Ash of vegetable waste used for economic production of low to high strength hydraulic binders

Fuel 78 (1999) 153–159

Ash of vegetable waste used for economic production of low to high strength hydraulic binders Piet Stroeven a,*, Danh Dai Bui b, Ely Sabuni c a

Delft University of Technology, Stevinweg 4, 2628 CN Delft, The Netherlands b Hanoi University of Civil Engineering, Street Giai Phong, Hanoi, Vietnam c ACT-Incofin (T) Ltd., Arusha, Tanzania Received 12 November 1997

Abstract Some vegetable wastes contain relatively large amounts of silica. One of the most promising examples is rice husk. Since rice is a major crop among many of the developing countries, it is available in large quantities. Its ash can be produced without using expensive fuels—such as in the case of Portland cement. During incineration, heat is released which could eventually be transformed into mechanical energy for grinding the ash. An artificial pozzolan is obtained, which can be combined with lime to yield a hydraulic binder of lower quality. Kaolin clay, when available, could be added during incineration to improve the binder quality. At the other end of the spectrum, Portland cement can be blended with finely-ground rice husk ash to produce high strength concretes with a water reducing agent at low water to cement ratios. Blending with the incinerated kaolin can further increase cost effectiveness. This study concentrates on RHA technology but also gives a survey of obtained strength data. 䉷 1998 Elsevier Science Ltd. All rights reserved. Keywords: Appropriate technology; Concrete; Rice husk ash

1. Introduction Technologies should be developed which can provide means for upgrading shelter within the scope of the socioeconomic and cultural environment. This is commonly referred to as ‘Appropriate Technology’. The tempting philosophy would be to base the appropriate housing solution entirely on indigenous resources, including waste components. Among the indigenous resources of high quality, timber (and to a lesser degree bamboo) takes a dominant position. Unfortunately, the pressure of overpopulation has resulted in a dramatic reduction in timber suitable for house construction, despite reforestation programs. The common upgrading concept for the wattle and daub type of shelter is the mud block system, which eliminates the need for timber as a load-bearing skeleton. Even wooden window and door frames are sometimes avoided because of scarcity. Timber cannot easily be replaced however in roof construction, which preferably is covered by a vegetable waste (grass, illuk), providing for insulating properties unattainable by corrugated iron sheets—a cheap but inappropriate solution to improve shelter. * Corresponding author. Tel.: ⫹ 31-15-2784035; Fax: 2611465; e-mail: [email protected].

⫹ 31-15-

A nationwide enquiry on living conditions executed in the late eighties by the Building Research Unit in Dar es Salaam among Tanzanian villagers—including women—learned that upgrading of the cow-dung floor should receive the highest priority, the next choice being improved erosion resistance of the walls. The thatched roof was not on the priority scheme. The Portland cement (PC) component in the concrete or mortar should be minimized, or even eliminated for this purpose, however. Instead, cement-like binders based on indigenous resources should be employed. The improved durability of the floor would reduce the intensive labour investment by women and could provide for better living conditions and reduce the health hazards. It has been known for quite some time that a highly reactive pozzolanic material can be obtained from controlled burning of rice husks (a FAO review mentions two German patents dealing with the use of RHA in concrete from 1924). Since those days, a considerable amount of effort has been invested in this field and major applications have been realized in South America as well as in Asia. A full session was devoted to RHA during the recent CANMET/ACI Conference in Milwaukee, showing the world-wide interest in this field [1]. Special reference should be given here to the work of Mehta [2], based on a fundamental insight into the chemistry involved in this

0016-2361/98/$ - see front matter 䉷 1998 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(98)00143-4

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P. Stroeven et al. / Fuel 78 (1999) 153–159

