Degradation of lime-pozzolan mortar exposed to dry deposition of SO 2 pollutant gas: Influence of curing temperature

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1 Materials and Structures/Matériaux et Constructions, Vol. 32, June 1999, pp Degradation of lime-pozzolan mortar exposed to dry deposition of SO 2 pollutant gas: Influence of curing temperature S. Martinez-Ramirez 1 and G. E. Thompson Corrosion and Protection Centre (UMIST), P.O. Box 88, M60 1QD Manchester (U.K) (1) to whom correspondence should be addressed, in the current address: Instituto de Ciencias de la Construcción Eduardo Torroja, C/Serrano Galvache s/n Madrid (Spain) Paper received: May 19, 1998; Paper accepted: June 26, 1998 SCIENTIFIC REPORTS A B S T R A C T The reaction of lime-pozzolan mortars with SO 2 gaseous pollutant using laboratory exposure chambers, with realistic presentation rates of pollutants, has been examined. Two different curing temperatures were used in order to promote microstructural alteration in the lime-pozzolan mortars. However, independent of curing temperature, the calcium aluminate hydrated phase formed in the hydration reaction of the pozzolan mortar was C 3 AS 2 H 2. For dry deposition of SO 2 pollutant gas, the important roles of water and water plus oxidant in increasing loss of calcium from the carbonate component in the mortar are revealed, with little influence on the processes of the curing temperature of mortar. For the lime-pozzolan mortars exposed to SO 2 in the additional presence of oxidant and water, the extent of chemical reaction of the mortar with the low ph surface water was about 80 times greater than that in the presence of SO 2 alone. R É S U M É On a étudié la réaction des mortiers à base de chauxpouzzolane en présence de SO 2 dans des enceintes de laboratoire sous atmosphère gazeuse utilisant des taux réels de gaz polluant. Pour caractériser la modification des mortiers, ceux-ci ont été vieillis à deux températures différentes. Cependant, indépendamment de la température de cure, la réaction d hydratation du mortier conduit à la formation du même aluminate de calcium de formule C 3 AS 2 H 2. Dans le processus de déposition du SO 2, on a observé que l eau et surtout l eau avec l oxydant augmentent la perte de calcium provenant du composant carbonaté du mortier. La température de vieillissement a une faible influence dans le processus. Pour les mortiers exposés au SO 2 en présence d oxydant et d eau, l importance de la réaction chimique du mortier avec une eau de surface de faible ph, est de l ordre de 80 fois plus grande que celle en présence de SO 2 seul. 1. INTRODUCTION The addition of pozzolan to lime mortars to improve properties, including strength, has been known since Roman times. Pozzolans are comprised of similar oxides to clinker and ordinary Portland cement, but with different proportions and mineralogical compositions. In the pozzolanic reaction, pozzolan reacts with Ca(OH) 2 to produce calcium silicate hydrate, gehlenite hydrate (C 2 ASH 8 ) or calcium aluminate hydrates (CAH 10, C 2 AH 8, C 3 AH 6 ), depending on factors such as curing temperature, presence of alkalis, structure, composition and physical properties of the reactive phase [1-3]. Gehlenite hydrate formation inhibits the transformation of hexagonal aluminate hydrates (CAH 10, C 2 AH 8 ) into cubic hydrate (C 3 AH 6 ) which is responsible for loss of strength. Shi and Day [2] have shown increases in strength of lime-pozzolan mortars with curing tempera- ture between 20 and 60 C, although no structural information was provided. In this paper two curing temperatures in the same range than the previous authors [2] have been used, in order to study the microstructure of the samples. Presently lime-pozzolan mortars are used for conservation and restoration purposes. In previous studies [4, 5], lime mortars for restoration have been exposed to SO 2 and NO x pollutant gases in laboratory-based chambers, enabling the influence of exposure conditions on degradation to be revealed. For example, SO 2 was shown to be more chemically reactive than NO or NO 2, particularly with the additional presence of oxidant and water. In the present study, the reaction of lime-pozzolan mortars with SO 2 pollutant gas is considered, with the role of curing temperature of the mortar, possibly influencing microstructure, being examined /99 RILEM 377

