GB2468588A - A process for producing a dry lime and aggregate mix - Google Patents

Abstract

A process for producing a dry Lime and sand mix comprising aggregates and a lime-based binder is disclosed which is characterised by the successive steps of:a) admixing a pre-determined quantity of calcium oxide (CaO) containing compound with wet aggregates, said aggregates having a predetermined free water content;b) providing a period of storage; andC) admixing at least one hydraulic component or binder wherein the amount of calcium oxide used is in the range of 109 to 280 wt% of said free water content or, alternatively, of 35 to 90 mol% of said free water content. The lime may be in the form of quicklime. The hydraulic binder may be Portland cement, ground granulated blast furnace slag, fly ash, silica fume, metakaolin or natural hydraulic lime.

Description

I

Process for Producing a Dry Lime Mortar Mix The present invention relates to a process of producing a dry lime mortar mix, characterised by the use of a calcium oxide containing compound (such as quick lime) to act as a drying and binding agent for a mortar aggregate. In particular, but not exclusively, the present invention provides a controlled low energy process of drying an aggregate with quick lime, in such a way that degradation of the resulting lime mortar product is minimised.

Mortar is a workable paste formed by mixture of a suitable binder (such as Portland cement and air-, hydraulic-or formulated-lime), water and fine aggregates, together with the possible inclusion of admixtures. It is applied as a wet paste which sets hard by a combination of hydration and carbonation. Mortar is used to bind construction blocks together and fill the gaps between them. Alternatively, it is used as a technical or decorative render to walls.

Lime mortars are well known and have been used for millennia. One traditional technique for their production is known as hot lime slaking' in which quicklime (calcium oxide, CaO) produced from the calcination of calcium carbonate, is mixed with a wet sand aggregate and worked during the slaking period or whilst the material is hot. In this slaking process the aim is to mix the calcium oxide (CaO) lime with water to produce calcium * 25 hydroxide (Ca(OH)2) known as slaked lime. In traditional lime mortar production the slaking process is carried out over a period of several days, * after which the slaked lime is viewed as the binder for the mortar aggregate, which is typically quarried sand. This technique can be used to S..

* produce either a dry mixture or a fluid mortar depending on the dosage rate of water. However the quality of the product can vary greatly and result in frequent unsoundness of the mortar. Whilst lime mortars were common until the late I 9th or early 20th century their use has fallen out of favour in more recent history.

Portland cement based mortars, usually referred to as cement-based mortar, incorporate a cement produced by grinding the clinker produced from the high temperature burning of calcareous and argillaceous materials. Such cement based mortars have been used almost exclusively in building projects in more modern times. This has led to lime mortars being reserved almost exclusively for restoration and conservation projects of older buildings of historical interest. One reason for this has been a perception that lime mortars do not have the properties, such as reliability, strength etc., required for modern construction techniques.

Portland cement based mortars were traditionally produced on site using volumetric batching procedures. However, these have poor quality control due to variability of volume batching which leads to a lack of consistency in the mortar composition. These problems were alleviated by the production of factory-based mortars for subsequent delivery to site in a silo.

The use of such pre-blended dry silo cement mortars is common practice throughout Europe. Pre-blended dry silo cement mortars are produced in computer controlled facilities which allow accurate gravimetric batching of fine aggregate, binders and appropriate admixtures. Hence, a wide range of site specific mix designs may be created and many of the potential * problems arising from on-site' volumetric batching are eliminated. Further : *. advantages of a pre-blended mortar system include reduced wastage, lower labour costs and cleaner, quieter construction practice. Water is then added to the dry mix to obtain a workable paste. * S

Unfortunately, the advantages of dry pie-blended silo mortars are somewhat offset by an increase in the energy required in the manufacturing process. Usually mortars utilise quarried sands as the aggregate component, and these sands typically contain significant amounts of water (5-10 % by weight). This water must be removed prior to mixing with hydraulic binders in order to prevent deleterious hydration reactions occurring during storage of the mortar.

The drying of the wet aggregate usually takes place in large diesel fired kilns; this is an energy intensive process which imposes an additional environmental cost upon the final cement mortar material. This is of particular concern in the context of the current drive for greater sustainability in construction materials.

There is still the need for a lime mortar mix which is stable for a given period of time, does not degrade easily and can therefore be stored in a silo whilst not requiring an extensive energy investment to dry the aggregates.

There is also a need for a lime mortar mix which provides reliable and sound mortar and a method of manufacture thereof.

According to the present invention there is provided a process of producing a dry lime and sand mix comprising aggregates and a lime-based binder, characterised by the steps of: S.....

* a) admixing a pre-determined quantity of calcium oxide (CaO) : *. containing compound with wet aggregates, said aggregates S...

:. having a predetermined free water content, b) providing a period of storage **..* . . . c) admixing at least one hydraulic component or binder.

wherein the amount of calcium oxide used is in the range of about 109 wt % to 280 wt% of said water content or, alternatively, of about 35 to 90 mol % of said water content.

The lime and sand mix thus obtained according to the invention can be used as a base for a dry mortar mix. The calcium oxide-containing compound is preferably quicklime and advantageously acts as a drying and binding agent for a mortar aggregate.

In particular, but not exclusively, the present invention provides a controlled low energy process of drying a lime mortar aggregate, in such a way that the resulting mortar is sound and the degradation of the hydraulic components within the lime mortar product is minimised.

In addition, such a process provides the opportunity to include at least one hydraulic component to control the setting of the fresh mortar and the performance of the hardened mortar such that the final binder:aggregate ratio is appropriate to yield high quality and high performance mortars. The final binder aggregate ratio to be achieved would lie, typically, in the range 1:0.5 to 1:4 by volume ratio, preferably in the range 1:2 to 1:3.

The hydraulic component or binder which can be used includes a cement such as Portland cement, ground granulated blastfurnace slag, fly ash, silica fume or metakaolin, or natural hydraulic lime or a mixture thereof.

The use of granulated blast furnace slag and pulverised fuel ash allows the recycling of these otherwise waste products and advantageously *�.*** * lowers the carbon footprint of the product. * ** * * .

The process according to the invention advantageously provides an alternative route to obtain a dried mortar aggregate, mitigating the need to S....

