WO2008000085A1

WO2008000085A1 - Process for producing a concrete additive from an agricultural residue

Info

Publication number: WO2008000085A1
Application number: PCT/CA2007/001164
Authority: WO
Inventors: Ashraf El-Dammaty
Original assignee: The University Of Western Ontario
Priority date: 2006-06-30
Filing date: 2007-06-29
Publication date: 2008-01-03

Abstract

A process for the treatment of agricultural waste, particularly from rice farming operations. The process may be used to produce a rice straw ash that is particularly suitable for use as a concrete additive. The process employs leaching of alkali metals from the rice straw prior to conversion of the straw to ash in order to attain these desirable concrete additive properties. The alkali metals, particularly potassium, may be used to produce a fertilizer. The ash produced has a high amorphous silica content and a low carbon content. The concrete additive made using the ash produces concrete having particularly high compressive strength and resistance to chloride penetration.

Description

PROCESS FOR PRODUCING A CONCRETE ADDITIVE FROM AN AGRICULTURAL

RESIDUE

Field of the Invention

[01] The invention relates to a process for the treatment of agricultural residue, for example residue produced during the farming and/or processing of rice. More particularly, the invention relates to a process for producing a concrete additive from rice straw that employs an aqueous solvent to reduce the equivalent alkali content of the straw, thereby increasing the quantity of amorphous silica in the ash produced during oxidation of the rice straw.

Background

[02] The disposal of agricultural residue is a significant problem in farming and food processing operations. In the case of cereal crop residue such as straw and husks produced during the harvesting and separation of seed kernels from plant matter, the residue is often left in the field, which represents an under-utilized potential income stream for the farmer. For some crops, the residue is burned. The burning of rice straw, in particular, poses a significant environmental hazard due to smog formation. Since rice is grown throughout the world and is an especially significant crop in the developing world, a solution is needed that allows farmers to realize the economic value of crop residue while at the same time disposing of it in an environmentally friendly manner.

[03] In the making of concrete, silica-based additives are often employed to increase compressive strength and resistance to chloride penetration. The most common source of silica is silica fume, which is a product that is mined from deposits of limited size using sometimes environmentally questionable practices. Another source of silica- based additives is fly ash produced from coal-fired power plants; this source sometimes also contains sulfur, which is undesirable, and is in jeopardy of being phased-out as conventional coal-fired power plants are replaced by coal gasification and nuclear power plants.

[04] It has been reported in the literature at least as early as 1974 that ash produced from combustion of rice hulls, rice husks and/or rice straw may be used as a high-silica concrete additive. However, in order to be effective as an additive, the silica must be in an amorphous state, and this is difficult to achieve unless combustion is carefully controlled. The ash must also have a low carbon content, expressed as Loss on Ignition (LOI), and this is difficult to achieve using the combustion conditions required to render the silica amorphous. In particular, a phenomenon called "centering" occurs in the combustion of rice straw having a high alkali content (potassium and sodium). Centering causes the straw to form a ball or plug within the furnace that prevents the effective conversion of the straw to ash that is high in amorphous silica while also being low in residual carbon content. As a result, environmentally friendly ash-production processes are needed that produce ash without centering.

[05] WO 0187793 discloses a process for making stabilized agglomerates from ash of rice husk or rice straw. This patent discloses the mixing of rice straw ash with agglomerating agents to make an aqueous dispersion and the grinding of the mixture to destroy the silica structure. The mixture is then filtered and atomized. This process does not focus on the production of the rice straw ash, but rather on downstream processing operations. In addition, this process is too complicated and expensive to be readily employed in the developing world.

[06] US 3,889,608 and US 3,959,007 disclose an apparatus and process, respectively, for the preparation of siliceous ashes. The ashes produced from rice hulls are high in amorphous silica and are particularly useful in hydraulic cement. The apparatus comprises a furnace having certain internal features. No pre-treatment of the rice hulls prior to combustion is disclosed.

[07] JP 11323752 discloses a process for the production of a material with high content of silica from rice straw. The material is useful as a cement additive. The process involves the use of two stages; the first at 300-800 ⁰C to produce a charcoal and the second at 550-900 ⁰C to produce the ash. An acid of a buffer solution is preferably added to increase hydrogen ion concentration. No leaching pre-treatment of the rice straw is disclosed.