2 Materials and Structures/Matériaux et Constructions, Vol. 32, June MATERIALS AND METHODS 2.1 Preparation of samples Traditional hydraulic mortar specimens, of dimensions cm., were employed. The mortars were composed of lime, volcanic pozzolan and sand in the ratio 1:1:6 with a water/binder ratio of Powders of hydrated lime (Ca(OH) 2 ), natural Italian pozzolan (KAlSi 2 O 6 ) and silica sand (99% quartz) were used; to increase reactivity, the pozzolan was ground to dimensions of less than 90 microns. In order to promote the pozzolanic reaction and then carbonation, the samples were cured using the following procedure: After moulding, the samples were placed in chambers at 100% relative humidity; curing of one-half of the samples was undertaken in a chamber 40 C, with curing of the remainder at 60 C. After curing for 72 hours, the specimens were removed from the chambers and dried for 3 hours at 40 and 60 C respectively; all samples were then placed in a chamber at 50% R.H. and subjected to daily CO 2 addition for 2 hours over a 21-day period. The percentage of free CaO was determined according to the ethilenglicol method [6]. Prior to exposure in the rigs, the samples were dried at 40 C for 24 hours. Three weighed samples of each lime-pozzolan mortar were exposed to the dry deposition of SO 2 pollutant gas, under conditions given in the following sections. The accessible porosity of unexposed samples was determined according to RILEM recommendations [7]. 2.2 Dry deposition The laboratory-based rigs employed for the dry deposition of pollutants have been described elsewhere [8]; typical pollutant concentrations utilised in the individual chambers are given in Table 1. As indicated in the Table, SO 2 gaseous pollutant at a concentration of 2.5 ppm SO 2 was used at a relative humidity of 84%. The influence of additional surface water was also examined, with 8 ml of water passed over the mortar surface for 1 hour every day. Further, the influence of ozone as oxidant, at a concentration of 4 ppm, was examined for mortars exposed to 84% relative humidity and those additionally wetted with water. The air flow in all chambers was 2.5 l air/min. The presentation rates of the pollutant gas and the oxidant were 8.5x10-6 mg SO 2 /cm 2 s and 4.2x10-6 mg O 3 /cm 2 s respectively. Table 1 Exposure conditions for dry deposition studies Chamber ppm SO 2 ppm O 3 ml. H 2 O/day Analyses After 4 weeks exposure in the rigs, morphological and mineralogical composition of the samples were examined by XRD, FTIR and SEM/EDX. During exposure, the daily run-off was collected from appropiate chambers and SO 3 2- and SO4 2- concentrations were determined on a weekly basis by HPLC. Once the runoff was collected, fructose was added to limit oxidation of any SO 3 2- anions [9]. The calcium and potassium contents of the respective solutions were determined by atomic absorption spectroscopy. After 4 weeks exposure, soluble salts, retained in the dried mortar, were extracted with water and cation and anion analyses were undertaken. Again, in order to limit any sulphite oxidation, fructose was added to the aqueous solution. Three replications of each sample were analysed. 