Assessment of strength development in blended cement admixed Bangkok clay

Fly ash and biomass ash have been widely accepted as waste materials substituting Portland cement. In this paper, the role of these two ashes on the strength development of cement admixed low-swelling Bangkok clay is investigated via unconfined compressive (UC) test and thermal gravity (TG) analysis. Fly ash and biomass ash are dispersing materials, increasing the reactive surface of the cement grains. The pozzolanic reaction does not play any significant role on the strength development with time since the amount of Ca(OH)2 is insufficient to react with the ashes. The contribution of the dispersing effect to the strength development is regarded akin as an addition of cement. Based on this premise, the clay–water/cement ratio hypothesis for blended cement admixed clay is proposed for analyzing and assessing the strength development. Even with the difference in water content, cement content and ash content, the blended cement admixed clay samples having the same clay–water/cement ratio, wc/C possess practically the same stress–strain response and strength. The relationship among strength, clay–water/cement ratio, and curing time for the blended cement admixed Bangkok clay is finally developed and verified. It is useful to assess the strength at any curing time wherein water content, cement content, and ash content vary over a wide range by using the test result of a single laboratory trial. For the economic mix design (the most effective dispersing effect), an addition of 25% ash is recommended. It can save on the input of cement up to 15.8%.