replaced by one having a Ca/Si ratio of approximately 1.3. The proportion of the Si in the hydration products that was present as polymeric ions was greater than that found in pure C3S or C3S-fly ash pastes of similar age, but the anion size distributions within the polymer were all broadly similar. Introduction The possible utilization of rice husk ash (RHA) in portland cement concrete has received attention recently, especially in countries of S.E. Asia and the Pacific, primarily for economic, environmental and energy considerations, and also because of their superior acid-resisting properties. Although a number of studies on RHA cements have been made (1-4) , little detailed information on their hydration chemistry exists. The present work was therefore undertaken. Experimental Methods and Results The C3S was triclinic, with a Blaine specific surface of 300 m 2 kg -I, free CaO below 0.I %, and no impurities detected by X_RD. The R H A w a s described (3) as containing 80-95 % Si02 and 1 - 2 % K20, the residue being mainly carbon, and as having a BET specific surface of 60 m 2 g-l; a partial analysis of the present sample gave K20 2.3, CaO 0.2, H20 4.2, C 11.5, SiO 2 (by difference) 82 %. Pastes with weight ratios C3S : RHA = 3 and water : total solids = 0.4 were hand-mixed and cured in sealed vials at 25~C. Samples
*Permanent Address: School of Natural Resources, University of the South Pacific, Laucala Bay, Suva, Fiji 89
Vol. 15, No. l M.H. Khan, et a l .
were rer,oved from the pastes, crushed and dried by evacuation with a rotary pump before being examined. XRD of the pastes showed the only crystalline phases to be unreacted C3S and small anounts of calcium hydroxide. These phases were determined using a modified internal standard method, in which 8 peaks of the standard (corundum) , 8 of the C3S and 1 of the CH were employed. A H~gg Guinier camera was used, with monochromatized Cu radiation; 4 films were made of each specimen, with different sample mounts, and peak heights were measured with a microdensitometer. The results are given in Table i, lines 2 and 3. They are probably reliable to _+5 % absolute on the percentages of C3S reacted and to _+0.5 % absolute on the contents of CH. A 2-year old paste gave a strong pattern of unreacted C3S, with no detectable peaks of CH. A weak, diffuse peak at 0.182 nm was attributable to C-S-H; the 0.305 r~ peak of this phase oould have been present but masked by C3S. No other peaks in the 5 - 50 ~ 28 range were observed.
TABLE 1 Results for Pastes of Tricalcium Silicate with Rice Husk Ash I. 2. 3. 4. 5. 6. 7. 8. 9. i0.
Curing Time (days) 1 % of C3S Reacted (XRD) 5 Calcium Hydroxide, % (a) 1 Loss at IO00°C (a) 7.2 % of RHA Reacted 4 Ca/Si Ratio of C-S-H (Calc.) (c) Mcnsmer (from XRD, %; b) 45 Monomer (from ~IMS, %; b) 35 Dimer (from ~ , %; b) 5 Dimer / (Dimer + Polymer) 0.60
3 15 2 13.4 14 (C) 40 30 i0 0.50
7 25 3 16.4 40 0.9 35 25 iO 0.25
14 30 2 17.8 48 1.0 35 25 i0 0.26
28 35 1 17.5 52 i.i 30 20 iO 0.27
39 40 1 17.2 58 1.2 30 25 15 0.31
90 40 1 19.8 67 1.0 30 25 15 0.26
245 50 1 16.1 69 1.2 25 20 15 0.25
(a) Referred to ignited weight. (b) Si as monomer or dimer as % of total Si. (c) Precision too low to justify calculation.