3. RESULTS AND DISCUSSION 3.1 Lime-pozzolan mortar characterisation before exposure Percentages of free CaO in the lime-pozzolan mortars prior to exposure were 0.06% and 0.04% after curing at 40 and 60 C respectively, indicating reaction with most of the Ca(OH) 2. Percentage of water accessible porosities of the samples before exposure, were: 28 ± 2% for lime-pozzolan mortar cured at 40 C and 30 ± 2% for the lime pozzolan mortar cured at 60 C. The water accessible porosity is defined as the ratio of the volume of the pores accessible to water to the bulk volume of the sample [7]. The value depends on the relative volume of the small diameter pores that are completely filled with water. For the differently cured lime-pozzolan mortars, the water accessible porosity values are similar, close to 30%, indicating that curing temperature has little influence on the relative volume of the small diameter pores. Mineralogical composition of the samples was examined by FTIR and XRD; with FTIR spectra for unexposed lime-pozzolan mortars cured at 40 C and 60 C similar. In both spectra absorption bands at 1420 and 873 cm -1 arising from CO 3 2- vibrations from calcite are observed. The broad band at 1080 cm -1, together with the sharp doublet at 793 and 776 cm -1, arises from the quartz sand component. The broad band at 1000 cm -1 suggests a Si-O stretching frequency. According to Grutzeck et al. [10], tobermorite and poorly crystalline calcium silicate hydrate have peaks in this region of the spectra, suggesting that a calcium silicate hydrate phase is present; the peaks are related to the modifying effect of calcium upon the Si-O stretching frequency in the developing C-S-H structure. Finally, bands from unreacted pozzolan are identified, indicating incomplete pozzolanic reaction. XRD of the lime-pozzolan mortars, cured at 40 C and 60 C, indicated the presence of quartz from the sand, calcite from external carbonation, hydrated calcium silicate from pozzolanic reaction and unreacted pozzolan. For the unexposed mortars, two additional peaks at d = 2.96 and 378

3 Martinez-Ramirez, Thompson 2.68 Å, arising from C 3 AS 2 H 2, represent a further product of the pozzolanic reaction. No Ca(OH) 2 was detected, in agreement with the previously determined low percentages of free CaO in the cured mortars. 3.2 Dry deposition studies Fig. 1 Variation of the calcium, potassium and sulphate concentrations in the run-off with time for wetted lime-pozzolan mortars cured at 40 C and 60 C and exposed to SO 2 pollutant gas. Fig. 2 Variation of the calcium, potassium and sulphate concentrations in the run-off with time for wetted lime-pozzolan mortars cured at 40 C and 60 C and exposed to SO 2 pollutant gas and ozone as oxidant Run-off analysis Over the exposure period of 4 weeks to SO 2 pollutant gas, the concentrations of sulphate, calcium and potassium ions in the run-off from the wetted samples were determined on a weekly basis. The values are summarised in Figs. 1 and 2 with, in all cases, no sulphite ions detected. Each value is the average from three samples, with reproducibility between samples being about 10%. From the figures, it is evident for wetted samples that potassium concentrations are very low in all cases. For both mortars, in the absence of oxidant (Fig. 1), sulphate and calcium concentrations were relatively low, less than 0.04 mmol/l; further, the sulphate concentrations are less than the calcium concentrations. However, in the additional presence of oxidant (Fig. 2), for wetted mortars, sulphate and calcium concentrations are relatively high (about 0.8 mmol/l) with greater concentrations of sulphate than calcium. This suggests that not all the low ph surface water reacts with the mortar, but is lost to run-off. For both mortars, the data are generally similar, indicating, at face value, little significant effect of curing temperature on reaction of the mortars exposed to SO 2 + O 3 + H 2 O conditions. From previous studies [12], SO 2 (g) dissolution in the moisture film present on the mortar and subsequent oxidation, assisted by appropriate catalysts, generates an acid solution. Chemical reaction of protons with CaCO 3 from lime-pozzolan mortar produces Ca 2+ ions, which may run-off the wetted surface, or generate calcium sulphate and ultimately gypsum which is retained in the near surface regions or on the surface. If gypsum is the sole reaction product, then the sulphate concentration in mmol/l should be equivalent to the calcium concentration. For the lime-pozzolan samples exposed to SO 2 + H 2 O, the reaction of SO 2 with the mortar is limited, with the resultant sulphate concentrations being less than 0.01 mmol/l. Additionally, the calcium concentration is slightly higher than sulphate, suggesting that the total calcium content of the run-off does not arise solely from gypsum. According to Hutchinson et al. [12], calcium concentra- 379

