The impact of the water/lime ratio on the structural characteristics of air lime mortars

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1 Structural Analysis of Historic Construction D Ayala & Fodde (eds) 2008 Taylor & Francis Group, London, ISBN The impact of the water/lime ratio on the structural characteristics of air lime mortars R.M.H. Lawrence & P. Walker BRE Centre for Innovative Construction Materials, Department of Architecture and Civil Engineering, University of Bath, UK ABSTRACT: Lime based mortars are now widely acknowledged to be generally superior to cement based mortars in the repair of appropriate historic infrastructure. Increasingly the benefits of hydraulic lime mortars are also being realised in new masonry construction. In order to standardise the expected performance of mortars, designers will specify the type of lime, the type of filler (aggregate), the proportions of each and quantity of water or the required workability. Limes can be non-hydraulic (calcium or dolomitic) or hydraulic (natural or artificial). It is well known that the water/binder ratio has a marked effect on the structural performance of cement-based mortars. This relationship is known as Abrams rule, which states that when a cement mortar is fully compacted, its strength is inversely proportional to the water/cement ratio. Abrams rule has also been demonstrated to apply to hydraulic lime mortars. The reason for this is that both cement and hydraulic limes require a minimum quantity of water to produce the chemical set resulting from the hydration of calcium silicates and calcium aluminates. Surplus water eventually dries out, leaving micropores in the matrix which weaken the resulting set mortar. It has generally been assumed that the same relationship applies to non-hydraulic lime (air lime) mortars. This paper reports on results of tests conducted on air lime mortars at early stages of curing. It is known that the form of air lime and the physical and chemical characteristics of the aggregate have a strong impact on the structural performance of air lime mortars. Results to date show that the water/lime ratio has a minimal impact on the structural performance of air lime mortars compared with the impact of lime and aggregate type. 91 day compressive strengths for air lime mortars with a water/lime ratio of 0.56 (a stiff mix) are identical to those with a water/lime ratio of (a loose slurry). Whereas Abrams rule is a key consideration for designers of cement and hydraulic lime mortars, it has been demonstrated that it requires modification in the case of air lime mortars. A relationship between form of lime, type of aggregate, water/lime ratio, and age of mortar is proposed. The resultant equation allows the compressive strength of air lime mortars to be predicted taking into account these factors. The insights gained from this study will allow practitioners to more confidently design and specify air lime mortars. 1 INTRODUCTION Many of the mechanical properties of hardened cement are associated with the physical structure of the hydration products, viewed at the level of colloidal dimensions [Neville, 1995]. The pore structure of hydrated cement paste contains two distinct ranges of pore size gel pores of about 3nm in diameter and capillary pores which are two or three orders of magnitude larger. Cement requires sufficient water to fully hydrate the various constituents, and water in excess of this required amount produces capillary pores. Thus the greater the water/cement ratio above the minimum required for complete hydration, the greater the amount of capillary pores created and therefore the higher the porosity of the hardened paste. When concrete or a cement mortar is fully compacted, its strength is inversely proportional to the water/cement ratio according to Abrams rule. where w/c represents the water/cement ratio of the mix by volume, and K 1 and K 2 are empirical constants. K 1 relates to aggregate strength, particle shape, size, grading and surface texture, and K 2 relates to the compressive strength of the cement paste [Nagaraj & Banu, 1996]. Abrams rule could well be valid for hydraulic lime mortars, since they also gain at least part of their 885