S S

use energy intensive kiln drying techniques. This has advantageous energy saving, and environmental sustainability advantages.

It is preferable that the calculated amount of CaO used in the present process be less than 90 mol%, more preferably less than 80 mol% and even more preferably less of 75 mol% of the free water content. It has been found that to dry a wet aggregate an equimolar amount of quicklime is not actually required. This is due to the fact that the reaction of the calcium oxide and free water present in the mortar aggregate results in an exothermic reaction, according to the following equation; GaO + H20 -) Ca(OH)2 + 62.5KJ The exothermic nature of the reaction results in the evaporation of some water content of the aggregate.

In fact, it has surprisingly been found that a stoichiometric amount of CaO with respect to free water is actually undesirable, as, although all the free water present in the mortar aggregate is bound, and therefore the aggregate is rendered dry, the aggregate is actually over dried, resulting in the occurrence of residual unslaked CaO and water being removed from internal pores within aggregate particles. This results in unsound final mortar products.

As such, the mortar aggregate is mixed with controlled amounts of CaO-containing compound, such as quicklime, and water is removed from the * *. ..* * S aggregate via the hydration of the CaO. In addition, further water is : *. removed by evaporation as a consequence of the heat generated by the *I** exothermic hydration reaction; thus, there is no need to utilise the full stoichiometric amount of quicklime to fully combine with the moisture *.**I* * * within the sand. In addition, the produced Ca(OH)2 is porous and may absorb additional water from the wet sand.

"Free water" is a term used to describe the surface water present on, and between particulate materials. It does not include any water contained in the particles internal pore structure. Water present in particulate pores, is effectively locked up in the particle such that degradation during storage is avoided. However, moisture content measurements will typically indicate the total water content of a sample tested. All references to moisture content in this application refer to free water, or moisture, content.

Conversely, it is preferred that the amount of GaO used in the process be no lower than 30 mol%, more preferably no lower than 40 mol% and even more preferably no lower than 55 mol% of free water content. Although lower stoichiometric amounts have been found to result in material which visually appears to be dry, and flow freely, this drying process using a low amount of GaO results in lime mortar mixes which are easily degraded on storage. Therefore, such process is not suitable for the production of dry silo products.

Preferably the CaO-containing product is a powdered quicklime. An example of suitable commercially available product is Superfine GL, manufactured by Lhoist, Derbyshire, UK. Typically, commercially available quick lime compounds have a calcium oxide (CaO) purity of at least 90% byweight.

* .. ** * * : *. Desirably, the aggregate to be used in the process of the invention is sand having a free moisture content not higher than 15% by weight, and most preferably the aggregate has a free moisture content of between 4% and 30 8%. As such, commercially available construction grade sands can be readily used in the present invention, this includes typical graded, sharp, pit sands as received from quarries. These readily available construction grade sand aggregates fall within the latter moisture content range.

However, sub optimal storage, i.e. outside in uncovered heaps, can result in the aggregate moisture being outside the above range.

Preferably the process of producing the lime mortar product is a batch or continuous process which comprises the following method steps; -determining the free water content of the mortar aggregate; -providing an amount of quick lime required to provide a CaO amount ranging between 55 to 75 mol% of said free water content; -mixing said aggregate and quick lime for a given period of time; -storing the dry lime mortar product thus obtained for a minimum period of time of 1 hour, suitably less than 3 days, desirably less than 24 hours and most preferably less than 12 hours and, -mixing the dry lime mortar product with a suitable hydraulic component or binder.

For example, preferred amount of mol% of calcium oxide (+/ 2%) can be selected as a function of both the moisture content of the sand (rn/c %) and the mixing time of the wet mixture using the following equations: For a 5 minute mixing time: Mol% CaO = -2.9 x rn/c (%) + 83 * * *** .** * For a 10 minute mixing time: : *.** Mol% CaO = -1.6 x rn/c (%) + 69.5 ****

I S..

Additionally, for a 15 minute mixing time: 30 Mol% CaO = -1.15 x m/c (%) + 64 There is the potential for an increased margin on the maximum value but only if the mortar is stored for prolonged periods before use.

More preferably the process is a batch process, and more preferably the batch process is performed in an open design mixing vessel. Use of an open mixing vessel allows the heat generated during the reaction of the quick lime and aggregate to escape, facilitating the removal of non-reacted water, as water vapour.

Alternatively, the batch process may be performed in a rotary drier. This advantageously provides a means of forcing a flow of material through a flow of air, which will promote removal of the non-reacted water.

In addition, it is preferable that the time period of the mixing step of the calcium oxide containing compound with the wet aggregate be at least 5 minutes but no-more than 30 minutes, and advantageously less than 15 minutes. This period is sufficiently short so that the energy saving made by using a non-kiln drying method is not negated by overtly long mixing times. More preferably, the amount of quick lime used in the process and the period of time available for the mixing step are optimised. A shorter mixing time will require a higher amount of lime, as in the former cases there will be less time opportunity for evaporation of water vapour generated. ** * *

It is further preferred that the dry lime mortar product is stored for a I.....

* minimum period of one hour and preferably 8 hours or more. It is however : **. preferable that the storage period does not exceed 3 days and most *q** preferably does not exceed 24 hours. *

Preferably the process is controlled by a computer. This eliminates the dangers of miscalculation by plant process operatives, and is typically employed in modern production plants for other pre-blended construction products.

According to a further embodiment of the present invention there is provided a pre-blended dry lime mortar product suitable for silo storage without degradation comprising an aggregate, a binder, and a hydraulic phase.

Preferably such a product shows no unacceptable degradation after up to 3 months of storage and more preferably after 6 months of storage such that the mortar fails to comply with its designated performance criteria as typified by a strength classification.

The present invention will now be further described by way of non-limiting examples and with reference to the following Tables and Figures and the use of 6 quicklimes (A -F) and 3 sands (1 -3): Figure 1 shows the amount of free lime (CaO), as contained within quicklime F, to be added to sand S2 of free moisture content in the range of 3 to 7% for three mixing times of 5 (6), 10 (.) and 15 (A) minutes.

Figure 2 shows how to determine the optimum combination of the e...

parameters of moisture content of the aggregate, mol% of quicklime and S.....