4 Materials and Structures/Matériaux et Constructions, Vol. 32, June 1999 tions close to 0.1 mmol/l are expected from the dissolution of calcium carbonate, thus contributing to the increased calcium content over the sulphate content in the run-off. For the wetted mortars, exposed to the additional presence of oxidant, SO 4 2- formation is increased, with significant gypsum formation through reaction of calcium carbonate with the low ph surface water. Further, the mortars contain C 3 AS 2 H 2 from the pozzolanic reaction; this compound is also susceptible to acid attack, increasing the calcium content in the run-off Retained soluble salts After 4 weeks exposure, the dried samples were ground and weighed. The retained soluble salts, in the variously exposed mortars, were extracted into water. The solid was weighed again and the percentage of soluble salts retained in the mortar was calculated (Fig. 3). From the aqueous solution, the concentrations of sulphite, sulphate, calcium and potassium were determined (Tables 2 and 3). Sulphite anions were detected for the lime-pozzolan mortars exposed to SO 2, with 0.1 ppm determined for the mortar cured at 40 C and 0.5 ppm for the mortar cured at 60 C; potassium was not detected for all exposure conditions. From the retained soluble salts, contrasting behaviour for the variously cured lime-pozzolan mortars is observed. For mortars cured at 40 C (Table 2), the percentage Fig. 3 Percentage soluble salts retained in the mortar after 4 weeks exposure; in the lime-pozzolan mortar cured at 40 C and 60 C and exposed 4 weeks in SO 2 conditions. retained soluble salts increases in the following order: SO 2 < SO 2 + O 3 SO 2 + H 2 O < SO 2 + O 3 + H 2 O. On the other hand, calcium and particularly sulphate contents in the retained soluble salts decrease for the mortar exposed to SO 2 + O 3 and SO 2 + H 2 O and increase for exposure to SO 2 and SO 2 + O 3 + H 2 O conditions. From these values it is evident that the soluble salts retained in the lime-pozzolan mortar exposed for 4 weeks to SO 2 + O 3 and SO 2 + H 2 O are low in SO 4 2- anion concentrations. For the mortars cured at 60 C (Fig. 3), the percentage retained soluble salts is basically similar for all exposure conditions except for the low values observed for exposure to SO 2 + H 2 O. The latter suggests that reaction products from the mortar exposed to SO 2 + H 2 O have run-off from the surface. Calcium and sulphate concentrations in the retained soluble salts are similar for all the mortars except for exposure to SO 2 + H 2 O. In general, as indicated previously in the run-off analyses, calcium concentrations are higher than the sulphate anion concentrations after analyses of extracted soluble salts, resulting from solubility of the calcium carbonate constituent of the mortar. In the present study, the extent of total reaction is comparatively low, as expected from the low, but realistic, presentation rates of SO 2 to the mortar surface. Tables 2 and 3, give the total calcium and sulphate concentrations for the mortars exposed to various conditions; the importance of water in the reaction of SO 2 with limepozzolan mortar is again revealed, particularly with the additional presence of ozone as oxidant. Compared with exposure to SO 2 at 84% relative humidity, the sulphate concentration increased by about times for exposure to SO 2 plus water and oxidant; further, for calcium concentration has increased but only by about 10 times. For SO 2 + O 3 + H 2 O exposure conditions, the SO 2 oxidation to sulphate proceeds in higher degree due to water, oxidant and catalyst presence. Comparing values from retained soluble salts and runoff, it is also generally evident that the contributions of