2 Table 1. Water/lime ratios used [by volume] Specimen Designation Water/Lime ratio Figure 1. Relationship between water/lime ratio and strength (Allen et al. 2003). strength through hydration products.allen et al. [2003] have shown this relationship in Figure 1. The data presented in Figure 1 cannot, however, be taken to be truly representative of Abram s rule. This is because the data are based on the compressive strengths of different binder:aggregate ratios each mortar requiring a different quantity of water to produce a specified flow. This means that two variables are present in the graph, and it is not clear what proportion of the compressive strength is affected by which variable. There has been conflicting evidence about the applicability of Abram s rule to air lime mortars. Schäfer & Hilsdorf [1993] and Winnefeld & Böttger [2006] present data which show that increased water content in air lime mortars does not reduce compressive strength. It has been shown that higher porosity in air lime mortars allows greater access to atmospheric carbon dioxide (CO 2 ), which promotes carbonation and therefore can produce greater compressive strengths [Lanas & Alvarez, 2003]. This paper describes a systematic evaluation of the impact of the water/lime ratio on the unconfined compressive strength of air lime mortars up to 91 days after manufacture. 2 EXPERIMENTAL 50 mm 50 mm 250 mm prisms of mortar were prepared with 1 part of dry hydrated high calcium lime (CL90) and 3 parts of silicate sand by volume using a range of different water/lime ratios. 9 specimens of each mortar type were prepared and 2 cubes were taken from each specimen for testing at the appropriate time intervals. For comparison purposes a further set of 9 prisms were prepared using 1 part NHL3.5 lime and 3 parts silicate sand with a range of different water/lime ratios. The amount of water added to each lime type ranged from the minimum quantity needed to make a workable mortar to the amount required to make a loose slurry. The water/lime ratios used were as shown in Table 1. Air Lime A1 0.5 A A A A A Hydraulic Lime H H H3 0.5 H H It was found that air lime required more water in order to make a workable mix than hydraulic lime, and could accommodate more water before becoming a loose slurry. This was likely to be a function of the greater capacity of air lime to absorb water than hydraulic lime as a result of having finer particles and therefore a greater surface area. Both mortars were de-moulded after 5 days and cured in a controlled environment of 60% RH at 20 C until testing. Compressive tests on six 50mm cubes were conducted after 28, 56 and 91 days from the date of manufacture. 3 RESULTS The results of compressive tests on the mortars are shown in Figures 2 4. Error bars are included showing the range of results of the six tests used to produce each data point. These data compare well with the data produced both by Schäfer & Hilsdorf [1993] and Winnefeld & Böttger [2006]. 4 ANALYSIS AND DISCUSSION The data for hydraulic lime mortars appear to follow Abrams rule with the relationship between strength and water/lime ratio following an approximate hyperbolic curve. The data for the air lime mortars, apart from the lowest water/lime ratio, show very little variation in compressive strength when the water/lime ratio is varied. It is conceivable that the data points for the air lime mortar are all to be found at the lower end of the hyperbolic curve, where there would be very little 886

3 Figure 2. Compressive test results on specimens 28 days an hydraulic set, which takes up to 28 days, depending on the hydraulicity of the lime. Subsequent to this the strength gains are due to carbonation. Van Balen [1994] has proposed a model for carbonation represented by a differential equation with a sink term (R(w, c)). The factors involved in the equation include time, the porosity and diffusivity of the material, the construction method and the presence of cracks, and the geometrical shape of the surface exposed to air. Carbonation depth (x) is proportional to the square root of time (t) ± a constant (e) in the form x = k t or x = e + k t, where k is a factor which does not necessarily correspond to a property of the material. It has been shown that the compressive strength of air lime mortars varies in proportion to the extent of carbonation (Lawrence, 2006a), and it is therefore reasonable to expect that development of compressive strength will also be proportional to the square root of time. The following formula has been developed to model the variation in compressive strength of air limes as the water/lime ratio varies. Figure 3. Compressive test results on specimens 56 days Figure 4. Compressive test results on specimens 91 days difference to be seen from an increase in the water/lime ratio. The factors involved in the strengthening of air lime mortars are different from those involved in hydraulic lime mortars. In air lime, after an initial strength gain achieved from the drying out of the mortar, subsequent strength gain is achieved over extended periods as a result of carbonation. In hydraulic lime mortars, there is also an initial strength gain achieved from drying, combined with a gain achieved through where f l is the compressive strength of the air lime mortar, K m is an empirical constant which varies according to the nature of the aggregate, K l is an empirical constant relating to the form of air lime, and d is the age of mortar in days since manufacture. The factor of 150 is an empirical constant derived to provide a best fit with actual data. It is known that differences in the mineralogy and granulometry of an aggregate will have a significant impact on the compressive strength of air lime mortars [Lawrence et al. 2006], even at a very early stage after manufacture. K m represents this effect. This constant will not vary for a given aggregate whatever the time from manufacture. As air lime mortar increases in age, so carbonation has an increasing impact on the compressive strength of the mortar. This effect occurs across the whole range of water/lime ratios, and the expression d represents this effect. The value of this expression will increase as the time from manufacture increases up to the point where carbonation is virtually complete. This expression appears to be valid up to values of 180 for d, beyond this value once the mortar has carbonated, the expression would not vary. Different forms of air lime carbonate to a greater or lesser extent and at a greater or lesser rate, mainly dependent on the size, shape and integrity of portlandite crystal present in the lime. K l represents this effect. This constant will not vary for a given lime whatever the time from manufacture. 887