Fig. 1. Combined BEI and X-ray images revealing porous husk structure and silica concentration at outer surface

material’s technology. Kaolin clay is another potential pozzolanic material. Upon incineration, metakaolin is obtained which is the main reactive component. In combination with rice husk ash and lime it can yield an effective, low-cost binder [3]. The same technology can be made appropriate for other socio-cultural environments. By using small fractions of PC, a better quality binder would allow production of stabilized mud/clay/sand blocks. Moreover, plasters could reduce erosion damage. By further increasing the PC content, a RHA-blended PC is obtained, which could compete with a high-quality PC blended with condensed silica fume (CSF) to attain high strength [4]. This type of technology is required for urban and infrastructural developments in Vietnam. The quality of aggregates is of increasing importance when also binders of higher quality are being developed. In Northern Vietnam, the coarse aggregate is of indigenous origin (crushed granite). In delta areas of long rivers, such as met in Vietnam, the fluvial sands are commonly too fine according to building codes. A mixture of coarse aggregate and such very fine sands leads to gap-graded mixtures, which have proven suitable for the production of high quality concretes [5]. In a series of research projects, part of the technological possibilities have been studied. Table 1 Chemical composition of two types of rice husk ashes Main oxides

Vietnamese RHA (% by weight)

Tanzanian RHA (% by weight)

SiO2 Al2O3 Fe2O3 CaO K2O MgO

96.7 0.08 0.03 0.30 0.73 0.16

88.9 0.30 0.19 0.43 3.67 2.07

2. Rice husk ash technology The rice husks used in the present investigations were from the Arusha region in Northern Tanzania. The raw material ranged in particle size from less than 0.038 mm to 2 mm. Chemical analysis revealed the material to contain 21.2% silica (a further 26.2% cellulose, 21.2% hemi-cellulose, 11.6% lignin and 19.8% pigments, pectine and protein). A combined study using back scattered electron (BEI) and X-ray images of the husk (Fig. 1) showed the silica to be distributed mostly under the husk’s outer surface [6, 8]. This confirms the general concept of a soluble form of silica transported through the plant, and concentrated at the outer surface of straw and husk through evaporation, whereupon it polymerizes into an opaline cellulose-silica membrane [7]. The composition of the RHA was determined by means of an ICP 200 emission spectrometer (ICP-200). Table 1 presents average data for the chemical analysis in comparison with similar data obtained on a Vietnamese rice husk type [8]. Based on a DTA analysis, a series of ashes was produced in a laboratory oven under different temperature– time regimes, whereupon the carbon content (the loss on ignition, LOI) was determined. Some results are presented in Table 2; note that ash percentages are by weight of the husk. Another sample of husks was subjected to 600⬚C for 10 h and a further one to 900⬚C for 15 h, the latter ash being ‘as white as lime’. All ashes were ground for 160 min in an agate mortar. Thereupon, the particle size distribution (psd) was determined by Malvern 2600 equipment. Differences between curves were not dramatic, with 50% of the particles under 5–10 mm. Examples of psd’s of ashes representing the 350⬚C–67 h and 600⬚C–10 h treatments have been given elsewhere [6]. The fineness of the ashes was additionally determined by nitrogen adsorption. The data averaged over

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Table 2 Effect of temperature–time regimes on LOI of RHA Temperature

Duration (h)

Ash ⫹ C (%)

Colour

LOI (%)

Ash (%)

350⬚C 400⬚C 500⬚C 600⬚C

67 24 20 6

24.1 23.8 23.5 32.4

Grey Grey Light grey Blackish

5.62 6.25 4.86 27.15

22.7 22.3 22.4 23.6

two analyses, which involved preheating at respectively 150 and 360⬚C, are presented in Table 3. An ash produced in a specially designed kiln (Fig. 2) under laboratory conditions, but supposedly representative of production of rural binder qualities (temperatures around 700⬚C), contained 23% unburnt carbon. For higher binder qualities, the LOI was reduced to 12% in a final stage of the research program, focusing on urban and coastal developments in Vietnam. The ‘rural’ ashes were ground in a 2-l laboratory ball mill to various degrees of fineness. BET data (obtained by nitrogen adsorption) are presented in Table 4. The grinding time in hours is shown in parenthesis. The ash particles had a large internal porosity before grinding, as revealed by secondary electron images of the ash. Initially, the surface area increased with grinding time. But upon collapse of the porous structure, the surface area dropped. The addition of a naphthalene type of superplasticizer during grinding ( ⫹ in Table 4) proved efficient. The ashes were also investigated by SEM, TEM and X-ray diffraction [6, 8]. Up to 500⬚C, the secondary electron images of the ash revealed particles as spherical or globular in shape and with a porous structure (Fig. 3). Partially crystallized ash was found at 600⬚C, whereas at 900⬚C this feature was dominant. This was confirmed by X-ray diffraction patterns. The SEM pictures revealed the globular structure to increase in size with rising combustion temperatures from 5–10 mm up to 10– 50 mm. Individual particle and pore sizes were up to 1 mm. Agglomerates seemed to become more compacted at higher combustion temperatures. Moreover, at 600⬚C fine porous crystalline grains, smaller than 1 mm, were displayed, possibly manifesting the transformation between the amorphous and the crystalline state. This process was completed at 900⬚C (Fig. 4).