[08] JP 59146594 discloses the treatment of plant containing lignocellulose (eg: rice straw) by enzymatic hydrolysis and microwave heating. Although hydrolysis is employed, no ash is generated.

[09] US 5,346,548, US 4,105,459, JP 03040947, BR 8404410, GB 2050341 , IN 74- CA2328 and IN 142246 all disclose the use of silica-containing rice straw or rice hull ash as cement additives. The focus of these disclosures is on the use of the additive rather than its preparation.

[10] All of the approaches in the prior art have drawbacks and there is still a need for an improved processes for treating agricultural residue and/or preparing concrete additives from agricultural residue, particularly using rice straw ash.

Summary of the Invention

[11] It has been found that pre-treating agricultural residue to remove alkali metals prior to conversion of the residue to ash reduces or eliminates the centering phenomenon, thereby allowing substantially all of the residue to be converted. In the combustion of rice straw, this allows an ash to be created that has a high amorphous silica content and low carbon content (LOI). This ash is particularly suitable for use as a concrete additive when its particle size is reduced to less than 10 microns. Concrete made using the additive exhibits superior strength and resistance to chloride permeability as compared with conventional silica fume.

[12] According to the present invention, there is provided a process for the treatment of an agricultural residue comprising: providing a supply of the agricultural residue; leaching the agricultural residue using an aqueous solvent to reduce an equivalent alkali content of the residue to less than or equal to 2.5%; drying the leached residue to a moisture content of less than or equal to 35% by weight; converting the leached residue to ash.

[13] An example of a suitable agricultural residue includes the residue of rice farming and processing operations, particularly rice straw. To improve the recovery of products of economic value from the process, the process may further comprise collecting an aqueous leachate and separating alkali metals from the leachate. This is particularly useful in the separation of sodium or potassium using, for example, ion exchange processes in order to make use of the potassium as an agricultural fertilizer, either as a product recovered from the leachate or, when sodium alone is removed, as a leachate based liquid fertilizer rich in potassium. A chopping and/or grinding operation may be employed prior to the leaching operation to reduce the particle size of the residue in order to increase the efficacy of leaching operations. The leaching may take place at elevated temperatures (eg: 50-100 ⁰C or 80-99 ⁰C), may take place in more than one leaching stage, and may employ an aqueous solvent (eg: water) having a pH of 6.5 or less. [14] According to another aspect of the invention, there is provided a process for the preparation of a concrete additive comprising: providing a supply of rice straw; leaching the rice straw using an aqueous solvent to reduce an equivalent alkali content of the rice straw to less than or equal to 2.5%; drying the leached rice straw to a moisture content of less than or equal to 35% by weight; converting the leached rice straw to rice straw ash; reducing the average laser diffraction particle size of the ash to less than or equal to 10 microns, thereby producing the concrete additive. The rice straw ash produced according to the above process may have an amorphous silica content of at least 80% and an LOI of less than or equal to 6%, making it especially suitable for use as a concrete additive.

[15] According to yet another aspect of the invention, there is provided a concrete additive made according to the above process and a concrete made therewith. Due to the unexpected improvement in concrete strength and resistance to chloride penetration provided by the invention, a relatively small amount of the additive is needed in order to attain desirable improvement in the concrete. The additive may be present in an amount of from 5 to 15 wt%, preferably from 7.5 to 12.5 wt%. The additive may be used to produce a concrete having a 7 day compressive strength of at least 49 MPa and/or a 7 day rapid chloride penetration of less than or equal to 1500 Coulombs. The concrete additive provides equivalent or improved properties when compared with silica fume having a similar silica content provided in similar proportions. In one embodiment, the concrete has a compressive strength after 7 days at least 5 % greater than a concrete having an equivalent proportion of silica fume of similar amorphous silica content.

Brief Description of the Drawings

[16] Having summarized the invention, preferred embodiments thereof will now be described with reference to the accompanying figures, in which:

[17] Fig. 1 shows the particle size distribution of ground straw;

[18] Fig. 2 shows the particle size distribution of ash before grinding;

[19] Fig. 3 shows the particle size distribution of ground ash;

[20] Fig. 4 shows the X-Ray Diffraction results for the ash;

[21] Fig. 5 shows the compressive strength results for concrete made using the ash; [22] Fig. 6 shows the rapid chloride penetrability results for concrete made using the ash;

[23] Fig. 7 shows a schematic process flow diagram of a process according to the present invention.

