Think Brick Australia - Technical Manual 10 - provides guidance for the construction of clay masonry in buildings. Page 14 of the Manual refers to the problem of using Fire Clay as a mortar workability additive and states the following;
"Finely powdered clay, which can be referred to as fire clay, builders’ clay, brickies’ clay or plasterers’ clay, is sometimes marketed as an admixture for masonry mortars, to enhance workability. While bricklayers find it produces a readily workable mix, this type of admixture can have severe effects on bond strength.
As the dose of clay increases, the bond strength reduces markedly (see Figure 3). Hence, the use of fire clay is not permitted as a mortar additive under Clause 220.127.116.11 of AS 3700-2011"