Effect of ultrafine fly ash on mechanical properties of high volume fly ash mortar

• The addition of ultra fine fly ash increased the 7-day compressive strength of HVFA mortars.
• The above addition also increased the 28-day compressive strength of HVFA mortars.
• Ultra fine fly ash is effective in consumption of C-H in high volume fly ash system.

This paper presents the effect ultrafine fly ash (UFFA) on compressive strength development of mortars containing high volume class F fly ash as partial replacement of cement. The experimental works are divided into two parts. Part one is conducted in binary blended cement mortar where Portland cement (PC) type I is replaced by UFFA at level of 5%, 8%, 10%, 12% and 15% (by wt). In this part, cement mortar and high volume fly ash (HVFA) mortars containing 40%, 50%, 60% and 70% of class F fly ash are also prepared and used as control mortars. The UFFA level which exhibited highest compressive strength is then selected and used in part two where the effect of UFFA in high volume fly ash replacement is evaluated. The study reveals that the cement mortars with 8% UFFA of cement replacement exhibited higher compressive strength at 7 and 28 days than control mortars. There is also a great improvement on compressive strength of HVFA mortars, particularly at early age. The large surface area of the UFFA promotes the hydration process and enhances the microstructure of the cement mortars to yields better strength and mechanical properties. In this study, the microstructure and phase identification after 28 days are also presented based on backscattered electron (BSE) image and x-ray diffraction (XRD) analysis of paste samples. The results indicate the effectiveness of UFFA in producing high packing density and in accelerating the pozzolanic activity to produce more C-S-H gel by consuming calcium hydroxide (CH) in order to improve the mechanical properties of HVFA mortars.