retained salts to the total percentage conversion of SO 2 is relatively small for the exposure conditions employed. As observed previously, the calcium concentration is lower than the sulphate concentration only for the mortars exposed to SO 2 + O 3 + H 2 O, implying incomplete reaction of the low ph surface water with the mortar. Table 2 Sulphate and calcium concentrations in the retained soluble salt and in run-off for lime-mortar cured at 40 C after 4 weeks exposure to different conditions Exposure Soluble salts Run-off Total conditions (mmol/l) (mmol/l) (mmol/l) SO 4 2- Ca SO SO 2 + O SO 2 + H 2 O SO 2 + O 3 + H 2 O Table 3 Sulphate and calcium concentrations in the retained soluble salt and in run-off for lime-mortar cured at 60 C after 4 weeks exposure to different conditions Exposure Soluble salts Run-off Total conditions (mmol/l) (mmol/l) (mmol/l) SO 4 2- Ca SO SO 2 + O SO 2 + H 2 O SO 2 + O 3 + H 2 O

5 Martinez-Ramirez, Thompson Mineralogical composition of the mortar after exposure The mineralogical composition of the lime-pozzolan mortars, after exposure for 4 weeks, was determined by FTIR and XRD. In the FTIR spectra of the mortars cured at 40 C apart from the presence of SiO 2, CaCO 3 and unreacted pozzolan, an additional small peak at 762 cm -1 is revealed. For the mortar exposed to SO 2 + O 3 + H 2 O, a broad band at 1100 cm -1, arising from SO 4 2- was detected. FTIR from the surface of this sample confirmed gypsum formation. Similar FTIR spectra were obtained for the lime-pozzolan mortars cured at 60 C. XRD analyses for both limepozzolan mortars confirm previous results, indicating the presence of quartz, calcite, calcium hydrate silicate, unreacted pozzolan and C 3 AS 2 H 2 in the samples. Additionally, for the mortars exposed to SO 2 + O 3 + H 2 O, gypsum was identified. Comparing XRD and FTIR results, the unidentified band in the FTIR spectra at 762 cm-1 possibly arises from hydrated calcium aluminate silicate. Several complex silicates have vibrations in this region as well as in the region about 1000 cm -1 [13]. According to Taylor (2), at 20 C, reaction between lime and kaolin produces hydrated ghelenite (C 2 ASH 8 ). At increased temperatures, i.e. 50 C, the stable compound is C 3 AS 1/3 H 5 1/3. From the present results, lime-pozzolan mortars cured at 40 or 60 C revealed the same hydrated calcium aluminium silicate, C 3 AS 2 H 2. Sulphite concentration was relatively low, about 0.5 ppm and the sensitivity of both techniques, XRD and FTIR, is not sufficient for detection and revelation of the contributing phases Microestructural analysis Observations of unexposed lime-pozzolan mortars by scanning electron microscopy indicated the presence of various components (Fig. 4). EDX ele- Fig. 4 Scanning electron micrograph of unexposed lime-pozzolan mortar, showing: 1) hydrated calcium aluminium silicate; 2) unreacted pozzolan and 3) pozzolanic calcium silicate gel. Fig. 5 Scanning electron micrograph of lime-pozzolan mortar exposed 4 weeks in SO 2 + O 3, showing cubic crystals with Ca as the main element. Fig. 6 Scanning electron micrograph of lime-pozzolan mortar exposed 4 weeks in SO 2 + O 3 + H 2 O, showing gypsum crystals. 381

6 Materials and Structures/Matériaux et Constructions, Vol. 32, June 1999 mental analysis at point 1 in Fig. 4 revealed the presence of Ca, Si, Al, indicating a calcium aluminate silicate from the pozzolanic reaction. EDX elemental analysis at point 2 in Fig. 4, reveals Ca and Si as the main species, suggesting the presence of pozzolanic calcium silicate gel; unreacted pozzolan is also revealed in the micrograph. For mortars exposed to SO 2, SO 2 + O 3 and SO 2 + H 2 O, an amorphous gel, formed in the hydration reaction, is observed (Fig. 5). Additionally, crystals of highly regular cubic form are revealed; Ca is the main elemental species detected by EDX, suggesting calcite. For the mortars exposed to SO 2 + H 2 O, the number of such cubic crystals decreases. The calcite crystals, present in the mortars, are the source of calcium ions through both dissolution in, and reaction with, the low ph solution. Concerning the mortars exposed to SO 2 + O 3 + H 2 O, a high population density of crystals is revealed (Fig. 6); elemental analyses detected Ca and S, confirming the presence of gypsum; in this case no cubic calcite crystals were evident probably through their masking by the gypsum crystals. Similar results were evident for both lime-pozzolan mortars exposed to SO 2 for 4 weeks, indicating that curing temperature did not influence the microstructure of the mortar. 