Detailed Description

[24] Referring to Fig. 7, agricultural residue comprising rice straw is provided first to a chopper 1 and then to a grinder 2 in order to reduce its particle size. The residue preferably has a low moisture content prior to size reduction and may need to be dried to a moisture content of less than 50% by weight, preferably less than 35% by weight, in order for size reduction to work effectively. The average residue particle size is reduced to less than 15 cm, preferably less than 10 cm, more preferably less than 5 cm, yet more preferably less than 3 cm, even more preferably less than 2.5 cm prior to conducting the leaching operation. The particle size of residue provided to the leaching vessel may be reduced to less than 2.5 cm, for example down to an average particle size of about 1 mm, in order to improve the rate of leaching, particularly when lower temperatures are used.

[25] The rice straw is then conveyed to a leaching tank 3. The leaching tank is of a suitable design known to persons skilled in the art and can employ batch, semi- continuous, or continuous leaching techniques. In one embodiment, the leaching tank 3 includes agitation means to keep the contents of the tank in a well-mixed state in order to ensure that the rate of leaching is not hampered by bulk-phase mass transfer limitations. In another, the leaching tank employs a screen at the bottom for accumulation of the residue in response to a continuous downward flow of the aqueous solvent through the tank. The leaching operation is conducted until the equivalent alkali content of the residue is less than or equal to 2.5 wt%.

[26] The conditions employed during the leaching operation are selected in order to reach the target equivalent alkali content within a specified retention time and associated volume of the leaching vessel. The pH of the aqueous solvent can be made acidic, for example less than or equal to 6.5, through addition of strong acids such as HCI. The temperature of the aqueous solvent is from 1 to 25 ⁰C, or alternatively may be elevated, for example in the range of 50 to 100 ⁰C or 80 to 99 ⁰C. Elevated temperatures are preferably obtained using energy recovered from downstream ash production processes. The leaching is conducted in at least one stage or in several stages, for example in two or more stages, in order to achieve the desired efficacy of the leaching operation. Each stage employ a retention time of at least 15 minutes, which is desirably reduced depending upon the foregoing conditions and the tank volume. The solids to water mass ratio (S/W) is at least 1 :35 and in some embodiments may be increased to 1 :50 in order to obviate the need for a second leaching stage.

[27] In order to increase the economic value of the process, the collected leachate is optionally treated using ion exchange 4 to separate sodium or potassium from the leachate. This allows for production of a fertilizer having agricultural value, particularly in the developing world. In one embodiment, the potassium is recovered from the ion exchange resin and utilized as a fertilizer feedstock. In another embodiment, sodium is removed from the leachate using the ion exchange resin and the purified leachate is then useful as a liquid fertilizer rich in potassium.

[28] The leached residue is then dried to a moisture content of less than or equal to 35%, preferably less than or equal to 25%, in a rotary solids dryer, screw press or drying oven 5. The dried residue is then converted to ash in a suitable reactor 6. In one embodiment, the reactor 6 is an oxidation reactor. The oxidation reactor 6 is any suitable reactor known to persons skilled in the art that accomplishes substantially complete and uniform oxidation of the residue to form ash. When rice straw is employed, uniform combustion produces an ash having a high amorphous silica content. One example of a suitable oxidation reactor 6 is disclosed in United States patent 6,139,313, which is incorporated herein by reference. Another example of a suitable oxidation reactor 6 includes a fluidized bed reactor. The fluidized bed preferably employs a siliceous sand media as a heat transfer medium and as an aid in ensuring substantially uniform mixing of the relatively low density agricultural biomass throughout the reactor without escaping into the freeboard with the fluidizing medium.

[29] Following conversion of the residue to ash, the particle size is reduced to less than 10 microns through a grinder 7. The average particle size of the ash is preferably less than 7 microns, more preferably less than 6 microns. The ground ash is then suitable for use as a concrete additive.

