Question

a) What is insoluble residue (IR) of cement? b) What is its cause? c) The IR of portland fly ash composite cement is considerably higher than of ordinary portland cement produced from the same clinker. Why?

Answer 1

Answer : -

(a) Insoluble residue (IR) measures adulteration of cement coming from impurities in gypsum (calcium sulphate) and high values have been shown to reduce strength. The high values of loss of ignition could be attributed to prehydration and it seems the cement had been stored for a long time.

(b) IR of cement is perhaps due to the fact that progressive addition of calcined clay (in PCC40 < PCC45 < PCC50) resulted to increased quantity of insoluble materials. The insoluble residues represents the unreactive hence did not participate in the hydration reactions as well as pozzolana reaction of the blended cement. The insoluble material could have acted as a filler material in the hydrated cement and subsequently slowed down the pozzolanic reaction.

(c) IR of portland fly ash composite cement is considerably higher than ordinary portland cement because the composite cements are composed of constant proportion of OPC (80%) with variable amounts of fly ash and homra. The addition of fly ash accelerates the initial and final sitting time, whereas the free lime and combined water contents decrease with fly ash content. The fly ash acts as nucleation sites which may accelerate the rate of formation of hydration products which fill some of the pores of the cement pastes. The fire resistance of composite cement pastes was evaluated after firing at 250, 450, 600, 800 °C with rate of firing 5 °C/min with soaking time for 2 h. The physico-mechanical properties such as bulk density and compressive strength were determined at each firing temperature. Moreover, the phase composition, free lime and microstructure for some selected samples were investigated. It can be concluded that the pozzolanic cement with 20 wt% fly ash can be used as fire resisting cement. And in ordinary portland cement the insoluble residue to be not higher than 0.75%. This limitation is much lower than the allowance provided by the British standard which is 1.5%. To verify the effect of insoluble residue on the properties of Portland cement, artificial insoluble residue was prepared and replaced in Portland cement type I. Finely crushed sand was extracted to represent artificial insoluble residue. Setting times and compressive strengths of cement mortar mixed with insoluble residue were investigated. The Portland cement was replaced by insoluble residue which varied in amounts of 0%, 0.5%, 1.0%, 1.5%, 2.0%, 3.0%, 5.0% and 7.0% by weight.

The results showed that the addition of the insoluble residue from 0.0% to 7.0% by weight in Portland cement did not affect the normal consistency or setting times of cement. However, the compressive strength of cement mortar was affected during the early age, but the figure reduced as the cement mortar was older. With 7.28% of insoluble residue in the mortar at 1 day, the compressive strength was reduced by 11.5%, but after 60 days, the strength of the same mortar was only reduced by 5.5% as compared to the control mortar. It was also found that the compressive strength of Portland cement mortar with insoluble residue provided by ASTM standard or British standard was still higher than the compressive strength of Portland cement mortar type I allowed by the standards. The limit of insoluble residue given by ASTM standard as 0.75 is rather low and can possibly be increased to 1.5% according to British standard, or even slightly higher, without significantly reducing the compressive strength of cement.