* mixing time, for quicklime F and sand S2 using the graph of figure 1, to : .*. yield mortars of either 1 MPa (A) or 5 MPa (B).

I *5S

Figures 3a to 3d show the temperature profiles and weight loss due to evaporation, for the dry lime product for each quick lime mixes of 1 kg of sand Si at 5, 7.5,10 and 12.5 % (Figures 3a, 3b, 3c and 3d, respectively) moisture mixed with quick lime samples (A to E), at 50% stoichiometric amount.

Figure 4 shows the nitrogen adsorption isotherms, corresponding to adsorption (solid lines) and desorption (dashed lines) for quicklime E (,) and the slaked lime () produced from it after mixing with sand S3 at 10% free moisture content.

Figures 5a to 5d show the results of evaporated water for the combination of quicklime E and sand Si for mol% amounts of quicklime in the range 55 -70%, for three different mixing times 5 minutes, 10 minutes and 15 minutes and for four different initial moisture content of the sand SI (5%, 5.5%, 6.5% and 7.5%; Figures 5a to 5d respectively).

Figure 6 shows the optimal drying conditions for Quicklime E and sand 1 using a 15 minute mixing time with additional annotation to indicate both sub-optimal and super-optimal drying conditions. The experimental parameters used for sub-optimal and super-optimal drying of sand Si used in Examples 5 and 6 are indicated.

Figures 7a and 7b show the results of nitrogen adsorption on slaked for quicklime E which had been optimally dried with sand S3 for lime which was freshly slaked (open symbols and for a sample which was stored for 12 weeks following slaking (solid symbols). Figure 7a shows the * S. 55.

* adsorption (solid lines) and desorption (dashed lines) plots and Figure 7b ,* shows the pore size distribution plots derived from the data in Figure 7a. **5 a..

S * a S *

Figure 8 shows the influence of quicklime and sand types on the amount of quicklime required for optimal drying for sands in the range 3 -7.5% moisture content.

Figure 9 shows the experimental parameters for optimal drying of sand Si used in Examples I to 4 for three mixing times, 5, 10 and 15 minutes.

Table I below shows the key properties of the quicklimes used to illustrate this invention.

Quicklimes A B C 0 E F Free lime (%) 92 92.9 93.5 88 92.2 93.9 Insolubles (wt%) 6 6.7 6 11.9 6.1 -CaO (wt%) 87.6 91.1 92.4 87.5 90.9 92.5 Ca(OH)2(wt%) 5.8 2.4 1.4 0.7 1.7 1.9 CaCO3 (wt%) 3.7 0.9 1.3 5.8 3.8 2.1 ABET (m2/g) 2.556 1.492 1.244 1.817 2.219 2.068 2 2 9 4 2 5 V101 (crn3/g) 0.006 0.004 0.004 0.005 0.007.007 Bulk Density (kg/rn3) 929 950 1250 1049 942 -Table 1: Quicklime properties e.. * . I...

* *.*** * * * S.

S p S..

* S pa * S Table 2 shows the gradings of the sands as mass % passing each sieve.

Sand sample Si S2 S3 Mesh Size (mm) 4 99.3 95 100 2 95.8 83.8 100 1 91.7 67.4 98.4 0.5 74.8 45.6 3.8 0.25 31.3 25.2 0.2 0.125 7.6 13.6 0 0.063 1.9 7.1 0 Table 2: Sand properties As an initial trial several mortar mixes were produced using quicklime drying. However, the drying phase was halted when the mixture was' visually dry and free flowing. These were compared with similar mixes produced with sands dried to saturated surface dry (SSD) conditions.

Whilst the SSD materials could be stored without degradation, those produced using visually assessed lime-dried sands exhibited degradation; the mortars had the consistency of wet sand and possessed excessive bleeding in the plastic state, whilst subsequently yielding low strengths in the hardened state. Thus, a more sophisticated assessment of the drying process is essential and forms the basis of the invention described herein. *...

On the basis of the current invention Figure 1 shows the relationship between the required mol% of free lime (CaO), as contained within : ** quicklime F, to remove the free moisture from the sand and the free S. moisture content of the sand S2 as it is affected by the mixing time of the combined quicklime and wet sand mixture. In this case the weight of wet

S

S

sand being dried is 1 kg. This determination takes account of the purity of the quicklime as shown by the free lime contents in Table 1.

The strength of such lime dried mortars depends on the nature of the hydraulic component, the ratio of the hydraulic component to the slaked lime (i.e. the binder phase) and the ratio of the weight of water in the mortar and the weight of the binder phase. When the binder comprises slaked lime and ground granulated blastfurnace slag strengths following 28 days curing at 65% relative humidity of 5 MPa and 1 MPa can be obtained from mixes of slag:lime weight ratio of approximately 1.0 and 0.2 respectively for mortars of equal workability as measured by flow values of 17 cm.

Figure 2 shows the specification of the control parameters needed to obtain mortar strengths of 5 MPa and I MPa as a function of moisture content of the aggregate, mol% of quicklime and mixing time. The moisture content of the aggregate may be "brought into range" for the desired slag:lime ratio and the range of commercially specified mixing times by either drainage in a stockpile or the addition of extra water.

The construction of Figure 1 is based upon the following observations: 1. the rate of evaporation is a function of the source of quicklime 2. the rate of evaporation is a function of the moisture content of the wet sand S. * 3. the rate of evaporation is a function of the mol% of quicklime added . to the wet sand to be dried S...

* 4. the drying process is a function of the sand type :: 30 These principles are illustrated in Figures 3, 4, 5, 6, 7 and 8.

In order to assess the performance of the quick lime products as agents for drying sand, initial tests have been carried out using sand, Si. 1kg of sand was fully dried and then soaked in ordinary tap water to yield a free moisture content of 5%, 7.5%, 10% and 12.5% by weight. Subsequently all 5 of the quicklimes (A -E) were dosed at 50% of the stoichiometric amount for full conversion of the CaO to Ca(OH)2 (i.e. 50 mol%) and were mixed in a Kenwood Chef, mixer set at speed 1, for a period of 1 hour. A thermocouple was secured within the charge and the temperature logged every second. At intervals of 5 minutes the mixing was momentarily halted and the system weighed so that the weight loss due to evaporation could be determined. In this test no allowance was made for the purity of the quicklime which was assumed to comprise 100% GaO with no impurities.

