Biogenic wastes in the Malaysian cement industry

Modern constructions demand a lot of cement, but OPC has a large negative impact on the environment, producing large amounts of CO₂. With increasing demand, raw material prices increase and fuel resources are falling. As there is currently no viable alternative to OPC, research centres around the use of alternative fuels and supplementary cementitious materials. Here researchers from Malaysia and Bangladesh present ways to use biogenic wastes as an alternative fuel and as a novel binder.

OPC is a versatile and widely-used construction material but burning CO₂-emitting materials at 1450°C produces a lot of CO₂,¹ which contributes to global warming.² To get a sustainable solution to this problem, much research has been conducted towards the use of biogenic wastes in cement production. Examples include palm oil fuel ash (POFA)^3,4, rice husk ash (RHA),^5,6 sawdust ash/ash from timber (AFT)⁷ and bagasse ash⁸ as supplementary constituents of cement and concrete.

RHA is generated from the rice husk processing industry. POFA is a waste from the palm oil milling industry. AFT is a waste from wood mills and the fibreboard industry. All of these types of ash can be produced by burning the relevant material in a kiln or furnace at 500-600°C. Large amounts of POFA, RHA and AFT are produced in Malaysia every year but they are not used to a significant extent at present. In spite of the potential financial, technical and environmental benefits, these materials are currently dumped locally with no commercial return. It has already been proven that all of these waste materials contain a high amount of amorphous silicon dioxide (SiO₂). This means that they could be used as pozzolanic materials in cement and concrete production.

Additionally, these biogenic agricultural wastes contain different proportions of cellulose, hemicellulose, lignin, ash, resin, wax/oils, water-soluble substances and moisture. Carbon, nitrogen, oxygen and hydrogen are the major constituent elements of these compounds. Small amounts of sulphur and chlorine and traces of others element are also present. Their volatile matter content is around 60%-75%. High ash content lowers the calorific value of residues while the presence of oil, resins and wax raises the calorific value. The heat potential of some of the selected agricultural wastes from the biomass are given in the literature.^9-12

Energy from biomass plays a big role in energy demand worldwide, supplying 10% of the total energy demand.13 In addition, in this study it is presented that the proper direct burning of the biogenic waste can be used as alternative fuel in the cement industry without quality problems or performance deterioration. This has the potential to save energy compared to OPC production.

Experimental programme

The waste collection, ash (POFA and RHA) production and utilisation procedure of the ashes used in this study are shown in Figure 1.

Materials and procedure

To perform this research, local wastes were collected from different palm oil mills and rice processing plants. For the preliminary investigation, rice husks and palm fibres were burnt in a UKM-designed furnace at a concrete laboratory at UKM. A schematic of the furnace is shown in Figure 2. During the production of POFA and RHA, the temperature was maintained between 500°C and 600°C.

RHA and POFA were produced in a similar procedure. Either rice husk or palm oil milling waste was put into the furnace through an opening at the top. Gas burners were placed inside the furnace under the air ducts. Combustion was carried out for two hours, after which the gas burners were taken out. To supply air and cooling, an electric fan was placed near the entrance of the furnace. The fan supplied air and maintained embers. After a few minutes the ash was allowed to cool at room temperature. When the ash reached room temperature, it was emptied from the bottom of the cylinder, falling into a container.

Experimental tests

The physical and chemical properties of the produced materials (RHA, POFA) have been investigated by an automatic Blaine machine and a Le Charteliar flask for specific gravity. The chemical composition was found by X-ray fluorescence (XRF) analysis.

At the preliminary stage, to compare energy requirements, the produced materials were used as an alternative fuel in a local cement plant as partial replacement of the coal ash where coal is used as the primary fuel. The ash was also used for the production of a binder as a cement additive at a concrete lab at UKM. The total energy/fuel requirements for cement concrete production were calculated.