[30] Not wishing to be limited by theory, it is believed that the leaching of alkali metals from the rice straw prior to conversion to ash increases the uniformity and efficacy of combustion to produce an ash having a higher amorphous silica content and lower carbon content (expressed as Loss on Ignition, or LOI) than in conventional rice straw ashes. Amorphous silica is more desirable than crystalline silica in concrete additive applications. A low LOI is desirable in concrete additive applications. The ash produced according to the present invention has an LOI of less than or equal to 6%, preferably less than or equal to 5%, more preferably less than or equal to 3% by weight. The amorphous silica content of the ash is preferably at least 80%, more preferably at least 84%, yet more preferably at least 85%, even more preferably at least 86%, still more preferably at least 87%.

[31] When used as a concrete additive, the ash according to the present invention is added in a surprisingly low amount relative to silica fume in order to produce a concrete having comparable compressive strength and chloride penetrability. The additive is added in an amount of from 5 to 15 wt%, preferably from 7.5 to 12.5 wt%. This desirably results in a concrete having equivalent or greater compressive strength and/or rapid chloride penetrability as compared with concrete made using silica fume at comparable levels. Concrete made using the additive preferably has a compressive strength of at least 49 MPa and a rapid chloride penetrability of less than or equal to 1500 Coulombs.

[32] The invention will now be further described with reference to the following examples. It is to be understood that these examples are illustrative and not to be construed in a limiting sense.

Examples

Chemical Composition of Rice Straw

[33] A chemical analysis for a sample of rice straw from Egypt was obtained by conducting a complete oxide analysis and the results are provided in Table 1.

Table 1 : Inorganic Composition of Rice Straw

[34] The analysis revealed that the ash represents about 15% of the total mass of the straw while the rest of the mass is in the form of organic materials. It can be seen from Table 1 that the rice straw is rich in silica. However, for use as a concrete additive, it is important that this silica is provided in an amorphous state with low carbon content. If uniform combustion of the straw is conducted under controlled conditions, the silica can be converted to an amorphous state with low carbon content , and in this case, it can have a positive effect in enhancing the mechanical properties of concrete.

[35] A preliminary combustion test for rice straw was previously conducted using a pilot scale reactor and a phenomenon described as "centering" was observed during this test. During combustion, the tested straw accumulated together and blocked the reactor, preventing completion of the test. This phenomenon was attributed to the presence of a high percentage of alkali in the straw composition in the form of potassium and sodium. A hydrolysis process to remove such minerals from the composition of the straw prior to combustion was subsequently investigated. The equivalent alkali content of the straw, defined as Na₂O + 0.658 K₂O, is provided in Table 1 , which shows a value of 10.14% for the sample.

Laboratory Scale Hydrolysis Tests

[36] A large number of tests were conducted on samples of rice straw in order to determine the optimum hydrolysis conditions for application in a full-scale process. The objective was to develop an economical and practical hydrolysis process for use in reducing the equivalent alkali content of the straw to less than 2.5% of the total ash. The first tests involved leaching straw in water for a specified duration. The leaching process was repeated over a number of stages. These tests involved varying the following hydrolysis variables: a) the ratio between the mass of the straw and the mass of water (S/W); b) the water condition (hot or cold and, in some cases, an acid was added to the water); c) the duration of each leaching stage; and, d) the particle size of the straw.

[37] The tests were conducted using both ordinary (tap) water and de-ionized water. However, there was no evidence that the efficiency of the hydrolysis process is influenced by using either type of water. Results of the chemical analysis of the water are shown in Table 2.

Table 2 Chemical Analysis of Water Used in the Hydrolysis (in ppm)

Hydrolysis Test Conducted on Chopped Straw

[38] The first set of laboratory tests was conducted on straw chopped to small size before conducting the hydrolysis. The average size of the chopped straw was about 4 inches (101.6 mm). A description of eight trial tests conducted in this set of experiments is provided in Table 3 All the tests included three leaching stages, except test C3, which included four stages. The leaching conditions applied in all stages following the first stage were identical.

