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2013FallShotcreteEMag

Limestone cement in Shotcrete By Jonathan E. Dongell tslag) with portland cement is common- limestone filler, or where an equivalent perfor-be made for tertiary blends or blended cementsthat incorporate combinations of pozzolan andoday, the use of supplemental cementitiousmaterial (SCM) (fly ash, microsilica, or place. The advantages range from ease of mance can be demonstrated. Unfortunately, pumping and application to increased rheology existing standards and codes do not provide and durability. Recently, ultra-fine (micro-fine) specific requirements for this essential aspect of limestone “filler” additions, beyond the limestone fillers added to portland cement. Until amounts used as a grinding aid, are being intro- they do, shotcrete companies should consider the duced into cements produced in the United following information. States. Many countries allow limestone replace- Cement companies cannot be expected to ment for certain types of cement. Some coun- produce cement that is ideal for every usage. For tries allow as much as 35% replacement for shotcrete, the applicator must ensure that the “general purpose” cement.1 Replacing a portion concrete mixtures with limestone cement produce of the cement with limestone fines produces a the specific characteristics and properties of the more energy-efficient product and lessens in-place material that provides the required ser- greenhouse gases, but the challenge is to do so viceability and durability in the service environ- “while succeeding in maintaining the funda- ment. It is recommended that prior to using mental characteristics of hydraulic cement.”2 limestone cements in the field, various mixture Perhaps a better goal is to do so while main- designs incorporating limestone cement be pro- taining or improving the fundamental charac- duced for lab testing. These should then be shot teristics of the “parent” portland cement; in on a number of test panels to ensure the mixture other words, producing cement that is “green” produces an acceptable end product and to famil- without sacrificing the performance, character- iarize the field crew with the unique material istics, and durability of the “parent” cement. characteristics and workability traits. In the United States, several ASTM Stan- dards,3-5 ACI Codes,6,7 and guides regulate the Use in Dry, Nonaggressive composition of limestone fines and its usage in concrete mixture designs. However, current exposures ASTM Standards do not require that limestone There are significant advantages and disadvan- fineness be fine enough to ensure a similar per- tages that should be considered when using formance to that of the “parent” portland cement limestone cement in relatively dry exposures. In (or “optimizing” the fineness). Instead, current general, increased amounts of optimized micro- standards focus on requiring a minimum strength, fine limestone—up to 15% replacement of the which is designed to ensure that limestone cement cement—have been shown to slightly improve performs its primary function as a hydraulic certain characteristics and properties of portland cement binder. Limestone additions that are not cement, including increased rheological perfor- “optimized,” or ground finer as the limestone mance (pumping, placing, and finishing ability); replacement increases, can increase permeability decreased water demand; and decreased drying and sorptivity, reduce abrasion resistance, and shrinkage. Therefore, a shotcrete placement where have a higher water demand; thus, producing an overall loss in durability when compared to using Table 1: Current allowable limestone limits portland cement alone.8-12 in cement It seems logical that if “optimizing” fineness Current cement standard Maximum is an essential factor in producing limestone specifications Allowable cement with similar or equivalent properties to ASTM C150 Up to 5% the parent portland cement, more stringent fine- ASTM C595 From 5 to 15% ness requirements should be mandated. Excep- tions to the optimized fineness requirement could ASTM C1157 No limits 10 Shotcrete • Fall 2013


2013FallShotcreteEMag
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