Table 3 BET specific surface area of samples of RHA Temperature (⬚C)

Duration (h)

SBET (m 2/g)

350 400 500 600 600 900

67 24 20 6 10 15

102 76 57 120 104 2

3. Materials 3.1. Low-strength mortars for rural applications To assess the practical possibilities with this ‘quality’ of RHA, it was used in combination with PC (RHA–PC), with PC and lime (RHA–lime–PC) and with lime (RHA–lime). The blending component(s) were added in the correct proportion to the RHA which had been ground for 65 min. The mixture was then ground for another 65 min. The fineness was checked by air permeability and found to be in the correct range of the ASTM C 595 (i.e. ranging from 0.27 to 0.30 m 2/g). About equal proportions of two fractions of sand were added to this binder. The size range of the sand fractions was 0.1 to 0.25 mm, and 0.25 to 0.5 mm, and the sand to binder ratio was 3. A minimum amount of water was added to achieve a certain level of workability, which was measured by the slump and flow test. The water to binder ratio was derived from the compositional data. As a result of this practical set up (adapting rural conditions), the slump and flow rate as well as the water to binder ratio fluctuated among ‘similar’ mixes of about 6 l. Standard prisms (40 × 40 × 160 mm) were made following laboratory procedures; i.e. moulds were filled in two separate layers, followed by 10 s compaction on the vibration table. After compaction the excess of material was removed. Specimens were stored before testing in a climatized room (20⬚C, 99% RH). Demoulding was carried out very carefully after 48 h. Initial and final setting times were determined using the Vicat apparatus. The well-known phenomenon of delayed setting times was displayed, in fact well above the levels indicated in ASTM C 595 (for PC blended by a pozzolana) [6]. Only PC blended for a smaller part with RHA would fulfil these requirements. Flow values (ASTM C 109) and slump of the various mixes varied slightly (slump between 2 and 5 mm). However, mixes had a considerably different water to binder ratio of 0.5 for the PC mortar and between 0.8 and 1.0 for the blended mixes. 3.2. High-strength concretes for urban/coastal applications The testing program was set up in the following way: 1. First Stage of Testing: Optimization of gap-graded concrete mixtures; no water reducing agent; blending of PC by RHA; fineness modulus sand of 1.72.

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P. Stroeven et al. / Fuel 78 (1999) 153–159 Table 4 BET specific surface area of RHA. Duration of grinding (h) in parentheses; only the last ash is ground in combination with a superplasticizer RHA-quality

SBET (m 2/g)

DSBET (m 2/g)

RHA(0) RHA(14) RHA(18) RHA(18)⫹

123 137 151 58

2 11 11 6

2. Second Stage of Testing: Optimization of some gapgraded RHA-blended concretes from stage 1; use of superplasticizer; fineness modulus sand of 0.98. 3. Third Stage of Testing: Optimization of some gap-graded RHA-blended concretes from stage 2; use of crushed rock as coarse aggregate, higher cement contents and reduced carbon content in RHA. 4. Fourth Stage of Testing: Optimization of pastes and mortars with kaolin-blended Portland cement.

Fig. 2. Section of kiln used for ashing the rice husks under laboratory conditions

The aggregate gradings of the gravel and sand are given in Ref. [10]. Fineness moduli amounted respectively to 6.89 and 1.72 (close to the reference sand of Vietnamese origin). A maximum grain size of 16 mm was selected. An ASTM type I normal PC was employed for the experiments, because it was the only available quality in Vietnam. Water to cement ratio was varied between 0.4 and 0.6, and sand to aggregate ratio between 0.19 and 0.4. Cement content ranged from 300 to 525 kg/m 3. Mixes were designed by the absolute volume method. Hereby, it was acknowledged that design methods for continuously graded mixes can also be employed for gap-graded ones containing very fine sand [10]. The latter was already indicated by the experiments by Bloem [11]. In the first and second test series the PC was blended by RHA produced in a specially designed oven [8, 10, 12]. The ash used in the testing program had a carbon content of 23%. This high carbon content was reduced in a third testing series to 12% [9].