Figures 3a to 3d show the temperature profiles and weight loss due to evaporation for each combination. It is apparent that for any given quicklime and sand mixture, the peak temperature increases with moisture content, as does the initial rate at which the water is evaporated when expressed as a percentage of free water evaporated after 5 minutes.

Results obtained in this regard are clearly shown in Table 3, below, where 20' the percentage values relates to the free water content of the sand sample, as described above.

It is apparent that as the moisture content increases a small reduction in the mol% required for optimal drying is observed and shown on Figure 1.

25 This would be anticipated on the basis of the above observation that * increased moisture contents of the sand increased the reactivity of the : *. lime and increased the initial rate of evaporation. *..* S.

S

The data also shows that as the mixing time is increased, the optimum :: 30 mol% decreases. Again, this is to be expected as a greater percentage of the free water is removed by evaporation; hence leaving less water to be removed by chemical reaction.

Table 3

Quicklime % of free water evaporated after 5 % of free water evaporated after I sample minutes mixing time hour mixing time %offreewater 5.00% 7.50% 10.00% 12.50% 5.00% 7.50% 10.00% 12.50% in weight in SI sand before the admixture of quicklime A 26.2 29.5 36.3 36.7 62.0 57.7 56.9 55.0 B 23.2 27.7 35.7 36.6 62.2 58.0 56.7 56.2 C 16.4 21.3 23.8 27.0 54.2 54.1 52.5 52.9 0 18.4 21.2 26.5 29.2 55.2 55.6 52.6 53.7 E 23.8 30.1 36.5 -59.8 56.7 56.2 -The more reactive quick limes (A, B and E), as assessed by rate of evaporation in the first 10 minutes and rate of temperature rise to the peak temperature show an increasing peak temperature and evaporation up to a moisture content of 10% by weight; the values at 12.5% by weight show only marginal increases. Since an evaporation of approximately 54% by weight is required to remove water which cannot be combined by slaking of the lime (the precise figure depends upon the purity of the individual limes) it can be seen that a 5 minute drying time yields a wet sand.

However, a drying time of 1 hour yields dry sands although those produced using limes C and D are marginal whilst the more reactive limes would appear to have caused the evaporation of such quantities of water so as not to provide sufficient for the complete slaking of the quicklime, this is particularly so for the 5% free moisture content. This data highlights the need for a high element of control over the aggregate drying process.

Sand S3 was used to facilitate the separation of the lime from the aggregate following the mixing period consequent upon the coarse and single size grading of the sand, thus allowing a detailed analysis of the resulting slaked limes formed. Mixes of 1 kg of sand S3 at 5, 7.5,and 10 % moisture content samples were prepared, and mixed with each quick lime sample (A to E), at 50 mol% (78 g, 117 g and 156 g respectively) and mixed for 20 minutes. Subsequently, the mixtures obtained were dried at 110°C until constant weight achieved and then placed over a 0.15 mm sieve to separate the sand and lime fractions.

Both the original quicklime and resulting slaked lime products were analysed for chemical species, surface area, porosity and density. As detailed above, the atypical laboratory grade nature of Sand 83 allows for a detailed accurate analysis of the properties of the final sand or quick lime mixture products since the particle sizes of the lime and sand are very different which they can be separated by simple sieving.

Table 4 shows the analyses of each original quick lime sample (A to E) together with those of the slaked limes produced in the experiment as detailed above for sand S3.

Table 4

Quick lime Quicklime Slaked lime Product S...

5% rn/c 7.5% rn/c 10% rn/c * A Free lime (%) 92 68.7 68.3 69 * *. Insolubles (wt%) 6 8.1 8 8.2 CaO(wt%) 87.6 0 0 0 Ca(OH)2 (wt%) 5.8 90.7 90.4 91.2 * CaCO3 (wt%) 3.7 5.9 5.5 5.6 *.*S.S ABET (m2Ig) 2.5562 16.3 19.01 23.13 VTOT(cm/g) 0.006 0.116 0.127 0.161 Bulk Density (kg/rn3) 929 547 545 542 B Free lime (%) 92.9 70.6 70.8 70.2 lnsolubles (wt%) 6.7 6.1 6 6.2 CaO (wt%) 91.1 0.3 0.8 0 Ca(OH)2 (wt%) 2.4 92.9 92.5 93.1 CaCO3 (wt%) 0.9 2.7 2.5 2.8 ABET (m2/g) 1.4922 19.62 20.54 23.2 VTOT (cm3lg) 0.004 0.125 0.133 0.135 Bulk Density (kgIm) 950 544 540 540 C Free lime (% CaO) 93.5 70.8 71 70.9 Insolubles (wt%) 6 5.7 5.9 5.8 CaO (wt%) 92.4 0.5 0.9 0.3 Ca(OH)2 (wt%) 1.4 92.9 92.6 93.2 CaCO3 (wt%) 1.3 3.1 2.9 2.9 ABET (m2/g) 1.2449 14.46 16.23 15.19 VTOT (cm3lg) 0.004 0.088 0.094 0.098 Bulk Density (kg/rn3) 1250 580 574 585 D Free lime (%) 88 65.8 65.5 65.1 Insolubles (wt%) 11.9 13.4 13.9 14.8 CaO (wt%) 87.5 0.5 0.4 0.3 Ca(OH)2 (wt%) 0.7 86.2 85.9 85.6 CaCO3 (wt%) 5.8 6.3 6.3 6.4 ABET (m2/g) 1.8174 18.37 19.04 23.06 VTOT (cm3lg) 0.005 0.101 0.104 0.121 Bulk Density (kgIm) 1049 559 549 555 E Free lime (%) 92.2 68.7 e**,I Insolubles (wt%) 6.1 4.8 CaO (wt%) 90.9 0 Ca(OH)2 (wt%) 1.7 90.8 CaCO3 (wt%) 3.8 6.9 ABET (m2/g) 2.2192 18.16 VTOT (cm3lg) 0.007 0.109 Bulk Density (kg/rn3) 942 559 It can be seen that the purity of the limes is reduced by the presence of insoluble residues, probably clays and sand, and small amounts of slaked and carbonated lime. Following the drying process it is apparent that the insoluble residue is commonly greater than that present in the original quicklime whereas it might be expected to reduce. This is attributed to a small fraction of sand which has been ground to <0.15 mm during the drying process. Consequently, the surface areas of the slaked product cited in Table 4 above are likely to be a slight underestimate of the key hydrated material essential to the hydration of the hydraulic phases such as ground granulated blastfurnace slag, fly ash, silica fume etc i.e. Ca(OH)2. Nevertheless, it can be seen that the surface area of the slaked lime is at least 1 order of magnitude greater than that of the original quicklime and, further, increases with increasing moisture content of the sand reflecting the increasing reactivity with increases in moisture content as shown by the increased maximum temperatures (see Figure 3a to 3d).