Table 3. Description of Samples Used in the Hydrolysis of the Chopped Straw

^* Included four leaching stages

[39] Oxide analyses were conducted for all samples of chopped straw collected at the end of the hydrolysis process. The results of these oxide analyses are shown in Table 4 and indicate that only the hydrolysis conditions applied in tests C1 , C2 and C4 achieved the target level of less than 2.5% for equivalent alkalis. From Table 3, it can be noted that the first stage of these three tests involved leaching the chopped straw in boiling water. In a full-scale process, this would involve boiling a large amount of water, which might be impractical in terms of the amount of consumed energy. However, it is expected that warm water (50-100 ⁰C or 80-99 ⁰C) could be used with similar effect. Leaching conducted using cold water did not achieve the desired results, even when the leaching was prolonged, as in test C8.

Table 4: Results of Oxide Analysis (wt%) of Chopped Rice Straw After Hydrolysis

[40] Chemical analysis was conducted for samples of the liquid that resulted from different stages of leaching in tests C1 and C2. Results are provided in Table 5 below.

Table 5: Chemical Analysis of Liquid (mg/l) Resulting from Hydrolysis of Chopped

Straw

[41] A mass balance analysis for these two tests was conducted using this data together with the chemical composition of the straw before and after hydrolysis. Details of the mass balance analysis are provided in Tables 6 and 7, below. The analysis focused on silica, which is the largest component of the straw, and the alkali minerals potassium and sodium, which are removed using the hydrolysis process. Table 6: Mass Balance Calculation for Silica- Test Ci

Table 7: Mass Balance Calculation for Equivalent Alkalis (Na?O+0.658 K?O) - Test Ci

Hydrolysis Test Conducted on Ground Straw

[42] Since the hydrolysis of the chopped straw using cold-water condition did not achieve the target alkalinity, it was decided to grind the straw into powder and then apply the leaching process. This would increase the surface area of the straw, which might result in an easier removal of the Potassium and Sodium minerals. The particles size distribution of the ground straw used in the study is provided in Figure 1 below. It can be seen from the figure that 50% of the material has a particle size less than 0.3 mm while 90% has a particle size less than 0.6 mm.

[43] Nine hydrolysis tests were conducted in this experimental phase. The hydrolysis was conducted in two stages all involving leaching the ground straw in cold water. The ratio S/W and the leaching durations used in all tests are provided in Table 8.

Table 8: Description of Samples Used in the Hydrolysis of the Ground Straw

Sample Stage I Stage Il

S/W Duration S/W Duration (Minutes) (Minutes)

G1 1 20 5 1 20 5

G2 1 20 20 1 20 15

G3 1 20 30 1 20 15

G4 1 35 10 1 35 10

G5 1 35 20 1 35 15

G6 1 35 30 1 35 15

G7 1 50 10 1 50 10

G8 1 50 20 1 50 15

G9 1 50 30 1 50 15

[44] Results of oxide analyses conducted for the ground straw after being leached are shown in Table 9. For the first four tests (G1 to G4), the oxide tests were conducted after the second leach, while they were conducted after two leaching stages for the rest of the tests. In view of the test results, it was decided that the hydrolysis conditions used in test G6 provided the best results for the ground straw. The equivalent alkali content after the second leach was 1 03%, which is considered satisfactory As such, the conditions associated with test G6 were then applied in subsequent pilot scale experiments Table 9: Results of Oxide Analysis (% Minerals) of Ground Rice Straw After Hydrolysis

Pilot Scale Test

[45] The study then proceeded by conducting a pilot scale test that simulates various stages of the process for producing a mineral additive for concrete from rice straw. The following steps were conducted a) An amount 100 Kg of straw was ground to achieve the particle size distribution shown in Fig. 1. b) A large steel tank was provided to conduct hydrolysis of the ground straw. A steel screen with very fine openings (about 0.5 cm) was constructed and installed inside the tank. c) Hydrolysis was conducted to the ground straw by applying two leaching stages and using the conditions associated with test G6 described above. d) It was found that post hydrolysis, the straw had a moisture content of 80%. The straw was dried using a heated oven to reduce the moisture content to 35% or less. e) Uniform combustion of the straw was then conducted, leading to ash rich in amorphous silica with low carbon content. f) The produced rice straw ash (RSA) was then ground such that its average particle size was about 7 microns.