Energy requirements for blended cement (with ash) and only clinker (without ash) were evaluated by an energy meter at the cement plant. For the production of non-cement binder, the selected materials (RHA, slag and POFA) were ground in a ball mill to a fineness of 7250cm²/g Blaine (±100cm²/g).

The chemical and physical properties of the materials are presented in Table 1. At the preliminary stage, flow properties and mortar (cured at room temperature) tests were performed. Compressive strength was determined according to EN196-1 standard: 2005 using 160mm × 40mm × 40mm prisms. The water/binder ratio (W/B) was fixed at 0.40:1 the mortar flow spread was tested according to ASTM C109.

Above - Table 1: Chemical and physical properties for POFA, RHA, slag and OPC. (LOI = Loss on ignition).

Results and discussion

The chemical and physical properties of materials are presented in Table 1. It is seen from the chemical and physical test results that both ashes contain a high percentage of silica (more than 50%) and show similar properties to OPC. These types of ash could be used as a supplement to OPC clinker in cement plants and for research work into novel cementitious binders.

OPC clinker is produced at 1450°C. On the other hand the burning temperature of ashes (RHA and POFA) is about 500-600°C, around 850-950°C lower. Replacing a given percentage of clinker with RHA or POFA will therefore enable significant energy savings. An optimum percentage will be confirmed after testing the properties of blended cements produced with the ash.

The incorporation of ashes in cements would be an alternative and useful way of saving energy. The exact amount of saving fuel energy could be finalised by experimental measurement.

The findings of Ehrenberg and Geiseler regarding energy consumption for cement production are shown in Table 2.9 It is seen from this table that for CEM I the total energy requirement is 1587kWh/t, but after adding slag, energy consumption is reduced to 1206kWh/t, 938kWh/t and 602kWh/t for slag substitution levels of 30%, 50% and 75% respectively.

Therefore, in the case of 50% slag replacement, the energy requirement is reduced to 59.1% of that of CEM I, a saving of 40.9%. For 75% slag replacement the energy requirement drops to just 37.9% of CEM I.

RHA and POFA exhibit chemical composition and pozzolanic properties similar to slag. It can therefore be expected that using RHA and POFA as alternative fuels in cement production would be a potential step for saving energy and could contribute to reducing CO2 emissions as well.

The energy values of different agricultural and other wastes are presented in Table 3. This shows that wood and saw dust contain a higher calorific value compared to other wastes. All of these wastes could be used as an alternative fuel/energy in various sectors for energy and hence could serve both purposes: as an energy source and as an ash. After burning, POFA produces the most ash (50% by weight). RHA produces 20% of its original husk weight in ash.

The results given in the Table 4 for the direct incorporation of RHA in a cement plant show that the RHA is compatible with common standard raw materials mixes, e.g. coal ash in cases where coal is the primary fuel. The substitution of RHA up to 1% of coal ash could be used without deterioration effects on the various moduli values and on the quality of the resultant clinker. The compressive strength and flow values are also good. Incorporation of RHA in cement plant could therefore be a positive and valuable step in terms of saving energy and clinker.

The strength development of the non-cement binders are shown in Table 5. In the case of slag and POFA, the strength of mortar is very low compared to OPC, but with addition of RHA into the binder the mortar test result exhibits better results at all stages with respect to OPC. Indeed the strength was 17.8% higher than that of OPC at 28 days. Thus it can be said that RHA acts as an activator for the non-OPC binder and the potassium hydroxide (KOH) has worked as a chemical activator. The greater fineness of RHA could be another reason for improved performance. The flow property of the new binders is slightly lower than that of OPC sample. It is well known that the inclusion of pozzolanic materials raises the water demand. Therefore flow is reduced for the same W/B ratio, but the result is reasonable.

Above - Table 2: Total per-tonne energy consumption for OPC and different types of slag-containing cement.

Above - Table 3: Typical chemical properties of various wastes.

Above - Table 4: Chemical and physical properties for clinkers made with different supplementary cementitious materials.