Fig. 3. SEI of RHA sample subjected to 500⬚C for 20 h

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Fig. 4. SEI of RHA sample subjected to 900⬚C for 15 h

Herein, the PC content was increased in accordance to the ACI 363R-92 recommendations for HSC (High Strength Concrete). A still finer sand with a fineness modulus around 1 was used. 4. Experimental 4.1. Low-strength mortars for rural applications Assessment of mechanical properties was accomplished in three-point bending on 40 × 40 × 160 mm prisms with a span of 107 mm (ASTM C 348). Further, the compressive strength was recorded on pieces of the broken prisms introducing the load via 40 × 40 mm platens (ASTM C 349). A total of 173 prisms was used for this investigation [8]. The development of compressive strength over a period of 1 year is shown for the best performing mixes in Table 5. Performance in bending developed proportionally [6]. 4.2. High-strength concretes for urban/coastal applications In the first stage of testing [13], 100 mm cubes were employed for the determination of compressive strength at 7 and 28 days. Specimens were compacted on the vibration table using a frequency of 2800 cycles/min and an

amplitude of 0.35 mm. The moulds were carefully filled in two successive layers, which were both compacted for 36 s. The specimens were cured under climatized conditions (20⬚C; 99% RH) until the day of testing. In the second stage of testing the compaction time was reduced to 2 × 10 s. Moreover, a series of specimens was tested at 182 days. The superplasticizer improved the workability of the gap-graded concrete with fine sand. This was reflected in an increased slump value and a reduction in the compaction energy. Compressive strength was also improved in most cases. Replacing part of the PC by an equal mass of RHA reduced the slump, so that combination with a superplasticizer was obligatory. To summarize, gap-graded concretes of high strength could be obtained by combined use of a superplasticizer and fine-grained RHA, replacing up to 40% of the PC. The PC content was between 240 and 300 kg/m 3 in these cases. In doing so, compressive strength was found to exceed 50 MPa after 7 days and 70 MPa after 28 days [10, 12]. A selection of the results obtained is presented in Table 6. For increased PC contents in accordance with ACI 363R92, a compressive strength after 28 days approaching 100 MPa was found for blending percentages up to 40 and a reference cement content around 500 kg/m 3 (Ref. [9]). A selection of data obtained is presented in Table 7.

Table 5 Development of compressive strength of mortars based on various binders. R, RHA; l, lime; P, PC; W, water; B, binder Code

4 9 11 12 14 15

Mixture composition

0.5(R)–0.5(l) 0.7(R)–0.2(l)–0.1(P) 0.6(R)–0.3(l)–0.1(P) 0.5(R)–0.4(l)–0.1(P) 0.65(R)–0.35(P) 0.5(R)–0.5(P)

W/B (g/g)

0.92 0.90 0.90 0.90 0.90 0.875

Slump (mm)

5.0 2.0 2.0 2.5 0.5 3.5

Compressive strength (MPa) 5 days

1 week

4 weeks

28 weeks

52 weeks

1.30 1.17 1.38 1.37 2.32 2.29

1.72 1.53 2.11 2.46 5.85 6.50

2.55 1.83 2.29 3.10 8.59 10.38

2.29 1.63 2.47 3.47 7.81 12.60

2.48 1.55 2.31 3.27 9.08 13.60

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P. Stroeven et al. / Fuel 78 (1999) 153–159

Table 6 Composition of gap-graded concretes of second testing stage (kg/m 3) containing coarse river aggregate of 16 mm maximum grain size, very fine sand (fineness modulus of 1.74) and RHA-blended Portland cement, and corresponding average compressive strength data (MPa). First three mixes had a slump of 5–15 mm, other two of 200 mm Code

C2 C4

Composition

Compressive strength (MPa)

C

W/RHA ⫹ C

RHA/C

S/S ⫹ G

7 days

28 days

182 days

240 210 400 400 300

0.40 0.40 0.32 0.35 0.35

0.25 0.43 0.25 0.25 0.67

0.19 0.19 0.20 0.19 0.19

53.5 49.1 53.8 51.6 52.5

72.3 64.1 69.1 70.1 70.5

74.8 78.5 81.6 83.6 90.1

Table 7 Composition (kg/m 3) of gap-graded concretes of third testing stage containing crushed granite as coarse aggregate (16 mm maximum grain size), very fine sand (fineness modulus 0.98) and RHA-blended Portland cement, and corresponding average compressive strength data (MPa) Code