[46] The same steps were also repeated using chopped straw instead of ground straw. In this pilot test, the straw was chopped to an average size of 1 inch (25.4 mm), which is significantly less than the size used in the small-scale hydrolysis test For both tests, oxide analyses were conducted for samples taken after each leaching stage. The results of those oxide analyses are provided in Table 10. In the first column of this table, the letter "P" refers to pilot test, while "G" and "C" denote ground and chopped material, respectively In the same column, the number "1" and "2" refer to tests conducted after the first and second leaches, respectively. It is clear from the test results that the amount of alkali was reduced to an acceptable level after the second leach.

Table 10: Results of Oxide Analysis (% Minerals) Resulting from Hydrolysis in Pilot

Test

[47] Oxide analyses were conducted for the ash resulting from the combustion of both the leached ground and chopped straw materials. The results are provided in Table 11. The ASTM C1240-01 standards provide specifications for the use of Silica Fume as a mineral admixture in concrete. Rice Straw Ash (RSA) provides a similar function to Silica Fume and therefore its specifications can be compared to those provided in the ASTM C1240-01 standards. The chemical requirements specified in those standards state that a minimum ratio of SiO₂ of 85.0% and maximum LOI of 6.0%. It is clear from the results reported in Table 11 that the ash resulting from the processing of the ground straw achieves these criteria For the ash resulting from the chopped straw, the LOI was less than the 6% limit and the silica content was very close to the value specified in the standards Table 11 : Results of Oxide Analysis of Ash (% Minerals) Resulting from Combustion

[48] A laser diffraction particle size analyzer was used to determine the particle size distribution of the ash resulting from the processing of the ground straw and the results are shown in Figure 2. According to the figure, the average particle size of the ash is about 25 microns (0.025 mm).

[49] The raw ash was then ground using a vibratory ring pulverizer. The particle size distribution of the ground ash resulting from a laser diffraction analysis is provided in Figure 3. According to the figure, the average particle size was reduced to about 6.6 microns (0.06 mm), a value considered adequate for RSA to act as an efficient concrete additive.

[50] An X-ray diffraction was conducted to examine the presence of crystalline silica in the ash. The results are provided in Figure 4. Generally, the ash is shown to be amorphous with a trace amount of Quartz and possibly Hematite.

Description of Concrete Mixtures

[51] A total of 5 concrete mixtures including a control mixture was made. The concrete mixtures incorporated proportions of a commercial Silica Fume (SF) and ground Rice Straw Ash (RSA) resulting from the pilot test conducted on the ground straw. All mixtures had a constant water-to cementitious materials ratio (w/c) of 0.40. ASTM type I cement was used in all mixtures. Natural washed gravel with a maximum particle size of 10 mm along with silica sand conforming to ASTM C33 were used in the concrete mixtures. A naphthalene sulfonate superplasticizer with a solid content of 42% was used to achieve the desired workability for all concrete mixtures. The superplasticizer dosage was tailored in each mixture to achieve a slump of 90+ 10 mm. The proportions of all concrete mixtures are shown in Table 12. The fresh concrete properties including slump and the superplasticizer dosage expressed using the high- range water-reducing admixture (HRWR) are provided in Table 12.

Table 12: Mixture Proportions of the Various Concrete Mixtures

[52] Based on data provided in Table 12, it can be seen that the superplasticizer requirement for concrete mixtures incorporating SF was higher than that of concrete mixtures incorporating RSA.

Compressive Strength Tests

[53] Compressive strength results for the various concrete mixtures at 1 , 7 and 28 days are shown in Figure 5. An increase in the 1-day compressive strength was achieved for all mixtures incorporating RSA. The highest 1-day strength improvement was obtained for the 10% RSA level. A slight decrease in the1-day strength in shown for the mixture with SF. At 7 and 28 days, the strength of all mixtures incorporating RSA and SF outperformed that of the reference mixture. At 28 days, using 7.5%, 10% and 12.5% cement replacement with RSA increased the compressive strength of concrete by up to 12.7%, 18.18% and 23.2%, respectively.

