Cement is most widely used as a soil binder in the field. Limestone, which is used in cement, is abundant in the world. However, the mining limestone and its processing causes environmental damage and large carbon dioxide emission. It is one of a major contribution to global warming as a typical greenhouse gas. In this study, two different eco-friendly soil cementation methods are developed without using cement.
First of all, this paper presents an environment-friendly sand cementation method by precipitating calcium carbonate using plant extract. The plant extract contains urease like Sporosarcina pasteurii, which can decompose urea into carbonate ion and ammonium ion. It can cause cementation within sand particles where carbonate ions decomposed from urea combine with calcium ions dissolved from calcium chloride or calcium hydroxide to form calcium carbonate. The effect of amounts of plant-extract or urea and different calcium sources (calcium chloride, calcium hydroxide and calcium nitrate) on the sand strength was tested in terms of unconfined compressive strength. As the amounts of plant extract or urea increased, the unconfined compressive strength of sand increased up to 10 times those without plant extract because calcium carbonate precipitated more, regardless of calcium source.
Secondly, a blast furnace slag with latent hydraulic property is used to cement sand. When the blast furnace slag reacts with an alkaline activator, it can cement soils. The effect of amount of blast furnace slag and types of alkaline activator on soil strength was investigated for resource recycling. Four different amounts of slag and six different activators (two naturals and four chemicals) were used for preparing specimens. The specimens were air-cured under limited condition and then tested for unconfined compressive strength (UCS). The UCS of cemented sand with slag increased, in the order of specimens mixed with potassium carbonate, calcium hydroxide, sodium hydroxide and potassium hydroxide. Chemical alkaline activator was better than natural alkaline activator. As the amounts of slag increased, the UCS and dry density of a specimen increased for all alkaline activator cases. As the curing time increased, the UCS increased up to 97%.
In addition to chemical activator, alkaliphilic microorganism was tested as a new alkaline activator. The alkaliphilic microorganism was added into sand with a blast furnace slag and a chemical alkaline activator. Four different ratios of blast furnace slag (4, 8, 12, 16%) and two different chemical alkaline activators (Ca(OH)2 and NaOH) were used for preparing cemented specimens with or without the alkaliphilic microorganism. The specimens were air-cured for 7 days and then tested for unconfined compressive strength (UCS). As blast furnace slag increased, the water content, dry density and UCS of a specimen increased. When a microorganism was added into the specimen, the UCS of a specimen with CaOH2 decreased but one with NaOH increased.
At current stage, the two environment-friendly soil cementation methods developed in this study are more expensive compared to current cementation method using ordinary Portland cement. However, such environmental-friendly soil cementation methods will be required in the future because the cost of cement will be gradually increased due to carbon tax.