4. CONCLUSIONS 1.- Microstructural analysis of lime-pozzolan mortars, cured at 40 C or 60 C, revealed amorphous gel formation. Calcium aluminate hydrated phase, formed in the hydration of the lime-pozzolan mortars cured at 40 C and 60 C, was C 3 AS 2 H 2. Unreacted pozzolan was also detected, indicating incomplete pozzolanic reaction during curing. 2.- In terms of sulphate formation, the reactivity of the lime-pozzolan mortars exposed to the dry deposition of SO 2 pollutant gas is relatively low. However, in the additional presence of oxidant and water, the reactivity increases by about 80 times. This behaviour is not influenced by the curing temperature of the mortars. 3.- Dissolution and reaction of the calcium carbonate component of the mortars proceeds in the moisture film, or in the surface water, present on the mortar. For mortars exposed to SO 2, SO 2 + O 3 and SO 2 + H 2 O, dissolution is more extensive than reaction. However contrasting behaviour is observed for mortars exposed to SO 2 + O 3 + H 2 O, where SO 2 oxidation to sulphate is enhanced due to the combined presence of water, oxidant and catalyst; consequently, significant loss of calcium to solution through reaction with the low ph surface water is evident. ACKNOWLEDGEMENTS The authors wish to express their thanks to the Dirección General de Investigación y de Tecnología from Spain for financial support of the work. REFERENCES [1] Taylor, H. F. W. The Chemistry of Cements, Vol. II. (Academic Express, London 1964). [2] Shi, C. and Day, R., Acceleration of strength gain of lime-pozzolan cements by thermal activation, Cement and Concrete Research 23 (4) (1993) [3] Day, R. and Shi, C., Influence of the fineness of pozzolan on the strength of lime natural-pozzolan cement pastes, Ibid. 24 (8) (1994) [4] Martínez-Ramírez, S., Puertas, F., Blanco-Varela, M. T. and Thompson, G. E., Studies on degradation of lime mortars in atmospheric simulation chambers, Ibid. 27 (5) (1997) [5] Martínez-Ramírez, S., Puertas, F., Blanco-Varela, M. T. and Thompson, G. E., Effect of dry deposition of pollutants on the degradation of lime mortars with sepiolite, Ibid. 28 (1) (1998) [6] UNE , Métodos de ensayo de cementos. Análisis químico. Determinación del óxido de calcio libre; método del etilenglicol. [7] RILEM Recommendations. Commission 25-PEM Protection et érosion des monuments, Recommended test to measure the deterioration of stone and to assess the effectiveness of treatment methods, Mater. Struct. 13 (75) (1980) [8] Haneef, S. J., Johnson, J. B., Dickinson, C., Thompson, G. E. and Wood, G. C., Effect of dry deposition of NO x and SO 2 gaseous pollutants on the degradation of calcareous building stones, Atmospheric Environment 26A (1992) [9] Gobbi, G., Zappia, G. and Sabbioni, C., Anion determination in damage layers of stone monuments, Ibid. 29 (6) (1995) [10] Grutzech, M. W., Atkinson, S. and Della Roy, M., Mechanism of hydration of condensed silica fume in calcium hydroxide solutions, in Fly Ash, Silica Fume, Slag and Pozzolans in Concrete, Proceedings of an International Conference, (1983), [11] Martínez-Ramírez, S. and Thompson, G. E. Dry and wet deposition studies of the degradation of cement mortars, Materiales de Construcción 48 (250) (1998) [12] Hutchinson, A. J., Johnson, J. B., Wood, G. C., Sage, P. W. and Cooke, M. J., Stone degradation due to wet deposition of pollutants, Corrosion Science 34 (1) (1993) [13] Gadsden, J. A. Infrared spectra of minerals and related inorganic compounds (Buttherworth, London, 1975). 382