The Long Term Effect on Cement Mortar by Admixture of Spray Drying Absorption Products

Abstract

ABSTRACTPreliminary investigations have shown that the substitutions of up to 10% fly ash (FA), with spray drying absorption products (SDA), in cement mortars (cement: 80% rapid portland cement (RPC), 10–20% FA, 0–10% SDA) results in low early strength of the same magnitude as in mortar with 80% RPC + 20% FA. After storing for 3 months under wet or moist conditions the strengths of some of the SDA-containing mortars had surpassed the strength of the (80% RPC + 20% FA) mortar. Dry stored SDA-containing mortars showed a higher tendency to desiccate. Determination of SO32− showed quick oxidation to SO42− and XRD revealed the formation of ettringite. After a year the strengths of mortars made of 100% RPC, (80% RPC + 20% FA) and (80% RPC + 10% FA + 10% SDA) stored wet or moist were equal. Attempts to use synthetic CaSO3 · ½H2O (CS·½H) as a retarding agent for cement hydration failed, but conductivity measurements of tricalcium aluminate (C3 A)-CS · ½H mixes showed reaction. SEM and XRD investigations identified calcium aluminate sulphite hydrate (C4ASH8). DTA–DTG examinations of synthetic C4ASH8 showed the formula to be C4ASH11. A modification of the instructions for preparation of mortar prisms resulted in the desired retarding reaction between the C3A from portland cement (PC)-clinker and synthetic CS · ½H and SDA-products as well. The resulting C4ASH11 proved to be very unstable and changed to ettringite within a very short time. Use of the modified instructions for preparation of mortar prisms containing SDA resulted in satisfactory early strengths. A series of mortar prisms with increasing content of SDA (x% RPC, (100-x)% SDA in cements; 0<x<80) were prepared and stored under wet, moist and dry conditions. These mortars, now one year old, show a gradual development of properties. Mortars with a SDA content in cement >80% cannot be stored wet. The effects on mortars of the individual constituents of the SDA-products are studied by XRD, development in strength and density. Fragments of 2 year old SDA containing cement mortars and SDA containing concrete from a parking place have been studied.