Effect of compaction pressure and wheat straw inclusion on geopolymer-stabilized rammed earth behavior

This paper presents the effect of compaction pressure level and wheat straw addition on the durability, mechanical behavior, and thermal conductivity of geopolymer stabilized rammed earth. Rammed earth specimens were prepared by static compaction to 5, 10, and 25 MPa of mixtures containing predefined amounts of sand, silt, clay, and wheat straw, stabilized with fly ash geopolymer. A number of unstabilized specimens made of the raw materials were also prepared for comparison. These specimens were cured inside plastic bags at 35 °C so that the least energy consumption is achieved. The durability was investigated by performing a dip test and spray test. The results of the geopolymer stabilized specimens demonstrated an excellent resistance to erosion by water, unlike the unstabilized specimens, which almost failed completely. The mechanical behavior was evaluated by performing unconfined compression test. The results indicated that material’s stiffness and strength increased considerably with increasing compaction pressure and curing age, with the majority of the increase occurring during the first month of curing. Compressive strength values of 4.2 and 10 MPa were recorded from tests on stabilized specimens compacted to 5 and 25 MPa, respectively, then cured for 60 days. These figures are promising, keeping that, a threshold unconfined compressive strength of 1−2 MPa is typically acceptable by many building codes. A relatively low thermal conductivity of about 0.35−0.5 W/ (K.m) was recorded from the hot wire method on specimens prepared from various mixtures, suggesting that the stabilized rammed earth outperforms most of the traditional building materials such as concrete. The inclusion of wheat straw improved material’s ductility by increasing strains, at which shear failure occurs. However, this inclusion led to undesirable reduction in strength and stiffness over the first two months of curing and almost no change in thermal conductivity, with respect to those obtained on the stabilized specimens. This response was attributed to material wise incompatibilities.