Rapid Chloride Penetrability

[54] Rapid chloride penetrability tests (ASTM C 1202) at 7 days were carried out for the five mixtures. The results are illustrated in Figure 6 along with the ASTM C1202 classification ranges. It is shown that using RSA reduced the rapid chloride penetrability of concrete from a high rating to a low or very low rating, depending on the addition level of RSA. The reduction for concrete mixtures incorporating RSA was higher than that of concrete mixtures incorporating similar proportions of SF. Conclusions

[55] A technique to produce a high quality mineral additive for concrete from rice straw was developed in this study. A large number of laboratory tests were conducted to optimize a hydrolysis procedure applied to reduce the alkalinity of the straw. Uniform combustion under controlled conditions was then conducted for the straw resulting from the hydrolysis. The produced rice straw ash (RSA) was shown to have a silica content of about 85%, with carbon content less than 6%. Moreover, the silica is shown to be in an amorphous state. The produced ash was then ground to reduce its average particle size from 25 microns to 6.6 microns. A number of concrete mixtures were made using different proportions of RSA. For comparison purposes, a control mixture with no additive and another mixture incorporating Silica Fume were prepared. The advantages of adding RSA to concrete were assessed by conducting compressive strength tests, as well as standard rapid chloride penetrability tests. These provided a measure of the durability of the concrete. Depending upon the addition rate, RSA enhanced the compressive strength of concrete by up to 23.2% at 28 days. RSA reduced the chloride penetrability of concrete from a high to a low or very low ASTM C1202 classification, depending upon the addition rate. These results indicate that the addition of RSA has a very positive effect in enhancing the durability of concrete. RSA was also more effective than SF in enhancing the strength and durability of concrete.

[56] The foregoing are examples of preferred embodiments of the invention and are provided for purposes of illustration. It will be apparent to persons skilled in the art that various embodiment and additional features of the invention may be provided without affecting the way in which the invention works. The inventor intends all aspects and sub-combinations of the invention to be encompassed by the following claims.

Claims

1) A process for the treatment of an agricultural residue comprising: a) providing a supply of the agricultural residue; b) leaching the agricultural residue using an aqueous solvent to reduce an equivalent alkali content of the residue to less than or equal to 2.5%; c) drying the leached residue to a moisture content of less than or equal to 35% by weight; d) converting the leached residue to ash.

2) The process according to claim 1 , wherein the agricultural residue comprises rice straw.

3) The process according to claim 1 , wherein the process further comprises: e) collecting an aqueous leachate; f) separating alkali metals from the leachate.

4) The process according to claim 3, wherein the alkali metals comprise sodium and wherein the process further comprises separating the sodium from the leachate through ion exchange.

5) The process according to claim 1 , wherein the residue has an average particle size less than or equal to 25 mm.

6) The process according to claim 1 , wherein the residue has an average particle size less than or equal to 1 mm.

7) The process according to claim 1 , wherein the aqueous solvent has an initial pH less than or equal to 6.5.

8) The process according to claim 1 , wherein the leaching takes place in at least two stages.

9) The process according to claim 1 , wherein the residence time in each stage is at least 15 minutes.

10) The process according to claim 1 , wherein the mass ratio of solids to water is at least 1 :35.

11)The process according to claim 1 , wherein the conversion to ash takes place through oxidation.

12)The process according to claim 1 , wherein the process further comprises recovering heat energy released during the oxidation of the residue and utilizing the heat energy to pre-heat the aqueous solvent and/or the residue.

13)A process for the preparation of a concrete additive comprising: a) providing a supply of rice straw; b) leaching the rice straw using an aqueous solvent to reduce an equivalent alkali content of the rice straw to less than or equal to 2.5%; c) drying the leached rice straw to a moisture content of less than or equal to 35% by weight; d) converting the leached rice straw to rice straw ash; e) reducing the average laser diffraction particle size of the ash to less than or equal to 10 microns, thereby producing the concrete additive.

13)The process according to claim 13, wherein the rice straw ash has an amorphous silica content of at least 80% by weight and an LOI of less than or equal to 6%.

14)The process according to claim 13, wherein the conversion to ash takes place through oxidation.

15)A concrete additive made according to the process of claim 13. 16)A concrete comprising the concrete additive according to claim 15.

17)The concrete according to claim 16, wherein the additive is present in an amount of from 5 to 15% by weight.

18)The concrete according to claim 17, wherein the compressive strength of the concrete is at least 49 MPa after 7 days.

19)The concrete according to claim 17, wherein the rapid chloride penetrability is less than or equal to 1500 Coulombs after 7 days.

20)The concrete according to claim 16, wherein the compressive strength after 7 days is at least 5% more than a concrete having an equivalent proportion of silica fume.