The use 12 M NH gave the highest splitting tensile and
flexural strengths. GL12 and GR12 concretes gave the maximum
splitting tensile concrete of 4.0 MPa, while GR12 concrete gave the
maximum flexural strength of 5.3 MPa. The average splitting tensile
strength was about 10% of their respective compressive strengths
and about 78% of their respective flexural strengths. The results
were similar to those of conventional concrete. With regards to the
use of LS and RCA, GR concretes with 12 and 16 M NH gave slightly
higher splitting tensile and flexural strengths than those of GL
concretes. This was due to the increased calcium hydroxide (CH)
and other calcium products in the system (from RCA as shown in
Fig. 1). The high calcium content, the additional CSH and CASH gels
were formed, which modified the microstructure of the paste and
acted as micro-aggregate (Phoo-ngernkham et al., 2013). The increase
in reaction products at the interface transition zone between
normal concrete and geopolymer material enhanced the strength
at contact zone. In addition, Shi et al. (2012) reported that the
physical and chemical characteristics of geopolymer concrete with
recycled aggregate was changed and caused a decrease in the volume
of void and led to overall improvement the density and homogeneity
of matrix.