Lightweight aggregates have been used in construction since before the days of the Roman Empire. The earliest types of lightweight concrete were made by using Grecian and Italian vesicular volcanic aggregates as lightweight aggregates. Ordinary hydrated burned lime was used as the cementitious material in the mix. These early lightweight concretes, by reason of the obviously weak materials, fell short in structural strength and performance of what we expect and achieve today. They were, however, amazingly durable, and existing examples of these early lightweight concretes can still be found in various early structures in the Mediterranean area.
The Romans, in their militaristic expansion and colonization, introduced and established the use of concrete wherever and whenever suitable local materials were available, such as limestone for preparation of quick lime as a cementing material.
Many different materials have been used for lightweight concrete since Roman days, and some have shown remarkably better structural qualities than the Roman materials. Up to the 20th century, however, such improvements were associated with the strength of the cementing materials used rather than with improvements to the lightweight aggregates.
The first such improvement to the strength of the concrete came about in the course of Roman colonization when their need for widely scattered building activity forced the use, on some occasions, of various impure grades of limestone for the preparation of required burned lime. In those instances where the impurities happened to be silica, alumina and iron oxides, the strength of concrete was found to be substantially greater than where pure limes were used. This superior material was referred to as gray lime, and subsequently became known throughout Europe as Roman cement.
This material was the first cement to demonstrate hydraulic properties, or the first capable of hardening under water, but it was not a consistent performer. The strength varied widely between production localities and the particular composition of the impure limestone's used. Even so, Roman cement was used in all concrete work where hydraulic properties and water-tightness were required from that time until the first half of the nineteenth century.
In 1824 Joseph Aspdin, an English bricklayer, conceived the idea of intermixing pulverized raw limestone and pulverized impure siliceous materials in varying percentages. He then subjected these mixtures to the high temperatures of coke fires, and pulverized the resultant clinkers. Aspdin thereby determined the approximate raw compounding required for maximum strength, and likewise removed for all the time the dangerous inconsistencies of the older Roman cements. This was the birth of that highly valuable building material that we rely upon today for all major construction: Portland cement, so called because when hardened it resembled a popular building stone quarried on the Isle of Portland, off the coast of England.
Before Aspdin's time, cements were relatively weak, so weak in fact, that the aggregates, both heavy and light, had greater structural strength than the cementing binder used. And heavyweight and lightweight concretes had been roughly comparable in performance.
Aspdin's Portland cement provided a cementing medium that for the first time was capable of exceeding the structural strength of lightweight aggregates used at that time. Because of their low stiffness and their consequent tendency to shear and crush under compressive stress, the lightweight concretes produced from natural lightweight aggregates could not achieve the strength of heavyweight rock and sand concretes. This differential in performance was intensified as additional cement improvement came with refinements in grinding equipment, the introduction of the rotary kiln to cement production, and closer scientific control in the processes of manufacture.
Inasmuch as very substantial economies, greater versatility in concrete uses, and many other advantages can result from decreasing the weight of concrete, innumerable new materials were used as lightweight aggregates in an attempt to capitalize on the greater strength of cement. Pumice, scoria, volcanic cinders, and vesiculated lava were among the natural lightweight aggregates used. None, however, was capable of providing structural qualities matching that of heavyweight concrete.
