Black cotton soil (BCS) forms a major soil group in Kenya and is characterized by high shrink/swell potential when exposed to water. A comprehensive series of laboratory tests were performed on BCS treated with lime (0–9%), volcanic ash (VA, 0–25%), and their combinations in order to study the physical–mechanical properties and mineralogical changes of the stabilized BCS. Moreover, a test road which replaced the BCS with the lime–VA-stabilized BCS was constructed to investigate the moisture change and soil movement in the BCS foundation. The results revealed that BCS stabilized with combinations of lime and VA shows larger California bearing ratio (CBR) and unconfined compressive strength (UCS) values when compared with a single stabilizer. BCS stabilized with 3% lime + 15% VA meets the performance requirements of roadbed materials in accordance with JTG D30-2015. The increase of pH and electrical conductivity (EC) in the stabilized soil promotes chemical reactions between the stabilizers and BCS to form new cementing agents, which are confirmed by X-ray diffraction (XRD) and transmission electron microscope (TEM) findings. The replacement of BCS with 3% lime + 15% VA-stabilized BCS shows an obvious effect on controlling the moisture change and soil movement in the foundation BCS. This research provides a low-cost strategy for making use of the vast resources of BCS in Kenya obtained from foundation excavation.

This research provides a strategy for partially replacing cement with municipal solid waste incineration (MSWI) bottom ash (BA) to improve the performance of concrete against sulphate attack. Mortar strength tests were performed firstly to evaluate the hydration activity of the ground BA. Concrete specimens were cured in standard conditions and immersed in a solution that contained 10% sodium sulfate. Then, the compressive strength of these specimens was measured to investigate the mechanical properties and durability of the concrete. Next, the capillary porosity of the concrete was determined from the volume fractions of water lost in specimens. Finally, the transport of the sulphate solution in concrete was analyzed using capillary rise, crystallization rate, and solution absorption tests. The results indicated that BA had a certain hydration activity. The equivalent replacement of cement by BA decreased the compressive strength of the specimens but increased the durability of the concrete. There was an excellent correlation between capillary rise height, sulfate solution absorption amount, crystallization rate, and coarse capillary porosity. The addition of BA can decrease the coarse capillary porosity and further slow the capillary transport and crystallization of sulfate solution in concrete. Overall, the replacement of cement with BA can improve the durability of concrete and actualize the utilization of MSWI residues as a resource.

The hydration products and strength of cement pastes incorporated with ground blast furnace slag (GBFS) (0% and 20% replacement) have been investigated under steam curing condition (80 °C for 7 h and 7 d) in comparison with normal curing condition (moist curing for 28 d). The results show that, during the initial 80 °C steam curing for 7 h, in addition to the filler effect, GBFS is still involved in cement hydration. The abundant available Al phase and Mg phase in GBFS promote the formation of flake-like hydrotalcite, foil-like C-(A)-S-H gels, as well as equant grain-shaped C-(A)-S-H gels. Prolonging the steam curing time to 7 d further improves the formation of hydrogarnet. Since the formation of both hydrogarnet and hydrotalcite can consume the available Al, steam curing for 7 d seems to favor the formation of low Al C-(A)-S-H gels. In addition, due to the formation of a large amount of hydration products, the influence of 20% GBFS addition on the demolding strength of initial steam-cured cement mortar (80 °C for 7 h) is almost negligible. However, further extending the steam curing time to 7 d increases the strength gap between 20% GBFS blended cement mortar and pure cement mortar, and the related mechanism is discussed.

To explore the feasibility of the application of steel slag powder (SSP) in steam-cured precast concrete, 0% and 20% SSP were used to replace cement and prepare cement paste, and the early age performance of steam-cured (80 °C for 7 h and 7 d) SSP-blended cement paste, including different types and amounts of hydrates, the microstructure and mechanical properties were investigated and compared with those of 28 d standard-cured SSP sample. The results show that SSP addition promotes the generation of laminar C-S-H gels and granular C-S-H gels after an initial 7 h steam curing. Further extending the lasting time of 80 °C steam curing to 7 days favors the production of hydrogarnet and crystalline C-S-H, of which the amount of formation of hydrogarnet in SSP composite cement paste is less and the particle size is smaller than those in the control sample. However, steam curing increases the gap between the number of hydrates formed in SSP-blended cement paste and the control paste. The delayed hydration effect of SSP on cement offsets the promoting effect of steam curing on the hydration of cement; in consequence, the incorporation of SSP seems to be detrimental to the hydration of steam-cured cement paste.