Admixtures are substances added to another substance usually a base substance which react with the base substance to improve some desirable properties. Admixtures are commonly used in concrete. In recent years, admixtures have found usefulness in soil stabilization where they can combined with lime or cement to stabilize soil. There are different types of admixtures used in concrete which include: water reducing admixtures, retarding admixtures, air-entraining admixtures, bonding admixtures, pozzolanic admixtures etc. In this post, I would focus on pozzolanic admixtures.
Pozzolans
Pozzolans are a broad class of siliceous or siliceous and aluminous materials which possess little or no cementitious properties but when they are in finely divided form and subjected to hydration reaction with water, they react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties such as calcium silicate hydrate. Pozzolans or pozzolanic materials used as admixtures could be naturally occurring or manufactured artificially. Naturally occurring ones are shale, pumicite etc. Artificial pozzolans include rice husk ash (RHA), bone ash (BA), groundnut shell ash (GSA), oil palm empty fruit bunch ash (OPEFBA), coconut coir ash (CCA) etc. Most of these products have found usefulness in concrete production and soil-cement or soil-lime production. Their advantages are that they are cheap and help to reduce the carbon effect of waste disposal and concrete production/use.
To determine whether a material is pozzolan or not, the material would first be subjected to composition determination test using methods like X-ray fluorescence or X-ray diffraction test. The composition of the compounds present would be shown in percentage. Next, pozzolanicity test would be carried out using some necessary compounds. The compounds used in pozzolanicity test are SiO2, Al2O3 and Fe2O3.
If, SiO2 + Al2O3 + Fe2O3 ˃ 70%, the material is a pozzolan
Bogue’s model
The Bogues’ equation provide simple and convenient method to find out the final composition of a clinker. It is used to calculate the approximate proportions of the four main minerals in Portland cement clinker. These minerals are C3S, C2S, C3A and C4AF. The Bogue’s model is very useful in the cement industry.
Tricalcuim silicate (3CaO.SiO2 or C3S) – This compound is responsible for the early strength gain and high heat of cement. It is usually more in rapid hardening Portland cement and less in low heat Portland cement.
Dicalcuim silicate (2CaO.SiO2 or C2S) – This compound is responsible for later strength gain of cement and produces less heat.
Tricalcuim aluminate (3CaO.Al2O3 or C3A) – This compound contributes little strength to cement. It produces great heat, very unstable and decomposes to lime in the presence of water or air. It is usually attacked by sulphates to form calcium sulphoaluminate.
Tetra-calcuim aluminoferrite (4CaO.Al2O3Fe2O3 or C4AF) – This contributes little strength to cement and produces little heat
The Bogue’s equation for potential composition given as:
C3S = 4.07 (CaO) -7.60 (SiO2) – 6.72 (Al2O3) – 1.43 (Fe2O3) – 2.85 (SO3)
C2S = 2.87 (SiO2) – 0.753 (C3S)
C3A = 2.65 (Al2O3) – 1.69 (Fe2O3)
C4AF = 3.04 (Fe2O3)
can be used to determine the composition of a pozzolan. Example, chemical analysis of rice husk ash (RHA) produced the following oxide composition (Table 1). Check whether the ash is pozzolan and determine the Bogue compound composition.
Table 1; Chemical composition of rice husk ash (RHA)
Check for pozzolanity
SiO2 (67.3) + Al2O3 (4.90) + Fe2O3 (0.95) = 67.3+4.90+0.95 = 73.15% ˃ 70%, the material is pozzolan.
Calculation of Bogue composition
C3S = 4.07 (1.36) – 7.6 (67.3) – 6.72 (4.90) – 1.43 (0.95) – 2.85 (2.8) = 5.5352 – 511.48 – 32.928 – 1.3585 – 7.98 = – 548.21%
C2S = 2.87 (67.3) – 0.753 (548.21) = 193.151 – 412.80 = -219.65%
C3A = 2.65 (4.90) – 1.69 (0.95) = 12.985 – 1.6055 = 11.38%
C4AF = 3.04 (0.95) = 2.89%
