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Concrete In Australia : June 2008
The above fl exibilities in the heat-curing regime of geopolymer concrete can be exploited in practical applications and precast concrete products can be manufactured ready for use within 24 hours after casting. Design of geopolymer concrete mixtures Concrete mixture design process is generally based on performance criteria. Based on the information given in Sections 3 to 6 above, some simple guidelines for the design of heat-cured low-calcium fl yash-based geopolymer concrete are proposed. The role and the infl uence of aggregates are considered to be the same as in the case of Portland cement concrete. The mass of combined aggregates may be taken to be between 75% and 80% of the mass of geopolymer concrete. The performance criteria of a geopolymer concrete mixture depend on the application. For simplicity, the compressive strength of hardened concrete and the workability of fresh concrete are selected as the performance criteria. In order to meet these performance criteria, the alkaline liquid-to-flyash ratio by mass, water-to-geopolymer solids ratio by mass, the wet-mixing time, the heat-curing temperature and the heat- curing time are selected as parameters. With regard to alkaline liquid-to-fl yash ratio by mass, values in the range of 0.30 and 0.45 are recommended. Based on the results obtained from numerous mixtures made in the laboratory over a period of four years, the data given in Table 3a are proposed for the design of low-calcium flyash-based geopolymer concrete. Note that wet-mixing time of four minutes, and steam-curing at 60o C for 24 hours after casting is proposed. The data given in fi gures 1and 5 may be used as guides to choose other curing temperatures and curing times. Sodium silicate solution is cheaper than sodium hydroxide solids. Commercially available sodium silicate solution A53 with SiO2 Na2 and sodium hydroxide solids (NaOH) with 97-98% purity are recommended. Laboratory experience suggests that the ratio of sodium silicate solution-to-sodium hydroxide solution by mass may be taken approximately as 2.5 (Hardjito and Rangan, 2005). O = 14.7%, SiO2 -to-Na2 The design data given in Table 3a assumes that the aggregates are in saturated-surface-dry (SSD) condition. In other words, the coarse and fi ne aggregates in a geopolymer concrete mixture must neither be too dry to absorb water from the mixture nor too wet to add water to the mixture. In practical applications, aggregates may contain water over and above the SSD condition. Therefore, the extra water in the aggregates above the SSD condition must be estimated and included in the calculation of water-to-geopolymer solids ratio given in Table 3a. Example The mixture design process is illustrated by the following example: A mixture proportion of heat-cured low-calcium flyash-based geopolymer concrete with design compressive strength of 45 O ratio by mass of approximately 2 – ie. = 29.4%, and water = 55.9% by mass, MPa is needed for precast concrete products. Assume that normal-density aggregates in SSD condition are to be used and that the unit-weight of concrete is 2400 kg/m3 . Take the mass of combined aggregates as 77% of the mass of concrete – ie 0.77x2400 = 1848 kg/m3 (15%) of . The combined aggregates may be selected to match the standard grading curves used in the design of Portland cement concrete mixtures. For instance, the aggregates may comprise 277 kg/m3 20mm aggregates, 370 kg/m3 kg/m3 (35%) of 7mm aggregates, and 554kg/m3 (20%) of 14mm aggregates, 647 (30%) of fine sand to meet the requirements of standard grading curves. The fi neness modulus of the combined aggregates is approximately 5.0. Table 3a. Data for design of low-calcium fl yash-based geopolymer concrete mixtures (Rangan, 2008, 2009). Water-to- geopolymer solids ratio, by mass 0.16 0.18 Workability Very Stiff Stiff 0.20 Moderate 0.22 0.24 High High Notes: • The fi neness modulus of combined aggregates is taken to be in the range of 4.5 and 5.0. • When cured in dry-heat, the compressive strength may be about 15% larger than the above given values. • When the wet-mixing time is increased from 4 minutes to 16 minutes, the above compressive strength values may increase by about 30%. • Standard deviation of compressive strength is about 10% of the above given values. The mass of low-calcium fl yash and the alkaline liquid = 2400 – 1848 = 552 kg/m3 . Take the alkaline liquid-to-flyash ratio by mass as 0.35; the mass of fl yash = 552/ (1+0.35) = 408 kg/ m3 and the mass of alkaline liquid = 552 – 408 = 144 kg/m3 and the mass of sodium . . Take the ratio of sodium silicate solution-to-sodium hydroxide solution by mass as 2.5; the mass of sodium hydroxide solution = 144/ (1+2.5) = 41 kg/m3 silicate solution = 144 – 41 =103 kg/m3 Therefore, the trial mixture proportion is as follows: combined aggregates = 1848 kg/m3 kg/m3 hydroxide solution = 41 kg/m3 = 14.7%, SiO2 -to-Na2 , sodium silicate solution = 103 kg /m3 . , low-calcium fl yash = 408 , and sodium To manufacture the geopolymer concrete mixture, select commercially available sodium silicate solution A53 with SiO2 O ratio by mass of approximately 2 – ie. Na2 = 29.4%, and water = 55.9% by mass. The O sodium hydroxide solids (NaOH) with 97% - 98% purity is Concrete in Australia Vol 34 No 2 41 Design compressive strength (wet-mixing time of 4 minutes, steam curing at 60o C for 24 hours after casting), MPa 60 50 40 35 30