TG curves, determined in N2 at 15 deg C mn-1 , were relatively featureless, showing gradual losses at 50 - 1 0 0 0 ° C attributable mainly to C-S-H. They showed no marked steps in the 400 - 500 ° C region; this is consistent with the conclusion from XRD that only small amounts of CH were present. In ssme cases, small weight gains in this region were observed. These were probably due to the presence in the N2 of small amounts of 0 2, which became fixed in the solid through a combination of combustion of the C, decomposition of CH and subsequent carbonation of the CaO. Table 3, line 4, gives ignition losses derived from the TG curves. Unreacted RHA was estimated by extraction with salicylic acid and methanol followed by H~I, as described by Mohan and Taylor (5) for determination of unreacted fly ash. This treatment dissolved C3S or C3S pastes (xmnpletely, and gave a loss of 3 % with the RHA, attributable to removal of moisture on drying the acid extracted residue. The percentages of RHA silica reacted (Table i, line 5) were calculated assuming that the acid extracted residues included all the carbon initially present. Analytical electron microscopy (A~4) was carried out using a Kratos (DORA instrument. In this technique (6), the sample is finely ground, dispersed on the specimen grid, and examined in transmission; 30 - 50 particles from each
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91 Ca3SiO5, RICE HUSK ASH, PASTES, TMS, REACTION
TABLE 2 Analytical Electron Microscopy: Frequency Distributions of Ca/Si Ratio Ca/Si Ratio < O.15 0.15-0.24 0.25-0.34 0.35-0.44 O. 45-0.54 0.55-0.64
Time (days) 1 14 245 I0 9 1 2 1 1
4 1 1 1 1 1
O 0 O 0 0 0
Ca/Si Ratio 0.65-0.74 0.75-0.84 0.85-0.94 0.95-1.O4 1.05-1.14 1.15-1.24
Time (days) 1 14 245 0 1 1 1 0 1
0 1 1 0 4 5
O O O O O 8
Time (days) 1 14 245
1.25-1.34 1.35-1.44 1.45-1.54 >i 1.55
1 1 O 2
iO 4 1 5
17 3 2 0
paste were analyzed. Each particle analyzed was tested for absence of crystalline phases by checking that it did not give a diffraction pattern. The specimens showed wide distributions of Ca/Si ratio, which tended to be bimodal, with peaks in the regions of 0.i - 0.2 and 1.2 - 1.5. Table 2 gives three, typical distributions. At short times, particles of the group of lower Ca/Si ratio predominated, but with increasing time, these were gradually replaced by ones of the group of higher Ca/Si ratio. The mean Ca/Si ratios observed at the longer times were around 1.29. Only a few particles virtually free from Ca were found; either all of the RK~ had taken up a little Ca, or they had been eliminated during the process of specimen preparation. Trimethylsilyl (~MS) derivatives were prepared, essentially as described by Tam~s et al. (7) and examined by GLC and in one case also by GPC, as described by Mohan and Taylor (8). The untreated RHA yielded negligible amounts of ~ derivatives. Table i, lines 8 and 9, gives the percentages of the total Si in the pastes that were recovered as monomer and dimer derivatives, respectively. The GPC data for the 39-day old paste gave the following results for the molecular weights of the TMS derivatives of the polymer fraction: mode, 1300; n~nber average, 1500; weight average, 2650. Ass~ing the silicate ions to be chains, this gives the follwing values for the ntm~ers of silicon atoms in the chain: mode, 5.1; number average, 6.0; weight average, 11.2. Discussion The percentages of C3S reacted at various times (Table i, line 2) are significantly below those found for the same C3S specimen alone or mixed with fly ash, in either case at w:s = 0.5 (5). The difference may be due to the lower w : s ratio, and the results therefore provide no evidence regarding the effect of RHA on the reaction rate of the C3S. The percentages of the total Si recovered as monomer in the ~MS procedure (line 8) are smaller than those attributable to C3S frcm the XRD results (line 7); if the TMS results are used as a measure of the amounts of C3S reacted, the values for the latter will therefore be greater (e.g., 59 % at 245 days). The RHA reacts relatively rapidly (Table i, line 5), 52 % of the silica having been consumed in 28 days; this may be contrasted with under 20 % for a good quality Class F fly ash under approximately similar conditions (5). The high reactivity of the RHA is also demonstrated by the fact that the contents of CH never exceed 3 % (Table i, line 3); the RHA takes up the CH almost as fast as it is produced by the C3S. This agrees with an earlier finding by Mehta (3).