4 Figure 5. Day 28 air lime data compared with the proposed Figure 8. Proposed equation applied to mortars made with different aggregates. 4SS3 = silicate sand; 4BN3 = crushed bioclastic limestone (Ham Hill stone); 4ON3 = crushed oolitic limestone (Stoke Ground Bathstone). Figure 6. Day 56 air lime data compared with the proposed Figure 7. Day 91 air lime data compared with the proposed As carbonation progresses through the depth of the mortar so the rate of carbonation will decrease. The expression 150 d represents this effect.as commented on above, this expression would become a constant once carbonation has completed. The proposed equation can only be considered to be valid only up to 180 days from manufacture for air lime mortars. Figures 5 7 represent models using the above formula for mortars at 28, 56 and 91 days, and compared with the data shown in Figures 1 3. For these calculations, the value of K m was taken as 0.05, and that of K l as 20. At 28 days the equation over estimates the compressive strength by up to 25%, improving with higher water/lime ratios. As the mortar ages, so the fit improves, until by 91 days, apart from the lowest water/lime ratio, the equation fits within the error bars of the actual data. The equation predicts a reduction in compressive strength in line with increases in water/lime ratio which the data do not entirely support, although experimental variations may explain small increases in strength at higher water/lime ratios. The equation does, however, go some way towards accounting for the more significant influence of the type of aggregate on the compressive strength. Figure 8 shows the proposed equation applied to different aggregate mortars keeping the water/lime ratio constant, but varying the time (d).the solid lines show the relationship between compressive strength for lime mortars made with oolitic aggregates (green), bioclastic aggregates (red) and silicate sand aggregates (blue), and the curves predicted by the proposed Compressive strength data are taken from Lawrence [2006b]. The water/lime ratios used to make the actual mortars were factored in. The constant K m which represents the impact of the aggregate on the compressive strength of the mortars was for the oolitic mortar, for the bioclastic mortar, and for the silicate sand mortar. It can be seen that up to 180 days there is a reasonable correlation between actual and predicted compressive strengths. The equation requires modification to take account of the completion of carbonation, but up to 180 days it seems to be able to predict with reasonable accuracy the compressive strength of air lime mortars as they are affected by water/lime ratio, type of aggregate and time from manufacture. The data for 888

5 mortars made with oolitic stone shows greater early strength than predicted by the Indeed early strength exceeds that of moderately hydraulic lime mortars. It has been shown by Lawrence (2006a) that no hydraulic effects are involved, and this phenomenon is the subject of ongoing research. be done to understand the mechanisms involved which produce such significant differences in compressive strength. Such an understanding will allow the formulation of a more developed equation than that which has been proposed. REFERENCES 5 CONCLUSIONS The experimental data demonstrate that, unlike cementitious or hydraulic lime mortars, the increase in porosity produced by higher water content does not result in the strength reductions predicted by Abrams rule. Indeed it can be seen that the choice of aggregate has a significantly greater impact on the strength characteristics of air lime mortars. One key implication of this are that for air-lime mortars, water/binder ratios can safely be selected to produce a mortar with suitable workability characteristics as demanded by the particular application, rather than based on the erroneous assumption that Abrams rule applies. Unlike hydraulic limes, with air-lime mortars apart from very stiff mortars made with water/lime ratios of 0.5 compressive strengths are shown to be remarkably unaffected by greater quantities of water in the initial mix. The proposed equation will allow a first approximation of compressive strength of different air lime mortars to be predicted. Continuation of the present study will allow the creation of empirical constants for types of lime and aggregate (K m and K l ). More work needs to Allen, G., Allen, J., Elton, N., Farey, M., Holmes, S., Livesey, P., Radonjic, M Hydraulic lime mortar for stone, brick and masonry. Shaftesbury, Donhead Publishing Ltd. Lanas, J., Alvarez, J.I Masonry repair lime-based mortars: factors affecting the mechanical behaviour. Cement and Concrete Research 33(11): Lawrence, R.M.H., Walker, P., D Ayala, D. 2006a. Nonhydraulic lime mortars. The influence of binder and filler type on early strength development. Journal of Architectural Conservation 12(2): Lawrence, R.M.H. 2006b A study of Carbonation in nonhydraulic lime mortars, unpublished PhD Thesis, University of Bath. Nagaraj, T.S., Banu, Z Generalisation of Abrams law. Cement and Concrete Research 26: Neville, A.M Properties of Concrete. Harlow, Longman. Schafer, H.R., Hilsdorf, H.K Ancient and new lime mortars the correlation between their composition structure and properties. In M. J. Thiel Conservation of Stone and other Materials. London, E. & F.N. Spon: Van Balen, K., van Gemert, D Modelling lime mortar carbonation, Materials and Structures, 27: Winnefeld, F., Böttger, K.G How clayey fines in aggregates influence the properties of lime mortars. Materials and Structures 39: