alternative materials for masonry units - sahf proceedings/papers/boschoff, billy... · +27 21 808...

Alternative Materials for Masonry Units

Author: WP Boshoff, MD de Klerk, G Coetzee, WI de Villiers, RD Tolêdo Filho 1

THE SOUTHERN AFRICAN HOUSING FOUNDATION

INTERNATIONAL CONFERENCE, EXHIBITION & HOUSING AWARDS

18-19 SEPTEMBER 2013

CAPE TOWN, SOUTH AFRICA

“PUBLIC PRIVATE PARTNERSHIPS”


William P Boshoff Civil Engineering Department

Stellenbosch University, [email protected] +27 21 808 4498

Marthinus D de Klerk, Gerrit Coetzee, Wibke I de Villiers

Civil Engineering Department Stellenbosch University, [email protected], [email protected]

+27 21 808 4498

Romildo D. Tolêdo Filho Department of Civil Engineering,

Universidade Federal do Rio de Janeiro, Brazil [email protected]

Keywords: Masonry Units, Alternative Materials, Sisal Fibres, Geopolymers Abstract There is currently no viable alternative to the use of cement-based masonry units in low-cost housing which is more economical, has a lesser impact on the environment and is still socially acceptable. There is however a need to produce such a material. Sisal fibre-reinforced, cement-based masonry units and geopolymers (alkali activated fly ash) are possible alternatives. The sisal reinforced masonry units could possibly use less cement and increase the R value of the material. For the geopolymer the material is made using only fly ash, which is an industrial by-product, sand and an alkali activator. This paper presents results of sisal fibre reinforcement on both low strength concrete and aerated concrete. Results of the effect of sisal reinforcement on hollow core masonry units and geopolymers are also reported. It has been found that sisal fibres could increase the strength of low strength cement-based materials as well as hollow core cement-based masonry units. The air content of the aerated concrete increased with the addition of fibres which in turn reduced the compressive strength but would however increase the R value. The water absorption of hollow core masonry units was improved significantly by using sisal fibres. It is also shown that geopolymers can also be a viable alternative material. The cost and strength of the geopolymer are similar to that of the low strength concrete and the carbon footprint of the geopolymer is also lower.



Introduction Cement based masonry units are the most commonly used building block of the low cost housing industry in South Africa (Laing 2011). It is not only economical, but the skills required to construct using these masonry units are normally readily available. Many alternative housing systems are available locally, e.g. wall panel systems, infill frame systems and light steel frame structures. However, only 0.6% of all low-cost housing units that have been built since 1994 were built using a type of alternative housing system (NDHS, 2010). The largest obstacles for the implementation of the alternative systems are the lack of social acceptance, the limitation that additions cannot be made easily to the structure and the limited availability of skills to construct these houses. There is a need to find alternative construction materials that not only have a lower environmental impact than the currently used cement masonry units, but are also more economical and still socially acceptable. Manufacturing masonry units using alternative materials is a possible solution as it would be accepted socially more easily compared to alternative housing systems. The skills to construct using masonry units are also more readily available. Conventional masonry units can then also be used in conjunction with the alternative masonry units, for example if additions are made. For a masonry unit manufactured from alternative materials to have a reduced environmental impact the embodied energy should be less, i.e. the energy required to manufacture the units should be less, and/or the R value should be higher. The R value is the thermal resistance with a higher value indicating a higher level of insulation. A higher R value would reduce the energy required to control the climate within the house or, if no heating or cooling is used, increase the standard of living. Two such alternative materials are sisal fibre reinforced concrete and geopolymer, also known as alkali activated fly ash binder. Sisal fibre is a leaf fibre that comes from the Agave sisalana plant. It is a labour intensive process to cultivate sisal and Brazil and China are responsible for 67 % of the world production of sisal fibres (Food and Agricultural Organisation 2009). To the authors’ knowledge there are currently no producers of sisal fibres in South Africa and where sisal is used it is imported from Brazil. Sisal fibres have a relatively low cost compared to synthetic fibres due to the low level of industrialisation that is required to produce the fibres. It has been used successfully in cement-based matrices (Pacheco-Torgal and Jalali, 2011) and many of the durability problems have been overcome (Bentur and Mindess, 2007; Silva et al., 2010, Tolêdo Filho et al., 2002). This makes sisal fibre reinforced cement-based mortar a promising material, especially in a developing country where the labour intensive cultivation could benefit the high unemployment typical of a developing country such as South Africa. The viability of sisal fibre reinforced masonry units was investigated and is reported in this paper. This includes the use of sisal fibres in a low strength concrete and aerated concrete as well as sisal fibre reinforcement in hollow core cement-based blocks. Another option is the use of geopolymer as an alternative material for masonry units. It consists of fly ash (ash from the coal burning process), fine aggregate (sand) and some type of alkali activator, typically sodium hydroxide, also known as caustic soda, and/or sodium silicate (Davidovits, 1989; Palomo et al., 1999; Silva de Vargas et al., 2011). The sodium hydroxide and/or sodium silicate is added in diluted form and the material is typically cured at a temperature between ambient conditions and 80 ˚C (Kong and Sanjayan, 2010). The material can reach its full strength within a day or two.



The advantage of this material is that it uses fly ash, a by-product, and contains no cement. Work has also been done on the use of PVA fibres to transform this quasi-brittle material to a more ductile material (Yunsheng et al., 2006). Even though geopolymer is a promising material, little research has been done to date on its viability as a material for masonry units. This paper reports on tests done on geopolymers to determine its suitability as material for masonry units. Sisal Fibre Reinforced Concrete. To investigate the possible use of sisal fibre reinforcement in masonry units, tests must first be done on the material. If an improvement is to be found, the addition of fibres should either increase the strength, increase the R value, increase the ductility or a combination of these. If the addition of sisal fibres increases the strength, the option exists that the cement content can be reduced while still obtaining the original strength. This would result in a material with a lower embodied energy and thus a lower environmental impact. To investigate this possible improved mechanical behaviour, a number of tests were done on a low strength concrete and aerated concrete. Test Setup and Program The mix designs of the two materials, the low strength concrete and aerated concrete, are shown in Table 1. The water/binder ratios were 0.9 and 0.39 for the low strength concrete and aerated concrete respectively. The cement was supplied by PPC while the fine aggregate was natural sand with a good grading, locally known as Malmesbury Sand. The reader is referred to Van Rooyen (2013) for more information on mix design and the use of aerated concrete. Table 1. Mix designs of the low strength concrete and aerated concrete Constituent Low Strength Concrete[kg/m3] Aerated Concrete [kg/m3] Water 170 184 Cement – CEM 1 52.5 - 472 Cement – CEM II 42.5 132.2 - Fly Ash 56.7 - Sand - Malmesbury 1215 944 Stone (9.5 mm) 810 - Air entraining agent - 1.3

Control mixtures (i.e. without fibres) were made for both material types and with an additional two mixtures per material containing 0.5% and 1.0% fibres by volume each. The fibres were washed, combed and cut to a length of 10 mm before being added to the concrete. More information on the preparation of the fibres for the use in concrete can be found in Coetzee (2013). Note that the addition of fibres did affect the workability, but all mixes had sufficient workability to be compacted properly. Six 100 mm cubes were cast and vibrated for each variation of the mix designs. The cubes were stripped the following day. The specimens were then water cured at a temperature of 23 °C until an age of 14 days when the specimens were tested. The specimens were still saturated with water when tested. Three of the cubes were tested in compression and three in tensile splitting. The compression test was done according SANS 5863 while the tensile splitting tests were done according SANS 6253. Test Results The compression and tensile tests of the low strength concrete are shown in Figure 1 while the results of the aerated concrete are shown in Figure 2. The minimum and maximum values are indicated with



error bars. Photos of the fracture surface of the aerated concrete were taken using a micro scope and are shown in Figure 3. It can be seen that the dominant failure mechanism is the pull-out of the fibres.

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Pa]

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Figure 1. The compression and tensile test results of the low strength concrete

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Figure 2. The compression and tensile test results of the aerated concrete

Figure 3. The fracture surface of a sisal fibre reinforced aerated concrete sample



It can be seen from Figure 1 that the addition of 0.5 % of fibres increased the compressive strength of the low strength concrete from 9.9 MPa to 10.3 MPa, however, with 1.0 % fibres the strength decreased to 9.8 MPa. The cubes of the mix without fibres fractured into pieces while the cubes with the fibres remained intact after failure. This indicates that the fibres increased the ductility. The tensile splitting strength however increased with both the 0.5% and 1.0% fibre variations of for the low strength concrete which is expected as the fibres affect the tensile strength more than the compressive strength (Hannant 1978). The compressive strength of the aerated concrete reduced with 43 % at 0.5 % fibre content as shown in Figure 2. The strength increased slightly again at the 1.0 % fibre content, but the strength is however still lower than the control mixture. The tensile splitting strength decreased with 22 % at 0.5 % fibre content which is not as significant as the compressive strength. The air content of the aerated concrete varied with an increase of fibre content as shown in Figure 4. The air content increased significantly from 23.0% to 29.4% with the addition of 0.5 % of fibres. The air content reduced slightly again with a further increase to 1.0% fibre content but was still higher than the control mixture. This increase of air content is an explanation for the decrease of compressive strength with the increase of fibre content for the aerated concrete. Even though the air content reduced the compressive and tensile strength, the increase of air content would increase the R value of the masonry unit which can make the sisal fibre reinforced aerated concrete a more viable option than sisal reinforced low strength concrete.

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tent [%

]

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Figure 4. The effect of the volume of sisal fibres on the air content of the aerated concrete

Discussion The test results of the low strength concrete reinforced with sisal fibres did show a slight increase of compressive strength, but not significant enough to make it a viable option in reducing the cement content. The interaction of the fibre with the mortar matrix can however still be improved and could thus result in either increased strength or the same strength using less cement. The addition of the fibres did however increase the ductility of the material. The aerated concrete units however showed a reduction in strength with an increase of fibres, but the fibres also facilitated an increase of air content which could be beneficial with regard to energy



efficiency. The R value of the aerated concrete with and without sisal fibres still needs to be investigated. Note that the required strength for solid masonry units for a single storey dwelling is 4 MPa and 10 MPa for the first storey of a double storey dwelling (NHBRC). All variations of the mix designs conformed to the requirement of a single storey dwelling, while only the 1.0% fibre variation of the low strength concrete and the 0 % and 1.0 % aerated concrete conformed to the double storey requirement.

Sisal Fibre Reinforced Masonry Units Hollow core sisal fibre reinforced cement-based masonry units were tested to ascertain whether there is an advantage in using sisal fibres as reinforcement. The fibres are the same as used for the low strength concrete and aerated concrete tests in the previous section. The compressive strength and water absorption were tested for the sisal fibre reinforced masonry units compared to a control unit without any fibres. Test Setup and Program Three mix designs were tested, namely 0%, 0.5% and 1.0% fibres by volume. The control mix design is shown in Table 2. The masonry units were manufactured using a pneumatic block making machine. All tests were done at an age of 28 days after the units were cured in a climate chamber at 23°C and relative humidity of more than 95 %. Table 2. The mix designs for the hollow core masonry units Weight [kg/m3] Water 170 Cement 164 Fly ash 82 Condensed silica fume 27 Sand 1150 4.8 mm stone 538 Crusher dust 231

The manufactured masonry units had dimensions of 390x140x190 mm with two hollow cores. The wall thickness was 20 mm at the top and 23 mm at the bottom. A typical unit is shown in Figure 5 a). The compression tests were done according to SANS 1215 with the exception that the loading rate was 0.5 mm/min. The specimens were capped using a sulphur-fly ash mixture 2 days before testing. The specimens were returned to the climate chamber directly after capping. Five specimens were tested per mix variation. The capillary water absorption tests were done in order to investigate the effect of the sisal fibres on the absorption of the masonry units. The specimens were first dried at 60 °C for 48 hours and allowed to cool to room temperature. The blocks were weighed and then placed in a large tank with the 390x140 mm sides face down, as shown in Figure 6 a). The blocks were placed on a thin rubber mesh to ensure the bottom surface is exposed to the water. The water level was kept 5 mm above the bottom surface of the blocks. Four specimens were tested per mix variation.

Test Results The compression test results are shown in Figure 7 with error bars indicating the minimum and maximum values. The compressive strength increased slightly with the increase of fibres to 0.5 %, but showed a strong decline when 1.0% fibres were added. These results showed a similar trend to the



sisal fibre reinforced low strength concrete shown in Figure 1, i.e. an increase with 0.5 % fibres and then a decrease with the 1.0 % fibres compared to the control mixture. The decrease of the samples at 1.0 % fibre volume was however more significant than the low strength concrete. A typical failure mode of the hollow core masonry units is shown in Figure 5 b) with the fibres clearly visible.

a) b) Figure 5: a) A typical hollow core masonry unit and b), a masonry unit after a compression test

a) b) Figure 6 a) The capillary water absorption test setup and b), the effect of the capillary absorption on

the masonry units

The capillary water absorption tests showed positive results for the sisal fibre reinforced masonry units. The results are shown in Figure 8 and it is clear that with an increase of fibre content the water absorption through capillary action is significantly reduced. The maximum reduction is found at a fibre volume content of 0.5 %. A typical sample showing the effect of the capillary water absorption is shown in Figure 6 b).



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Compressive Stren

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Volume Fibres Figure 7. The compression test results of the hollow core masonry units with error bars indicating the

maximum and minimum strengths

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Volume Fibres Figure 8. The capillary water absorption results of hollow core masonry units with the error bars

indicating the maximum and minimum values Discussion It can be seen from the results presented that there is an improvement of the performance of hollow core masonry units if sisal fibres are added at a volume of 0.5 %. Both the compressive strength and water absorption behaviour improved at a fibre volume of 0.5 %. The capillary water absorption was decreased with 37 % with the addition of 0.5 % fibres. The low strength of the masonry units, around 1.5 MPa, is however a concern. This is half the required 3 MPa (NHBRC) which is required for hollow core blocks for a single storey house and further development is required to increase the compressive strength of the sisal fibre reinforced hollow core masonry units.

Geopolymer A number of tests were done on geopolymers to investigate the viability of this material as an alternative material for masonry units. Geopolymers consists of aggregate, fly ash, water and an alkali



activator, sodium hydroxide used in this case. The mix was varied to investigate the effect of the ratio of constituents on the compression and tensile splitting strength of the material. Test Setup and Program A number of tests were done on variations of geopolymers to determine its sensitivity to different mix variations. The variations included the sodium hydroxide (NaOH) to fly ash ratio, fine aggregate to fly ash ratio, the curing temperature and curing time. Both compression and tensile splitting tests were done. All the tests were done using 100x100x100 mm cubes. All variations were tested at an age of 1, 7 and 28 days with the exception of the curing temperature, curing time and some of the sodium hydroxide ratios which were only tested on 1 and 7 days. Two cubes were tested per variation. The compression and tensile splitting tests were tested using the same setup as described in the section for the sisal fibre reinforced low strength concrete and aerated concrete tests. The specimens were cast and then cured for the specified time and temperature. Note they were not cured in water as for conventional concrete. After the curing of the specimens they were left at room temperature until the age of testing. Note that the specimens were not submersed in water. The mix variations are shown in Table 3. Note that tensile splitting tests were not done on all mix variations. The ratios are all per mass and 1 mol = 40 g. Table 3. The mix variations for the geopolymer tests

Mix Variation

NaOH / Fly Ash [mol/kg]

Fine Aggregate / Fly Ash [kg/kg]

Water / Fly Ash [kg/kg]

Curing Time

[hours]

Curing Temperature

[˚C]

Compression Test

Tensile Splitting

Test

Control 2.1 3 0.4 24 60 x x

Conc1 1.4 3 0.4 24 60 x Conc2 1.8 3 0.4 24 60 x

2.4 3 0.4 24 60 x x 2.8 3 0.4 24 60 x x 3.5 3 0.4 24 60 x

FineA 2.1 2.5 0.4 24 60 x x 2.1 3.5 0.425 24 60 x x 2.1 4 0.45 24 60 x

CTemp 2.1 3 0.4 24 70 x 2.1 3 0.4 24 80 x

CTime 2.1 3 0.4 6 60 x 2.1 3 0.4 8 60 x

Test Results The compression test results for the sodium hydroxide concentration, fine aggregated ratio, curing temperature and curing time are shown in Figures 9 to 12 respectively. The tensile splitting test results are shown in Figures 13 and 14 for the sodium hydroxide concentration and fine aggregate ratio variations respectively.



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Figure 9. The effect of the sodium hydroxide concentration on the compressive strength of the

geopolymer

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Figure 10. The effect of the aggregate / fly ash ratio on the compressive strength of the geopolymer

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Figure 11. The effect of the curing temperature on the compressive strength of the geopolymer

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Figure 12. The effect of the curing time on the compressive strength of the geopolymer

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Figure 13. The effect of the sodium hydroxide concentration on the tensile splitting strength of the

geopolymer The results showed that the compressive strength, Figure 9, and the tensile splitting strength, Figure 13, are strongly influenced by the concentration of the sodium hydroxide. The strength of the geopolymer is also shown to be sensitive to the aggregate / fly ash ratio, Figures 10 and 14. The higher the ratio, the less fly ash is added, thus resulting in a lower strength. The curing time also had an influence on the strength while the curing temperature, ranging from 60 ˚C to 80 ˚C, does have an influence, but not significantly. Discussion It can be seen from the results shown of the geopolymer tests that it can be a viable alternative for masonry units with regard to strength. Compression strengths in excess of 20 MPa can be achieved with the correct concentration of sodium hydroxide.



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Figure 14. The effect of the aggregate / fly ash ratio on the tensile splitting strength of the geopolymer Geopolymers are shown to be influenced strongly by the sodium hydroxide / fly ash ratio. There is a significant increase in strength at a concentration between 2 mol / kg fly ash and 2.5 mol / kg fly ash after which the strength does not increase significantly with an increase of concentration. A concentration of 2.5 mol / kg fly ash can thus be seen as an optimum after which the increase of sodium hydroxide does not influence the strength significantly. It can also be seen that the strength of the geopolymer increases with an increase of age, thus showing there is also a strength gain when stored at room temperature after the curing period. The strength is however reduced significantly if the fine aggregate / fly ash ratio is increased, i.e. less fly ash in the geopolymer. The strength of a geopolymer with aggregate / fly ash ratio of 4 is around 57 % weaker than with a ratio of 2.5. For the tensile splitting results the strength actually increases when the fine aggregate / fly ash ratio increases from 3 to 3.5, as shown in Figure 14. This could be due to the increase in the energy required to fracture the material in tension due to a higher volume of coarser material, i.e. sand. The curing temperature, Figure 11, did not show any significant change in strength with the variation from 60 ˚C to 80 ˚C. Temperatures below 60 ˚C should also be investigated as a viable option. The reduction of curing temperature could significantly reduce the cost and also the embodied energy. The curing time does however have an influence on the final strength as shown in Figure 12. With increasing the curing time from 6 hours to 24 hours the strength increased with around 51 %. It is proposed that the effect of the curing time is further investigated. The curing time should not be confused with the age of testing. The curing period is a short period at an elevated temperature and the specimens are then allowed to cool and are kept at laboratory conditions until the age of testing. From all the results shown in this section it is clear that the older the specimens, the higher the strength. This indicates the curing did continue, even at room temperature. The cost of the geopolymer is similar to that of the low strength concrete. The cement for a 10 MPa low strength concrete would cost around R220 / m3 at the current market price while the sodium hydroxide for a 10 MPa geopolymer would cost around R210 / m3. Heat generation is however required for the curing of the geopolymer which will result in an additional cost. The carbon footprint for the low strength concrete is typically 164 kg CO2eq / m3 while the geopolymer’s carbon footprint is significantly less at 98 kg CO2eq / m3 (Swiss Centre for Life Cycle



Inventories, 2013). Note that the heat generation for the curing is not included in the carbon footprint calculation. Based on the shown results it can be concluded that geopolymer is a viable option as an alternative material for masonry units. Attention should however be given to the curing time and temperature as this could will influence the cost and carbon footprint of producing geopolymer masonry units. Conclusions A large number of tests were done on a variety of alternative materials for masonry units. These materials include sisal fibre reinforced low strength concrete, aerated concrete and hollow core cement-based masonry units. The following conclusions can be drawn:

- Sisal fibres can increase the compressive and tensile strength of low strength concrete. The increase is however not significant enough to reduce the cement content. The strength obtained conforms to the minimum required for masonry units. Further research is required to improve the behaviour of sisal fibre reinforced low strength concrete before the material can be viable for the manufacturing of masonry units.

- Adding sisal fibres to aerated concrete resulted in a decrease of strength. This is due to the increase of the air content with the addition of fibres.

- The hollow core cement-based masonry units did show a slight increase of strength with the addition of 0.5 % sisal fibres by volume. The water absorption was also reduced significantly with an increase of fibres. The strength of the units was however below the required strength and further development is thus required before these masonry units can be viable.

- Geopolymer was shown to be a viable material for masonry units based on the mechanical properties. The strength is strongly influence by the ratio of the sodium hydroxide to the fly ash content as well as the ratio of the fine aggregates to the fly ash. The curing temperature did not have a significant influence in the range of 60 ˚C to 80 ˚C.

- The cost of the geopolymer is similar to that of the low strength concrete while the carbon footprint is less (excluding the heat curing) than for the low strength concrete which makes geopolymer an attractive alternative material for masonry units.

References Bentur, A., Mindess, S., 2007, Fibre reinforce Cementitious composites, second edition, Taylor & Francis. Coetzee, G., 2013, The mechanical and volumetric behaviour of sisal fibre reinforced concrete block, MEng Theses, Stellenbosch University, South Africa. Davidovits, J., 1989, Geopolymers and geopolymeric materials, Journal of Thermal Analysis and Calorimetry, Vol 35, pp. 429-441. Laing, H., 2011, The CMA House – Bringing detail and Durability to Affordable Housing, International Conference on Housing & Construction Conference & Exhibition, South African Housing Foundation. Food and Agricultural Organisation (FAO), 2009, Jute, kenaf, sisal, abaca, coir and allied fibres – statistics. Food and Agriculture Organisation of the United Nations. Hannant, D.J., 1978, Fibre cements and fibre concretes, Wiley Publishers. Kong, D.L.Y., Sanjayan, J.G., 2010, Effect of elevated temperatures on geopolymer paste, mortar and concrete, Cement and Concrete Research, Vol 40, pp. 334-339.



National Department of Human Settlements (NDHS), Chief Directorate: Research, 2010, The use of Alternative Technologies in LCH construction: Why the slow pace of delivery? Human Settlements Review 1 (1), pp 266-270. Pacheco-Torgal, F., Jalali, S., 2011, Cementitious building materials reinforced with vegetable fibres: a review, Construction and Building Materials, Vol 25, pp 575-581. Palomo, A., Grutzeck, M.W., Blanco, M.T., 1999, Alkali-activated fly ashes: A cement for the future, Cement and Concrete Research, Vol 99, pp 1323-1329. SANS 1215, 1984, Concrete Masonry Units, South African Bureau of Standards. SANS 5863, 1994, Concrete tests - Compressive strength of hardened concrete, South African Bureau of Standards. SANS 6253, 1994, Concrete tests - Tensile splitting strength of concrete, South African Bureau of Standards. Silva, F., Tolêdo Filho, R.D., Melo Filho, J.A., Fairbairn, E.M.R., 2010, Physical and mechanical properties of durable fibre-cement composites. Construction and Building Materials, Vol 24, pp 777-785. Silva da Vargas, A., Dal Molin, D.C.C., Vilela, A.C.F., José da Silva, F., Pavão, B., Veit, H., 2011, The effects of Na2O/SiO2 molar ratio, curing temperature and age on compressive strength, morphology and microstructure of alkali-activated fly ash based geopolymers, Vol. 33. Swiss Centre for Life Cycle Inventories, ecoinvent v3.0, 2013, EDIP Tolêdo Filho, R.D., Ghavami, K., England, G.L., Scrinever, K., 2002, Development of vegetable fibre-mortar composites of improved durability, Cement and Concrete Composites, Vol 24, pp 185-196. Van Rooyen, A.S., 2013, Structural lightweight aerated concrete, MSc Thesis, Stellenbosch University, South Africa. Yunsheng, Z., Wei, S., Zongjin, L., 2006, Impact behaviour and microstructural characteristics of PVA fibre reinforced fly ash-polymer boards prepared by extrusion technique, Journal of Material Science, Vol 41, pp 2787-2794.

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