The surface areas obtained, for the quicklimes and slaked limes, fall within the typical range for such materials.

Figure 4 represents the output from the measurement of the adsorption of nitrogen onto the surfaces of both a quicklime and the slaked lime produced from it, showing differences in the N2 adsorption indicating differences in the structure of the two materials. The "as-received" quick lime compound displays a typical type II isotherm indicative of a non-porous solid. The isotherm is completely reversible and hysteresis is absent. In contrast, the slaked lime displays a type lIb isotherm consisting * of a normal type II adsorption branch with type H3 hysteresis on desorption. This form of isotherm indicates the presence of aggregates of non-porous plate-like particles, the individual particles consisting of the crystallites of calcium hydroxide. The observed hysteresis loop may be *.**II * 30 assigned theoretically to intercrystalline adsorption of nitrogen within the aggregates, in ill-defined pores at the point where crystallites come into contact. The presence of such porosity raises the possibility of a third drying process, that being absorption of water within this space.

Figures 5a to 5d show the results of evaporated water for the combination of quicklime E and sand Si for the four mol% amounts of quicklime, for three different mixing times 5 minutes, 10 minutes and 15 minutes and for four different initial moisture content of the sand SI (5%, 5.5%, 6.5% and 7.5%).

The first step requires that the balance of chemically combined water, and evaporated water, needs to be controlled. To achieve this step the quantity of water removed via chemical combination (CaO to Ca(OH)2) can be calculated according to equation I: (1) Wr(MxC)/iOO where: Wr = Free water removed (%) M = mol% CaO C = CaO content of as-received quicklime available for slaking (%) Hence, to ensure an adequately dry material, the remaining water must be removed by evaporation during the mixing process (equation 2): *..* (2) We=100-Wr * * : *. where: S. We = Required evaporation (%) (and is shown as the linear line in Figures 5a -5d). S.... S 30

The intersection points of the required and measured evaporation (indicated by closed symbols on Figure 5a) indicates the conditions for optimum drying where all of the quicklime should be slaked and the free moisture removed for each period of mixing. The value of mol% quicklime content is determined for each combination of moisture content of the sand and mixing time to create the representations shown in Figure 1.

In order to illustrate the Influence of moisture content and mol% of quicklime, quicklimes B and C were used at 40%, 50% and 60% mol% calculated in relation to sand I at a free moisture contents of 5% and 10% and to the chemical reaction: CaO + H20 -Ca(OH)2 + 62.5KJ. One kg of each sand sample to be tested was dosed with the desirable mol% amount of quicklime of interest, and the two components were mixed in a Kenwood Chef mixer set at speed ifor 1 hour. All sands were oven dried at 105°C for 24 hours in an oven without a circulated atmosphere and allowed to cool before the addition of sufficient water to yield the required free moisture content. The wet sands were allowed to stand overnight in an air-tight box to ensure a sufficient moisture saturation before the addition of the quicklime.

The analysis of the slaked lime products obtained as above is shown in Table 5, below, and exemplifies the feasibility and optimal mol% amounts of quicklime to use with a given moisture content aggregate sample. s*ö

25 The proportional amount of free water remaining has been estimated after S.....

* 1 accounting for that absorbed in the sand particles, assuming that they * ** * * . remain saturated. I...

I S..

The degree of lime conversion has been estimated by first determining the * .S s5.

* 30 amount of water which has reacted by calculating the difference between the total amount of water added to each mix and the sum of the water evaporated during mixing and the residual water. The residual water was determined by heating the mixture at 1 05C until constant weight was achieved. Subsequently, the amount of Ca(OH)2 was calculated assuming equimolar combination of CaO and reacted water.

As shown in Figures 3a to 3d above, it is apparent that the maximum temperature increases with the moisture content of the aggregate sample as well as with increases in the amount of quick lime utilised. It can be seen that low stoichiometric additions of quicklime do not fully remove all the free water, despite the visual appearance of the mixture being of a free-flowing dry material, whilst high quicklime additions do not necessarily fully slake all of the quicklime. The former case could lead to degradation of the combined mixture of lime/dried sand and hydraulic binder during the storage period in a silo before use on site whilst the latter may yield an unsound product should the mix water of the final mortar not rapidly slake the remaining quicklime during the mixing process. The use of either quicklime at 60 mol% and 10% moisture content generates such reactivity that water is removed from the pores of the sand particles and is shown in Table 5 below as 0-% water remaining. Negative water content would result in a less optimal final product.

Table 5

Quicklime Mol% TMAX (°C) Free H20 remaining (%) CaO converted (%) 5% rn/c 10% rn/c 5% rn/c 10% rn/c 5% rn/c 10% rn/c 4*s*** __________ ___________ __________ ___________ __________ ____________ __________ ___________ * B 40 41.9 66.3 15 19 98 99 * *. 50 50.6 78.1 0 0 98 100 * . . 4.* 58.1 77.4 0 0-96 91 4** __________ ___________ __________ ___________ __________ ____________ __________ ___________ * 40 34.6 49.1 16 21 99 99 C 50 37.1 60.2 0 1 99 100 46 70.4 0 0-98 90 It is apparent that low mol% can lead to inadequate drying whilst high mol% can lead to incomp'ete slaking and the potential for unsoundness.