Vol. 15, No. l M.H. Khan, et a l .
The ~ results showed that a product of iowCa/Si ratio (0.i - 0.2) was formed initially and was subsequently replaced by one having a Ca/Si ratio slightly below 1.3. This observation agrees well with that made by Grutzeck et al. (9) on the reaction of CH with silica fume, which is a reactive form of silica broadly similar to RHA. The earlier literature on the CaO-SiO2-H20 system also contains references to the formation of a very low Ca/Si material, which was sometimes described as hydrous silica with adsorbed lime (i0). The mean Ca/Si ratio of about 1.3 found for the older pastes is lower than those found for the C-S-H of pastes made using fly ash (5), and is near to those found for C-S-H formed in pastes made using portland cement and silica fume (11,12). The Ca/Si ratio of the product can also be calculated from the amounts of C3S and RHA reacted and of CH formed (Table i, line 6). The precision of such results is low, especially at small degrees of reaction, because small experimental errors have large effects on the results; also, it is not known how the early product of low Ca/Si ratio behaves in the extraction procedure used to determine unreacted RHA. For the longer times, the results agree as well as could be expected with those determined by AEM. The XRD evidence shows that the C-S-H is of very low crystallinity; this contrasts with an observation (4) on a product obtained from RHA and CH. In Table i, line i0, the fractions of the Si in the hydration product present as dimer are calculated on the assumption that the remaining Si in the product occurs as polymer. For any given time, they are considerably lower than those found for pastes of C~S either alone or mixed with fly ash (5,8). This is consistent with thezr generally lower Ca/Si ratios. The distributions of anion size within the polymer fraction, in contrast, do not differ greatly from those found in pure C3S or C 3S-fly ash pastes. Acknowledqments We thank the Inter-University Council and the Pacific for financial support that allowed M.H.K. Professor K. Mehta for the sample of rice husk ash, GPC determination and Mr. J. Marr and Mr. M. Visa] for
University of the South to work in Aberdeen, Dr. M. Quereshi for the the chemical analyses.
References i. 2. 3. 4.
5. 6. 7. 8. 9. iO. ii. 12.
P.K. Mehta and N. Pitt, Resource Recovery and Conservation 2, 23 (1976). P.D. Cady and P.R. Groney, Cem. Technol. 7, 215 (1976). P.K. Mehta, J. Amer. Concr. Inst. 744, 440 (1977). D.J. Cook and P. Suwanvitaya, Proc. ist. Int. Conf. Use Fly Ash, Silica Ftmle and Other Mineral By-Products in Concrete, Montebello, 1983 (Amer. Concr. Inst. Publ. SP-79) 2, 831 (1983). K. Mohan and H.F.W. Taylor, Proc. Conf. Effects Flyash Incorp. Cement and Concrete, Boston, 1981, 54 (Materials Research Society, 1981). J.A. Gard, K. Mohan, H.F.W. Taylor and G. Cliff, J. Amer. Ceram. Soc. 6__3, 336 (1980). F.D. Tam~s, A.K. Sarkar and D.M. Roy, in Hydraulic Cement Pastes: Their Structure and Prq0erties, p. 55 (Cem. Concr. Assoc., Slough, UK, 1976). K. Mohan and H.F.W. Taylor, Cem. Concr. Res. i_22, 25 (1982). M.W. Grutzeck, S. Atkinson and D.M. Roy, Ref. 4, 2, 643. H.H. Steinour, Chem. Revs. 40, 391 (1983). E.J. Sellevold, D.H. Bager, E.K. Jensen and T. Knudsen, Proc. Nordisk Miniseminar Silica in Concrete, Trondheim, 1981. M. Regourd, B. Mortureux and H. Hornain, Ref. 4, 2, 847.