Setting time and standard consistency of Portland cement binders blended with rice husk ash, calcium carbide and metakaolin admixtures

Abstract

The use of pozzolanic materials to reduce carbon dioxide (CO2) emissions and enhanced properties of mortar and concrete has received increased interests in the last decades. In this study, admixtures such as metakaolin (MK), rice husk ash (RHA) and calcium carbide waste (CCW) partially replaced Portland cement (PC) at 5, 10, 20 % in multiple combinations of aforementioned admixtures as binders and the effect of setting times (initial and final) and standard consistency on the binders were evaluated. The Department of Environment (DOE) approach for mix proportions was employed. The mixes are in six groups consisting of control PC, each admixture replaced PC at 5, 10 and 20 % replacement level in binary mixes, then two of the admixtures replaced PC at same replacement level in ternary and the three admixtures replaced PC in quaternary mixes respectively. Results indicated that RHA significantly influenced the standard consistency of the binders incorporating the three admixtures as compared with CCW and MK. The requirement of water in the binder increased with increase in percentage replacement level of RHA within the binder due to its porous nature. At 5 % replacement level, it was 9 % above control value, 21 % and 39 % at 10 and 20 % replacement level subsequently. Similarly, setting times at the aforementioned replacement levels are 27 % and 6 % above the control value for initial and final setting time. For 20 % replacement level, it was 24 % and 38 % above the control value for the initial and final setting time respectively. This is due to reduction in PC content leading to less content of Tricalcium silicate aluminate (C3A) and amount of carbon content within the pozzolanic materials which retard the rate of reactions. By and large, the admixtures retard the setting times of quaternary binder, and increased its water of consistency which can be overcome by activation. However, they are useful in the construction of concrete structural elements or structures that requires longer time for placement. Furthermore, it will be worthwhile to investigate same properties at the nanoscale since these have been established at the micro level.