Pore gradation effect on Portland cement and geopolymer concretes

Geopolymers possess comparable properties to ordinary Portland cement (OPC) with less consumption of natural resources and lower carbon emissions. This paper investigated microstructural characteristics of geopolymer concrete (GPC) and OPC concrete (OPCC) and their correlation to strength and durability. The effects of the liquid to binder ratio and addition of silica fume/slag blend to mixtures were studied through compressive strength, ultrasonic pulse velocity (UPV), apparent volume of permeable voids (AVPV), pore detection image analysis (PDIA), and liquid nitrogen porosimetry (LNP). From the results, the strength development of geopolymers and OPC was strongly correlated to the void distribution and the pore structure. Like UPV and AVPV, PDIA showed that the void volume of OPC varying from 7% to 14% was significantly lower than geopolymers with 8%-32% of void volume. Additionally, LNP indicated the specific surface area of OPC distributed over 4.1 m2/g to 20.8 m2/g was 50%-66% lower than that of geopolymers. The tolerance of these parameters could control the mechanical and durability characteristics. The variations in the void and pore distribution of GPC illustrated a layer-based transition from the top to the bottom of sections. The UPV and PDIA results stated that the pore volume decreased gradually in the middle and bottom layers of GPC, and the LNP illustrated a shift and increase in nanometric gel pores in the pore size distribution of lower layers.