This permits the definition of three stages of drying by the addition of quicklime, i.e. sub-optimal, optimal and super-optimal as shown on Figure 6. Figure 6 shows the optimal drying conditions for Quicklime E and sand I using a 15 minute mixing time. Super-optimal conditions are to be found above this line where there is the potential for unsoundness as incomplete slaking of the quicklime will free CaO. Conversely, sub-optimal conditions are located below this line where there is the potential for premature hydraulic reactions to occur during the storage period before use.

Quicklime E and sand 3 were mixed in 1 kg batches with moisture contents and mol% quicklime to yield the 3 conditions of sub-optimal, optimal and super-optimal drying indicated on Figure 6 indicated by the closed symbols and mixed for 15 minutes. The mixtures were stored in an air-tight container for periods up to 12 weeks. At intervals a sample was taken and the lime extracted by sieving and analysed (see Tables 6, 7 and 8).

The first analysis was undertaken immediately after the prescribed mixing time in order to provide base-line data. The sand was removed by sieving over a 0.15mm sieve to leave the lime fraction. This was analysed for total free lime by the titration method and CaCO3 and Ca(OH)2 using TGA (thermal gravimetric analysis). Following dissolution of the lime fraction in IM HCL it is apparent that a small amount of ground sand was generated during the drying process which could be quantified. This allowed a correction to be applied to the amount of CaO present as CaCO3, Ca(OH)2 and GaO in the lime fraction alone. Soundness evaluations were made in accordance with BS EN 459-2 after various periods of storage; a maximum expansion of 2 mm is permitted for a sound lime.

It can be seen that immediately following mixing the CaO conversion had not progressed to completion resulting in an unsound lime. However, a 24 hour period of storage increased the quantity of Ca(OH)2 with a corresponding decrease in CaO, the CaO effectively acting as an in-situ desiccant. For optimal and sub-optimal conditions, the storage period also facilitated the production of a sound hydrated lime whereas the super-optimal material remained unsound. It is apparent that none of the mixtures yield complete slaking immediately following the mixing period.

The benefit of a storage period of 24 hours is clear and this forms an essential part of the invention described herein. Consequently, the hydraulic phase should not be added until the quicklime is sufficiently slaked, typically after 24 hours of commencement of the mixing process and more preferably after 8 hours.

The initial increase in Ca(OH)2 observed during the first 24 hours is accompanied by an increase in the BET surface area, in accordance with previous data. The long-term development of surface area is a function of the drying conditions. The storage of optimally dried material produces a reduction in surface area following the increase observed in the first 24 hours. The rate of decrease increases with period of storage.

The data shows a reduction in the adsorption capacity of the lime following storage and a large reduction in the volume of smaller pores (ca. <100 A), as can be seen in Figures 7a and 7b respectively.

". 25 Table 6 Sub-optimal E13 0* __________ ___________ ____________ H20 ABET Ca(OH)2 CaCO3 CaO Soundness Sample (wt.%) (m2Ig) (asCaO) (asCaO) Exp(mm) I... ___________ ___________ ___________ ___________ ___________ ___________ _____________ CaO 0 2.2 1.3 -2.2 92.8 >4 After mix 5.4 20.6 74.7 6 15.6 >2 24hr 5.1 23.7 89.5 -5.8 1.1 0.09 a 72hr 5.1 no value 89.6 5.8 1 - 96hr 4.8 no value 89.5 5.9 1 - lwk 5 23.2 89.3 5.8 1.2 - 2wk 4.7 22.9 89 6.1 1.2 - 4wk 4.4 22.9 89.3 5.9 1.1 - 6wk 3.2 21.1 89.1 6.4 0.8 - 8wk 1.7 20.3 89 6.5 0.9 - l2wk 1.2 16 88.4 6.9 1 -

Table 7 Optimal E13

H20 ABET Ca(OH)2 CaCO3 GaO Soundness Sample (wt.%) (m'41g) (as CaO) (as CaO) Exp (mm) CaO 0 2.2 1.3 2.2 92.8 >4 After mix 10 20.9 77.5 5.9 12.9 >2 24hr 12.5 23.3 89.3 5.4 1.6 0 lwk 12.4 24.2 90.6 4.7 1 - 2wk 12.8 25.9 90 5.3 1 - 4wk 10.6 24.9 89.2 5.8 1.4 - 8wk 9,6 24.6 89.1 6.3 1 - l2wk 7 23.1 88.6 6.9 0.9 -Table 8 Super-optimal E13 H20 ABET Ca(OH)2 CaCO3 GaO Soundness Sample (wt.%) (m'Ig) (as CaO) (as CaO) Exp (mm) CaO 0 2.2 1.3 2.2 92.8 >4 Aftermix 1.2 18.7 81 4.3 11 >2 24hr 1.2 20.5 85.9 4.4 6.1 >2 *1 lwk 0.9 20.2 89.4 4.3 2.7 2.29 2wk 1 20.8 89.4 4.4 2.6 2.19 * . 4wk 1 20.3 89.6 4.3 2.4 1.86 * ** _________ __________ 8wk 1.1 19.5 90.5 4.5 1.3 0.23 l2wk 1.0 18.9 90.4 4.5 1.4 0.22 S.C..I * . It is apparent that the surface area (ABET) of the lime increases by approximately an order of magnitude as a consequence of the drying process. The surface area shows a gradual reduction with storage.

Table 9 shows a comparison of the slaked lime produced from quicklime C and the commercial CL9O slaked lime produced by the same manufacturer. The lime drying process yields a higher surface area and porosity (VTOT) than commercial slaking. Finer slaked lime will increase the water retention capacity of the newly mixed mortar and improve the microstructure of the hardened mortar yielding improvements to mechanical properties.

Table 9 Comparison of slaked lime Quick lime CL9O Slaked lime Product 5% rn/c 7.5% rn/c 10% rn/c C Free lime (% GaO) 71.3 70.8 71 70.9 Insolubles (wt%) 3.8 5.7 5.9 5.8 CaO (wt%) 0.9 0.5 0.9 0.3 Ca(OH)2 (wt%) 93 92.9 92.6 93.2 CaCO3 (wt%) 2.3 3.1 2.9 2.9 ABET(m'/g) 11.26 14.46 16.23 15.19 VTOT (cmIg) 0.072 0.088 0.094 0.098 Bulk Density (kg/ms) 575 580 574 585 * * I...