Above - Table 5: Phyiscal properties for novel binders and OPC.

Conclusion

The burning temperature of ashes (RHA and POFA) is about 500-600°C but it is 1450°C for OPC clinker. It can therefore be said that incorporation of such ashes in the cement industry would be an effective way of saving energy. The exact amount of energy saved could be determined by trials at specific cement plants.

Based on preliminary investigations, it can be concluded that there is a possibility of saving energy/fuel using local biogenic wastes. Besides, from the test results it is found that with presence of RHA the strength property of new binder is greater (17.75% higher than OPC at 28 days) and flow property of the new binder is comparable to OPC.

Furthermore, effective utilisation of these waste materials as supplementary cementitious materials will help researchers to investigate sustainable ways of saving energy and material, particularly in cement production. The use of these waste materials in cement production is reasonable, valuable and can help to minimise the present energy consumption and reduced CO₂ emissions.

References

1. Neville, A. M. 'Properties of Concrete,' Longman, Singapore, 1994.

2. Nehdi, M. 'Ternary and quaternary cements for sustainable development,' Concrete International, Vol. 23, No. 4, pp. 35-42, 2001.

3. Basri, H. B.; Mannan, M. A. & Zain, M. F. M. 'Concrete using waste oil palm shells as aggregate,' Cement and Concrete Research, Vol. 29, pp. 619–622, 1999.

4. Tangchirapat, W.; Jaturapitakkul, C. & Chindaprasirt, P. 'Use of palm oil fuel ash as a supplementary cementitious material for producing high-strength concrete,' Construction and Building Materials, Vol. 23, Issue 7, pp. 2641-2646, July 2009.

5. Mahmud, H. B.; Majuar, E.; Zain, M. F. M. & Hamid, N. B. A. A. 'Mechanical properties and durability of high strength concrete containing rice husk ash,' Journal of advanced concrete technology, Vol. 79, pp. 21-30, 2009.

6. Zhang, M. H. & Malhotra, V. M. "High-performance concrete incorporating rice husk ash as a supplementary cementing material", ACI Material Journal, Vol. 93, No. 6, pp. 629–636, 1996.

7. Elinwa, A. C.; & Mahmood, Y. A. 'Ash from timber waste as cement replacement material,' Cement & Concrete Composites, Vol. 24, pp.219–222, 2002.

8. Chusilp, N.; Jaturapitakkul, C. & Kiattikomol, K. "Effects of LOI of ground bagasse ash on the compressive strength and sulfate resistance of mortars," Construction and Building Materials, Vol. 23, pp. 3523–3531, 2009.

9. Geiseler, E. A. "Jokologische Eigenschaften von Hochofenzement, Bston-Informationen," Vol. 37, No. 4 pp. 51-63, 1997. In : "Bensted and Barnes, Chapter 12, Structure and Performance of Cement," Second Edition, Spon press London and New York, pp.310-325, 1997.

10. Ravi, M. "Waste products from oil palm industry and their utilization potential," Presented at the World Renewable Energy Congress, Kuala Lumpur, Malaysia, 8-11 June 1999.

11. Menon, Ravi, Rahman, Zulkifli and Bakar, A, Nasrin. "Empty fruit bunches evaluation: mulch in plantation vs. fuel for electricity generation". Oil Palm Industry Economic Journal, Vol.3 (2), 2003. Malaysian Palm Oil Board 2003.

12. Suki, Anhar, Wok, Kamal & Nor, Mohd Tusirin. "Biomass utilisation: Prospects & challenges," Presented at International Palm Oil Congress (PIPOC), Kuala Lumpur, 25-29 September 2005.

13. Karstensen K.H. "IEA (International Energy Agency), 2006, Key World Energy Statistics 2006, Paris, France, "Cement Production in Vertical Shaft Kilns in China: Status and Opportunities for Improvement," United Nations Industrial Development Organisation, Vienna, Austria, 2006.