Research method

Development of Material Design Method for
Autoclaved Lightweight Aerated Concrete

X-ray diffraction/
scattering

Analysis of formation mechanism of crystalline component tobermorite

Role of SPring-8

Achievements

Clarification of formation mechanism of crystalline component tobermorite* byin-situ observation of synthesis reaction of autoclaved lightweight aerated
concrete (ALC)**
Paving the way for material design of ALC and control of its synthesis reaction
R&D facilities: Asahi Kasei Corporation and Asahi Kasei Construction Materials Corporation*Tobermorite: This is a crystalline calcium silicate hydrate, which forms the framework of ALC. It has high strength and durability in that it does notchemically change under heat or water. Tobermorite was first discovered in Tobermory (Scotland) in 1880 as a mineral, and it was named after thetown. After that, the crystals existing in concrete produced under steam were found to be the same as those of the naturally occurring tobermorite.
**Autoclaved lightweight aerated concrete (ALC): When a main material composed of silica, cement, and quick lime (calcium oxide) is mixed
with water and a foaming agent (aluminum), the mixture foams and hardens similarly to a sponge cake and produces a porous concrete. Theweight of ALC is one-fifth that of conventional concrete, and ALC has a high heat insulation and a high fire resistance. In Japan, ALC has become
widespread since the 1960s as a building material. Nowadays, it is used for various purposes in many buildings as wall, floor, and partition materials.

BackgroundResults

The quality and quantity of tobermorite
crystals determine the strength and durability ofALC. To grow platelike large crystals, the heat andwater should be uniformly distributed in thematerial, and a condition under which the many
air bubbles are generated as a space for crystal
growth is necessary. To this end, ALC as a building
material is produced using a high-temperatureand high-pressure steam sterilizer (autoclave).
Recently, significantly increased performanceof ALC has been demanded. However, as thechemical reaction occurring in the autoclave
cannot be observed from the outside, changing
the mixing ratio of the raw materials and synthesis
conditions through trial-and-error was the only
method adopted to improve the performance ofthe ALC.

An autoclave cell was placed in theexperimental hutch of a SPring-8 beamline. By
irradiating the high-intensity X-rays on the material
produced in the autoclave cell, the diffraction angle
was measured to examine the contents of thesynthesized material.
From the above examination, the formation
mechanism of the tobermorite has been graduallyrevealed. It was considered that tobermorite wasgenerated from calcium silicate hydrate (CSH)
synthesized as a result of the reaction between
silica and quick lime. In addition to this process,the existence of a route for generating tobermorite
from monosulfate, a sulfate compound, via hydroxyl
ellestadite (HE) was clarified. Furthermore, the timing
at which the raw materials change into tobermorite
was determined.
It became possible to proceed with thematerial design of ALC using the results obtainedin this study as a guideline.

Publication: J. Kikuma et al.; Journal of Synchrotron Radiation 16, 683-686 (2009)

Structure of ALC

X-ray diffraction experiment at SPring-8

In ALC, many air bubbles are dispersed and they are connectedby fine pores. The air bubbles and fine pores comprise 80%of the total volume. Because
ALC contains much air, similar
to a down jacket, it has a highheat insulation capability.
Tobermorite crystals grow at
the surface of the air bubbles
and fine pores.

Two-dimensional pixel detector, PILATUS

X-ray diffraction pattern observed in formation
reaction of ALC

PILATUS measures the X-rays diffracted by the material produced in theautoclave cell with a high time resolution.
PILATUS is a highly sensitive two-dimensional X-ray detector jointly
developed by the Japan Synchrotron Radiation Research Institute (JASRI)
and the Paul Scherrer Institute (PSI) in Switzerland.

Change in mineral phase in formation reaction of ALC

The X-rays diffract upon irradiation on a sample product. Each material
has a unique diffraction angle, which can be used for identification. By
observing the X-ray diffraction pattern, it is possible to identify when
tobermorite is generated.

PILATUS

Tobermorite (002)

Tobermorite crystal

Furnace

0.5 mm

2 μm

Autoclave cell

400

Diffraction intensity

Air bubble

Concrete component
containing tobermorite

Hydroxyl ellestadite (HE)
Anhydrite
Tobermorite (220)Tobermorite (002)slaked lime
Monosulfate
Silica (Quartz)

Diffraction angle

3002001000Time (min)Diffraction intensity

High-intensity
X-ray at SPring-8

Air bubble

As the amounts of silica and slaked lime (calcium hydroxide)
decrease, more tobermorite is generated. This finding supports theCSH synthesis route, which is known conventionally. In addition, theexistence of an intermediate product called monosulfate wasrevealed. A route in which the monosulfate disappears in anintermediate stage and tobermorite is then synthesized throughhydroxyl ellestadite (HE), was found.

Diffraction angle 2θ020

100200300

Time (min)40050021