Development of strong lightweight cementitious matrix for lightweight concrete simply by increasing a water-to-binder ratio in Ca(OH)2-Na2CO3-activated fly ash system

Reducing the specific gravity (SG) of structural binder pastes is one possible way to prevent material segregation between binder paste and lightweight aggregates, which often occurs in producing lightweight concrete due to their SG differences. To this end, this study suggests the use of a new lightweight fly ash binder paste, given that its SG can largely be reduced simply by increasing the water-to-binder (w/b) ratio without both excessive bleeding and significant strength reduction. Class F fly ash was activated with varying weights of water (i.e., w/b), Ca(OH)2, and Na2CO3 to prepare the samples. Compressive strength, SG, and water absorption were measured with microstructural study using XRD, TG, and MIP. The results showed satisfactory performances with strengths of 27.8-56.5 MPa, an oven-dried SG of 1.24-1.47, and water absorption of 13.3-24 wt% for selected representative mixtures at 28 days. In particular, the strongest sample, at 56.5 MPa, also obtained a low value of oven-dried SG of 1.47, which lay within the range of oven dried SG (∼ 0.6-1.5) of commercial lightweight aggregates. In this study, strength, SG, water absorption, reaction products, and pore characteristics were mainly determined by w/b ratio rather than Ca(OH)2 and Na2CO3 contents; thus, this binder would be more advantageous for quality control than other types of fly ash-based cementless binders in structural construction.