*....: 15 Figure 8 shows the influence of quicklime and sand types on the amount of quicklime required for optimal drying. Comparing combinations F/I and F/2 shows the coarser sand 2 requires slightly less quicklime than the finer sand 1. Quicklime C was sourced from the same supplier as quicklime F but is less reactive. It is apparent that the less reactive quicklime must be present in greater mol% quantities to achieve optimum drying.

It is thus possible to calculate the preferred amount of mol% calcium oxide as a function of both the moisture content of the sand and the mixing time of the wet mixture in order to obtain optimum drying conditions. The mol% calcium oxide should preferably be no more than 75% and no less than 55% depending on moisture content of the sand and the mixing time.

With reference to Figure 1 and quicklime F, specific amounts of calcium oxide may be calculated: For a 5 minute mixing time (3) Mol% CaO = -2.872 x rn/c (%) + 82.8 Additionally, for a 10 minute mixing time (4) Mol% CaO -1.633 x rn/c (%) + 69.5 Additionally, for a 15 minute mixing time (5) Mol% CaO = -1.149 x rn/c (%) + 63.7 Comparison of equations (3 -5) shows that the mol% CaO is more sensitive to moisture content of the wet sand for shorter mixing periods.

A 2% variation of the % of calcium oxide can be acceptable. There is the potential for an increased margin on the maximum value but only if the mortar is stored for prolonged periods before use to prevent the occurrence of an unsound lime.

* ** 25

Example 1.

* 9.000 kg of sand 1 (in the saturated surface dry condition) was mixed with 0.433 kg of water to yield a sand of 4.81% free moisture content. 0.939 kg of quicklime E (69.7% mol%) was added and mixed for 5 minutes in a Hobart mixer, speed 1. The mixture was subsequent'y stored in an air-tight container for 24 hours. This processing constitutes optimum conditions as defined by the current invention and shown in Figure 9.

Ground granulated blastfurnace slag (ggbs) was added to the mixture to yield a ggbs:calcium hydroxide ratio of 0.2. At this stage, the dry mortar mix was divided into two equal portions. One portion was stored in a sealed plastic container and retained under laboratory conditions (20°C, 50% RH) for 10 weeks. The other portion of the mix was prepared immediately. The mortars are designated "stored" and "fresh" respectively.

The materials for each mortar using either fresh or stored materials were mixed in a Hobart mixer, speed 1, with sufficient water to yield a mortar of flow 170 +1-5mm (according to BS EN 1015-3). The plastic mortars were cast into steel moulds of the required geometries and initially cured within the moulds for 3 days at 20°C and a relative humidity of 98%. The moulds were then stripped and the samples cured for a further 4 days under the same conditions. Subsequently, samples were cured at 20°C (� 2°C) and 65% RH (� 5%) prior to testing. The compressive strength (according to BS EN 1015-11) at an age of 91 days for the fresh mortar was 0.96 MPa and 1.18 MPa for the stored mortar.

Example 2

9.000 kg of sand 1 (in the saturated surface dry condition) was mixed with 0.482 kg of water to yield a sand of 5.35% free moisture content. 0.939 kg * * 25 of quicklime E (62.6 mol%) was added and mixed for 10 minutes in a Hobart mixer, speed 1. The mixture was subsequently stored in an air-tight container for 24 hours. This processing constitutes optimum conditions as defined by the current invention and shown in Figure 9.

Ground granulated blastfurnace slag (ggbs) was added to the mixture to yield a ggbs:calcium hydroxide ratio of 0.2. At this stage, the dry mortar mix was divided into two equal portions. One portion was stored in a sealed plastic container and retained under laboratory conditions (20°C, 50% RH) for 10 weeks. The other portion of the mix was prepared immediately. The mortars are designated "stored" and "fresh" respectively.

The materials for each mortar using either fresh or stored materials were mixed in a Hobart mixer, speed 1, with sufficient water to yield a mortar of flow 170 +1-5mm (according to BS EN 1015-3). The plastic mortars were cast into steel moulds of the required geometries and initially cured within the moulds for 3 days at 20°C and a relative humidity of 98%. The moulds were then stripped and the samples cured for a further 4 days under the same conditions. Subsequently, samples were cured at 20°C (� 2°C) and 65% RH (� 5%) prior to testing. The compressive strength (according to BS EN 1015-11) at an age of 91 days for the fresh mortar was 0.91 MPa and 1.18 MPa for the stored mortar.

Example 3

9.000 kg of sand 1 (in the saturated surface dry condition) was mixed with 0.512 kg of water to yield a sand of 5.69% free moisture content. 0.939 kg of quicklime E (58.9 mol%) was added and mixed for 15 minutes in a Hobart mixer, speed 1. The mixture was subsequently stored in an air-S..

tight container for 24 hours. This processing constitutes optimum conditions as defined by the current invention and shown in Figure 9.

* *. 25 Ground granulated blastfurnace slag (ggbs) was added to the mixture to ::. yield a ggbs:calcium hydroxide ratio of 0.2. At this stage, the dry mortar **.

mix was divided into two equal portions. One portion was stored in a sealed plastic container and retained under laboratory conditions (20°C, 50% RH) for 10 weeks. The other portion of the mix was prepared immediately. The mortars are designated "stored" and "fresh" respectively.

The materials for each mortar using either fresh or stored materials were mixed in a Hobart mixer, speed 1, with sufficient water to yield a mortar of flow 170 +1-5mm (according to BS EN 1015-3). The plastic mortars were cast into steel moulds of the required geometries and initially cured within the moulds for 3 days at 20°C and a relative humidity of 98%. The moulds were then stripped and the samples cured for a further 4 days under the same conditions. Subsequently, samples were cured at 20°C (� 2°C) and 65% RH (� 5%) prior to testing. The compressive strength (according to BS EN 1015-11) at an age of 91 days for the fresh mortar was 0.96 MPa and 1.21 MPa for the stored mortar.

