Use of diatomaceous earth as a siliceous material in the formation of alkali activated fine-aggregate limestone concrete
Authors:   , Miller Sean Anthony , Barsoum Michel W. 
Published: July 2021

The motivation behind this research has a historical, environmental, and developing world aspect. From a historical stand point the goal is to further the research established by Barsoum et al. claiming that parts of the Ancient Egyptian Pyramids were cast from a reconstituted limestone containing a binding phase of amorphous silica and/or calcium/magnesium silicates. From an environmental stand point the goal is to create an alternative cement to Portland cement, which has an enormous carbon footprint. For every 1 ton of Portland cement produced, roughly 0.8 tons of CO₂ are released into the environment. By creating an environmentally friendly concrete based on ubiquitous materials, the final goal is to some day be able to use such a concrete in developing countries where proper building materials are difficult to acquire. Experiments were developed to investigate and understand the role of diatomaceous earth (DE) as a source of silica in the formation of alkali activated fine aggregate concrete with lime as the alkali. In the 6 month trial four formulas were developed to investigate how hydraulic lime mortar using DE as the source of silica differs in strength and properties from naturally hydraulic lime mortar and non-hydraulic lime mortar. Formulas with low and high DE contents were created and compared with the naturally hydraulic and non-hydraulic lime mortar controls. The strength and binding phase properties of the four formulas were investigated over a 180 day testing period using compressive strength, XRD, TGA, SEM, and phenolphthalein tests. The results from the 6 month trial showed that the high DE formula had the best compressive strength of the four formulas at 7 MPa, and the low DE formula had the second best strength at 5 MPa after 180 days. A major issue was discovered with the high DE formula however, as it was observed that its' humidity chamber and container cured concrete samples lost up to 50% of their strength when left to dry for 7 days in air. The low DE formula did not have this issue and thus it was hypothesized and supported by SEM images that the strength retention issue in the high DE formula resulted from the existence of undissolved diatoms throughout the sample. Cement paste experiments were set up to further investigate the binding phase properties of the formulas using XRD and TGA. The cement paste XRD showed that Ca1.5SiO3.5·xH₂O was the specific type of C-S-H that formed in the DE based cements and provided its strength. Celite 266 and Celite 400 DE were then used to investigate how changing the type of DE used affected the properties of the concrete. The results showed that the original Perma-Guard DE had the best strength properties. They also showed a counterintuitive correlation that the lower the surface area of the DE, the higher the compressive strength. The validity of this relationship was questioned and possible explanations provided. Finally, a Ca/Si ratio experiment was conducted to explore the relationship between Ca/Si ratio in the concrete, compressive strength, and the strength retention issue seen in the original high DE formula. Conclusions were made on the role of DE as a source of silica in alkali activated fine aggregate concrete, its viability as a building material in developing countries, and its cost and environmental competitiveness with Portland cement. Finally, future work and further optimization testing was discussed.