A-4 A-12 A-16 B-28

Composition

Compressive strength (MPa)

C

RHA/C

S/S ⫹ G

W/RHA ⫹ C

7 days

28 days

182 days

508 450 434 522

0.20 0.20 0.20 0.15

0.19 0.25 0.25 0.25

0.273 0.33 0.35 0.35

81.1 68.6 61.9 64.6

98.8 86.2 79.0 82.2

108.1 90.6 92.7 95.6

5. Conclusions RHA–lime mixes are at the lower end of the technology range. Among the studied mixes the one with minimum amount of RHA (i.e. 50%) performed properly (5). It should be remarked in this context that the availability of laterite clay (containing the required alumina component) can significantly improve the quality of the technology output as was demonstrated by Cabrera et al. [3], using activated red tropical soils from the Southern Indian Shield in Sri Lanka. For the primary purposes of densification of a floor’s surface and of stabilization of the mud for walling, this would be sufficient.

Fig. 5. Normalized values of compressive strength of a Portland cementbased 90-day-old mortar as a function of blending percentage by weight of an incinerated kaolin clay. SP ˆ superplasticizer

In stage four, the same profit was experimentally revealed for higher quality mortars aiming for the Vietnamese market [15]. The economic range of blending percentages is illustrated in Fig. 5, showing the normalized compressive strength of mortars at 90 days versus metakaolin-blending percentage. Table 8 gives a survey of the types of clay available in Northern Vietnam. The one used for the investigations (available on the Dutch market) is given in the last column for comparison. The compressive strength of RHA–lime–PC mixes was found to fulfil the requirement of ASTM C 91 for masonry work even for PC contents under 50%. So, here a better quality material is obtained, which can be used for the production of building blocks, replacing those using only

Fig. 6. Field ovens constructed in Suduwatura Ara, Sri Lanka, for the rural production of RHA-based binders

P. Stroeven et al. / Fuel 78 (1999) 153–159 Table 8 Compositional data on some of the clays from different regions in Vietnam, and on the clay used in the fourth testing stage Component

SiO2 Al2O3 Fe2O3 MgO CaO LOI at 800⬚C

Origin of clays Tuyen Quang

Hai Hung

Vinh Phu

Czech Republic

50–60 36–42 2–4 2 0.5 9–11

55–60 36–40 4–5 0.3 0.35 7–11

44–46 34–36 1–3 0.5 0.7 14

46.6 35.7 0.85 0.35 0.35 11.9

PC. The local stage of development should be more advanced in this case. The ash can be produced in simple field ovens of baked bricks, such as the one constructed in Suduwatura Ara (Sri Lanka) and shown in Fig. 6. Also other appropriate solutions have been assessed, such as old oil drums. The conditions in open heaped-up burning of the rice husk material are for most of the material probably similar to or even better than in the specially designed kiln used in the present experiments [6, 10, 14]. Thus, open heaped-up burning could provide a cheaper solution for the primary applications of RHA in rural villages. In the combined concept of a gap-graded aggregate with a minimum of very fine sand and a RHA-blended Portland cement binder, a superplasticizer should be employed. With PC contents of about 250 kg/m 3 or higher, high strength concretes can be produced (exceeding 69 MPa at 28 days), as illustrated by Table 6. For improved durability the PC content should be brought in accordance with ACI 363R-92. Compressive strength values as high as 90–100 MPa after 28 days can be obtained, even when using a type I normal PC quality. This is illustrated by experimental data collected in Table 7. Economical solutions for urban developments, infrastructural facilities and coastal structures can be achieved in this way (in delta areas) in developing countries where rice is the main crop. This will be countries with a tropical climate, so application of air-entrainment for achieving proper freeze–thaw resistance can be omitted. The relatively high LOI values in the present investigations had no detrimental effect on the rheological features of the cementitious composites. Hence, limitation of these values the in case of the high-quality binders used in Vietnam would only require optimization in economical respect, because higher LOI values lead to an increased demand for superplasticizers.

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