With an annual production of 4 million tons, sugar cane bagasse ash (SCBA) has potential to be used as an alternative supplementary cementitious material (SCM) for Thailand. Prior research has reported satisfactory improvements in compressive strength when SCBA was used to partially replace cement. Such improvements partly result from the filler effect and the pozzolanic reaction between reactive SiO2 from the SCBA and Ca(OH)2 from cement hydration. Four samplings of SCBAs obtained from a sugar factory (Dan Chang Bio-Energy Co., Ltd., Suphanburi, Thailand) were analyzed for their pozzolanic activities using various methods in this work. The SCBAs were each ground to two different finenesses such that the filler and pozzolanic effects can be elucidated. From chemical extraction, the amorphous (or reactive) SiO2 contents of the SCBAs were found to be in the range of 42-49 and 34-43 wt% for fine and coarse SCBAs, respectively. These are considered low compared with typical rice husk ash (> 85%). Nevertheless, the agreements between the amorphous SiO2 content, the Δσ2 min from the conductometric method of Luxán et al. (1989), and the 28-day strength activity index (SAI) suggest that the former two methods can potentially be used to predict the pozzolanic effect on the SAI. Despite the low amorphous SiO2 contents, cement mortars made with 10% SCBA replacement still show satisfactory gains in compressive strength, suggesting that the amorphous SiO2 content is not a primary factor contributing to strength in this system. The results should help promote the use of industrial SCBAs as an SCM, given that their particle sizes are reduced prior to use, to increase both the filler and the pozzolanic effects and to make the effect of the amorphous SiO2 content on the compressive strength gain less discriminating.