From Examples I -3 it can be seen that the optimum process produces mortars of the same strength (95% significance) which do not deteriorate in storage when the ggbs:calcium hydroxide ratio is kept constant.

Example 4

9.000 kg of sand 1 (in the saturated surface dry condition) was mixed with 0.363 kg of water to yield a sand of 4.03% free moisture content. 0.689 kg of quicklime E (60.9 mol%) was added and mixed for 15 minutes in a S...

Hobart mixer, speed 1. The mixture was subsequently stored in an air-tight container for 24 hours. This processing constitutes optimum * *, 25 conditions as defined by the current invention and shown in Figure 9.

Ground granulated blastfurnace slag (ggbs) was added to the mixture to S..

yield a ggbs:calcium hydroxide ratio of 1.0. At this stage, the dry mortar mix was divided into two equal portions. One portion was stored in a sealed plastic container and retained under laboratory conditions (20°C, 50% RH) for 10 weeks. The other portion of the mix was prepared immediately. The mortars are designated "stored" and "fresh" respectively.

The materials for each mortar using either fresh or stored materials were mixed in a Hobart mixer, speed 1, with sufficient water to yield a mortar of flow 170 +1-5mm (according to BS EN 1015-3), The plastic mortars were cast into steel moulds of the required geometries and initially cured within the moulds for 3 days at 20°C and a relative humidity of 98%. The moulds were then stripped and the samples cured for a further 4 days under the same conditions. Subsequently, samples were cured at 20°C (� 2°C) and 65% RH (� 5%) prior to testing. The compressive strength (according to BS EN 1015-11) at an age of 91 days for the fresh mortar was 6.84 MPa and 7.19 MPa for the stored mortar. As before, the optimum process has yielded a mortar which does not deteriorate in storage.

Example 5

9.000 kg of sand I (in the saturated surface dry condition) was mixed with 0.512 kg of water to yield a sand of 5.69% free moisture content. 1.068 kg of quicklime E (67.0 mol%) was added and mixed for 15 minutes in a Hobart mixer, speed 1. The mixture was subsequently stored in an air-tight container for 24 hours. This processing constitutes super-optimum conditions as defined by the current invention and shown in Figure 6.

Ground granulated blastfurnace slag (ggbs) was added to the mixture to yield a ggbs:calcium hydroxide ratio of 0.2. At this stage, the dry mortar * .. 25 mix was divided into two equal portions. One portion was stored in a sealed plastic container and retained under laboratory conditions (20°C, S..

50% RH) for 10 weeks. The other portion of the mix was prepared *:*" immediately. The mortars are designated "stored" and "fresh" respectively.

The materials for each mortar using either fresh or stored materials were mixed in a Hobart mixer, speed 1, with sufficient water to yield a mortar of flow 170 +1-5mm (according to BS EN 1015-3). The plastic mortars were cast into steel moulds of the required geometries and initially cured within the moulds for 3 days at 20°C and a relative humidity of 98%. The moulds were then stripped and the samples cured for a further 4 days under the same conditions. Subsequently, samples were cured at 20°C (� 2°C) and 65% RH (� 5%) prior to testing. The compressive strength (according to BS EN 1015-11) at an age of 91 days for the fresh mortar was 1.66 MPa and 1.88 MPa for the stored mortar. There is no statistical difference between these results (95%). In this instance the lack of soundness (Table 8) of the binder (expansion greater than 2 mm according to BS EN 459-1) in the fresh mortar has not had an adverse influence on strength. * * **** * * . * ** * * * * * *

*.b... * *

Claims (15)

1. CLAIMS1. A process for producing a dry lime and sand mix comprising aggregates and a lime-based binder, characterised by the successive steps of: a) admixing a pre-determined quantity of calcium oxide (CaO) containing compound with wet aggregates, said aggregates having a predetermined free water content; b) providing a period of storage; and c) admixing at least one hydraulic component or binder wherein said quantity of calcium oxide used in step a) is in the range of about 109 to 280 wt% of said free water content or, alternatively, of about 35 to 90 mol% of free said water content.
2. 2. The process of claim 1, wherein said calcium oxide-containing compound is quicklime.
3. 3. The process of claim I or 2, wherein said additional hydraulic component or binder is added in proportion such that the final binder:aggregate ratio lies, typically, in the range 1:0.5 to 1:
4. 4 by volume ratio. S..S..... 4. The process of claim 3, wherein said hydraulic component or binder is chosen in the group selected from Portland cement, ground * *. 25 granulated blastfurnace slag, fly ash, silica fume, metakaolin, or natural hydraulic lime or a mixture thereof.

   S
5. 5. The process according to any one of claims ito 4, wherein the calculated amount of CaO is less than 80 mol% of said free water content.
6. 6. The process according to claim 5, wherein said calculated amount of CaO is less than 75 mol% of said free water content.
7. 7. The process according to anyone of claims 1 to 6, wherein said amount of CaO is no lower than 40 mol% of free water content.
8. 8. The process according to claim 7, wherein said amount of CaO is no lower than 55 mol% of free water content.
9. 9. The process according to anyone of claims 1 to 8, wherein said aggregates have a free water content not higher than 15% by weight.
10. 10. The process according to claim 9, wherein said free water content is chosen in the range of 4% to 8%.
11. 11. The process according anyone of claims 1 to 10, wherein said process is a batch or a continuous process which comprises the following successive steps: -determining the free water content of the mortar aggregate; -providing an amount of quick lime required to provide a CaO amount ranging from 55 to 75 mol% of said free water content; -mixing said aggregate and quick lime for a given period of *.....* . Lime, -storing the dry lime mortar product thus obtained for a e.* minimum period of time of 1 hour; and *** -mixing the dry lime mortar product with a suitable hydraulic component or binder.
12. 12. The process according to anyone of claims ito 11, wherein said mixing time of the calcium oxide containing compound with the wet aggregate is at least 5 minutes.
13. 13. The process according to claim 12, wherein said mixing time does not exceed 30 minutes.
14. 14. The process according to anyone of claims ito 12, wherein the duration of said storage step b) is at least one hour.
15. 15. The process according to claim 13, wherein the duration said storage step b) does not exceed 3 days. S... * S *Sp*S** * S. * S r **-S S e.SS**.. SI S