Effects of Treatment of Rice Husk Ash on a Ternary Concrete with Cement and Fly Ash

Abstract

In this paper, rice husk obtained from the same source with no special pretreatment was burnt in an oven within a controlled temperature of 500 – 600 ◦C for 30 minutes and cool in a metal container for several days. During cooling the amount of air supplied into the container was varied and this lead to different rice husk ash (RHA) products with different unburnt carbon content and appearance in color. However, the main physical and chemical properties of the RHA with lower unburnt carbon content (LRHA) and RHA with higher unburnt carbon content (HRHA) were relatively the same. LRHA and HRHA were prepared in a ternary concrete mix with cement and fly ash (FA) each with a total replacement ratio of 20%, 30%, and 40%  and the ratio of FA:RHA maintained at 3:1through out all mixes. To understand more about the compatibility of RHA and FA, Class C and Class F FA were separately used in one similar mix design. All ternary mixes were prepared along with normal concrete, cast in cylindrical molds and then pond cured for up to 28 days. In preparing the normal concrete mix, viscosity enhancing admixture (VEA) was used to increase the cohesiveness and control the slump of the normal mix case. VEA was used to compensate the low workability of ternary mixes influenced by the addition of LRHA and HRHA. On another comparison, the LRHA was reduced to smaller particle size by grinding (GRHA) and prepared in a ternary mix with cement and class F FA but this time superplasticizer was added to all mixes. These mixes were compared to normal concrete but without VEA. The objective of these comparisons was to identify the most compatible ternary mix with cement, RHA, and FA and determine the effect of particle size of RHA on concrete. The particle size of RHA was analyzed using laser diffraction method and the compressive strength of different mixes were compared on 7, 14 and 28 day after pond curing. To determine the effect of VEA on concrete, two similar normal mixes with and without VEA were prepared and compressive strength compared on 7, 14 and 28 days. From the result of these experiments, LRHA was found to produce better compatibility with the ternary mix than HRHA. However, both LRHA and HRHA showed lower strength after 7, 14 and 28 days than normal concrete which can be attributed to the high particle size of LRHA and HRHA. After grinding, GRHA ternary mix produced a strength that is comparable to normal concrete. Also, it was found that when VEA of the amount 6% equivalent to total cement content was added to normal concrete the compressive strength on 7, 14 and 28 days were not significantly affected.